Quantum Chemical Studies of the Fukui Function as a reactivity … · 2019. 2. 6. · Quantum...

Quantum Chemical Studies of the Fukui Function as a reactivity index, Conformation and Light Harvesting Efficiency of 1-Methoxy-4-Propylbenzene as -Linker with Donor-Acceptor variations effect for DSSCs performance

V. Sivagamia, *M. Karnanb and M. Anuradhaa

aAssistant Professor, bAssociate Professor, PG & Research Department of Physics Srimad Andavan Arts & Science College, Trichy-620 005, Tamil Nadu, India

[email protected] [email protected]*

ABSTRACT: The FT-IR and FT-Raman spectra of 1-Methoxy-4-Propylbenzene (1M4PB) were recorded and analyzed. The vibrational frequency calculations and molecular geometry optimization of 1M4PB were carried out with Gaussian 0W software packages developed by Frisch and co-workers. The Fukui function fˉ is studied as reactivity indices for electrophilic substitution reactions on 1-Methoxy-4-Propylbenzene. The molecular electrostatic potential (MEP) and Mullikan atomic charges were obtained from DFT calculations using the B3LYP/ 631G+d and 6311G++d,p basis set. The NLO property of the titled compound is obtained. The correlations between the statistical thermodynamics and temperature are also obtained. It is seen that the heat capacities, entropies and enthalpies increase with increasing the interactions of the molecular vibrations. Several approaches have recently been proposed to increase the efficiency of solar cells above the theoretical limit. The calculated HOMO and LUMO energies shows the charge transfer occurs within the molecule. The electronic transitions and the UV-Vis absorption spectra of 1M4PB are obtained theoretically. The application of methoxybenzene compounds in solar cells technology for improvement of sun-light harvesting and their efficiency (LHE) using series of organic sensitizers including donors and binary linker conjugated bridges are investigated using DFT. The LHE of the titled molecule and their dyes are predicted using TD-DFT.

Keywords: 1-Methoxy-4-Propylbenzene, HOMO - LUMO, Fukui function, Light Harvesting Efficiency, FT-IR and FT-Raman

I. INTRODUCTION

Methoxybenzene [1] is more electron rich than benzene because of the resonance effect of methoxy group upon the aromatic ring and it reacts with electrophiles in the electrophilic aromatic susbstitution reaction more quickly than benzene. Methoxybenzene is a recognized monosubstituted benzene derivative, which has an asymmetric substituent attached to the phenyl ring. It is of considerable interest owing to the environmental concern and also as a model compound for a lot of chemically and biologically interesting system [2]. Methoxybenzene and many of its derivatives are found in natural and artificial fragrances. It is used in perfumery, an insect pheromone, as flavouring in food and in the manufacture of other chemicals [3, 4].

It is known that -bonded molecules containing methoxy groups can give rise to a back-donation of electrons, that is the oxygen releases part of the electronic charge of its lone pairs and injects it in the * orbitals of the adjacent C-H bonds [5]. As a consequence, the charge on the hydrogen decreases, the bond lengthens and its strength weakens. Experimental and theoretical evidence suggests that the process is mostly effective with the hydrogen atoms in trans-conformation with respect to the lone pair orbitals. The vibrational frequencies and the infrared intensities of the C-H modes involved in back-donation are strongly affected. If the intensity is interpreted in the light of the ECCF (Equilibrium Change and Charge Fluxes) [6, 7] these changes due to back-donation cause the softening in the frequency of one of the C-H stretching modes and an increase in its intensity. However, literature survey reveals that to the best of our knowledge, no experimental and computational vibrational spectroscopic study using DFT (B3LYP) with 6-31+G (d) and 6-311++G (d,p) in 1-M4-PB is published so far.

The functional groups present in methoxybenzene leads to the variation of charge distribution in the molecule and consequently affect the structural, vibrational and electronic parameters. In this work, we present a detailed spectral investigation of the title compound. The reactivity descriptors, defined within the framework of DFT are global hardness, global electrophilicity, chemical potential, local softness, Fukui functions etc [8]. The reactive nature of the titled compound is understood by calculating the global reactivity descriptors. This study is also focus on complete description of excitation energy, HOMO-LUMO energies and first order

JASC: Journal of Applied Science and Computations

Volume VI, Issue II, February/2019

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hyperpolarizability and molecular electrostatic potential (MEP) of 1-M4-PB. Recently, several new materials used in organic solar cells have been studied and developed. Among them, dye sensitized solar cells (DSSCs) are currently receiving significant attention, due to their potentially low production cost, flexibility and high energy conversion efficiency [1]. The electronic properties, optical properties and thermodynamic properties at different temperatures (heat capacity, Gibb’s free energy, entropy and enthalpy) of the titled compound are also investigated.

II. EXPERIMENTAL DETAILS

The titled compound 1M4PB was purchased from Sigma Aldrich Chemical Company with a purity of %. The FT-IR and FT-Raman spectra of 1M4PB are recorded without any further purification. The FT-IR spectrum of the compound is recorded in the range of 4000 – 400 cm-1 using Bruker IFS 66V spectrometer with a MCT detector, KBr beam splitter and global source at a resolution of 2 cm-1. The FT-Raman spectrum of 1M4PB is also recorded using the same instrument with FRA -106 Raman accessories in the region of 3500 – 100 cm-1 with the source of Nd:YAG laser operating at 106 nm excitation wavelength at 200 mW power.

III. COMPUTATIONAL DETAILS

The vibrational frequency calculations and molecular geometry optimization of 1M4PB were carried out with Gaussian 0W software packages developed by Frisch and co-workers [9]. The Becke’s three parameter hybrid functional with the Lee-Yang-Parr correlation functional method (B3LYP), one of the most robust functional of the hybrid family, was herein used for all the calculations, combined with standard 6-31+G (d) and 6-311++G (d,p) basis sets [10, 11]. All the parameters were allowed to relax and all the calculations converged to an optimized geometry which corresponds to a true minimum, as revealed by the lack of imaginary values in the wave number calculations. The Cartesian representation of the theoretical force constants has been computed at the fully optimized geometry. The symmetry of the molecule was also helpful in making vibrational assignment. By combining the results of the GAUSSVIEW program with symmetry considerations, vibrational frequency assignments were made with a high degree of confidence. There is always some ambiguity in defining internal coordination. However, the defined coordinate form complete set and matches quite well with the motions observed using the GAUSSVIEW program. The systematic comparison of the results from DFT theory with results of experiments has shown that the method using B3LYP functional is the most promising in providing correct vibrational wave numbers [12].

IV. RESULT AND DISCUSSIONS

A. Molecular Geometry

The optimized geometrical parameters were calculated by Becke3-Lee-Yang-Parr (B3LYP) with 6-31+G (d) and 6-311++G(d,p) basis sets using GAUSSIAN 09. The maximum number of potentially active observable fundamentals of non-linear molecule having N atoms is equal to (3N-6), apart from the three translational and three rotational degrees of freedom [13]. The molecule does not possess any rotational, inversion or reflection symmetry, the molecule is considered under C1 point group symmetry. The most optimized structural parameters such as bond length, bond angle and dihedral angle of 1M4PB are also calculated and it is shown in Table I in accordance with the atom numbering scheme is shown in Fig.1. By comparing the computed bond lengths and bond angles at B3LYP with 6-31+G(d) and 6-311++G(d,p) these values are more or less same with the experimental values. But the theoretical calculations are executed upon isolated molecule in the gaseous phase and the experimental results are executed to the molecules in solid state [14].

The phenyl ring appears to be little distorted and the angles are slightly out of the perfect hexagonal structure due to the substitution of O-CH3 group instead of H atom. The optimized bond lengths of the six C-C bonds of the ring are in the order of C4-C5>C1-C6>C2-C3>C1-C2>C3-C4>C5-C6. The calculated bond length of C1-O7 is 1.368Å which is just 0.002 Å lower than the reported experimental value of 1.370 Å [15]. The optimized bond angles are in the order of C3-C4-C5<C2-C1-C6<C1-C2-C3<C1-C6-C5<C4-C5-C6<C2-C3-C4. The bond angle of C2-C1-C6 is 2.279 Å compressed than the bond angle of C4-C5-C6, due to the effect O-CH3 at C1 [16].



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Table I:

The optimized geometrical parameters of 1-Methoxy-4-propylbenzene by DFT/B3LYP method using 631G+d and 631G++d,p basis sets.

*Ref. [14]

Parameters

Bond Length (A˚)

Parameters

Bond Angle (A˚)

Parameters

Dihedral angles(A˚)

6-31+ G(d)

6-311++ G(d,p)

Exp.* 6-31+G(d) 6-311++ G(d,p)

Exp.*

6-31+G(d) 6-

311++G(d,p)

C1-C2 1.395 1.399 1.362 C2-C1-C6 119.361 119.416 120.7 C6-C1-C2-C3 0.203 0.163

C1-C6 1.400 1.404 1.384 C2-C1-O7 124.756 124.745 C6-C1-C2-H12 -179.617 -179.665

C1-O7 1.368 1.370 1.370 C6-C1-O7 115.883 115.838 O7-C1-C2-C3 179.934 179.902

C2-C3 1.399 1.402 1.427 C1-C2-C3 119.564 119.522 120.8 C2-C1-C6-H25 179.441 179.487

C2-H12 1.082 1.085 1.08 C1-C2-H12 121.179 121.224 O7-C1-C6-C5 -179.970 -179.901

C3-C4 1.394 1.397 1.385 C3-C2-H12 119.257 119.254 C6-C1-O7-C8 -179.836 179.970

C3-H13 1.086 1.088 C2-C3-C4 121.995 122.032 119.9 C1-C2-C3-H13 -179.739 -179.755

C4-C5 1.404 1.407 1.363 C2-C3-H13 118.581 118.544 H12-C2-C3-C4 179.889 179.893

C4-C14 1.514 1.516 C4-C3-H13 119.424 119.424 C2-C3-C4-C14 178.981 179.145

C5-C6 1.386 1.390 1.440 C3-C4-C5 117.330 117.292 H13-C3-C4-C5 179.492 179.517

C5-H24 1.086 1.089 C3-C4-C14 121.885 121.915 H13-C3-C4-C14 -1.216 -1.041

C6-H25 1.084 1.086 C5-C4-C14 120.781 120.790 C3-C4-C5-H24 -179.341 -179.365

O7-C8 1.420 1.420 C4-C5-C6 121.640 121.675 121.4 C14-C4-C5-C6 -179.003 -179.127

C8-H9 1.096 1.098 C4-C5-H24 119.478 119.453 C14-C4-C5-H24 1.359 1.187

C8-H10 1.089 1.092 C6-C5-H24 118.881 118.871 C3-C4-C14-C15 18.771 17.863

C8-H11 1.096 1.098 C1-C6-C5 120.109 120.062 118.5 C3-C4-C14-H16 134.622 133.789

C14-C15 1.096 1.099 C1-C6-H25 118.619 118.695 C3-C4-C14-C17 -103.648 -104.531

C14-H16 1.096 1.099 C5-C6-H25 121.271 121.242 C5-C4-C14-C15 -161.961 -162.714

C14-C17 1.544 1.546 C1-O7-C8 118.451 118.430 C5-C4-C14-H16 -46.109 -46.788

C17-H18 1.096 1.099 O7-C8-H9 111.452 111.362 C5-C4-C14-C17 75.621 74.892

C17-H19 1.096 1.099 O7-C8-H10 105.900 105.804 C4-C5-C6-H25 -179.687 -179.726

C17-C20 1.531 1.533 O7-C8-H11 111.483 111.381 H24-C5-C6-C1 179.601 179.579

C20-H21 1.094 1.096 H9-C8-H10 109.282 109.393 C1-O7-C8-H9 61.113 61.178

C20-H22 1.093 1.098 H9-C8-H11 109.366 109.430 C1-O7-C8-H10 179.853 179.945

C20-H23 1.095 1.097 H10-C8-H11 109.271 109.387 C1-O7-C8-H11 -61.403 -61.284

Fig. 1 Optimized molecular structure of 1-Methoxy-4-Propylbenzene



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B. Vibrational Assignments:

The spectroscopic signature of the titled compound is obtained by performing a frequency calculation analysis. The compound 1-M4-PB consists of 25 atoms and its has 69 normal modes which are distributed as,

= 47 A’ (in plane) + 22 A” (out of plane)

The geometry of the titled compound is possessing C1 point group symmetry. The vibrational assignments of 1-M4-PB along with the calculated FT-IR and FT-Raman frequencies are presented in Table II. The experimental and the calculated FT-IR and FT-Raman spectrum are shown in Fig. 2 and Fig. 3.

1) Methyl group Vibrations:

For the assignments of CH3 group frequencies, basically nine fundamental can be associated with each CH3 group, namely CH3 ss (symmetric stretching), CH3 ass (asymmetric stretching), CH3 ips (in-plane stretching), CH3 ipb (in-plane bending), CH3 opb (out-of-plane bending), CH3 sb (symmetric bending), CH3 ipr (in-plane rocking), CH3 opr (out-of-plane rocking) and CH3 (twisting) modes. Generally methyl groups are referred as electron donating substituent in the aromatic ring. For the methoxy group compounds [17, 18], the asymmetric and symmetric stretching modes of vibrations are appear in the range of 2860 – 2935 cm-1

and 2825 - 2870 cm-1

respectively. The FTIR bands are observed at 3016, 2964, 2948 and 2898 cm-1 and the Raman

spectrum are observed in 3015, 2950, 2934 and 2902 cm-1 for CH3 stretching vibrations of 1M4PB. The

theoretically computed values of 3130, 3084, 3079, 3057, 3016 and 3001 cm-1 by DFT/B3LYP/6-311++G (d,p) for asymmetric and symmetric stretching vibrations of methoxy group respectively.

The bending vibrations are usually observed at 1450 cm-1 for methyl substituted benzenes [19]. As expected, the bands at 1599, 1516 cm-1 in FT-IR and 1593, 1531 cm-1

in FT-Raman are observed due to the CH3 bending vibrations. The calculated values by DFT/B3LYP/6-311++G (d,p) for bending vibrations are obtained in 1505, 1499, 1495 and 1492cm-1 respectively. These theoretically calculated values also coincide well with the experimental values. In the FT-Raman spectrum, a band observed at 1418 cm-1 is assigned to CH3 in-plane rocking vibration. The calculated value by DFT/B3LYP/6-311++G (d,p) method at 1475 and 1411 cm-1 shows good agreement with the experimental data.

2) CH2 Vibrations:

The each CH2 group have six fundamental vibrations namely symmetric stretching, asymmetric stretching, scissoring, wagging, twisting and rocking modes. In titled compound 1M4PB, the methylene bridge gives four (2 symmetrical, 2 asymmetrical) stretching and the couple of scissoring, wagging, rocking and twisting modes [20]. Generally for CH2, the asymmetric stretching modes are appearing in the range of above 3000 cm-1 and the symmetric stretching modes appear in the range of 3000-2900 cm-1[21, 22]. In FT-IR the band is observed at 2632cm-1. The theoretically computed values 3055, 3033, 3020 and 3009 cm-1 by DFT/B3LYP/6-311++G (d,p) for asymmetric and symmetric stretching vibrations of CH2 respectively. The CH2 scissoring mode has been observed at 1488 cm-1 in FT-IR and 1609 cm-1 in FT-Raman spectra. The band at 1305 cm-1 and 1273 cm-1 in FT-Raman are assigned to CH2 wagging and twisting vibrations. The peak observed at 742 cm-1 in FT-IR is for CH2 rocking vibration.

3) C-H Vibrations:

The aromatic structure shows the presence of C-H stretching vibration in the region 3000 - 3100 cm-1 [23]. The expected stretching vibrations are assigned at 3083, 3062, 3060, 3049 and 3033 cm-1. These assignments are in good agreement with the literature data. The C-H in-plane and out-of-plane bending vibrations are generally observed in the range 1000 - 1300cm-1 and 1000 - 750 cm-1 [24]. In the titled compound, the in-plane bending vibrations are observed at the bands 1193 and 1000 cm-1 and the out-of-plane bending vibration are observed at 848 cm-1. The theoretically computed frequencies for C-H vibrations by DFT/B3LYP/6-311++G (d,p) method shows excellent agreement with the recorded spectral value as well as the literature data.

4) C-C Vibrations:

The (C=C) vibrations are all more fascinating if the double bonds are in conjugation with the ring. The genuine positions are resolved not such a great amount by the idea of substituent but rather by the type of the substitution around the ring [25]. The ring C=C and C-C stretching vibrations are generally occurs in the region



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1625-1400 cm-1 and 1380-1280 cm-1 [26-28]. The C=C stretching vibration is observed at 1671, 1450 cm-1 in Raman spectrum. The C-C stretching vibrations are observed at 1249cm-1 in IR spectrum and the corresponding Raman spectrum are observed at 1370, 1241 cm-1. The theoretically computed frequencies by DFT/B3LYP/6-311++G (d,p) method shows excellent agreement with the experimental value as well as the literature data. The in-plane bending vibrations are observed at 798, 765, 656 and 649 cm-1 and the out-of-plane bending vibrations are observed at 288 cm-1.

5) O-CH3 Vibrations:

The O-CH3 stretching vibration is generally appeared in the region 1100-1000 cm-1 for methoxybenzene and its derivatives. In this titled compound, the computed value by DFT/B3LYP/6-311++G (d,p) method for O-CH3 stretching is observed at 1061 cm-1. Raman Rao et al., [29] have proposed an assignment for the bending vibration of C-O-CH3 is in the region 670-300 cm-1 for methoxybenzene and its derivatives. The theoretically calculated value for in-plane and out-of-plane bending vibration of C-O-CH3 are obtained in 566 and 225 cm-1. The C-O stretching, in-plane vibrations are observed at 1282 cm-1, 588 cm-1 in FT-IR spectrum and the corresponding out-of-plane bending vibration is observed at 112 cm-1 in FT-Raman spectrum. These observed values are in good agreement with the literature data [30] and well coincides with the theoretically calculated values.

Tra

nsm

itta

nce

(%

)

B3LYP/cc-pVDZ

1500 4000 3500 2500 2000 1000 500 3000

B3LYP/6311G++D,P

B3LYP/631G+D

Wavenumber (cm-1)

Fig. 2 Experimental and calculated FT-IR spectra of 1-Methoxy-4-Propylbenzene

OBSERVED



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Table II: Observed FT-IR, FT-Raman and calculated (unscaled and scaled) frequencies and vibrational assignments of 1-methoxy-4-

propylbenzene using DFT/B3LYP-631G+d and 6311G++d,p basis set

No

Observed frequencies (cm1)

Calculated frequencies (cm1)

IR Intensity (km mol1)

Raman Intensity (km mol1)

Vibrational assignments /PED

(≥ 10%) a b

FT-IR FT-

Raman Unscaled Scaled Unscaled Scaled a b a b

1 3083 3220 3091 3201 3088 12.300 9.224 99.038 97.020 -CH (97) 2 3060 3062 3207 3066 3189 3065 9.095 6.324 143.437 140.759 -CH(96) 3 3049 3171 3057 3155 3054 17.865 14.832 76.459 71.595 -CH (98) 4 3033 3170 3040 3154 3038 19.199 15.202 75.406 69.498 -CH (95) 5 3016 3015 3155 3023 3130 3019 26.877 24.480 140.583 135.304 -CH3 as (96) 6 2964 2950 3102 2969 3084 2961 44.228 40.400 112.576 109.110 -CH3 as (94) 7 2948 2934 3097 2952 3080 2945 68.008 64.850 37.618 35.445 -CH3 as (73),-CH2as (25) 8 2898 2902 3081 2906 3058 2904 43.766 40.572 60.351 61.414 -CH3 as (93)

9 2632 3070 2638 3055 2635 16.922 11.972 4.780 2.254 -CH2 as (65),-CH3as(33)

10 3048 2589 3033 2583 1.563 1.001 124.722 125.990 -CH2 as (86),-CH3as(13) 11 3035 2576 3020 2569 84.041 81.131 144.367 46.646 -CH2 s (75), -CH3 s (23) 12 3032 2545 3017 2539 21.323 17.109 87.184 208.886 -CH3 s (66), -CH2 s (32) 13 3023 2450 3009 2447 53.620 4.720 211.016 168.550 -CH2 s (82), -CH3 s (12) 14 2128 3022 2137 3001 2131 21.415 61.218 73.635 160.222 -CH3 s (94) 15 1898 1665 1902 1653 1899 47.822 45.772 69.284 67.755 -CC(ring)(85) 16 1765 1626 1771 1616 1767 10.352 10.838 9.033 9.421 -CC(ring)(82)

17 1671 1557 1672 1542 1676 123.863 134.424 1.898 1.520 -CC(ring)(73), CH3(18)

18 1609 1531 1614 1510 1611 7.561 8.504 3.351 2.308 -CH2(68), -CH3(27) 19 1526 1601 1505 1599 37.851 37.304 6.045 5.234 -CH3(2) 20 1599 1593 1521 1604 1499 1593 8.709 8.202 9.853 8.605 -CH3(88), -CH2(8) 21 1531 1516 1539 1495 1536 0.296 0.379 2.123 1.613 -CH3(68), -CH2(30) 22 1516 1515 1519 1492 1515 7.729 8.863 14.798 13.767 -CH3(86) 23 1488 1508 1492 1487 1487 0.410 0.309 23.828 22.999 -CH2(78), -CH3(12) 24 1492 1478 1475 1475 10.301 7.500 5.143 3.787 - CH3(88), -CH(8) 25 1450 1460 1458 1449 1452 1.208 1.777 1.307 1.205 -CC (85), -CH3(13) 26 1418 1432 1429 1411 1424 1.804 2.210 0.664 0.328 -CH3(97) 27 1396 1395 1380 1390 0.279 0.291 16.380 16.306 -CH2(92)

Fig. 3 Observed and simulated FT-Raman spectra of 1-Methoxy-4-propylbenzene

Wavenumber (cm-1)

B3LYP/6311G++d,p

4000 3000 2000 1000 0

OBSERVED

500 1500 2500 3500

Ram

an i

nte

nsi

ty (

a.u

.)

B3LYP/631G+d



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a – B3LYP/6-31 G+d b- B3LYP/6-311++ G(d,p)

Abbreviations: s-symmetrical, as-asymmetrical, -stretching, -in-plane bending, -out of plane bending, -torsion, -scissoring, -wagging, -rocking and t-twisting

V. CHARGE DISTRIBUTION ANALYSIS

A) Molecular Electrostatic Potential:

The Molecular Electrostatic Potential (MEP) maps are very useful three dimensional diagrams of molecules. They enable us to visualize the charge distribution of molecules and charge related properties of molecules [31]. The MEP at a given point p(x,y,z) in the vicinity of a molecule is the force acting on a positive test charge (a proton) located at p through the electrical charge cloud generated through the molecules electrons and nuclei. The electrostatic potential of a molecule is a good guide in assessing the molecules reactivity towards positively or negatively charged reactants. It is typically visualized through mapping its values onto the surface reflecting the molecules boundaries. MEP was calculated by using the basis set B3LYP/631G+d to

28 1370 1369 1379 1355 1376 21.101 18.873 5.636 4.715 -CC(ring) (66), t-CH2(28)

29 1346 1355 1332 1350 13.280 16.909 3.617 4.791 -CC(ring) (87),t-CH2(12)

30 1330 1324 1320 1316 1.295 1.267 12.313 10.078 t-CH2(82), -CH(17)

31 1305 1325 1318 1314 1312 0.373 0.212 5.746 4.461 -CH2(85), -CH3(12)

32 1282 1286 1279 1271 1275 275.471 261.721 14.663 13.630 -CO(84), - CH3(10)

33 1273 1259 1277 1247 1271 5.642 8.830 0.882 1.135 t- CH2(78), -CH3(13)

34 1249 1241 1233 1242 1226 1239 0.265 0.131 26.422 26.778 -CC(83)

35 1213 1226 1202 1222 17.837 12.421 10.237 7.699 -CH3(86)

36 1193 1208 1197 1197 1191 17.186 25.904 6.151 8.692 -CH(93)

37 1166 1161 1179 1164 1168 1161 1.123 0.650 2.156 2.740 -CH3(88)

38 1149 1143 1139 1139 9.535 10.495 0.337 0.322 -CH(73), -CH2(15)

39 1107 1116 1105 1108 1101 6.609 6.444 10.084 9.180 -CC(66), -CH3(27)

40 1092 1089 1083 1082 2.423 2.277 0.817 0.626 -CH(63), -CH2(18)

41 1071 1068 1062 1061 54.414 56.939 3.714 3.858 -CH3(85)

42 1016 1045 1025 1038 1019 0.537 0.500 9.635 8.206 -CC(86)

43 1000 1030 994 1027 990 0.358 0.276 1.061 0.905 -CH(85)

44 967 968 968 967 0.161 0.142 0.059 0.111 -CH(76)

45 945 941 943 940 0.006 0.003 0.060 0.138 -CH(78)

46 903 901 898 895 0.166 0.179 8.764 8.246 -CH3(72)

47 883 878 877 873 0.442 0.454 0.438 0.470 -CH2(63), -CH3(34)

48 848 854 852 852 848 22.147 21.657 12.724 11.286 -CH (74)

49 835 830 833 826 23.733 24.531 21.401 23.155 -CH (67)

50 798 820 808 817 806 16.222 12.158 4.655 3.081 Ring deformation (71)

51 765 761 759 762 756 9.951 9.434 2.180 2.797 -CC (ring)(65)

52 742 749 745 745 741 1.423 1.488 0.147 0.162 -CH2(68), -CH3(12)

53 656 649 720 659 726 647 2.815 4.282 1.038 1.035 -CC(ring) (69)

54 588 583 653 592 653 586 0.485 0.490 5.496 5.465 -CO(58)

55 566 558 567 553 15.750 16.112 0.893 1.013 -O-CH3(65)

56 529 520 528 518 13.631 14.563 0.552 0.487 -CC(ring) (71)

57 461 459 461 448 0.833 0.988 1.458 1.318 -CC(ring) (67)

58 426 416 425 413 0.048 0.058 0.098 0.117 -CC (59)

59 385 374 385 368 2.054 2.033 3.456 3.445 -CC(62), -Ring (34)

60 348 341 348 340 1.070 1.022 0.680 0.715 -CC (58)

61 288 292 291 291 290 0.234 0.240 3.206 3.205 -CC (51)

62 261 254 251 246 0.259 0.416 0.380 0.486 t-CH3(49)

63 244 243 240 241 238 0.130 0.161 0.254 0.283 t-CH3(53)

64 227 198 225 193 1.815 1.769 1.202 1.235 -O-CH3 (48)

65 160 195 156 192 155 0.450 0.464 1.499 1.468 t-CH3(57)

66 112 103 109 100 104 2.694 2.493 0.725 0.716 -CO (43)

67 83 79 82 78 0.024 0.021 0.061 0.071 -CC(ring) (41)

68 63 59 62 57 1.222 1.302 1.208 1.151 -CC (46)

69 39 36 40 34 0.462 0.483 1.905 1.861 -CC (39)



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indicate the reactive sites of electrophilic and nucleophilic attack for 1-M4-PB molecule. The minimum and maximum limits (9.252e-3) of electrostatic potential of the titled molecule are observed. In the MEP map, the red and yellow regions (negative) were related to electrophilic reacitivity and the blue regions (positive) were to nucleophilic reacitivity and green represents the areas of zero potential. The electrostatic potential value ascends in the order of red < orange < yellow < green < blue [32]. The MEP surface is plotted for the titled compound is shown in Fig.4.

B.Mulliken Atomic Charges:

The Mulliken atomic charges are the total and finest population analysis method. The electron population of each atom of the molecules is identifying, because of calculating the Mulliken charges as explained by the density functional methods. The atomic charge plays an important role in the application of quantum mechanical calculation to molecular systems [33]. It has been used to describe the process of electro negativity equalization and charge transfer in chemical reactions [34], and to model the electrostatic potential outside molecular surfaces [35-37]. The Mulliken atomic charges by different basis sets B3LYP / 631G+(d) and 631G++(d,p) are calculated and also we done a comparison and it is shown in Table III. It is decent to mention that the C2 and C4 atoms of 1M4PB show the positive charge while C1, C3, C14 and C20 atoms shows the negative charges. However, the results can better to represent in graphical form which is shown in Fig. 5.

Table III:

Mullikan population analysis of 1-Methoxy-p-propylbenzene performed at B3LYP/ and B3LYP/6-311++g(d,p)

Atoms

Atomic charges (from basis set)

B3LYP/ 6-

31G+(d)

B3LYP/ 6-

31G++(d,p) C1 -0.086 -0.497

C2 0.528 0.209

C3 -1.043 -0.490

C4 0.470 1.338

C5 -0.255 -0.198

C6 -0.120 -0.406

O7 -0.383 -0.160

C8 -0.325 -0.315

H9 0.202 0.152

H10 0.207 0.175

H11 0.202 0.151

H12 0.174 0.168

H13 0.172 0.161

C14 -0.434 -0.830

H15 0.195 0.145

H16 0.204 0.146

C17 -0.400 -0.141

H18 0.192 0.135

Fig. 4 Molecular Electrostatic Potential surface of

1-Methoxy-4-Propylbenzene



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C) Local reactivity descriptors:

Fukui function [38] is one of the widely used local density functional descriptors to model chemical reactivity and site selectivity. The atom with the highest Fukui function is highly reactive compared to the other atoms in the molecule. Fukui function is defined as the derivative of the electron density (r) with respect to the total number of electrons N in the system, at constant external potential v(r) acting on an electron due to all the nuclei in the system where is the chemical potential of the system.

The electronic chemical potential is the derivative of the total energy E with respect to the electron density. It is more convenient to represent the fukui function values around each atomic site into a single value that characterizes the atoms in a molecule. In a chemical reaction, a change in the number of electrons involves the addition or subtraction of at least one electron in the frontier orbital. Thus, calculating Fukui function helps us to determine the active sites of a molecule, base on the electron density changes experience by it during a reaction. Depending on the electron transfer, three types of Fukui function are defined,

, for nuclephilic attack,

, for electrophilic attack,

, for radical attack

Where q(N) is the charge on the kth atom for neutral molecule while q(N+1) and q(N-1) are the same for its anionic and cationic species respectively. These calculations are performed at the equilibrium geometries

of the neutral charge state of the molecule. The Fukui functions and will give the regions at which the

molecule is most able to accommodate the addition and removal of an electron, respectively.

H19 0.200 0.138

C20 -0.674 -0.635

H21 0.218 0.146

H22 0.197 0.129

H23 0.196 0.131

H24 0.176 0.165

H25 0.188 0.184

Fig. 5 Mulliken atomic charge of 1-Methoxy-4-Propylbenzene



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The large values of will point out the molecular regions most susceptible to nucleophilic attacks,

while large values will be couple with regions susceptible to electrophilic attacks. In a molecular system,

the atomic site which posses highest condense Fukui function favours the higher reactivity. Lee et al. [39] have studied the condensed Fukui function and concluded that the most reactive site during the chemical reaction has the higher value of fk.

Morel et al., [40] have recently proposed a dual descriptor (f(r)), which is defined as the difference between nucleophilic and electrophilic fukui function. It is given by the equation,

]

If fr > 0, then the site is favoured for a nucleophilic attack, whereas if fr < 0, then the site is favoured for an electrophilic attack. According to dual descriptor fr provides a clear difference between nucleophilic and electrophilic attack at a particular site with their sign. It provides positive value for sited prone for nucleophilic attack and negative value prone for electrophilic attack.

The calculated values are listed in Table IV. According to the condition for dual descriptor, the

nucleophilic site for 1M4PB Positive i.e. fr > 0 is C2, C3, C4, C5, C6, O7, C8, C14, C17 and C20. Similarly the electrophilic site Negative i.e. fr < 0 is C1, H9, H10, H11, H12, H13, H15, H16, H18, H19, H21, H22, H23, H24 and H25. During the reaction, behaviour of electrophilic and nucleophilic attack depends on the local behaviour of molecule.

Table IV:

Condensed Fukui functions for 1-Methoxy-4-propylbenzene calculated at B3LYP/6-311G++(d,p) method

V. FIRST ORDER HYPERPOLARIZABILITY

Non linear optics deals with the interaction of applied electromagnetic fields in various materials to generate new electromagnetic fields, altered in wavenumber, phase or other physical properties [41]. Organic molecules are able to manipulate signals efficiently of importance in technologies such as optical communication, optical computing and dynamic image processing [42, 43]. In the fields telephoning, signal transferring and fiber optic cables, NLO enhance the functions for developing technologies like frequency modulation, optical changing, optical controlling and optical logic circuits. The electronic and vibrational contribution to the first order hyperpolarizabilities have been studied theoretically for organic and inorganic

Atom qk(N+1) qk (N) qk(N-1) fkn fk

e fkr Δfr skn

ske sk

r ωk+

ωk- ωk°

C1 0.277 0.188 0.261 -0.016 0.089 0.037 -0.104 -0.006 0.034 0.014 -0.051 0.290 0.119

C2 -0.133 0.091 -0.208 -0.076 -0.224 -0.150 0.148 -0.029 -0.086 -0.058 -0.247 -0.732 -0.490

C3 -0.176 -0.002 -0.239 -0.063 -0.174 -0.119 0.110 -0.024 -0.067 -0.046 -0.207 -0.568 -0.388

C4 0.124 0.387 0.139 0.014 -0.262 -0.124 0.276 0.005 -0.101 -0.048 0.046 -0.858 -0.406

C5 -0.165 -0.077 -0.221 -0.056 -0.088 -0.072 0.031 -0.022 -0.034 -0.028 -0.185 -0.287 -0.236

C6 -0.122 0.137 -0.202 -0.080 -0.259 -0.169 0.179 -0.031 -0.100 -0.065 -0.261 -0.847 -0.554

O7 -0.563 0.230 -0.589 -0.026 -0.793 -0.410 0.767 -0.010 -0.305 -0.158 -0.086 -2.593 -1.339

C8 -0.168 -0.017 -0.126 0.042 -0.151 -0.055 0.193 0.016 -0.058 -0.021 0.136 -0.494 -0.179

H9 0.147 0.012 0.110 -0.037 0.134 0.049 -0.171 -0.014 0.052 0.019 -0.120 0.439 0.160

H10 0.167 0.000 0.104 -0.063 0.167 0.052 -0.231 -0.024 0.064 0.020 -0.208 0.547 0.170

H11 0.147 0.012 0.112 -0.035 0.134 0.050 -0.169 -0.013 0.052 0.019 -0.114 0.439 0.163

H12 0.126 -0.005 0.029 -0.097 0.131 0.017 -0.228 -0.037 0.050 0.006 -0.317 0.427 0.055

H13 0.121 -0.002 0.014 -0.107 0.123 0.008 -0.230 -0.041 0.047 0.003 -0.349 0.401 0.026

C14 -0.318 -0.020 -0.309 0.010 -0.299 -0.144 0.308 0.004 -0.115 -0.056 0.032 -0.977 -0.472

H15 0.127 0.004 0.082 -0.045 0.123 0.039 -0.168 -0.017 0.047 0.015 -0.147 0.403 0.128

H16 0.132 0.015 0.085 -0.047 0.118 0.035 -0.165 -0.018 0.045 0.014 -0.154 0.385 0.116

C17 -0.229 0.046 -0.208 0.021 -0.276 -0.127 0.297 0.008 -0.106 -0.049 0.070 -0.902 -0.416

H18 0.123 0.003 0.067 -0.057 0.120 0.032 -0.176 -0.022 0.046 0.012 -0.185 0.391 0.103

H19 0.128 -0.001 0.116 -0.012 0.130 0.059 -0.142 -0.005 0.050 0.023 -0.039 0.424 0.193

C20 -0.406 0.002 -0.397 0.009 -0.408 -0.200 0.417 0.003 -0.157 -0.077 0.030 -1.335 -0.653

H21 0.146 0.000 0.167 0.021 0.146 0.084 -0.125 0.008 0.056 0.032 0.069 0.479 0.274

H22 0.125 0.000 0.094 -0.031 0.125 0.047 -0.156 -0.012 0.048 0.018 -0.103 0.409 0.153

H23 0.130 0.001 0.079 -0.051 0.129 0.039 -0.180 -0.020 0.050 0.015 -0.168 0.421 0.127

H24 0.122 0.001 0.016 -0.106 0.121 0.007 -0.227 -0.041 0.047 0.003 -0.348 0.396 0.024

H25 0.137 -0.007 0.026 -0.112 0.144 0.016 -0.256 -0.043 0.055 0.006 -0.366 0.471 0.052



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systems. The values of the first order hyperpolarizability were found to be quite large for the pi-conjugated molecules with electron donating and the electron accepting substituent attached to a ring, compared to the monosubsituted molecule [44]. This type of functionalization of organic materials, with the purpose of maximizing NLO properties is still followed by the common way.TED MA

The first order hyperpolarizability of titled compound is calculated using B3LYP/6-31G+(d) and 6311G++(d,p) basis set, based on the finite field approach. In the presence of an applied electric field, the energy of a system is the function of electric field. First order hyperpolarizability () is a third rank tensor that can be described by 3 3 3 matrices. The components are defined as the coefficient in Taylor’s series expansion of energy in an external electric field. When an external electric field is weak and homogeneous, this expansion becomes,

E= E0 - F - FF - FFF+.....

where, E0 is the energy of an unperturbed molecule, F is the field at the origin, , and are the components of dipole moment, polarizability and the first order hyperpolarizabilities respectively. The total static dipole moment (), the mean polarizability (0), the anisotropy of the polarizability () and the mean first order hyperpolarizabilty (0), are defined by using the x, y and z components. It is as follows,

The total static dipole moment is,

The isotropic polarizability is,

The anisotropic polarizability is,

The mean first order hyperpolarizability is,

Where,

Since the output values of the polarizabilities () and first order hyperpolarizability () using Gaussian 09 are reported in atomic units (a.u.). The calculated values are converted into electrostatic units (e.s.u.) (Note: 1 a.u. = 8.639 10-33 e.s.u.)

The total molecular dipole moment and first order hyperpolarizability is 1.1908 Debye and 4.051410-

30 e.s.u., respectively for the monosubstituted molecule and are depicted in Table V. Total dipole moment of 1M4PB compound is slightly lesser than those of Urea and first order hyperpolarizability of 1M4PB is 11 times greater than those of Urea ( and of urea are 1.3732 Debye and 0.372810-30 e.s.u) for the basis set B3LYP/6-31+G(d). By using different donors and acceptors and the titled compound is as - linker, the better results are obtained. The values of total dipole moment and first order hyperpolarizability for 1-M4-PB1 is 4.709 Debye and 1.93310-29 e.s.u, 1-M4-PB2 is 2.1348 Debye and 8.021610-30 e.s.u., 1-M4-PB3 is 3.6027 Debye and 8.313510-30 e.s.u., 1-M4-PB4 is 4.6815 Debye and 2.044210-29 e.s.u., 1-M4-PB5 is 7.1626 Debye and 1.157610-29 e.s.u., and for 1-M4-PB6 is 2.7021 Debye and 1.110110-29 e.s.u., By comparing the values of the total dipole moment and first order hyperpolarizability with other dyes and the monosubstituted molecule, the better results were obtained in 1-M4-PB4 (ie. Thiophene and Cynate are substituted as donor and acceptor) which is 4.6815 Debye (4 times greater than those of Urea) and 2.044210-29 e.s.u., (55 times greater than those of Urea) respectively.



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Table V:

The DFT/ B3LYP/6-31G+(d) and 6311G++(d,p) calculated electric dipole moments (Debye), Dipole moments compound, polarizability (in a.u), β components and βtot (1030esu) value of 1-Methoxy-4-propylbenzene

(NON LINEAR OPTICAL PROPERTIES)

VII. THERMODYNAMIC PROPERTIES

The total energy of a molecule is the sum of translational, rotational, vibrational and electronic energies, ie., E = Et+Er+Ev+Ee. The statistical thermo chemical analysis of 1M4PB is carried out. On the basis of vibrational analysis, the thermodynamic parameters such as heat capacity (Cp), enthalpy (H-E)/T, Gibb’s free energy (G-E)/T and entropy (S) for different temperature ranges (50K – 500K) are determined and the results are shown in Table VI. As the temperature is increases for various ranges (50K – 500K), the values of thermodynamic parameters are increases. Due to this fact the molecular vibrational intensities of the titled molecule are also increases with temperature. The correlation equations between heat capacity, enthalpy, Gibb’s free energy, entropy and temperatures were fitted by parabolic equations and the corresponding fitting factors (R2) for these thermodynamic properties are 0.9996, 0.9995, 0.9859 and 0.9948 respectively. The regression coefficient is also given in the parabolic equation.

(Cp0) total = 8.07663 + 0.11701 T + 1.251x10-5 T2 (R2 = 0.99967)

(H0-E00) / T = 7.10502 + 0.01689T + 6.29863 x10-6 T2 (R2 = 0.9995)

(G0-E00) / T = - 49.4632 - 0.08763 T + 9.08376 x10-5 T2 (R2 = 0.9859)

(S0) total = 56.56841 + 0.10451 T - 8.45344 x10-5 T2 (R2 = 0.99482)

All these thermodynamical data gives the helpful information for further study of the titled compound. The variations of such thermodynamic parameters relative with temperature are graphically shown in Fig 6.

Parameters

1-M4-PB B3LYP/

6-31+G(d)

1-M4-PB B3LYP/

6-311++G(d,p)

1-M4-PB1 631G+d

(Ben+CN)

1-M4-PB2 631G+d

(Ben+COOH)

1-M4-PB3 631G+d

(Ben+NO2)

1-M4-PB4 631G+d

(Thio+CN)

1-M4-PB5 631G+d

(Thio+COOH)

1-M4-PB6 631G+d

(Thio+NO2)

x -0.2406 -0.2566 -4.387 1.2263 -3.5964 4.5372 7.1121 2.056

y -1.1272 -1.2337 1.693 -1.646 0.182 1.1531 -0.8395 1.6317

z -0.2989 0.0823 -0.259 0.5866 0.1105 0.0069 0.1265 0.6418

tot 1.1908 1.2628 4.709 2.1348 3.6027 4.6815 7.1626 2.7021

xx -61.2715 -61.3905 -119.900 -119.49 -138.2568 -120.3716 -116.1288 -130.5252

yy -65.4652 -64.4982 -80.159 -104.04 -98.1405 -77.6084 -114.9101 -120.0995

zz -71.0304 -71.5178 -94.886 -121.74 -127.2529 -99.239 -117.1787 -120.6129

xy 5.7003 5.4196 7.391 5.2924 6.5739 -6.1278 9.4011 -18.3975

xz -0.2912 -0.9013 -1.098 -3.7461 0.4284 -0.0079 -1.7616 2.4247

yz 1.7949 -0.0401 1.894 1.3094 -1.6652 -0.0108 -0.7117 -0.4226

Δ(esu) -65.922367 -65.802167 -98.3146 -115.09 -121.21673 -99.073 -116.07253 -123.74587

xxx -43.1131 -54.4837 -257.259 24.2517 -127.6841 275.0819 145.7147 120.3232

yyy 4.1315 2.9119 31.987 -62.11 -57.3227 25.3412 -56.8341 -18.4725

zzz 4.1476 4.6182 -3.040 1.2212 1.8908 0.0077 -1.0582 4.2225

xyy -6.9792 -4.8298 11.508 -19.005 8.0242 -2.7981 -35.0692 4.6039

xxy -17.5342 -18.3532 33.087 -33.162 32.8549 21.093 -28.6515 89.7202

xxz -1.8178 4.3758 -4.255 9.9696 1.908 0.0547 3.418 10.4915

xzz -2.3886 -2.8059 -2.286 -10.994 12.2574 -4.4092 15.2394 7.6574

yzz -1.943 -0.9924 7.937 -10.248 -4.2854 10.0896 -4.9012 -4.9615

YYZ 0.605 -0.611 -5.757 8.1524 2.0948 0.1175 2.0318 -3.2713

XYZ 1.1321 1.0009 -14.698 11.5403 3.0328 -0.1671 -0.5186 4.8391

tot(esu) 4.0514E-30 4.8385E-30 1.933E-29 8.0216E-30 8.3135E-30 2.0442E-29 1.1576E-29 1.1101E-29



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Table VI:

Calculated thermodynamical parameters of 1-methoxy-4-propylbenzene for 631G+(d) and 6311G++(d,p) basis set

VIII. ELECTRONIC AND UV-VIS SPECTRAL PROPERTIES

A) Electronic Properties:

Among electronic applications of these materials is their use as organic solar cells, we note that theoretical knowledge of the HOMO and LUMO energy levels of the components is studying organic solar cells. Both HOMO and LUMO are the main orbital taking part in chemical reaction. The HOMO and LUMO energy levels of the donor and the acceptor components for photovoltaic devices are very important factors to determine whether the effective charge transfer will happen between donor and acceptor [45]. Here HOMO energy indicates the ability of electron giving and LUMO indicates the ability of electron accepting.

The calculated frontier orbitals HOMO, LUMO and band gaps by using B3LYP/6-311++G(d,p) basis set of the titled compound and also for six dyes are listed in Table.7 and the corresponding HOMO-LUMO images are shown in Fig.7. The values of energy gaps are 5.82365 eV for 1M4PB, 4.6114 eV for 1M4PB1, 4.2942eV for 1M4PB2, 4.0575eV for 1M4PB3, 3.8398 eV for 1M4PB4, 3.5761 eV for 1M4PB5, 3.2501 eV for 1M4PB6. The calculated energy gap (Eg) of the studied compounds increases in the order of 1M4PB6 < 1M4PB5 < 1M4PB4 < 1M4PB3 < 1M4PB2 < 1M4PB1 < 1M4PB. The much lower band gap of 1M4PB6 is compared to that of 1M4PB indicates a significant effect of intramolecular charge transfer, which would make

Temp.

(K)

631G+(d) 6311G++(d,p)

Heat

Capacity(Cp)

(cal/mol-Kelvin)

Enthalpy

(H-E)/T

(cal/mol-K)

Gibb’s Free Energy

(G-E)/T

(cal/mol-Kelvin)

Entropy

S (cal/mol-

Kelvin)

Heat

Capacity(Cp)

(cal/mol-Kelvin)

Enthalpy

(H-E)/T (cal/mol-

Kelvin)

Gibb’s Free Energy

(G-E)/T

(cal/mol-Kelvin)

Entropy

S (cal/mol-

Kelvin)

50 14.075 8.085 -52.387 60.472 14.166 8.092 -52.385 60.477

100 20.116 8.775 -58.141 66.916 20.240 8.803 -58.140 66.943

150 25.846 9.691 -61.481 71.172 25.946 9.738 -61.482 71.220

200 31.556 10.687 -63.835 74.522 31.662 10.753 -63.837 74.590

250 37.680 11.720 -65.652 77.372 37.833 11.804 -65.655 77.459

298.5 44.202 12.773 -67.131 79.904 44.425 12.875 -67.135 80.010

350 50.856 13.837 -68.379 82.216 51.148 13.957 -68.383 82.341

400 57.356 14.909 -69.458 84.366 57.705 15.047 -69.462 84.509

450 63.508 15.985 -70.407 86.393 63.896 16.142 -70.412 86.554

500 69.213 17.066 -71.256 88.322 69.626 17.241 -71.261 88.502

Fig. 6 Correlation graphics of heat capacity, enthalpy, Gibb’s free energy and

entropy at various temperatures for 1-Methoxy-4-Propylbenzene



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the absorption spectra red shifted. However, the band gap value of 1M4PB6 has the most outstanding photo physical property (smaller than that of other Dyes and the titled compound) than the others.

B) Global Reactivity Descriptors

The global reactivity descriptors are widely used to understand the global nature of the molecules in terms of their stability and it is possible to obtain knowledge about the reactivity of the molecules. Based on density functional descriptors and global chemical reactivity descriptors of compounds such as hardness, chemical potential, softness, electronegativity and electrophilicity index as well as local reactivity have been defined. From Koopman’s theorem, the ionization potential (I) and electron affinity (A) are the Eigen values of HOMO and LUMO with the change of sign [46] Ionization potential (I) = - E HOMO

Electron affinity (A) = - E LUMO Where I and A are the ionization potential and electron affinity of the compounds respectively.

Electron affinity refers to the capability of a ligand to accept precisely one electron from a donor. However in many kinds of bonding viz. covalent hydrogen bonding, partial charge transfer takes place. Ionization energy is a fundamental descriptor of the chemical reactivity of atoms and molecules. High ionization energy indicates high stability and chemical inertness and small ionization energy indicates high reactivity of the atoms and molecules. Absolute hardness and softness are the important properties to measure the molecular stability and reactivity. It is apparent that the chemical hardness fundamentally signifies the resistance towards the deformation or polarization of the electron cloud of the atoms, ions or molecules under small perturbation of chemical reaction.

Using Koopman’s theorem for closed shell compounds, hardness (), softness (S) and chemical potential () can be defined as,

----------1 ----------2 and ----------3

EHOMO = -5.7257eV ELUMO = 0.09795eV E = 5.82365eV

Fig. 7 a) HOMO – LUMO plot of 1-Methoxy-4-Propylbenzene



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Fig. 7 b) HOMO- LUMO plot of D--A

1-M4-PB-2

EHOMO = - 5.91812eV E = 4.2942eV ELUMO = -1.623952eV

EHOMO = - 6.0974eV ELUMO = -1.4860eV E = 4.6114 eV

EHOMO = -6.29236eV E =3.5761 eV ELUMO = -2.71624eV

1-M4-PB-1

1-M4-PB-3

1-M4-PB-4

EHOMO = -5.84477eV E = 4.0575eV ELUMO = -1.78724eV

1-M4-PB-5

EHOMO =-5.6948 eV E =3.8398 eV ELUMO = -1.855eV

EHOMO =--6.03984eV E =3.2501eV ELUMO = -2.7897eV

1-M4-PB-6



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Chemical hardness is useful in studying the stability and reactivity of compounds. It is in terms of the HOMO and LUMO energies [47]. The large chemical hardness have large excitation energies or their electron densities are difficult to alter, while the small chemical hardness has small excitation energies ie., the electron densities are easily altered. Chemical hardness can be calculated by using the equation 1 and the values are listed in Table VII. It is shown that the value of 1-M4-PB has the largest chemical hardness compared with the others. The hardness value is decreased by substituting the donor and acceptor to the titled molecule. By comparing with all those dyes, the Dye 6 Thiophene as donor and NO2 as acceptor have the lowest hardness value.

Equation 3 [48, 49] indicates the electrochemical potential which shows the escaping tendency of electrons and it can be associated with the molecular electronegativity in a compound [50]. The values of electrochemical potential are calculated and it is listed in Table VII. If the value of chemical potential is more negative, it is more difficult to lose an electron but easier to gain one. From Table VII, it shows that the titled compound 1-M4-PB is less stable and more reactive among all other compounds.

Parr et al. have defined a new descriptor to quantify the global electrophilic power of the compound as electrophilicity index () as a measure energy lowering due to maximal electron flow between donor and acceptor. It is a combined descriptor involving electronic chemical potential and chemical hardness which expresses propensity of a species to accept electron. They defined electrophilicity index () as follows

----------4

Electrophilicity index provides an idea of the stabilization energy when the system gets saturated by electrons, which come from the external environment. These reactivity information shows if a molecule is capable of donating charge. The helpfulness of this new reactivity quantity has been recently demonstrated in understanding the toxicity of various pollutants in terms of their reactivity and site selectivity. A good, more reactive, nucleophile is characterized by a lower value of, while higher values indicate the presence of a good electrophile. All these calculated values of electrophilicity index are shown in Table VII. From this table it is noticeable that 1M4PB-6 has the high value of electrophilicity index which indicate that it have strong electrophiles than others.

Maximum amount of electronic charge that an electrophile system may accept is given by the following equation.

-------(5)

The maximum charge transfer Nmax in the direction of the electrophile was predicted using the equation (5). Thus, while the defined quantity describes the tendency of the molecule to acquire additional electronic charge from the environment, the quantity defined by electrophilicity index describes the charge capacity of the molecule. The maximum charge transfer is obtained in 1-M4-PB6 (whose energy gap is very low) compared to the others.

The two new reactivity indices nucleofugality (En) and electrofugality(Ee) are proposed by Ayers and co-workers. It is used to quantify the nucleophilic and electrophilic capabilities of leaving group. They can be defined as follows,

---------(6)

---------(7)

From the equation (6) & (7) the values of nucleofugality and electrofugality are calculated and it is shown in Table VII. It is clear that 1-M4-PB6 has the highest (ΔEn) and (ΔEe) values.

Gomez et al. proposed the simple charge transfer model for donation and back-donation of charges [51]. An electronic back-donation process is an interaction between the inhibitor molecule and the metal surface. The concept establishes that if both processes occur, namely charge transfer to the molecule and back-donation from the molecule. The energy change is directly proportional to the hardness of the molecule. It can be represented by the following expression

----------(8)

The E back-donation denotes that when >0 and E back-donation <0 the charge transfer to a molecule followed by a back-donation from the molecule is energetically favoured. In this background, it is possible to balance the stabilization among inhibiting molecules. The calculated value of E back-donation is listed in Table VII. From this table, 1-M4-PB6 is the best inhibitor (ie. it has the highest value) than the others.



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Table VII:

Quantum Chemical Parameters HOMO-LUMO energies, Energy gap and Global reactivity descriptors and Back donation of 1-Methoxy-4-propylbenzene calculated B3LYP/6-311++G(d,p) basis set.

C) UV-Visible Absorption Spectra:

The longest wavelength of absorption spectrum (max), oscillator strength (f) and the vertical excitation energy (E) values for 1M4PB and its derivatives are calculated. These values are calculated by using TD-DFT/ B3LYP/6-31G+(d) level. The obtained results are listed in Table.8 which demonstrates that the lowest singlet electronic excitation is characterized as a typical - transition. They are due to electron motions between the frontier molecular orbital; like the promotion of an electron from the HOMO to the LUMO. Of all transitions from ground state to excited state (HOMO to LUMO), the most probable transition is which has the larger oscillator strength [45]. The absorption spectra are shown in Fig.8. The maximum absorption wavelength of 1-M4-PB6 dye is 652.82 nm. As shown, the maximum absorption wavelength shows a bathochromic shift in the following order 1-M4-PB 1-M4-PB1 1-M4-PB4 1-M4-PB2 1-M4-PB5 1-M4-PB3 1-M4-PB6. The simulated absorption spectra of 1-M4-PB and the theoretically designed dyes are shown in Fig. 8.

Parameters Values (eV)

1-M4-PB 1-M4-PB1 1-M4-PB2 1-M4-PB3 1-M4-PB4 1-M4-PB5 1-M4-PB6

HOMO energy -5.7257 -6.0974 -5.91812 -6.29236 -5.84477 -5.6948 -6.03984

LUMO energy 0.09795 -1.4860 -1.623952 -2.71624 -1.78724 -1.855 -2.7897

Energy gap 5.82365 4.6114 4.2942 3.5761 4.0575 3.8398 3.2501

Hardness(η) 2.912 2.3057 2.1471 1.7881 2.0288 1.9199 1.625

Softness(S) 0.172 0.2169 0.2329 0.2796 0.2465 0.2604 0.3076

Chemical potential(μ) -2.814 -3.7917 -3.7710 -4.5043 -3.8160 -3.7749 -4.4147

Electrophilicity index(ω) 1.360 3.1177 3.3116 5.6734 3.5889 3.7111 5.997

Charge Transfer (ΔNmax) 0.966 1.6445 1.7564 2.5191 1.8809 1.9662 2.717

Nucleofugality (ΔEn) 0.001 0.4788 0.6141 2.0631 0.7872 0.8961 2.394

Electrofugality (ΔEe) 5.629 8.0623 8.1562 11.0717 8.4192 8.4459 11.224

Back donation (ΔEback-don.) -0.727 -0.5764 -0.5368 -0.4470 -0.5072 -0.4800 -0.406

Wavelength nm

Ep

silo

n

Fig. 8(a)



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D) Light Harvesting Efficiency for DSSCs:

Dye-sensitized solar cells (DSSCs) have attracted a lot of interest for the conversion of sunlight into electricity because of their high efficiency and low cost [52]. The performance of DSSC strongly depends on the following factors: i) the absorption efficiency of the sensitizing dye for the solar light spectrum, ii) electron transfer and iii) probability of electron transfer from the electron donor to the dye. All these factors are closely associated with the structure of view, it is imperative to investigate the electronic structures of the sensitizing dye molecule for an understanding of the mechanism of the charge separation and the electron transfer, which are the key processes in this type of solar cells.

The most extensively studied organic dyes usually adopt the donor-pi-spacer-acceptor (D--A) structural motif in order to improve the efficiency. The photovoltaic properties of such dyes can be clearly tuned by selecting suitable groups within the D--A structure. The density functional theory (DFT) has emerged as a reliable standard tool for the theoretical treatment of structures as well as electronic and absorption spectra. Its time-dependent extension, called time-dependent DFT (TD-DFT), can give reliable values for the valence excitation energies with standard exchange correlation functional. The computational cost of TD-DFT calculation has maintains a uniform accuracy for open-shell and closed-shell systems. In recent years, TD-DFT has been extensively used to study the structures and absorption spectra of sensitizing dyes for DSSCs. The Light Harvesting Efficiency (LHE) is very important factor for the organic dyes considering the role of dyes in the DSSC. It can be expressed as,

LHE = 1 – 10-A = 1 – 10-f Where, f is the oscillator strength of the dye associate to the wavelength max. We observed that the

larger value of f, obtained the higher LHE value. The values of LHE are calculated for all the dyes and it is shown in Table.8. The D--A structure scheme is shown in Fig. 9a and chemical structure of 1-methoxy-4-propylbenzen for newly designed dyes is shown in Fig. 9b. The LHE value for 1M4PB in gas phase is 0.52543. The derivatives 1M4PB1, 1M4PB2, 1M4PB3, 1M4PB4, 1M4PB5 and 1M4PB6 are also calculated in gas phase. Out of seven, including 1M4PB dyes, 1-M4-PB-4 is producing most efficient LHE than other derivatives are studied.

Wavelength nm

Ep

silo

n

Fig.8(b)

Ep

silo

n

Wavelength nm

Fig. 8(c)

Fig. 8 (a), (b) (c) Theoretical UV-Vis spectrum of 1-Methoxy-4-propylbenzene and its derivatives



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Table VIII: Calculated light harvesting efficiency (LHE) of 1-Methoxy-4-propylbenzene and its derivatives calculated

using TD-DFT/B3LYP/6-31G+(d) basis set.

System Wavelength (λmax) nm

Excitation energy (E) eV

Oscillator Strength (f)

LHE

1-M4-PB 178.52 6.9449 0.3237 0.52543

1-M4-PB1(Benzene + CN) 282.46 4.3895 0.4405 0.63734

1-M4-PB2(Benzene + COOH) 298.95 4.1473 0.4060 0.60735

1-M4-PB3(Benzene + NO2) 600.76 2.0638 0.0036 0.00825

1-M4-PB4(Thiophene + CN) 297.00 4.1746 0.4766 0.66627

1-M4-PB5(Thiophene+COOH) 313.62 3.9534 0.4547 0.64900

1-M4-PB6(Thiophene + NO2) 652.82 1.8992 0.0020 0.00459

e

D

A

h

Fig. 9 (a)

R1

R2

Fig. 9 (a): Different parts of Donor- spacer-Acceptor system 9 (b): Chemical structure of 1-Methoxy-4-propylbenzene

R1-Benzene, Thiophene; R2 – CN, COOH, NO2

Fig. 9 (b)



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IX. CONCLUSION

In this study, the vibrational properties of 1-methoxy-4-propylbenzene are calculated using DFT/B3LYP by 631G+d and 6311G++d,p basis sets on the basis of potential energy distribution. The results are compared with the frequencies obtained from experimentally observed FT-IR and FT-Raman spectra. The theoretical values are in good agreement with the experimental frequencies. The predicted Molecular Electrostatic Potential figure revealed the negative and positive regions of the molecule. The active sites for the electrophilic and nucleophilic reactions are also observed by the local reactivity descriptors (Fukui function) of 1-methoxy-4-propylbenzene.

The Non-linear optical property of the titled compound is calculated theoretically by the determination first order hyperpolarizability. From the results, it have been seen that 1M4PB4 has the greater value than those of Urea, and then the titled compound is good candidature for NLO study. The correlations between the thermodynamical parameters at various temperatures were also calculated. In this study, we have seen that the temperature increases for various ranges (50K – 500K), the values of thermodynamic parameters are increases. Due to this fact the molecular vibrational intensities of the titled molecule were also increases with temperature. All the theoretically designed dyes are in bathochromic shift as compared to the titled compound. Out of seven, including 1M4PB dyes, 1-M4-PB-4 (Thiophene as Donor, CN as Acceptor) is producing most efficient LHE than other derivatives were studied.

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