ATOMIC AND MOLECULAR PHYSICS

DFT, FT-IR AND FT-RAMAN INVESTIGATIONS OF 2-CHLORO-5-NITROBENZYL ALCOHOL

S. RAMACHANDRAN, G. VELRAJ*

Department of Physics, Periyar University, Salem - 636 011, Tamilnadu, India. *E-mail: gvelraj@yahoo.co.uk

Received June 15, 2012

The experimental and theoretical study on the structure and vibrations of 2-Chloro-5-Nitrobenzyl Alcohol (2C5NBA) are presented. The Fourier transform infrared spectra (4000-400cm-1) and the Fourier transform Raman spectra (4000-100cm-1) of the title compound in the solid phase have been recorded. The geometry was optimized with C1 symmetry constraints using the DFT/B3LYP method with 6-31G(d) basis set. The vibrational frequencies determined experimentally were compared with DFT gradient calculations which was obtained theoretically employing the B3LYP/6-31G(d) basis set method for the optimized geometry of the compound. The vibrations obtained from DFT method were found in good agreement with the experimental data. The study is extended to the HOMO - LUMO analysis to calculate the energy gap (∆), Ionization potential (I), Electron Affinity (A), Global Hardness (η), Chemical Potential (µ), Global Electrophilicity (w). The calculated HOMO and LUMO energy reveals shows that the charge transfers occurring within the molecule. In addition, thermodynamic properties were obtained in the range of 100-700 K. Key words: DFT, FT-IR, FT-Raman, 2-chloro-5-nitrobenzyl alcohol, Vibrational analysis,

HOMO LUMO.

1. INTRODUCTION

Benzyl Alcohol (BA) is commonly used in pharmaceutical products as an antimicrobial preservative at levels of 3-5% [1]. It is well established when exposed to air, BA oxidizes slowly to benzaldehyde and subsequently to benzoic acid. The chemical oxidation of benzyl alcohol to benzaldehyde has been widely studied [2-4]. The use of aromatic alcohols has been restricted to their antimicrobial activity. That is probably because these molecules have been reported to cause aggregation of proteins, and are hence not ideal candidates as protein stabilizers. Protein aggregation not only has an adverse effect on the functionality of a protein but can also trigger a severe immune response in some patients [5-7]. BA is considered to be one of the least toxic preservatives and can be used up to 2% v/v

Rom. Journ. Phys., Vol. 58, Nos. 3–4, P. 305–317, Bucharest, 2013

S. Ramachandran, G. Velraj 2 306

for parenteral applications [8]. It also possesses potential antioxidative properties [9]. BA, also referred to as benzene methanol or phenylmethanol, is a naturally occurring organic compound that is commonly found in many plants, fruits and teas. Due to its polarity and low toxicity, BA is used as a general solvent and as a carrier solvent for flavoring substances in some foods and beverages and also as a preservative in cosmetic industries. Benzyl alcohol is mainly used in flavor industries, perfumery, artificial essences, cosmetics, paints, lacquers, soap and industrial solvents [10]. It is used as a local anesthetic [11, 12]. However, BA and m-cresol have been shown to accelerate aggregation by binding to proteins and favoring the formation of the aggregation prone molecular population. This was observed as the lower melting temperature of proteins like recombinant human interleukin-1 receptor antagonist (rhIL-1ra), recombinant human growth hormone (rhGH), and monoclonal antibody in the presence of BA [13-15]. It has also been shown that the effect of benzyl alcohol-induced aggregation was pH-dependent; an acceleration in the rate of aggregation of recombinant human granulocyte colony stimulating factor (rh GCSF) was seen at pH 7.0, whereas, a high positive charge on the same protein at pH 3.5 resulted in strong protein-protein electrostatic repulsion, thus, inhibiting BA induced aggregation [16]. BA and its substituted are found in the natural products and used for mechanistic investigation [17-19].

In the present work, the DFT frequency calculations for 2C5NBA have been verified both experimentally and theoretically. Therefore, the present investigation was to study the vibrational spectra of the molecule completely with wave number accuracy. The Density Functional Theory (DFT) calculations have been performed to support wave number assignments, thermodynamic properties and HOMO LUMO properties. This study may provide useful information about the molecular geometry, HOMO LUMO and thermodynamic property of 2C5NBA.

2. EXPERIMENTAL METHODS

The compound 2C5NBA (99%) was purchased from Sigma-Aldrich Company and used as such for the spectral measurements. The room temperature Fourier Transform IR spectra of the title compounds were measured in the 4000-400cm−1 region at a resolution of ±2cm-1 using BRUKER Tensor 27FT-IR spectrometer equipped with a KBr beam splitter and globar source. The Fourier Transform-Raman spectra of 2C5NBA were recorded on a BRUKER IFS-66 V model interferometer equipped with an FRA-106 FT-Raman accessory. The spectra were recorded in the region 4000-100cm−1 stokes region using the 1064nm line of Nd:YAG laser for the excitation operating at 200mW power. The reported wave numbers are expected to be accurate within ±2cm-1.

3 DFT, FT-IR and FT-Raman investigations 307

3. COMPUTATIONAL DETAILS

The molecular structure optimization of the title compound and corresponding vibrational harmonic frequencies were calculated using the DFT with Beckee-3-Lee-Yag-Parr(B3LYP) with 6-31G(d) basis set using GAUSSIAN 03 program package without any constraint on the geometry. The optimized geometrical parameters, true rotational constants, fundamental vibrational frequencies, IR intensity, Raman activity, electronic polarizabiliy, atomic charges, dipole moment and other thermodynamic parameters were calculated using the Gaussian 03 package [20]. By combining the results of the GAUSSVIEW [21] program. We know that DFT potentials symmetrically overestimate (2.5%) the Vibrational wavenumbers. These discrepancies are corrected either by computing anharmonic corrections explicitly [22] or by introducing a scaled field or by directly scaling the calculated wavenumbers with a proper factor. We calibrated the vibrational wavenumbers calculated by B3LYP method. After scaling with a scaling factor, the deviation from the experiment is more reliable. Analytic frequency calculations at the optimized geometry were done to confirm the optimized structures to be an energy minimum and to obtain the theoretical vibrational spectra. The total energy distribution (TED) was calculated by using the scaled quantum mechanics (SQM) method [23] and parallel quantum solution (PQS) [24] program and the fundamental vibrational modes were characterized by their TED.

The Raman activities (SRa) converted to relative Raman intensities (IRa) using the following relationship derived from the intensity theory of Raman scattering

4( )

[1 exp( / )]o i i

ii i

f SI

hc kT−

=− −ν ν

ν ν (1)

Where oν is the laser exciting wavenumber in cm−1 (in this work, we have used the excitation wavenumber oν = 9398.5cm−1, which corresponds to the wavelength of 1064nm of a Nd: YAG laser), iν the vibrational wavenumber of the ith normal mode (cm−1), while Si is the Raman scattering activity of the normal mode iν . f (is a constant equal to 10−12) is a suitably chosen common normalization factor for all peak intensities. h, k, c and T are Planck and Boltzmann constants, speed of light and room temperature in Kelvin. However, the defined coordinates forms complete set and matches quite well with the motions observed using GAUSSVIEW program. In order, to analyze the very low frequency vibrations, the FT-IR and FT-Raman spectrum was taken in the range of 4000-100cm-1 in the solid phase.

S. Ramachandran, G. Velraj 4 308

4. RESULT AND DISCUSSION

4.1. MOLECULAR GEOMETRY

The structure and the scheme of numbering the atoms of 2C5NBA are shown in Fig.1. The geometry of the molecules under investigation is considered to possess C1 point group symmetry. The 48 fundamental modes of vibrations of each compound are distributed into the irreducible representation under C1 symmetry as Γ3N-6vib= 48. All vibrations are active in both IR and Raman. All the frequencies are assigned in terms of fundamental, overtone and combination bands. The optimized geometrical parameters (bond length and bond angles) by DFT/B3LYP with 6-31G(d) as the basis set are tabulated in Table 1.

Fig. 1 – Molecular structure of 2C5NBA along with numbering of atom.

4.2. VIBRATIONAL ASSIGNMENT

The observed FT-IR and FT-Raman spectra along with the theoretical infrared and Raman spectra of 2C5NBA are shown in Figs. 2-3. The observed and calculated frequencies using DFT/B3LYP with 6-31G(d) basis set along with their probable assignments of 2C5NBA are summarized in Table 2.

4.2.1. C-H vibrations

The aromatic C-H stretching vibrations were normally found between 3100 and 3000 cm-1 [25]. Accordingly, in the title compound, asymmetric and symmetric stretching vibrations were observed at 3090cm-1 and 2901cm-1 in the FT-IR spectrum whereas, in the FT-Raman, it is at 3087cm-1 and 2896cm-1, respectively. The in-plane bending and out-of-plane bending vibrations of the aromatic C-H group have also been identified for the title compound and they are presented in Table 2.

5 DFT, FT-IR and FT-Raman investigations 309

Table 1

Optimized geometric data for 2C5NBA using B3LYP/6-31G(d) Bond length Value( Å ) Bond angle Value (˚)

C1–C2 1.509 C2–C1–O12 108.1 C1–O12 1.424 C2–C1–H13 108.8 C1–H13 1.100 C2–C1–H14 109.4 C1–H14 1.099 O12–C1–H13 111.2 C2–C3 1.405 O12–C1–H14 111.3 C2–C7 1.395 H13–C1–H14 107.7 C3–C4 1.396 C1–C2–C3 122.4 C3–Cl11 1.752 C1–C2–C7 120.0 C4–C5 1.390 C3–C2–C7 117.5 C4–H15 1.084 C2–C3–C4 121.9 C5–C6 1.391 C2–C3–Cl11 120.3 C5–H16 1.082 C4–C3–Cl11 117.7 C6–C7 1.392 C3–C4–C5 119.8 C6–C8 1.470 C3–C4–H15 119.7 C7–H17 1.083 C5–C4–H15 120.4 N8–O9 1.230 C4–C5–C6 118.4 N8–O10 1.230 C4–C5–H16 121.5 O12–H18 0.969 C6–C5–H16 119.9 C5–C6–C7 121.8 C5–C6–N8 119.1 C7–C6–N8 119.0 C2–C7–C6 120.3 C2–C7–H17 120.4 C6–C7–H17 119.1 C6–N8–O9 117.6 C6–N8–O10 117.5 O9–N8–O10 124.7 C1–O12–H18 107.7

Fig. 2 – FT-IR spectrum: (a) Observed; (b) 6-31G(d).

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0

Tran

smitt

ance

%

W a v e n u m b e r ( c m -1 )

( a )

(b )

S. Ramachandran, G. Velraj 6 310

Fig. 3 – FT-Raman spectrum: (a) Observed; (b) 6-31G(d).

4.2.2. C-Cl vibrations

The C-Cl stretching gives generally strong bands in 770-505cm-1 region [26]. The sharp FT-IR and FT-Raman bands at 600cm-1 and 612cm-1, respectively observed in the spectrum of the title compound are assigned to C-Cl is stretching vibration.

4.2.3. C-C vibrations

The bands between 1650-1400cm-1in benzene derivatives were assigned to C-C stretching modes [27]. Accordingly, in the present study, the carbon-carbon vibrations of the title compound were observed at 1584 and 1714cm-1 in the FT-IR spectrum and the FT-Raman spectrum observed at 1612cm-1 and 1420cm-1.

4.2.4. Nitro group vibrations

The characteristic group frequencies of nitro group are relatively independent of the rest of the molecule, which makes this group convenient to identify. Aromatic nitro compounds have strong absorptions due to the asymmetric and symmetric stretching vibrations of the NO2 group at 1655cm-1 and 1603cm-1 respectively. The Hydrogen bonding has little effect on the NO2 asymmetric stretching vibrations [28, 29]. The FT-IR band at 1789cm-1 has been designated to scissoring modes of NO2. The FT-Raman band has been designated to the rocking mode of nitro group. The FT-IR and FT-Raman bands at 794cm-1and 805cm-1 in 2C5NBA have been designated to scissoring modes of the nitro group. The very weak FT-Raman band at 211cm-1 is assigned to NO2 torsion mode.

4000 3500 3000 2500 2000 1500 1000 500

Ram

an In

tens

ity (A

rbitr

. Uni

ts)

W a v e nu m b e r (cm -1)

(a )

(b )

7 DFT, FT-IR and FT-Raman investigations 311

Table 2 Detailed assignments of fundamental vibrations of 2C5NBA

Experimental frequency(cm-1)a Calculated frequency (B3LYP)(cm-1) Modes no FT-IR FT-Raman

6-31G(d) wavenumbers IR Intensity Raman

Intensity

Vibrational Assignment with TED maximum (%)b

υ1 3889vw - 3752 20.031 5.893 υsOH (100) υ2 - - 3256 2.167 3.691 υasOH (98) υ3 3509s - 3245 1.901 4.67 υsOH (98) υ4 - - 3231 0.923 5.414 υCH (88) υ5 - - 3049 31.073 8.298 υCH (86) υ6 - - 3013 52.313 5.916 υCH (84) υ7 3090m 3087m 1673 91.128 85.038 υasCH (61) υ8 2901m 2896m 1640 70.603 24.691 υsCH (55) υ9 1789w - 1616 82.416 11.038 δNO2 (63) υ10 1714w - 1547 2.231 0.666 υCC (77) υ11 1584m 1612w 1512 36.598 15.544 υCC (83) υ12 1513vs 1575m 1484 18.607 2.063 δCH (89) υ13 - 1514m 1442 6.421 371.897 δCH(84) υ14 - - 1397 371.379 6.324 wCH (79) υ15 - 1420m 1369 14.660 2.241 υ CC (53) υ16 1340vs 1347s 1301 10.196 15.532 υRing (58) υ17 - - 1287 37.327 79.130 υCN (67) υ18 - - 1246 28.020 80.183 τCH (74) υ19 - - 1219 31.938 9.730 υCN(52) υ20 - - 1165 4.199 24.092 βOH (63) υ21 - 1194s 1132 8.287 22.082 ρNO2 (82) υ22 1184s - 1076 122.833 47.785 βCH (67) υ23 - - 1062 43.288 2.395 βCH (63) υ24 - 1096s 1015 4.838 3.78 υCO (78) υ25 - 1070s 982 0.467 5.837 υCO (54) υ26 1027vs 1038s 945 31.312 8.743 ρCH (67) υ27 - - 933 31.464 8.51 υCN (55) υ28 - - 853 12.158 38.814 γCH (71) υ29 903s 925s 819 18.493 7.016 γCH (66) υ30 - - 754 25.193 2.511 γCH (61) υ31 794s 805m 740 11.511 41.53 δNO2 (81) υ32 740s - 708 2.030 13.792 βRing (77) υ33 - 712m 615 10.046 8.908 τRing (81) υ34 600w 612m 577 15.810 29.573 υCCl (47) υ35 - - 542 6.460 6.594 βCH (51) υ36 - - 522 9.385 6.704 γCN(59) υ37 491m 482wv 465 1.810 80.531 βRing (42) υ38 - - 391 1.661 25.62 τRing (47) υ39 - - 352 0.555 47.713 βCN (36) υ40 - 336vw 315 0.402 18.418 βRing (52) υ41 - 296vw 277 2.918 13.374 τRing (65) υ42 - - 245 1.690 87.97 βCCl (36) υ43 - 211vw 206 110.287 17.466 τNO2 (51) υ44 - - 193 2.248 136.556 γOH (41) υ45 - 149vw 136 1.860 33.089 βCC (38) υ46 - 112vw 94 3.615 104.361 τRing (37) υ47 - - 58 0.057 108.820 τRing (54) υ48 - - 55 0.157 5.893 τNO2 (36)

avs-very strong; s-strong; m-medium strong; w-weak; vw-very weak. bτ- torsion; γ- out of plane bending; β- in plane bending; ρ- rocking; δ- scissoring; υ- stretching; υs- symmetric stretching; υas- asymmetric stretching; w- wagging.

S. Ramachandran, G. Velraj 8 312

4.2.5. O-H and C-O vibrations

Hydrogen bonding alters the frequencies of the stretching and bending vibration. The O-H stretching bands move to lower frequencies usually with increased intensity and band broadening in the hydrogen bonded species. Hydrogen bonding if present in five or six member ring system would reduce the O-H stretching band to 3200-3550cm−1 region [30]. In the present study (for 2C5NBA), the stretching vibration of the hydroxyl group is observed at 3889cm-1 and 3509cm-1 in the FT-IR spectrum. The bending vibrations of the alcohol group are also identified and listed in Table 2. The strong band observed in the region 1300-1200 cm-1 may be due to C-O stretching vibrations for p-mono substituted phenols [31]. In the present study, strong FT-Raman bands are observed at 1070cm-1 and 1096cm-1.

4.3. HOMO-LUMO ENERGY GAP AND RELATED MOLECULAR PROPERTIES

The analysis of wave function indicates that the electron absorption corresponds to the transition from the ground to the first excited state and is mainly described by one electron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied orbital (LUMO). The HOMO, LUMO and HOMO-LUMO energy gap of 2C5NBA in the DFT level in 6-31G(d) basis set has been calculated. The HOMO-LUMO energy gap reveals that the energy gap reflects the chemical activity of the molecule. The atomic orbital compositions of the frontier molecule orbital are sketched in Fig.4.

Fig. 4 – Atomic orbital composition of the frontier molecule for 2C5NBA.

9 DFT, FT-IR and FT-Raman investigations 313

Associated within the framework of Self-Consistency Function (SCF) Molecular Orbital(MO) theory the ionization energy and electron affinity can be expressed through HOMO and LUMO orbital energies as I = -EHOMO and A = -ELUMO. The hardness corresponds to the gap between the HOMO and LUMO orbital energies. The larger the HOMO-LUMO energy gaps the harder the molecule [32].

The global hardness, is η=1/2(ELUMO-EHOMO). The hardness has been associated with the stability of chemical system. The electron affinity can be used in combination with ionization energy to give electronic chemical potential, µ=1/2(EHOMO + ELUMO). The global electrophilicity index, w = µ2/2η is also calculated and listed in Table 3.

Table 3

Comparison of HOMO, LUMO, energy gaps (HOMO –LUMO) and related molecular properties of 2C5NBA(eV)

Molecular Energy B3LYP/6-31G(d) EHOMO –6.7577 ELUMO –0.6054 Energy gap (∆) 6.1522 Ionisation Potential (I) 6.7577 Electron affinity (A) 0.6054 Global Hardness (η) 3.0761 Chemical potential (µ) –3.6815 Global Electrophilicity (ω) 2.2029

4.4. THERMODYNAMIC PROPERTIES

The thermodynamic parameters namely heat capacity, entropy, rotational constants, dipole moments, vibration and vibrational zero point energies of the compounds have also been computed at the DFT-B3LYP level using 6-31G(d) basis sets and are presented in Table 4. The energy of 2C5NBA has been calculated to -1010.86075197a.u. The thermodynamic data provide helpful information for the further study on the title compound, when these may be used as a reactant to take part in a new reaction. These standard thermodynamic functions can be used as reference thermodynamic values to calculate the changes of entropies (∆ST), changes of enthalpies (∆HT) and changes of Gibbs free energies (∆GT) of the reaction. There is no much deviation in the thermodynamic properties calculated from these methods. The dipole moment and its principal inertial axes strongly depend upon the molecular conformation. The 2C5NBA has higher dipole moments (4.0140Debye).

S. Ramachandran, G. Velraj 10 314

Table 4

The calculated thermodynamic parameters of 2C5NBA employing B3LYP/ 6-31G(d) methods.

Thermodynamic parameters (298K) B3LYP/ 6-31G(d) SCF energy (a.u.) -1010.86075197 Total energy (thermal), Etotal (kcal mol−1) 86.139 Heat capacity at const. volume, Cv (cal mol−1 K−1) 38.996 Entropy, S (cal mol−1 K−1) 104.545 Vibrational energy, Evib (kcal mol−1) 84.362 Zero-point vibrational energy, E0 (kcal mol−1) 79.35406 Rotational constants (GHz) A 1.58283 B 0.48348 C 0.38108 Dipole moment (Debye) µx 3.8598 µy -0.0170 µz -1.1017 µtotal 4.0140

On the basis of vibrational analysis at B3LYP/6-31G(d) level, the standard

statistical thermodynamic functions: heat capacity ( 0,p mC ), entropy ( 0

mS ), and

enthalpy changes ( 0mH ), for the title compounds were obtained from the theoretical

harmonic frequencies and listed in Table 5.

Table 5

Thermodynamic properties at different temperatures at the B3LYP/6-31G(d) level of 2C5NBA

T(K) 0,p mC (cal mol-1k-1) 0

mS (col mol-1k-1) 0mH (kcal mol-1)

100 11.701 9.054 79.899 200 22.671 20.591 81.626 298.15 33.035 31.583 84.362 300 33.226 31.788 84.423 400 43.006 42.716 88.245 500 51.263 53.231 92.972 600 57.920 63.188 98.444 700 63.250 72.532 104.512

11 DFT, FT-IR and FT-Raman investigations 315

Fig. 5 – Correlation graph of heat capacity and temperature for 2C5NBA molecule.

Fig. 6 – Correlation graph of entropy and temperature for 2C5NBA molecule.

Fig. 7 – Correlation graph of enthalpy and temperature for 2C5NBA molecule.

0,p mC = –1.51846 + 0.13452T –5.94593x10-5T2 (R2 = 0.99957) (2)

100 200 300 400 500 600 700

10

20

30

40

50

60

70

Hea

t cap

acity

(Cm

,0

p) (

cal m

ol-1 K

-1)

Temperature (K)

100 200 300 400 500 600 7000

10

20

30

40

50

60

70

80

Entro

py (S

0 m) (

cal m

ol-1 K

-1)

Temperature (K)

100 200 300 400 500 600 700

80

85

90

95

100

105

Enth

alpy

(H0 m) (

kcal

mol

-1)

Temperature (K)

S. Ramachandran, G. Velraj 12 316

0mS = –3.12514 + 0.1230T –2.10965x10-5T2 (R2 = 0.99997) (3)

0mH = 78.72648 + 0.00605T + 4.42841x10-6T2 (R2 = 0.99971) (4)

From Table 5, it can be observed that these thermodynamic functions are increasing with temperature ranging from 100 to 700 K due to the fact that the molecular vibrational intensities increase with temperature [33]. The correlation equations between heat capacities, entropies, enthalpy changes and temperatures were fitted by quadratic formulas, and the corresponding fitting factors (R2) for these thermodynamic properties are 0.99957, 0.99997 and 0.99971, respectively. The corresponding fitting equations are as follows and the correlation graphics from those shows in Figs. 5-7. All the thermodynamic data supply helpful information for the further study on the 2C5NBA. All thermodynamic calculations were done in gas phase and they could not be used in solution.

5. CONCLUSION

In the present work, we have performed both experimental and theoretical vibrational analyses of 2C5NBA. A comparison of the results of experimental and theoretical study has given a full description of the geometry, vibrational frequencies, electronic properties and thermodynamic properties of this molecule. The calculated geometrical parameters and vibrational frequencies obtained by density functional theory calculations B3LYP/6-31G(d) methods are in good agreement with the experimental values obtained for the investigated molecule. The HOMO-LUMO energy gap of the title compound corresponding to 6.1522eV by using B3LYP/6-31G(d) method respectively. The other molecular properties such as Ionization Potential (I), Electron affinity (A), Global Hardness (η), Chemical potential (µ) and Global Electrophilicity (w) were calculated. The thermodynamic data provide helpful information for the further study on the title compound. The correlations between the statistical thermodynamics and temperature are also obtained. It is seen that the heat capacities, entropies and enthalpies increase with the increasing temperature owing to the intensities of the molecular vibrations increase with increasing temperature.

Acknowledgments. The first author Acknowledge with thanks to Periyar University for providing University Research Fellow (URF).

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