CHAPTER-IV

MOLECULAR STRUCTURE AND VIBRATIONAL

SPECTRA OF PYRAZOLE AND 3, 5-DIMETHYL

PYRAZOLE BY DENSITY FUNCTIONAL THEORY

Abstract

This chapter deals with the vibrational spectroscopy of pyrazole (PZ)

and 3,5-dimethyl pyrazole (DMP). The FTIR and FT-Raman spectra of the title

compounds have been recorded in the region 4000-400 cm-1 and 4000-50 cm-

1, respectively. The molecular geometry and vibrational frequencies of PZ and

DMPZ molecules in the ground state have been calculated by using the

density functional methods (B3LYP) with 6-31G* basis set. The Total Energy

Distributions (TED) for the normal modes were computed for the minimum

energy structure of the molecules. Comparison of the observed fundamental

frequencies of PZ and DMPZ molecules with calculated results by density

functional B3LYP approach will give better result for studies molecular

vibrational problem.

CHAPTER-IV

MOLECULAR STRUCTURE AND VIBRATIONAL SPECTRA OF

PYRAZOLE AND 3, 5-DIMETHYL PYRAZOLE BY DENSITY

FUNCTIONAL THEORY

4.1 INTRODUCTION

Pyrazole is a simple aromatic organic compound of the heterocyclic

series characterized by a five-membered ring structure composed of three

carbon atoms and two nitrogen atoms in adjacent positions. Being so

composed and having pharmacological effects on humans, they are classified

as alkaloids, although they are rare in nature. In 1959, the first natural

pyrazole, 1-pyrazolyl-alanine, was isolated from seeds of watermelons [116].

The term pyrazole was given to this class of compounds by Ludwig knorr in

1883. It is commonly used as phenylbutazone and an anti-inflammating drugs

used in treatement of arthritics and a sense of dyes used as sensitizing agent

in colour photography.

In medicine, derivatives of pyrazoles are used for their analgesic, anti-

inflammatory, antipyretic, antianhythmic, tranquillizing, muscle relaxing,

psychoanaleptic, anticonvulsant, monoamineoxidase inhibiting, antidiabetic

and antibacterial activities [117-119]. Some of them are active ingredients of

products with potential antitumor activity [120-122]. In agriculture, they are in

the use as pesticides [123–125]. As pyrazoles readily form complexes, they

are suiTable agents for investigating the active sites of biomolecules [126]

and for modelling the biosystems of oxygen transfer. In living organisms,

metal ions are usually bonded to the imidazole part of hystidine, which is a

part of the proteins. In view of the similarity of pyrazole and imidazole, [127]

they are suiTable to mimic enzymatic reactions. 3, 5-dimethyl pyrazole is used

as intermediate for the manufacture of organic photochemical and dye stuffs.

The inclusion of a substituent group in pyrazole leads to the variation of

charge distribution in molecules and consequently this affects the structural,

electronic and vibrational parameters. The methyl and amino groups are

generally refered as electron donating substituents in aromatic ring systems

[128]. The CH3 interacts with nearby π – system via hyper conjugation, this

imply electronic delocalization and are taken into account by the molecular

orbital approach [129, 130].

In order to understand the vibrational properties and structural

characteristics of the compounds pyrazole and 3, 5- dimethyl pyrazole, the

density functional theory (DFT) calculation with B3LYP/6-31G* combination

was carried out and the observed bands were assigned based on the results

of normal coordinate analysis.

4.2 EXPERIMENTAL

The fine polycrystalline samples of pyrazole (pz) and 3, 5-dimethyl

pyrazole (DMPZ) were kindly provided by the Sigma chemical company

(U.S.A) and used as such for the spectral measurements. The room

temperature Fourier transform infrared spectra of the title compounds were

measured with KBr pellet technique in the 4000-400 cm-1 region at a

resolution of 1 cm-1 using BRUKER IFS 66V FTIR spectrometer equipped

with a cooled MCT detector for the mid-IR range. Boxcar apodization was

used for the 250 averaged interferograms collected for the sample and

background.

The FT-Raman spectra were recorded on a BRUKER IFS-66V model

interferometer equipped with an FRA-106 FT-Raman accessory. The spectra

were recorded in the 4000-50 cm-1 stokes region using 1064nm line of a

Nd:YAG laser for excitation operating at 200mW power. The reported

frequencies are believed to be accurate within cm-1.

4.3 COMPUTATIONAL DETAILS

Quantum chemical density functional calculations were carried out for

pyrazole (pz) and 3, 5-dimethyl pyrazole (DMPZ) with the 2003 window

version of the Gaussian suite programme [83] using the Becke-3-Lee-Yang-

Parr(B3LYP) functionals [81,131,132] supplemented with the standard 6-31G*

basis set (referred as DFT calculations). The normal grid (50,194) was used

for numerical integration. The 6-31G* basis set adds polarization function in

form of six d-type functions for each atom other than H to the split valence

6-31G basis. The Cartesian representation of the theoretical force constants

has been computed at the fully optimized geometry by assuming both the

molecules belong to CS point group symmetry. The theoretical DFT force field

was transformed from Cartesian into local internal coordinates and then

scaled empirically according to the SQM procedure [76, 133, 134].

1

32( )Scaled B LYP

ij i j ijF CC F …4.1

where Ci is the scale factor of coordinate i, 3B LYP

ijF is the B3LYP/6-31G* force

constant in the local internal coordinates, and 3B LYP

ijF is the scaled force

constant. The transformation of force field from Cartesian to internal local-

symmetry coordinates, the scaling [135, 136], the subsequent normal

coordinate analysis calculation of total energy distribution (TED) and

prediction of IR and Raman intensities were done on a PC with the version

V7.0-G77 of the MOLVIB programme written by Sundius [98, 137, 138]. To

achieve, a close agreement between the observed and calculated

frequencies, least square fit refinement algorithm was used. The force field

obtained by this way was then used to recalculate the normal modes, TED’s

and the corresponding theoretically expected IR and Raman intensities to

predict the full IR and Raman spectra. For the plots of simulated IR and

Raman spectra, pure Lorentzian band shapes were used with a band width

(FWHM) of 10 cm-1.

The prediction of Raman intensities was carried out by following the

procedure outlined below. The Raman activities (S i) calculated by the

GAUSSIAN-03 programme are adjusted during scaling procedure with

MOLVIB and were converted to relative Raman intensities (Ii) using the

following relationship derived from the basic theory of Raman scattering

[139-141].

Where o is the exciting wavenumber (in cm-1 units), i the vibrational

wavenumber of the ith normal mode, h, c, and k are the universal constants

and f is a suitably chosen common normalization factor for all peak intensities.

4.4 RESULTS AND DISCUSSION

4.4.1 Structural Properties

The labeling of atoms of the title compounds are shown in Figs. 4.1 (a)

and (b). The global minimum energies obtained by the DFT structure

... 4.2

optimization of pyrazole (pz) and 3, 5-dimethyl pyrazole (DMPZ) were found

to be –226.198 and –304.840 Hartrees for B3LYP/6-31G* basis set,

respectively. This energy difference is clearly understandable, since the

molecules are under different environments

The optimized values of bond lengths and bond angles are reported in

Table 4.1. The methyl groups substitution at C3 and C5 atoms in DMPZ

pushes the electrons away from itself and it exerts a +I effect or electron

releasing inductive effects. As a result, the bond length of the molecules is

slightly different.

4.5 VIBRATIONAL FORCE CONSTANTS

The output of the quantum chemical calculations contains the force

constant matrix in Cartesian coordinates in Hartree/Bohr2 units. These force

constants were transformed to the force fields in the internal local-symmetry

coordinates. The local-symmetry coordinates, defined in terms of the internal

valence coordinates following the IUPAC recommendations [142] are given in

Tables 4.2 - 4.5 for PZ and DMPZ, respectively. The force fields determined

were used to calculate the vibrational total energy distribution (TED) among

the normal coordinates. The most important diagonal force constants of PZ

and DMPZ are listed in Table 4.6.

The bonding properties of PZ and DMPZ are influenced by the

rearrangements of electrons during substitutions and addition reactions. The

values of the stretching force constants between nitrogen and carbon atoms

(N1-C5, N2-C3) in PZ are found to be less than DMPZ due to electron

releasing inductive effect of methyl group.

4.6 MOLECULAR VIBRATIONS AND SIMULATED SPECTRA

The compounds of PZ and DMPZ belong to CS point symmetry and their

21 and 39 fundamentals are distributed among the symmetry species as:

ГVib = 16A′ (in-plane) + 5 A″(out-of-plane) and

ГVib = 27A′ (in-plane) + 12A″ (out-of-plane)

respectively. All the vibrations are active in both Raman scattering and

infrared absorption. In the Raman spectrum the in-plane vibrations (A′) gives

rise to polarized bands while the out-of-plane ones (A″) to depolarized band.

The observed and calculated infrared and Raman spectra of PZ and DMPZ

are produced in common frequency scales in Figs.4.2 - 4.5.

The assignments of the normal modes of vibrations of the investigated

molecules along with the observed fundamentals, unscaled frequencies

obtained by B3LYP/6-31G* calculations and scaled frequencies as well as the

TED descriptions are reported in Tables 4.7 and 4.8 for PZ and DMPZ

respectively. Root mean square (RMS) values were obtained in this study

using the following expression,

RMS=n

i

i

calc

in

2exp

)1(

1 …4.3

Deviations between the unscaled and experimental frequencies for all

modes were found to be 30.6 cm-1 and 56.8 cm-1 for PZ and DMPZ,

respectively. This is quite obvious, since the frequencies calculated on the

basis of quantum mechanical force fields usually differ appreciably from the

observed frequencies. This is partly due to the neglect of anharmonicity and

partly due to the approximate nature of the quantum mechanical methods. In

order to reduce the overall deviation between the unscaled and observed

fundamental frequencies, scale factors were applied in the normal coordinate

analysis and the subsequent least square fit refinement resulted in a very

close agreement between the observed fundamentals and the scaled

frequencies (Tables 4.7 and 4.8). Refinement of the scaling factors applied in

this study achieved a weighted mean deviation of 6.6cm-1 and 10.6 cm-1

between the experimental and SQM frequencies of the title compounds.

Due to the low symmetry of the molecule, several internal coordinates

contribute to each normal mode of the title compounds. The detailed

vibrational assignments of fundamental modes of PZ and DMPZ by normal

mode analysis based on scaled quantum mechanical force field calculations

are listed in Tables 4.7 and 4.8.

4.6.1 Carbon-Hydrogen vibrations

The heterocyclic structure shows the presence of C-H stretching

vibrations around 3000 cm-1 [143, 144]. In PZ these bands are observed at

3166, 3148 and 3133 cm-1, while in DMPZ the band is identified at 3191

cm-1. Figs 4.2 - 4.4 show broad IR absorption from 3250 - 2750 cm-1, for the

title compounds. The strong absorption can be attributed to a tautomeric form

of pyrazole. A numbers of spikes observed throughout the broad absorption is

indication of Fermi Resonance.

The C-H in-plane deformations are obtained at 1263, 1034 and 1031

cm-1 in PZ and the same mode occurred at 1169 cm-1 in FTIR spectrum of

DMPZ [145-147]. The C-H out-of-plane bending modes are observed at 897,

840 and 754 cm-1 of FTIR spectrum in PZ and these FTIR bands are

appeared at 781 cm-1 of DMPZ [148, 145].

4.6.2 Nitrogen-Hydrogen vibrations

In all the heterocyclic compounds, the N-H stretching vibrations [149]

occur in the region 3500 – 3000 cm-1. The FTIR band appeared at 3430 cm-1

for PZ and 3302 cm-1 for DMPZ have been assigned to N-H stretching modes

of vibrations.

4.6.3 Methyl group vibrations

For the assignments of CH3 group frequencies, basically nine

fundamentals can be associated to each CH3 group namely, CH3

ss – symmetric stretch; CH3 ips – in-plane stretch (i.e. in-plane hydrogen

stretching modes); CH3 ipb – in-plane bending (i.e. in-plane hydrogen

deformation modes); CH3 sb – symmetric bending; CH3 ipr – in-plane

rocking; CH3 opr – out-of-plane rocking; tCH3 – twisting hydrogen bending

modes. In addition to that, CH3 ops – out-of-plane-stretch and CH3 opb –

out-of-plane bending modes of CH3 group would be expected to be

depolarized for A’’ symmetry species.

The CH3 ss frequencies are established at 2946 cm-1 and 2931 cm-1

in IR and Raman spectra of DMPZ, the CH3 ips are assigned at 3040 cm-1

and 3039 cm-1 in IR for DMPZ. These assignments are also supported by

literature [150] in addition to TED output.

The four in-plane methyl hydrogen deformation modes are also well

established. We have observed the symmetrical methyl deformation modes

CH3 sb at 1378 cm-1 and 1375 cm-1 in IR and Raman spectra, in-plane

methyl deformation modes CH3 ipb at 1485 cm-1 and 1466 cm-1 in IR. The

bands at 2993 cm-1, 2948 cm-1 and 1467 cm-1, 1466 cm-1 in IR are

attributed to CH3 ops and CH3opb respectively in the A’’ species. The

methyl deformation modes mainly coupled with in-plane bending vibrations.

The bands obtained at 1021 cm-1 in Raman and 960 cm-1 in IR and 1011

cm-1, 950 cm-1 in IR are assigned to CH3 in-plane and out-of-plane rocking

modes. The tCH3 (methyl twisting mode) vibrations are assigned within the

characteristic region and reported in Table 4.8.

4.6.4 Ring vibrations

Ring stretching modes (C=C, N-C, N-N) appears in narrow spectral

region 1640-1400 cm-1 and 1150-925 cm-1 in pyrazole [151]. In the present

work, the C=C stretching vibrations are attributed for PZ at 1558 and 1401

cm-1 in FTIR and FT-Raman spectrum and for DMPZ at 1598 and 1460

cm-1 in FTIR spectrum [151, 152]. The N-C stretching modes are observed at

1468, 1138 cm-1 in FTIR for PZ and 1491, 1424 cm-1 in FTIR and FT-Raman

spectrum for DMPZ [151, 153]. The N-N stretching vibrations are assigned

1156 and 1153 cm-1 in FT-Raman spectrum for PZ and DMPZ [154]. The

in-plane and out-of-plane deformations are assigned within the characteristic

region and reported in Table 4.8.

4.7 CONCLUSION

FTIR and FT-Raman spectra of pyrazole and 3, 5-dimethyl pyrazole were

recorded and the detailed vibrational assignments were presented. The

molecular geometry, vibrational frequencies, infrared intensities and Raman

scattering activities of the molecules have been calculated by using DFT

(B3LYP) method with 6-31G* basis set. The intramolecular charge transfer

and the π-electron delocalization have been investigated. The influence of

electronic effect such as electron releasing inductive effect, on the N-H

stretching vibrational wavenumbers decreases due to substitution of methyl

group in DMPZ has been extensively investigated. The vibrational frequencies

were calculated and scaled values (with 6-31G* basis set) have been

compared with experimental FTIR and FT-Raman spectra. The observed and

the calculated frequencies are in good agreement.

Table 4.1

Optimized geometrical parameters of PZ and DMPZ obtained by

B3LYP6/31G* density functional calculations.

Bond length

Value(Aº)

Bond angle

Value(º)

PZ DMPZ PZ DMPZ

N1-N2 1.350 1.355 N1-N2-C3 103.901 104.382

N2-C3 1.332 1.333 N2-C3-C4 112.138 111.053

C3-C4 1.413 1.419 C3-C4-C5 104.484 105.726

C4-C5 1.381 1.382 C5-N1-H6 127.899 127.657

N1-H5 1.008 1.008 N2-C3-H7(C7) 119.408 120.381

C3-H7(C7) 1.082 1.499 C3-C4-H8 128.263 127.771

C4-H8 1.080 1.081 C4-C5-H9(C9) 132.045 131.972

C5-H9(C9) 1.080 1.496 C3-C7-H10 − 109.682

C7-H10 − 1.063 C3-C7-H11 − 111.613

C7-H11 − 1.096 C3-C7-H12 − 111.613

C7-H12 − 1.096 C5-C9-H13 − 116.151

C9-H13 − 1.092 C5-C9-H14 − 111.867

C9-H14 − 1.097 C5-C9-H15 − 111.867

C9-H15 − 1.097

The atom indicated in the parenthesis belongs to DMPZ; for numbering of atoms refer Figs 4.1 (a) and (b).

Table 4.2

Definition of internal coordinates of PZ.

For numbering of atoms refer figs. 4.1 (a) and (b).

No.(i) Symbol Type Definition

Stretching

1 Ri N-N N1-N2

2-3 ri N-C N1-C5, N2-C3

4-5 Pi C-C C3-C4, C4-C5

6 Di N-H N1-H6

7-9 Ti C-H C3-H7, C4-H8, C5-H9

Bending

10-14 βi bRing N1-N2-C3, N2-C3-C4, C3-C4-C5,

C4-C5-N1, C5-N1-N2.

15 αi N-N-H N2-N1-H6

16 αi C-N-H C5-N1-H6

17-18 δi N-C-H N2-C3-H7, N1-C5-H9

19-22 δi C-C-H C4-C3-H7, C3-C4-H8, C5-C4-H8,

C4-C5-H9

Out-of-plane bending

23 ωi N-H C5-N2-N1-H6.

24-26 ψi C-H N2-C4-C3-H7, C3-C5-C4-H8, N1-

C4-C5-H9

Torsion

27-31 i Ring N1-N2-C3-C4, N2-C3-C4-C5, C3-

C4-C5-N1, C4-C5-N1-N2, C5-N1-

N2-C3.

Table 4.3

Definition of internal coordinates of DMPZ.

For numbering of atoms refer fig.4.1 (b).

No.(i) Symbol Type Definition

Stretching

1 Ri N-N N1-N2

2-3 ri N-C N1-C5, N2-C3

4-5 Pi C-C C3-C4, C4-C5, C3-C7, C5-C9

6 Di N-H N1-H6

7-9 Ti C-H C4-H8

10-15 Qi CH(methyl) C7-H10,C7-H11,C7-H12,C9-H13,C9-H14,

C9-H15

Bending

16-20 βi bRing N1-N2-C3, N2-C3-C4, C3-C4-C5, C4-C5-N1,

C5-N1-N2.

21 αi N-N-H N2-N1-H6

22 αi C-N-H C5-N1-H6

23-24 σi N-C-C N2-C3-C7, N1-C5-C9

25-26 σi C-C-C C4-C3-C7, C4-C5-C9

27-28 δi C-C-H C3-C4-H8, C5-C4-H8

29-34 Φi C-C-H(methyl) C5-C9-H13,C5-C9-H14,C5-C9-H15,C3-C7-

H10,C3-C7-H11,C3-C7-H12.

35-40 θi H-C-H H13-C9-H14,H13-C9-H15,H14-C9-H15,H10-

C7-H11,H10-C7-H12,H11-C7-H12

Out-of-plane bending

41 ωi N-H C5-N2-N1-H6.

42-43 πi C-C(methyl) N2-C4-C3-C7, N1-C4-C5-C9

44 ψi C-H C3-C5-C4-H8

Torsion

45-49 i Ring N1-C2-C3-C4, N2-C3-C4-C5, C3-C4-C5-N1,

C4-C5-N1-N2, C5-N1-N2-C3.

50-51 i CH3 (C4,N1)-C5-C9-(H13,H14,H15),(C4,N2)-C3-

C7-(H10,H11,H12).

Table 4.4

Definition of local symmetry coordinates of PZ.

a= cos 144˚; b= cos 72˚ a

These symbols are used for description of the normal modes by PED in Tables 4.6. b

The internal coordinates used here are defined in Table 4.2.

No.(i) Symbol a Definition b

1 NN R1

2-3 NC r2, r3

4-5 CC P4, P5

6 NH D6

7-9 CH T7, T8, T9

10 Rbend1 (β10+a(β11+β14)+b(β12+β13)

11 Rbend2 (a-b)(β11-β14 )+(1-a)(β12-β13)

12 bNH (α15- α16) /√2

13-15 bCH (δ17-δ18)/√2, (δ19-δ20)/√2, (δ21-δ22)/√2

16 ωNH ω23

17-19 ωCH ψ24, ψ25, ψ26

20 tRtorsion1 b( 27+ 31)+a( 28+ 29)+ 30

21 tRtorsion1 a-b( 31- 27)+(1-a)( 29- 28)

Table 4.5

Definition of local symmetry coordinates of DMPZ.

a= cos 144˚; b= cos 72˚ a

These symbols are used for description of the normal modes by TED in Tables 4.7. b

The internal coordinates used here are defined in Table 4.3.

No.(i) Symbol a Definition b

1 NN R1

2-3 NC r2, r3

4-7 CC P4, P5, P6, P7

8 NH D8

9 CH T9

10-11 CH3ss (Q10+Q11+Q12)/√3, (Q13+Q14+Q15)/√3

12-13 CH3Ips (2Q10-Q11-Q12)/√6, (2Q13-Q14-Q15)/√6

14-15 CH3Ops (Q11-Q12)/√2, (Q14-Q15)/√2

16 Rbend1 (β16 + a( β17 + β20 ) + b(β18 + β19)

17 Rbend2 (a-b) ( β17- β20 ) + (1-a) (β18 - β19)

18 bNH (α21- α22) /√2

19-20 bCC (σ23- σ24)/√2, (σ25- σ26)/√2

21 bCH (δ27 – δ28)/√2

22-23 CH3ipr (2Φ29-Φ30-Φ31)/√6, (2Φ32-Φ33-Φ34))/√6

24-25 CH3opr (Φ30- Φ31)/√2, (Φ33- Φ34)/√2

26-27 CH3sb (-Φ29- Φ30- Φ31+ θ35+ θ36+ θ37)/√6, (-Φ32- Φ33- Φ34+ θ38+ θ39+ θ40)/√6

28-29 CH3ipb (-θ35- θ36-2 θ37)/√6, (-θ38- θ39-2 θ40)/√6

30-31 CH3OPb ( θ 35- θ 36)/√2, ( θ 38- θ 39)/√2

32 ωNH ω41

33-34 ωCC π42, π43

35 ωCH ψ44

36 tRtorsion1 b( 45+ 49)+a( 46+ 48)+ 47

37 tRtorsion1 a-b( 48- 46)+(1-a)( 49- 45)

38-39 CH3 50, 51

Table 4.6

Diagonal stretching force constants of PZ and DMPZ

a The atoms indicated in the parenthesis belongs to CNB; for numbering of atoms refer Figs. 5.1 (a)

and (b).

b Stretching force constants are given in mdyn

0

A−1

.

Descriptiona

Force constantsb

PZ DMPZ

N1-N2 6.1 5.7

N2-C3 6.2 7.2

N1-C5 7.79 8.1

C3-C4 6.1 6.1

C4-C5 7.2 7.4

N1-H5 6.6 6.0

C3-H7(C7) 5.4 4.7

C4-H8 5.5 5.6

C5-H9(C9) 5.4 4.8

C7-H10 − 5.0

C7-H11 − 4.9

C7-H12 − 4.9

C9-H13 − 5.0

C9-H14 − 4.8

C9-H15 − 4.8

Table 4.7

Observed and B3LYP/6-31 G* level Calculated vibrational frequencies (cm-¹) of PZ

Sl.

No Symmetry

species

Observed

frequencies (cm-1)

Calculated frequencies (cm-1) with

B3LYP/6-31G* force field

TED(%) among type of internal

coordinatesc

FTIR Raman unscaled scaled IRa (Ai) Ramanb (Ii)

Q1 A' 3450 − 3667 3450 70.111 100.171 NH(99)

Q2 A' 3155 − 3291 3166 1.014 126.649 CH(99)

Q3 A' − 3148 3273 3148 2.57 22.967 CH(99)

Q4 A' 3144 − 3259 3133 5.34 97.321 CH(99)

Q5 A' 1558 − 1585 1551 6.623 1.196 CCar(29),bCH(24),bNH(22), NCar(21)

Q6 A' 1468 − 1500 1472 6.373 2.597 NCar (38), bNH (29),bCH(14), CCar(10)

Q7 A' − 1401 1441 1409 14.391 20.533 CCar(53),bCH(22),bRing(15)

Q8 A' − 1361 1397 1369 4.407 6.77 bCH(38), NCar (36),bNH(11)

Q9 A' − 1263 1292 1263 2.358 8.788 bNH(41), NCar(31),bCH(11)

Q10 A' − 1156 1187 1159 0.893 21.201 NNar(42), CCar(26),bCH(19)

Q11 A' 1138 − 1156 1129 17.423 6.609 NCar(47),bCH(21), CCar(20),bNH(10)

Q12 A' 1046 − 1066 1034 7.534 4.434 bCH(53), CCar(38)

Q13 A' 1035 − 1055 1031 34.72 1.968 bCH(41), NNar(40), CCar(11)

Q14 A' 938 − 940 934 5.747 1.303 bRing(74),bCH(21)

Abbreviations used: R, ring; b, bending; d, deformation; sym, symmetric; asy, asymmetric; ω, ωagging; t, torsion; trig, trigonal; , stretching. Contributions larger than 10% are given. a Relative absorption intensities normalized with highest peak absorption equal to 1.0.

b Relative Raman intensities calculated by Eq.4.1 and normalized to 100

c For the notations used see Table 4.4

Q15 A' 918 − 922 918 9.429 2.772 bRing(90)

Q16 A 893 − 889 897 3.589 1.447 ωCH(83),tRing(17)

Q17 A 840 − 824 840 8.463 0.833 ωCH(74),tRing(15)

Q18 A 760 − 753 754 66.505 1.272 ωCH(80),tRing(20)

Q19 A 656 − 692 664 27.608 0.291 tRing(96)

Q20 A 619 − 637 609 0.058 0.035 tRing(95)

Q21 A 498 − 498 496 57.425 2.148 ωNH(78), tRing(21)

Table 4.8 Observed and B3LYP/6-31 G* level Calculated vibrational frequencies (cm-¹) of DMPZ

Sl.

No Symmetry

species Observed

frequencies (cm-1)

Calculated frequencies (cm-1) with

B3LYP/6-31G* force field

TED(%) among type of internal

coordinatesc

FT IR Raman unscaled scaled IRa (Ai) Ramanb (Ii)

1 A' 3302 − 3659 3283 59.54 96.789 NH(99)

2 A' 3202 − 3264 3191 2.708 66.752 CH(99)

3 A' 3040 − 3142 3033 10.488 57.01 CH3ips(100)

4 A' 3039 − 3139 3029 10.904 70.216 CH3ips (100)

5 A 2993 − 3097 2989 23.115 84.547 CH3ops(100)

6 A 2948 − 3087 2979 19.993 101.644 CH3ops(100)

7 A' 2946 − 3046 2936 28.695 186.513 CH3ss(100)

8 A' − 2931 3040 2931 39.395 174.323 CH3ss(100)

9 A' 1598 − 1633 1606 38.583 6.316 CCar(38), NCar(18), CCm(15),bCH(11)

10 A' − 1491 1540 1506 2.037 21.520 NCar(45),CH3ipb(18),bRing(10)

11 A' 1485 − 1529 1489 2.207 15.139 CH3ipb(57), NCar(11), CCm(10), CCar(10)

12 A 1474 − 1515 1472 6.352 39.319 CH3opb(52), CCm(13)

13 A 1467 − 1511 1470 5.188 20.168 CH3opb(94)

14 A' 1466 − 1503 1466 6.777 18.073 CH3ipb(89)

15 A' 1460 − 1471 1436 24.047 10.054 CCar(32),bCH(31), NCar(17)

16 A' − 1424 1454 1417 5.947 20.419 NCar(26), CCar(25),bNH(22),bRing(10)

17 A' 1378 − 1435 1385 3.482 14.951 CH3sb(44), CH3ipb(40)

18 A' − 1375 1433 1375 0.254 7.033 CH3sb(46), CH3ipb(43)

Abbreviations; R, ring; ss, symmetric stretching; ass, antisymmetric stretching; ips, in-plane stretching; ops-out-of-plane stretching; b, bending; sb, symmetric bending; ipb, in-plane bending; opb, out-of-plane bending; ipr, in-plame rocying; opr, out of plane rocking; d, deformation; sym, symmetric; asy, asymmetric; ω, wagging; t, torsion; trig, trigonal; s, stretching. Only contributions larger than 10% are given. a

Relative absorption intensities normalized with highest peak absorption equal to 1.0. b

Relative Raman intensities calculated by Eq.4.1 and normalized to 100 c

For the notations used see Table 4.4

19 A' 1267 − 1312 1270 21.874 4.137 bNH(34), Ncar(23), CCar(13)

20 A' 1156 − 1185 1169 1.227 1.094 bCH(63), CCm(22)

21 A' − 1153 1178 1136 14.03 1.645 NNar(77)

22 A 1029 − 1078 1028 2.15 8.755 bRing(55), CCm(16), CCar(15), NCar(11)

23 A − 1021 1075 1012 2.430 0.714 CH3opr(81), bCHm (17)

24 A 1011 − 1050 1010 0.788 1.118 CH3opr(70), bCHm (25)

25 A' 982 − 1020 991 22.007 6.677 bRing(43), CCar(13),bCHm(10), NCar(10)

26 A' 960 − 999 958 1.53 3.142 CH3ipr(58), CCar(22)

27 A' 950 − 994 953 2.894 0.697 CH3ipr(73)

28 A 781 − 803 781 34.911 1.016 ωCH(74),tRing(25)

29 A' 738 − 746 731 3.583 1.556 CCm(48), bRing(38)

30 A 662 − 688 679 11.74 0.361 tRing(95)

31 A 645 − 662 650 13.391 0.04 tRing(99)

32 A' − 593 592 579 0.816 10.77 CCm(45),bRing(41)

33 A 450 − 455 468 50.692 2.659 ωNH(97)

34 A' − 343 395 340 3.424 0.933 bCCm(87)

35 A − 250 329 290 0.639 2.511 ωCCm(63),tRing(32)

36 A' − 217 260 225 1.618 0.441 bCCm(86)

37 A − 180 182 165 3.067 0.669 ωCCm(86),tRing(12)

38 A − − 95 94 0.002 0.29 tCH3(49),tRing(45)

39 A − − 48 47 0.061 0.331 tCH3(64),ωCCm(21),tRing(13)

(a) (b)

Fig. 4.1 Molecular structure of

(a) Pyrazole along with numbering of atoms.

(b) 3, 5-dimethyl pyrazole along with numbering of atoms.

Wavenumber (cm-1)

Fig 4.2 Comparison of observed and calculated FTIR spectra of

pyrazole

(a) calculated with B3 LYP/6-31G*

(b) observed with KBr disc

(a)

(b)

Absorb

ance (

Arb

itr.

Units)

Wavenumber (cm-1)

Fig 4.3 Comparison of observed and calculated FT-Raman spectra of

3, 5-dimethyl pyrazole

(a) calculated with B3 LYP/6-31G*

(b) observed with KBr disc

(a)

(b)

Absorb

ance (

Arb

itr.

Units)

Wavenumber (cm-1)

Fig 4.4 Comparison of observed and calculated FTIR spectra of

3, 5-dimethyl pyrazole

(a) calculated with B3 LYP/6-31G*

(b) observed with KBr disc

(a)

(b)

Absorb

ance (

Arb

itr.

Units)

Wavenumber (cm-1)

Fig 4.5 Comparison of observed and calculated FT-Raman spectra of

3, 5-dimethyl pyrazole

(a) calculated with B3 LYP/6-31G*

(b) observed with KBr disc

(a)

(b)

Ram

an

In

tensity (

Arb

itr.

Units)