Wavenumber (cm-1)

Fig 5.5 Comparison of observed and calculated FT-Raman spectra of p-

chloronitrobenzene

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

(b) observed with KBr disc

(a)

(b)

Ram

an

In

tensity (

Arb

itr.

Units)

CHAPTER-VI

HYDROGEN BONDING AND MOLECULER VIBRATIONS

OF 2-BROMO-4-CHLORO PHENOL AND

2, 4-DICHLORO PHENOL

Abstract

The vibrational spectra of 2-bromo-4-chloro phenol (BCP) and 2,

4-dichloro phenol (DCP) have been computed using B3LYP methodology and

6-31 G* basis set. The solid phase FTIR and FT-Raman were recorded in the

region 4000-4000 cm-1 and 4000-50 cm-1 respectively. The effects of

hydrogen bonding were studied between the OH and halogen (Br-, Cl-)

atoms. On the basis of SQm methodology, a normal coordinate analysis was

performed to assign the various fundamental frequencies according to the

potential energy distribution (PED). Simulation of infrared and Raman spectra

utilizing the results of these calculations led to a good overall agreement with

observed spectral patterns.

CHAPTER-VI

HYDROGEN BONDING AND MOLECULER VIBRATIONS OF

2-BROMO-4-CHLORO PHENOL AND 2, 4-DICHLORO PHENOL

6.1 INTRODUCTION

Phenols and their derivatives are biologically and industrially useful

compounds. The properties of phenol and its derivatives are determined by

their hydrogen and π-bonding systems [181]. Their principle use is in the

manufacture of phenol formaldehyde polymers [182]. It also used in the

manufacture of many products including insulation materials, adhesives,

paint, rubber, ink, dyes, illuminating gases, perfumes, soap and toys and

cosmetics including sunscreens, hair dyes and skin lightening preparations

[183, 184]. Phenol is used in embalming and research laboratories. It is a

product of the decomposition of organic materials, liquid manure and the

atmospheric degradation of benzene. It is found in some commercial

disinfectants, antiseptics, lotions and ointments. Phenol is active against a

wide range of microorganisms and there are some medical and

pharmaceutical applications including topical anesthetic and eardrops,

sclerosing agent. It is also used in the treatment of ingrown nails in the “nail

matrix phenolization method”. Another medical application of phenol is its use

as a neurolytic agent, applied in order to relieve spasms and chlonic pain.

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

charge distribution in molecules and consequently this greatly affects the

structural, electronic, and vibrational parameters. The bromo and chloro

groups are generally referred to as electron with drawing substituents in

aromatic systems. In BCP and DCP, the hydroxyl group is a powerful

activating group.

The modern vibrational spectrometry has proven to be an exceptionally

powerful technique for solving many chemistry problems. It has been

extensively employed both in the study of chemical kinetics and chemical

analysis. The problem of signal assignment however, as well as

understanding the relationship between the observed spectral features and

molecular structure, and reactivity can be difficult. Even identification of

fundamental vibrational wavenumbers often generates controversy.

Harmonic force fields derived from quantum mechanics are widely

used for the calculation of wavenumbers and the modes of normal vibrations.

Indeed, applying current quantum mechanical methods have opened up the

way for calculating the wavenumbers and intensities of spectral bands with a

minimum degree of arbitrariness (although the degree depends on the level of

the quantum-mechanical treatment) and finding rational explanation for a

number of chemical and physical properties of substances[24, 68, 69, 185].

However, for a proper understanding of IR and Raman spectra, a

reliable assignment of all vibrational bands is essential. Recently,

computational methods based on density functional theory are becoming

widely used. These methods predict relatively accurate molecular structure

and vibrational spectra with moderate computational effort. In particular, for

polyatomic molecules the DFT methods lead to the prediction of more

accurate molecular structure and vibrational wavenumbers than the

conventional ab initio restricted Hartree-Fock (RHF) and Moller-plesset

second order perturbation theory (MP2) calculation to understand the

structures and the fundamental vibrational wavenumbers of the title

compounds. DFT calculations are carried out to present a full description of

the vibrational spectra of these two important molecules, especially the

assignment of the vibrational modes, using B3LYP/6-31G*, to obtain the

geometrics, vibrational wavenumbers, IR intensities and Raman activities.

6.2 METHODS

6.2.1 Experimental Details

The commercial crystalline samples (99% purity) of BCP and DCP were

obtained from Lancaster chemical company, UK and used as such for the

spectral measurements. The Fourier trasform infrared spectrum of the title

compounds were recorded in the region 400–4000 cm-1 using Perkin-Elmer

spectrum RXI spectrophotometer equipped with He-Ne laser source, KBr

beam splitter and LiTaO3 detector. The samples were prepared by pressing

BCP and DCP with KBr into pellet form.

The FT-Raman spectra of BCP and DCP were recorded on a BRUKER

IFS–66V model interferometer equipped with an FRA-106 FT-Raman

accessory in the 4000-50 cm-1 Stokes region using the 1064 nm line of a

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

numbers are believed to be accurate within ± 1 cm-1.

6.2.2 Computational Details

In order to perform geometrical optimization and energy calculation of

the title compounds the GAUSSIAN 03 W software package [83] was used.

The calculations were carried out using the B3LYP functional [81, 82]

combined with standard 6-31 G* basis set. In order to express the normal

modes in a molecular fixed coordinate system, a set of local symmetry

coordinates for the title compounds were defined as recommended by Pulay

et al [186, 132]. The transformation of force field from Cartesian to internal

coordinates, the scaling, the subsequent normal coordinate analysis (NCA)

and the prediction of Raman intensities were done by employing the

procedure described in Chapter IV – section 4.3.

For the plot of spectra, digital versions of the observed and simulated

spectra of the title compounds were used. For visual comparison, these

spectra were plotted on a common frequency scale using ORIGIN software.

6.3 RESULTS AND DISCUSSION

6.3.1 Geometrical parameters

Geometrical optimization was first performed to establish reliable structures

using B3LYP/6-31G* basis set from the optimized structure were real and

positive and then the local minimum was defined to be a sTable conformer.

For BCP and DCP, the computationally predicted conformers are shown in

Figs 6.1(a)-6.1(b) and 6.2(a)-6.2(b), respectively. The total energies obtained

for these conformers are listed in Table 6.1. It is clear from the Table 6.1 that

the conformer in Fig 6.1(b) of BCP and Fig 6.2(b) of DCP produced the global

minimum energy and it forms the most sTable conformer as shown in Figs

6.1(c) and 6.2(c).

The calculated optimized geometrical parameters obtained in the study

for the title compounds presented in Table 6.2. The energy obtained in this

study is different which is clearly understandable. Since the environments of

the molecule are different. This clearly shows that the optimized structure

obtained for BCP and DCP are due to intramolecular hydrogen bonding. The

substitution of benzene with OH group and electronegativities (Br-, Cl-) in

ortho and para positions leads to intramoleculer hydrogen bonding.

In comparison BCP and DCP, both of the compounds having highly

electronegative atoms at the 2nd and 4th position, the electronic effect is

operating. Due to –I effect, the bond length is increased between C-Br and

C-Cl atoms. The bond length between carbon and chlorine atoms is shorter

than carbon and bromine atom due to chlorine highly electronegative than

bromine and hence the force constants are increased.

In para substitution both inductive and measomeric effects become

important and the domination of one over the other will decide the

wavenumber of absorption in the title compounds. In ortho substituted

compounds, the lone pair of electrons on two atoms influences each other

through space interactions and changes the vibrational frequencies of both

the groups. This effect is called field effect.

6.3.2 Force constants

The output of the quantum-mechanical calculations contains the force

constant matrix in Cartesian coordinates and 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

[187, 133] are given in Tables 6.3 and 6.4 for BCP and DCP, respectively.

The force fields determined were used to calculate the vibrational potential

energy distribution (PED) among the normal coordinates.

The bonding properties of BCP and DCP are influenced by the

rearrangements of electrons during substitutions and addition reactions. The

values of the stretching force constants between carbon atoms in BCP are

shorter than DCP due to bromine atom having high reduced mass so it

reduced the force constants. In the halogen compounds, the C-H stretching

shifts to higher wave number due to the –I effect of the halogen atom. Due to

the –I (inductive) effect of halogen, C-H part of the molecule becomes rich in

s-component and hence force constant increases. Since chlorine is more

electronegative, the bonded electrons between the carbon atoms are slightly

shifted towards the halogen atoms of the title compounds. The values of the

stretching force constants between carbon and chlorine atoms of BCP are

found to be lesser than the values of stretching force constant between

carbon and chlorine atoms of DCP due to chemical environmental changes.

The most important diagonal force constants (stretching only) of BCP and

DCP are listed in Tables 6.5

6.3.3 Molecular vibrations

The molecule BCP and DCP belongs to cS point group symmetry, 33

normal modes of vibrations of BCP and DCP are distributed among the

symmetry species as, Γvib = 23A′+10A″. The A′ and A″ represents the in-plane

and out-of-plane vibrations, respectively. All vibrations are active both in

infrared absorption and Raman scattering.

Detailed description of vibrational modes can be given by means of

normal coordinate analysis. For this purpose, the full set of (45 for BCP and

DCP) standard internal coordinates (containing 12 redundancies for both the

title compounds) and a non-redundant set of local symmetry coordinates were

constructed by suiTable linear combinations of internal coordinates following

the recommendations of Forgarasi and Pulay [132] et.al. The theoretically

calculated DFT force fields were transformed to this later set of vibrational

coordinates and used in all subsequent calculations.

The IR and Raman intensities and normal mode descriptions

(characterized by PED) for the title compounds (BCP and DCP) are reported

in Tables 6.6 and 6.7, respectively. For visual comparison the observed and

simulated FTIR and FT-Raman spectra of the title compounds are presented

in Figs 6.3 - 6.6, which help to understand observed spectral features.

6.3.4 SQM analysis and assignments

The unscaled frequencies obtained by B3LYP method are larger than

the experimental values of both BCP and DCP. The RMS errors between the

unscaled and experimental frequencies are 39.8 cm-1 and 35.1 cm-1

respectively for BCP and DCP. In order to reproduce the calculated

frequencies close to the observed frequencies, a selective scaling procedure

was employed. The calculated frequencies were scaled using a set of

transferable scale factors recommended by Rauhut and Pulay [136]. The

SQM treatment improved the agreement between the experimental and the

scaled frequencies considerately, leading to an RMS deviation of 9.3 cm -1 for

BCP and 10.4 cm-1 for DCP.

6.3.5 OH group vibrations

The free hydroxyl group stretching vibrations occur in the region

3690-3600 cm-1. Hydrogen bonding alters the frequencies of the stretching

and bending vibrations. The OH stretching bands move to lower frequencies

usually with increased intensity and band broadening in the hydrogen bonded

species. In the present study, the stretching vibration of hydroxyl group of

BCP was observed at 3616 cm-1 and 3439 cm-1 for DCP. This shift in

frequencies towards lower wavenumber reflects the strength of hydrogen

bond. The OH stretching vibrations are overlapped together with the weak

C-H stretching bands [151, 188, 189, 190]. The strong bands observed at

1105 cm-1 and 1409 cm-1 were assigned to the OH in-plane vibration of BCP

and DCP, respectively [191, 192].

The OH torsional vibration is very anharmonic and hence it is difficult to

reproduce this frequency with a harmonic approach. For BCP the frequency of

this vibration was observed at 389 cm-1 and for DCP 404 cm-1 [151].

6.3.6 C-H Vibrations

In Infrared spectrum, most of the aromatic compounds have strong

peaks in the region of 3100-3000 cm-1 due to C-H stretching vibrations. In

BCP these nodes are observed at 3088, 3071 and 3068 cm-1and in DCP

these modes are identified at 3092, 3081 and 3071 cm-1.

The C-H in-plane bending vibrations are usually weak and observed in

1000-1300cm-1 region [193,194]. The C-H out-of-plane bending modes are

observed in the region 1000 – 800 cm-1 [151, 195]. In the present work, the

bands observed at 1492, 1277 in FTIR and 1139 cm-1 in FT-Raman for BCP

and 1180, 1137, 1094 cm-1 in FTIR for DCP are assigned to C-H in-plane

bending vibrations. The C-H out-of-plane bending modes for BCP and DCP a

assigned within characteristic region [196] and are depicted in Tables 6.6 and

6.7.

6.3.7 Halogen vibrations

The vibrations belonging to the bond between the ring and the

halogen atoms are worth to discuss here, since mixing of vibrations are

possible due to the lowering of the molecular symmetry and the presence of

heavy atoms on the periphery of molecule. [174, 197]

(a) C-Cl vibrations

Most aromatic chloro compound has strong absorptions at 760-395

cm-1, which is due to a combination of vibrational modes and dihalogen-

substituted benzenes exhibit in the former band [174,151]. In the present

work, the C-Cl stretching vibrations are identified at 648 cm-1 in FTIR spectra

for BCP and 729 cm-1 and 452 cm-1 in FTIR and FT-Raman spectrum for

DCP. The C-Cl in-plane bending and out-of-plane bending modes for BCP

and DCP were also assigned within the characteristic region and were

presented in Tables 6.6 and 6.7.

(b) C-Br vibrations

The C-Br stretching vibration of the title compound has been

observed at 266 cm-1. The in-plane and out-of-plane bending vibrational

assignments of C-Br are shown in Table 6.6. These assignments are in good

agreement with the literature [151].

6.3.8 Ring vibrations

The ring stretching vibrations are very important in the spectrum of

benzene and its derivatives are highly characteristic of the aromatic ring itself.

The ring carbon-carbon stretching vibrations occur in the region 1430-1625

cm-1. For aromatic six membered rings there are two to three bands in this

region due to skeletal vibrations [151]. In the present work, the observed and

calculated wavenumbers are in excellent agreement with the literature. The

FTIR bands identified at 1590, 1403, 1323, 1100 and 844 cm-1 and FT-Raman

band at 1594 cm-1 in BCP are attributed to C=C stretching vibrations

[194,195]. The FTIR bands at 1595,1486,1371,1251 and 822 cm-1 and

FT-Raman band at 1596 cm-1 are assigned to C=C stretching vibrations in

DCP [194, 177]

The ring deformation vibrations are ascribed to the FTIR band at 1042

cm-1 and FT-Raman bands at 705 and 350 cm-1 in BCP and the FTIR band at

1054 and 653 cm-1 and FT-Raman band at 349 cm-1 in DCP [194, 198, 153,

199]. The out-of-plane deformations are established at 548 and 425 cm-1 in

FTIR spectra and 339 cm-1 in FT-Raman for BCP and 694 and 440 cm-1 in

FTIR spectra and 120 cm-1 in FT-Raman spectra for DCP [200, 194, 195]. The

calculated C-C out-of-plane and in-plane bending modes have been found to

be consistent with the recorded spectral values.

6.4 CONCLUSION

Complete vibrational analysis of BCP and DCP are performed on the

basis of DFT calculations at the B3LYP/6-31G* levels of theory. The

influences of hydroxyl group and in the vibrational wavenumbers of the title

compounds are discussed. The substituents of chlorine and bromine atoms in

the ortho position of phenol give rise to strong intramolecular hydrogen

bonding. The role of strong intramolecular hydrogen bonding on the molecular

geometry of the most sTable conformer and on the vibrational frequencies is

confirmed by the quantitative agreement between the calculated and the

observed band intensities and also polarization properties and it is believed to

be unambiguous.

Table6.1

Total energies (in Hartrees) based on B3LYP/6-31G* basis set for BCP

and DCP.

Conformers

Energy calculation

BCP DCP

a -3338.1613 -1226.6517

b -3338.1675 -1226.6560

Table 6.2

Optimized geometrical parameters of BCP and DCP obtained by

B3LYP6-31G* density functional calculations.

The atom indicated in tne parenthesis belongs to DCP; For numbering of atoms refer figs. 6.1 (c) and 6.2(c).

Bond length

Value(A°) Bond angles

Value(°)

BCP DCP BCP DCP

C1−C2 1.402 1.403 C1−C2−C3 121.895 121.730

C2−C3 1.391 1.392 C2−C3−C4 118.645 118.719

C3−C4 1.392 1.392 C3−C4−C5 120.816 120.845

C4−C5 1.395 1.395 C4−C5−C6 119.671 119621

C5−C6 1.390 1.390 C6−C1−O7 118.207 118.226

C1−O7 1.354 1.355 C1−C2−Br8(Cl8) 118.309 118.553

C2−Br8(Cl8) 1.916 1.762 C2−C3−H9 120.904 120.259

C3−H9 1.083 1.083 C3−C4−Cl10 119.872 119.284

C4−Cl10 1.757 1.757 C4−C5−H11 120.024 120.066

C5−H11 1.064 1.084 C5−C6−H12 120.937 120.937

C6−H12 1.085 1.085 C1−O7−H13 108.578 108.993

O7−H13 0.974 0.973

Table 6.3

Definition of internal coordinates of BCP and DCP.

The atom indicated in tne parenthesis belongs to DCP. For numbering of atoms refer figs. 6.1 (c) and 6.2(c).

No.(i) Symbol Type Definition

Stretching

1−6 ri C−C C1−C 2, C2−C3, C3−C4, C4−C5, C5−C6, C6−C1.

7−9 Ri C−H C3−H9, C5−H11, C6−H12.

10 Pi C−Br(Cl) C2−Br8(Cl8).

11 qi C−O C1−O7.

12 pi C−Cl C4−Cl10

13 Qi O−H O7−H13

Bending

14−19 δi bC−H C2−C3−H9, C4−C3−H9, C4−C5−H11, C6−C5−H11, C5−C6−H12, C1−C6−H12.

20−21 βi bC−Br(Cl) C1−C2−Br8(Cl), C3−C2−Br8(Cl8).

22−23 αi bC−O C6−C1−O7,C2−C1−O7.

24−25 γi bC−Cl C3−C4−Cl10,C5−C4−Cl10

26 ρi bO−H C1−O7−H13.

27−32 βi Ring C1−C2−C3, C2−C3−C4, C3−C4−C5, C4−C5−C6, C5−C6−C1, C6−C1−C2.

Out-of-plane bending

33−35 ωi gC−H H9−C3−C2−C4, H11−C5−C4−C6, H12−C6−C5−C1.

36 ωi gC−Br(Cl) C1−C3−C2−Br8(Cl8)..

37 ωi gC−Cl C3−C5−C4−Cl10.

38 ωi gC−O C2−C6−C1−O7

Torsion 39 i tO−H (C6)C2−C1−O7−H13.

40−45 i Tring C1−C2−C3−C4, C2−C3−C4−C5, C3−C4−C5−C6, C4−C5−C6−C1, C5−C6−C1−C2, C6−C1−C2−C3.

Table 6.4

Definitions of local symmetry coordinate of BCP and DCP.

a These symbols are used for description of the normal modes by PED in Tables 6.6 and 6.7.

b The internal coordinates used here are defined in Table 6.3.

No.(i) Symbol a Definition b

5 CC r1,r2,r3,r4,r5,r6

7−9 CH R7,R8,R9

10 CBr (CCl) P10 11 OH Q11

12 CCl p12

13 CO q13 14−16 bCH (δ14− δ15)/√2,( δ16− δ17)/√2,(δ18− δ19)/√2.

17 bCBr (bCCl) (β20−β21)/√2

18 bCO (α22 –α23)/√2

19 bCCl (γ24− γ25)/√2

20 bCOH ρ26

21 bRtrigd (β27− β28+ β29− β30+ β31− β32)/√6

22 bRsymd (−β27− β28+2 β29− β30+ β31−2 β32)/√12

23 bRasymd (β27− β28+ β29− β30)/√12

24−26 ωCH ω33, ω34, ω35

27 ωCBr (ωCCl) ω36

28 ωCO ω37 29 ωCCl ω38

30 OH 39

31 Rtrig ( 409− 41+ 42− 43+ 444− 45)/√6

32 Rsym (− 40+ 42+ 43− 44)

33 Rasym (− 40+2 41− 42− 43+2 44− 45)/√2

Table 6.5

Diagonal stretching force constants of BCP and DCP

a The atoms indicated in the parenthesis belongs to DCP; for numbering of atoms refer

Figs.6.1 (c) and 6.2(c)

b Stretching force constants are given in mdyn

0

A−1

.

Descriptiona

Force constantsb

BCP DCP

C1−C2 6.638 6.876

C2−C3 6.642 6.934

C3−C4 6.667 6.978

C4−C5 6.569 6.869

C5−C6 6.807 7.112

C6−C1 6.491 6.806

C1−O7 6.514 6.827

C2−Br8 (Cl8) 1.758 4.040

C3−H9 5.224 5.235

C4−Cl10 3.512 4.171

C5−H11 5.177 5.199

C6−H12 5.158 5.179

O7−H13 7.319 6.620

Table 6.6

Calculated frequencies (cm-¹) of BCP by B3LYP/6-31 G* method and vibrational assignment based on potential energy

distribution (PED).

Sl. No

Symmetry species

Observed frequencies (cm-1)

Calculated frequencies

B3LYP/6-31G* force field (cm-1)

IR Intensity

Raman activity

Characterization of normal modes with PED (%)

FTIR Raman unscaled scaled

Q1 A′ 3616 − 3663 3616 81.577 58.289 OH(100)

Q2 A′ 3088 − 3249 3087 0.561 56.296 CH(99)

Q3 A′ − 3071 3233 3077 1.723 146.678 CH(99)

Q4 A′ 3068 − 3218 3063 1.756 71.756 CH(99)

Q5 A′ − 1594 1649 1598 5.625 18.864 CC(65), bCH(16)

Q6 A′ 1590 − 1630 1592 15.568 11.742 CC(68), bCH(11)

Q7 A′ 1492 − 1523 1496 219.678 2.173 bCH(47), CC(36), CO(11)

Q8 A′ 1403 − 1454 1402 0.976 3.284 CC(53), bCH(24)

Q9 A′ 1323 − 1389 1329 3.767 2.914 CC(42), bCH(33), bOH(16)

Q10 A′ − 1292 1331 1296 80.798 6.603 CO(57), CC(24), bCH(10)

Q11 A′ 1277 − 1289 1265 20.293 2.427 bCH(42), CC(38), CO(13)

Q12 A′ 1175 − 1234 1174 156.597 1.684 bOH(39), bCH(29), CC(25)

Q13 A′ − 1139 1162 1134 48.337 5.670 bCH(37), CC(33), bOH(18)

Q14 A′ 1100 − 1115 1092 3.681 8.578 CC(42), CCl(22), bCH(14), Rtrigd(14)

Q15 A′ 1042 − 1049 1045 14.788 2.329 Rtrigd(52), CC(26), bCH(12)

Q16 A″ 863 − 957 896 0.206 0.897 ωCH(83),tRtrig(12)

Q17 A″ − 847 897 831 11.536 1.171 CH(79)

Abbreviations; R, ring; b, bending; d, deformation; sym, symmetric; asy, asymmetric; ω, ωagging; t, torsion; trig, trigonal; , stretching. Only contributions larger than 10% are given.

Q18 A′ 844 − 859 830 3.947 23.742 CC(38), CO(23), Rsymd(17), Rtrigd(11)

Q19 A″ 807 − 837 779 29.067 2.602 CH(75), tRasym(11)

Q20 A′ − 705 714 703 22.353 5.312 Rasym(70), CC(13), CBr(10)

Q21 A″ − 662 704 661 1.242 0.540 ωCO(66), tRtrig(10)

Q22 A′ 648 − 657 656 55.103 0.926 CCl(42), Rsymd(25)

Q23 A″ 548 − 561 559 4.476 0.560 tRasym(38), CCl(36)

Q24 A′ 497 − 500 496 21.251 1.198 bCO(61),bCBr(11)

Q25 A″ 425 − 454 422 0.000 0.105 tRsym(51), ωCBr(21)

Q26 A″ − 389 438 390 104.650 2.092 tOH(99))

Q27 A′ − 360 381 359 0.620 7.410 Rsymd59), CCl(27)

Q28 A″ − 339 341 333 0.293 0.804 tRtrig(43),ωCBr(17), ωCCl(16), ωCH(14)

Q29 A′ − 264 334 283 2.567 2.759 CBr(30), bCCl(30), bCBr(27)

Q30 A′ − 249 250 235 2.566 2.926 bCBr(46), CBr(32), bCO(10)

Q31 A′ − 167 164 143 0.168 3.114 bCCl(62),bCBr(22)

Q32 A″ − 140 160 141 0.034 0.794 ωCCl(45), tRasym(19) ,ωCBr(15),ωCH(14)

Q33 A″ − 125 126 122 0.046 3.256 ωCBr(50),tRsym(21), tRasym(20)

Table 6.7

Calculated frequencies (cm-¹) of DCP by B3LYP/6-31 G* method and vibrational assignment based on potential energy

distribution (PED).

Sl. No

Symmetry species

Observed frequencies (cm-1)

Calculated frequencies B3LYP/6-31G*

force field (cm-1)

IR Intensity

Raman activity

Characterization of normal modes with PED (%)

FTIR Raman unscaled scaled

Q1 A′ 3439 − 3700 3439 77.605 64.956 OH(100) Q2 A′ 3092 − 3240 3091 0.358 67.847 CH(99) Q3 A′ 3081 − 3232 3084 1.547 132.734 CH(99) Q4 A′ 3071 3071 3218 3069 1.883 69.569 CH(99) Q5 A′ 1596 1652 1602 3.998 20.075 CC(72), bCH(12) Q6 A′ 1595 − 1632 1595 29.096 9.904 CC(75) Q7 A′ 1480 − 1526 1486 243.508 3.367 CC(44), bCH(30), CO(18) Q8 A′ 1409 − 1454 1408 61.737 2.211 bOH(42), CC(36), bCH(12) Q9 A′ 1371 − 1380 1364 44.814 2.897 CC(69), bOH(15), bCH(13) Q10 A′ 1326 − 1331 1310 18.136 6.736 CO(43), CC(27), bCH(20) Q11 A′ 1251 − 1284 1256 101.055 2.152 CC(47), bOH(18), bCH(15)), CO(14) Q12 A′ 1180 − 1226 1190 67.098 1.985 bCH(56), CC(21), bOH(10)) Q13 A′ 1137 − 1165 1130 4.693 8.932 bCH(45), CC(26), CCl(19) Q14 A′ 1094 − 113 1092 12.283 3.179 bCH(46), CC(28), CCl(12) Q15 A′ 1054 − 1064 1044 20.283 1.199 Rtrigd(33), CC(23), bCH(19), CCl(18) Q16 A″ 955 − 950 948 0.206 0.754 ωCH(88) Q17 A″ 867 − 874 865 15.849 1.386 ωCH(86) Q18 A′ 822 − 864 844 17.312 16.080 CC(27), Rtrigd(23), CCl(18),Rsymd(14),

CO(13)

Abbreviations; R, ring; b, bending; d, deformation; sym, symmetric; asy, asymmetric; ω, ωagging; t, torsion; trig, trigonal; , stretching. Only contributions

larger than 10% are given.

Q19 A″ 816 − 831 825 29.518 2.948 ωCH(82)

Q20 A′ 729 728 739 61.458 9.620 CCl(38), CC(21), Rasymd(20)

Q21 A″ 694 − 694 694 1.426 0.263 tRtrig(64),ωCO(17), ωCCl(11)

Q22 A′ 653 − 658 646 12.626 4.616 Rasymd(39), CCl(27), Rsymd(11)

Q23 A″ 490 − 560 530 1.208 0.627 ωCO(28), ωCCl(22), tRsym(21), tRasym(19)

Q24 A′ 452 − 515 434 3.727 7.461 CCl(31), bCCl(23), bCO(17), Rasymd(14)

Q25 A″ 440 − 447 431 0.008 0.032 tRsym(41), tRasym(26), ωCCl(21)

Q26 A″ − 404 411 403 121.645 2.298 tOH(96)

Q27 A′ − 389 398 392 3.224 3.339 bCCl(57), Rasymd(14), CCl(12)

Q28 A′ − 349 379 336 0.097 5.794 Rsynd(76), CCl(11)

Q29 A″ − 341 340 312 0.047 0.818 ωCCl(49), tRasym(19), ωCO(16)

Q30 A′ − 203 282 209 5.221 0.421 bCO(67), bCCL(23)

Q31 A′ − 195 198 196 2.182 2.296 bCCl(79), bCO(10)

Q32 A″ − 139 174 154 0.071 2.396 ωCCl(75), tRsym(17)

Q33 A″ − − 125 120 0.056 0.525 tRasym(33), ωCCl(30), tRsym(18), ωCH(11)

(c)

Fig.6.1 (a) and (b) Two conformers of 2-bromo-4-chloro phenol

(c) STable conformers of 2-bromo-4-chloro phenol along with

numbering of atoms

(c)

Fig.6.2 (a) and (b) Two conformers of 2, 4-dichloro phenol

(c) STable conformers of 2, 4-dichloro phenol along with numbering of

atoms

Wavenumber (cm-1)

Fig 6.3 Comparison of observed and calculated FTIR spectra of

2- bromo- 4– chloro phenol

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

(b) observed with KBr disc

(a)

(b)

Absorb

ance (

Arb

itr.

Units)

Wavenumber (cm-1)

Fig 6.4 Comparison of observed and calculated FT-Raman spectra of

2- bromo-4- chlorophenol

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

(b) observed with KBr disc

(a)

(b)

Ram

an

In

tensity (A

rbitr.

Un

its)

Wavenumber (cm-1)

Fig 6.5 Comparison of observed and calculated FTIR spectra of

2,4 - dichlorophenol

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

(b) observed with KBr disc

(a)

(b)

Absorb

ance (

Arb

itr.

Units)

Wavenumber (cm-1)

Fig 6.6 Comparison of observed and calculated FT-Raman spectra of

2, 4- dichlorophenol

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

(b) observed with KBr disc

(a)

(b)

Ram

an I

nte

nsi

ty (

Arb

itr.

Un

its)