ft-raman and ftir spectra, normal coordinate analysis and ab initio computations of...

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Vibrational Spectroscopy 47 (2008) 10–20

FT-Raman and FTIR spectra, normal coordinate analysis and

ab initio computations of (2-methylphenoxy)acetic acid dimer

C. James a,e, C. Ravikumar a, Tom Sundius b, V. Krishnakumar c,R. Kesavamoorthy d, V.S. Jayakumar a, I. Hubert Joe a,*

a Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram 695015, Kerala, Indiab Department of Physical Sciences, University of Helsinki, P.O. Box 64, FIN-00014 Helsinki, Finland

c Department of Physics, Periyar University, Salem 636011, Indiad Material Division, Indra Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, India

e Department of Physics, Scott Christian College, Nagercoil 629003, India

Received 1 December 2006; received in revised form 4 January 2008; accepted 15 January 2008

Available online 1 February 2008

Abstract

The Fourier Transform Raman and infrared spectra of the crystallized herbicide (2-methylphenoxy)acetic acid (MPA) have been recorded in the

region 4000–400 cm�1. The geometry, intermolecular hydrogen bond, and harmonic vibrational frequencies of MPA have been investigated with

the help of B3LYP density functional theory (DFT) methods. The calculated molecular geometry has been compared with the experimental data

obtained from XRD data. The assignments of the vibrational spectra have been carried out with the aid of normal coordinate analysis (NCA)

following the scaled quantum mechanical force field methodology (SQMFF). The strong doubly hydrogen bonded interface of the dimerized

system is well demonstrated by the red shift in OH stretching frequency concomitant with the elongation of bond length. The most stable structure

of the dimer possesses center of symmetry and interaction energy of�83.642 kJ mol�1 after the basis set superposition error (BSSE) correction by

the counterpoise (CP) method. The natural bond orbital analysis (NBO) ascertains that the delocalization of unpaired electron of oxygen atom onto

the C–O bond causes double bond character.

# 2008 Elsevier B.V. All rights reserved.

Keywords: Vibrational spectra; DFT calculation; Normal coordinate analysis; Counterpoise method; NBO analysis

1. Introduction

Phenoxyacetic acid and its derivatives represent one of the

most widely used classes of herbicides [1–3] and pesticides [4].

Herbicidal phenoxyacetic acids are more active against broad-

leaved weeds as they cause cell elongation resulting to

abnormal growth and death of the plant [5]. Aryloxyacetic acid

compounds are proved to be potential candidates for the

treatment of insulin resistance and hyperglycemia [6–8].

Recently phenoxyacetic acids invoke much interest among

the spectroscopists owing to their novel bioactivities such as

anticancer, antitumour, anti-inflammatory, antimicrobial,

expectoration, plant growth regulation and inhibition of tillage,

etc. [9–11].

* Corresponding author. Tel.: +91 4712340767.

E-mail address: [email protected] (I. Hubert Joe).

0924-2031/$ – see front matter # 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.vibspec.2008.01.006

The advent of fast computers along with sophisticated

computational methods makes the task of solving various

structural chemical problems a simple. Ab initio DFT

computations have recently become an efficient tool in the

prediction of molecular structure, harmonic force fields,

vibrational frequencies and IR and Raman activities of

biological compounds [12,13]. Cox et al. has recently reported

[14] the crystal structure of (2-methylphenoxy)acetic acid

(MPA), C9H10O3, involving carboxylate groups of centrosym-

metrically related pairs of molecules. Although spectral studies

on related compounds are scant, vibrational spectral analysis of

phenoxyacetic acids and substituted phenoxyacetic acids have

been reported [15–17] in literature. In the present study, a

detailed spectral investigation of MPA has been performed

using the scaled quantum mechanical (SQM) force field

technique based on density functional theory (DFT) calcula-

tions [18]. The interaction energy is calculated with a view to

understand the salient features of intermolecular hydrogen

mailto:[email protected]

http://dx.doi.org/10.1016/j.vibspec.2008.01.006

C. James et al. / Vibrational Spectroscopy 47 (2008) 10–20 11

bonding interactions in the acetic acid cluster, which involve a

variety of chemical and biochemical processes [19]. The basis

set superposition error (BSSE) caused by the use of finite basis

sets in quantum chemical calculations has been corrected by the

popular counterpoise (CP) method of Boys and Bernardi [20].

The effect of CP correction on the geometries has been taken

into account and the correction is applied to recalculate the

interaction energy of the H-bonded dimer. The change in

electron density (ED) in the s* antibonding orbitals and E2

energies have been calculated by natural bond orbital (NBO)

analysis using DFT wave functions to give clear evidence of

stabilization originating in the hyperconjugation of hydrogen

bonded interactions.

2. Experimental

2.1. Sample preparation

The title compound (2-methylphenoxy)acetic acid (98%

Aldrich) was purchased from Sigma–Aldrich. Recrystallization

of MPAwas carried out as suggested by Cox and Hickey [14] by

slow evaporation method from the mixed solution of methanol

and ethanol. Colorless small crystals grew from the solution

within a day.

2.2. IR and Raman measurements

The infrared spectrum of the sample was recorded in the

region 4000–400 cm�1 using PerkinElmer Spectrum One FT-

IR spectrometer with the standard KBr pellet technique. The

resolution of the spectrum is 4 cm�1.

The Raman spectra of MPA were recorded with a single

stage imaging spectroscope [ISA Jobin-Yvon SPEX HR-320,

0.32 m, f4.1] fitted with liquid nitrogen cooled CCD detector

(2000 � 800 pixels with an active area of 30 mm � 12 mm)

and a 600 g/mm grating blazed at 1064 nm excitation with a

spectral slit width of 0.1 mm. NIR laser beam of power 100 mW

was focused on the sample kept in a capillary tube at an angle of

about 908 to the incident beam to a spot approximately 25 mm

in diameter. The scattered light was collected at an angle of 908relative to the incident laser beam with an f/1 lens and

collimated. The scattered laser light was removed using an

HSPF-5812 super notch filter (Kaiser Optical Systems Inc.) in

the collimated beam and Raman scattered radiation was

focused onto the spectroscope slit width with f/4 lens. Spectra

were collected for samples with 1000 scan accumulated for

over 30 min duration.

3. Computational details

The quantum chemical calculations of MPA monomer and

dimer have been performed using the B3LYP level of theory

supplemented with the standard 6-31G(d) basis set, using the

Gaussian 98 software package [21]. The optimized geometry

corresponding to the minimum on the potential energy surface

has been obtained by solving self-consistent field equation

iteratively. Harmonic vibrational wavenumbers have been

calculated using analytic second derivatives to confirm the

convergence to minima on the potential surface and to evaluate

the zero-point vibrational energies. The centrosymmetric

structure and mutual exclusion between the IR and Raman

spectra of the cyclic dimer have been confirmed by optimizing

the structure with C2h symmetry constraints. Multiple scaling of

the force field has been performed by the SQM procedure

[18,22] to offset the systematic errors caused by basis set

incompleteness, neglect of electron correlation and vibrational

anharmonicity [23]. Normal coordinate analysis on dimer has

been performed to obtain full description of the molecular

motion pertaining to the normal modes using the MOLVIB

program version 7.0 written by Sundius [24,25]. The Gaussian

calculated analytical force constants are used by MOLVIB in

the calculation of vibrational frequencies by diagonalization of

the dynamical matrix. The Raman activities (Si) calculated by

Gaussian-98 program have been suitably adjusted by the

scaling procedure with MOLVIB and subsequently converted to

relative Raman intensities (Ii) using the following relationship

derived from the basic theory of Raman scattering [26,27]:

Ii ¼f ðn0 � niÞ4Si

ni

�1� exp

��hcni

kT

�� ; (1)

where n0 is the exciting frequency (in cm�1 units), ni is the

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

universal constants, and f is the suitably chosen common

scaling factor for all the peak intensities. The simulated IR

and Raman spectra have been plotted using pure Lorentzian

band shapes with a bandwidth of (FWHM) of 10 cm�1.

The interaction energy on dimer formation through the

doubly hydrogen bonded interface was calculated using the

supermolecule method [28,29]. The interaction energy is

calculated using the formula:

DEi; j;... ¼ Ei; j;...ði j . . .Þ �X

i

EiðiÞ; (2)

where the terms in brackets indicate the basis set to be used. The

interaction energy calculated using this formula is slightly

overestimated due to BSSE. The BSSE is removed by using

the standard counterpoise method of Boys and Bernardi

[20,29], where all energies are calculated by considering the

basis set for the whole cluster. Another contribution added to

the interaction energy is the deformation energy which is the

energy change required to distort the molecules from their

geometry in isolation to that in the cluster [29,30]. The defor-

mation energy is of repulsive type and that can be calculated

using the formula:

Edeform ¼X

i

ðEcomplexi � Eisolated

i Þ; (3)

where the subscripts mention the geometry to be used.

The natural bonding orbitals (NBO) calculations [31] were

performed using NBO 3.1 program as implemented in the

Gaussian 98 package at the DFT/B3LYP level in order to

understand various second order interactions between the filled

Fig. 1. Optimized structure of MPA monomer calculated at B3LYP/6-31G(d).

Table 1

Optimized bond lengths (A) of MPA monomer and dimer by B3LYP/6-31G* in

comparison with XRD data

Bond length Monomer (m) Dimer (d) Expt. (e) Dd � m Dd � e

C1–C2 1.399 1.399 1.390 0.000 0.009

C2–C3 1.395 1.395 1.386 0.000 0.008

C3–C4 1.411 1.410 1.405 0.000 0.005

C4–C5 1.397 1.397 1.381 0.000 0.016

C5–C6 1.399 1.399 1.389 0.000 0.010

C6–H7 1.087 1.087 0.950 0.000 0.137

H5–H8 1.084 1.084 0.950 0.000 0.134

C2–H9 1.088 1.088 0.950 0.000 0.138

C1–H10 1.086 1.086 0.950 0.000 0.136

C3–C11 1.508 1.508 1.504 0.000 0.004

C11–H12 1.094 1.094 0.981 0.000 0.113

C11–H13 1.096 1.096 0.980 0.000 0.116

C11–H14 1.096 1.096 0.980 0.000 0.116

C4–O15 1.374 1.374 1.381 0.001 �0.007

O15–C16 1.403 1.403 1.416 0.000 �0.013

C16–H17 1.100 1.100 0.990 0.000 0.110

C16–H18 1.100 1.100 0.990 0.000 0.110

C16–C19 1.520 1.519 1.487 �0.001 0.032

C19–O20 1.357 1.322 1.322 �0.035 0.000

O20–H21 0.976 1.004 0.929 0.028 0.075

C19–O22 1.204 1.223 1.219 0.019 0.004

H21� � �O25 – 1.685 1.695 – �0.010

O22� � �H23 – 1.685 1.695 – �0.010

O25–C26 – 1.223 1.219 – 0.004

H23–O24 – 1.004 0.929 – 0.075

C. James et al. / Vibrational Spectroscopy 47 (2008) 10–2012

orbitals of one subsystem and vacant orbitals of another

subsystem, which is a measure of the intermolecular

delocalization or hyperconjugation. The hyperconjugative

interaction energy was deduced from the second-order

perturbation approach [32]

Eð2Þ ¼ �nshsjFjsi2

es� � es¼ �ns

F2i j

DE; (4)

where hsjF jsi2, or F2i j is the Fock matrix element between the i

and j NBO orbitals, es and es� are the energies of s and s*

NBO’s, and ns is the population of the donor s orbital.

4. Results and discussion

4.1. Optimized geometry

The optimized molecular geometry of the monomer and

dimer structures of MPA calculated using GAUSSIAN 98W

and GAUSSVIEW programs are shown in Figs. 1 and 2,

respectively. The complete optimized geometrical parameters

are given in Table 1 (bond angles and dihedral angles are

available in supplementary material) with the comparison of the

XRD data. The global minimum energy of MPA monomer

(Fig. 1) calculated by DFT structure optimization method is

�574.652 Hartrees. The molecules in dimer MPA (Fig. 2) are

bound together via doubly hydrogen-bonded interactions. The

total interaction energy (�81.391 kJ mol�1 with BSSE and

�83.642 kJ mol�1 without BSSE) estimated using Eqs. (2) and

Fig. 2. Optimized structure of MPA dim

(3) is comparable to the related values reported for acetic acid–

benzoic acid dimer [33]. This interaction arises largely through

the two equivalent stable hydrogen bonded O20–H21� � �O25 and

O24–H23� � �O22 contacts that result in increased stabilization.

This is well reflected as distortion in the molecular geometry

with respect to the isolated molecule. The O–H distances in the

groups involved in the hydrogen bonds are lengthened by

0.0283 A upon dimerization. The molecule apart from

hydrogen atoms is calculated to be planar with C3–C4–O15–

C16 torsion angle �1.018 as reported [14]. The X-ray data and

the calculated geometry agree well except the shortening of few

ring and methyl C–H bonds (�0.1 A) in crystals probably due

to the improper C–H� � �O hydrogen bonding in crystals. The

shortening of C19–O20 bond (0.035 A) upon dimerization is

due to the redistribution of partial charges on O20 atoms as the

unpaired electron is significantly delocalized and thereby the

C–O bond shows considerable double bond character typical of

er calculated at B3LYP/6-31G(d).

Table 2

Occupation numbers of the interacting NBOs with their respective energies

Parameters Occupancy (e) Energy (a.u.)

Monomer Dimer Docc. Monomer Dimer DE

n1(O25)a 1.977 1.952 �0.025 �0.6942 �0.6509 �0.0433

s* (O20–H21) 0.014 0.066 0.052 0.3831 0.5131 0.1300

n1(O22) 1.977 1.960 �0.017 �0.6942 �0.6931 �0.0011

s* (O24–H23)a 0.014 0.062 0.048 0.3831 0.5338 0.1507

s* (C19–O20) 0.104 0.080 �0.024 0.3386 0.3866 0.0480

s* (C16–O19) 0.076 0.065 �0.011 0.3571 0.3866 0.0295

s* (C19–O22) 0.021 0.026 0.005 0.6118 0.5776 �0.0342

p* (C19–O22) 0.206 0.249 0.043 0.0028 �0.0158 �0.0186

n1(O15) 1.959 1.966 0.007 �0.5438 �0.5548 0.0110

n2(O15) 1.849 1.861 0.012 �0.3121 �0.3091 �0.0030

n1(O20) 1.976 1.968 �0.008 �0.6257 �0.5754 �0.0503

n2(O20) 1.826 1.784 �0.042 �0.3434 �0.3441 0.0007

a Values for monomer are taken from identical NBOs of other unit.

Table 3

Second order perturbation theory analysis of Fock matrix in NBO basis

Donor (i) Acceptor ( j) E(2)a

(kcal mol�1)

E(j) � E(i)b

(a.u.)

F(i, j)c

(a.u.)

Within unit 1

n1(O22) s* (C16–O19) 0.53 1.08 0.022

n1(O22) s* (C19–O20) 4.77 1.08 0.065

n2(O22) s* (C16–O19) 20.05 0.69 0.108

n2(O22) s* (C19–O20) 22.98 0.69 0.115

n2(O20) p* (C19–O22) 58.33 0.33 0.125

n1(O15) s* (C3–C4) 0.62 1.13 0.024

n1(O15) s* (C4–C5) 6.53 1.15 0.077

n2(O15) s* (C4–C5) 27.36 0.35 0.093

From unit 1 to unit 2

n1(O22) s* (H23–O24) 9.21 1.23 0.095

n2(O22) s* (H23–O24) 18.97 0.84 0.116

n2(O22) s* (O24–C26) 0.06 0.72 0.006

From unit 2 to unit 1

n1(O25) s* (O20–H21) 10.46 1.16 0.099

n2(O25) s* (O20–H21) 15.60 0.82 0.105

n2(O25) s* (C19–O20) 0.06 0.69 0.006

Within unit 2

n1(O25) s* (O24–C26) 3.96 1.06 0.059

n2(O25) s* (O24–C26) 47.67 0.71 0.169

n2(O25) s* (C26–C27) 36.26 0.72 0.149

n2(O24) p* (C26–O25) 50.44 0.41 0.129

a E(2) means energy of hyperconjugative interactions; cf. Eq. (2).b Energy difference between donor and acceptor i and j NBO orbitals.c F(i, j) is the Fock matrix element between i and j NBO orbitals.

Table 4

Composition of H-bonded NBOs (in terms of natural atomic hybrids)

NBO Monomer Dimer DNBO

spn(O20–H21) sp3.73 sp2.38 +s

%s-char 21.71 29.56 +7.85

pol. O20% 75.66 77.54 +1.88

pol. H21% 24.34 22.46 �1.88

q(H21)/e 0.502 0.496 �0.006

q(O20)/e �0.717 �0.692 +0.025

spn(C19–O22) sp1.90 sp1.97 �s

%s-char 34.42 33.56 �0.86

pol. C19% 34.10 33.95 �0.15

pol. O22% 65.90 66.05 +0.15

q(C19)/e 0.812 0.834 +0.022

q(O22)/e �0.570 �0.628 �0.058


a carbonyl group. Similar effect is also noticed in bond angle

C19–O20–H21 with an increase of 3.88 over the isolated

molecule. The DFT calculation predicts that the O–H group is

energetically favored to be in cis position.

4.2. NBO analysis

NBO analysis is proved to be an effective tool for chemical

interpretation of hyperconjugative interaction and electron

density transfer (EDT) from filled lone electron pairs of the

n(Y) of the ‘‘Lewis base’’ Y into the unfilled antibond s* (X–H)

of the ‘‘Lewis acid’’ X–H in X–H� � �Y hydrogen bonding

systems [34]. NBO analysis has been performed on MPA

monomer and dimer in order to elucidate intermolecular

hydrogen bonding, intermolecular charge transfer (ICT),

rehybridization, delocalization of electron density and coop-

erative effect due to n(O)! s* (O–H). The intermolecular O–

H� � �O hydrogen bonding is formed by the orbital overlap

between the n(O) and s* (O–H) which results ICT causing

stabilization of the H-bonded systems. Hence hydrogen

bonding interaction leads to an increase in electron density

(ED) of O–H antibonding orbital. The increase of population in

O–H antibonding orbital weakens the O–H bond. Thus the

nature and strength of the intermolecular hydrogen bonding can

be explored by studying the changes in electron densities in

vicinity of O� � �H hydrogen bonds.

The NBO analysis of MPA in comparison between

monomer and dimer clearly manifests the evidences of the

formation two strong H-bonded interactions between oxygen

lone electron pairs and s* (O–H) antibonding orbitals. In

Table 2, the occupation numbers with their energies for the

interacting NBOs are given. The magnitudes of charges

transferred from lone pairs of n(O25) and n(O22) of the

hydrogen bonded O atoms into the antibonds s* (O20–H21) and

s* (O24–H23) being the H-donors, respectively was signifi-

cantly increased (0.05099e and 0.0479e) upon dimerization

providing unambiguous evidence about the weakening of both

bonds, their elongation and concomitant red shifts of their

stretching frequencies. Similar conclusion can be obtained

while considering the energy of each orbital. The stabilization

energy E(2) associated with hyperconjugative interactions

n1(O25)! s* (O20–H21), n2(O25)! s* (O20–H21),

n1(O22)! s* (O23–H24) and n2(O22)! s* (O23–H24) are

obtained as 10.46, 15.60, 9.21 and 18.97 kcal mol�1,

respectively (Table 3) which quantify the extend of

intermolecular hydrogen bonding. The differences in E(2)

energies are reasonably due to the fact that the accumulation of

electron density in the O–H bond is not only drawn from the

n(O) of hydrogen-acceptor but also from the entire molecule.

Unusually, rehybridization plays a negative effect in O20–H21

bond. It is observed in Table 4 that the s-character of O20–H21

hybrid orbitals increases (7.85%) from sp3.73 to sp2.38 that

leads to a conspicuous strengthening of O20–H21 bond and its

contraction. This shows the existence of a mesomeric structure


characterized by delocalization of electron density from the s*

(O20–H21) antibonding orbital to the remaining part of the

molecule. This is quite possible because the energy of s*

(O20–H21) antibonding orbital (0.12996 a.u.) is higher than the

energy of s* (C19–O20) antibonding orbital (0.04801 a.u.)

which supports the likelihood of the delocalization of ED from

the O–H to the C–O region. This is clearly reflected in the

geometry as bond C19–O20 contracts to an amount of 0.035 A

with respect to the monomer. Further, the second order

perturbation theory analysis of Fock matrix in NBO basis

shows that the n2(O22) and n2(O25) can readily interact with

the s* (O24–C26) and s* (C19–O20) antibonding orbitals,

respectively. H-bonded NBO in terms of natural atomic

hybrids (Table 4) also demonstrates that the redistribution of

natural charges in the O–H bonds as the hydrogen side of

the bond becomes less positive (�0.00585e at H21) which

destabilizes the H-bond. Because hyperconjugation and

rehybridization act in opposite directions, the compression

and elongation of the bond O–H is a result of a balance of

the two effects. However the hyperconjugative interaction

is dominant and overshadows the rehybridization effect

resulting a significant elongation in O–H bond (0.0283 A)

and a concomitant red shift (�67 cm�1) in stretching

frequency.

The ED in the carbonyl C O antibonding orbitals s* (C19–

O22) and p* (C19–O22) are increased significantly (0.00488e

and 0.04266e, respectively) upon dimerization which yields to

weakening the bond and its elongation (0.0191 A) associated

with the downshift of stretching frequency (�14 cm�1). This is

evident from the E(2) energy (58.33 kcal mol�1) of the

hyperconjugative intramolecular interaction n2(O20)! p*

(C19–O22). In addition, the s character of spn hybrid orbital

for the C19–O22 bond decreases from sp1.90 to sp1.97 upon

dimerization, which substantiates the bond weakening.

4.3. Vibrational spectral analysis

The assignment of fundamentals has been made based on

normal coordinate analysis following a force field calculation

with the same ab initio method that was employed for the

geometry optimization of the centrosymmetric dimer molecule.

The non-redundant set of internal coordinates for MPA dimer

has been defined in Table 5 (definition of internal valence

coordinates are available in supplementary material) more

similar to the ‘natural coordinates’ recommended by Pulay

et al. [35]. The ‘‘virtual bond’’ coordinates listed in Table 5

were used to express the bend and torsions motions of the H-

bonded ring of the dimer, as described in the butyrolactam

dimer example given by Pulay and coworkers [36]. In addition,

the selective scaling was incorporated according to the SQM

scheme using a set of 12 transferable scale factors (given in the

last column of Table 5) recommended by Rauhut and Pulay [18]

with the RMS frequency error 12 cm�1. The SQM frequencies

related to the observed peaks are presented in Table 6a (infrared

active modes) and Table 6b (Raman active modes) along with

detailed assignments showing symmetry species under C2h

point group. The observed FT IR, FT Raman spectra and

simulated theoretical spectra calculated at B3LYP/6-31G* level

are given in Figs. 3 and 4 for visual comparison. The DFT

calculation shows that the MPA monomer and dimer have

almost similar vibrational contributions except that associated

with intermolecular O–H� � �O hydrogen bonds owing to their

spectral equivalence. While comparing the IR and Raman, it is

found that most of the IR active bands are either weak or

inactive in Raman and vice versa due to possession of center of

symmetry in MPA dimer. The symmetry species under C2h

point group clearly demonstrates mutual exclusion between the

IR and Raman spectra and their corresponding activities

provide a better approximation to the IR and Raman spectra

observed in polycrystalline solid state. The assignments of

frequencies for the different functional groups are discussed

below.

4.3.1. Phenyl ring vibrations

The selection rule for ortho-disubstituted phenyl ring

allows four C–H stretching vibrations, viz. 2, 7b, 20a and 20b

in the range 3120–3010 cm�1 [37]. Although the DFT

predicts these bands, these are observed as inseparable in

IR. Usually Raman has one strong band in this zone [38]. The

medium intense band in IR at 3032 cm�1 and the strong band

in Raman at 3040 cm�1 are assigned to this mode. There are

five C–C stretching vibrations (8a, 8b, 19a, 19b and 14),

which are more substituent dependant. The degenerate mode

8a of o-disubstituted ring is expected to be in the range 1609–

1565 cm�1 and 8b extends from 1625 to 1586 cm�1 with 8a is

smaller than 8b. The 8b mode is observed as strong band in

Raman at 1600 cm�1 and as medium at 1593 cm�1 in IR

which is coupled with CH bending mode. The 8a mode

appears only in IR as weak at 1554 cm�1. There is

considerable percentage of C–H bend character involved in

these vibrations as the hydrogen and its carbon moving

oppositely while C–C stretching. The intensities of these

bands increased substantially due to the methyl substituent

having electron-donor properties. The strong hyperconjuga-

tive interaction between the lone pair electron of the O15

substituent and the s* (C4–C5) antibond orbital predicted by

NBO analysis (Table 3) is clearly reflected in both IR and

Raman spectra showing a downshift of C–C stretching mode

ca. 1308 and 1306 cm�1, respectively. Another important ring

mode is the in-plane C–H bend that are expected to have small

amount of C–C stretch interaction, which usually appear in

the region 1300–1000 cm�1. Of the four C–H in-plane

bending modes appear in IR, viz. 1308, 1242, 1169,

1136 cm�1 (3, 9a, 18b and 18a, respectively), 9a mode is

coupled with substituent oxygen stretching mode resulting

intensity enhancement. The C–H out-of-plane bending

vibrations are usually observed in the region 1000–

675 cm�1. These modes are designated as gauche in internal

coordinates definition and are identified as medium Raman

band at 994 cm�1 (17b), strong IR bands at 924 and 765 cm�1

(17a) and medium band in Raman at 850 cm�1 (11). Other

fundamentals of ring that show similar characteristics are ring

torsions, viz. trigonal deformation, asymmetric deformation,

out-of-plane asymmetric deformation and puckering.

Table 5

Definition of local symmetry coordinates (much like the natural internal coordinates) and the corresponding force constant (mdyn/A) with scale factors used

No. Symbol Definition Scale factorsa Force constant

Stretching

1–8 CHar r1, r2, r3, r4, r5, r6, r7, r8 0.9185 5.20

9–10 CH3ss (r9 + r10 + r11)/H3, (r12 + r13 + r14)/H3 0.9948 5.13

11–12 CH3ips (2r9 � r10 � r11)/H6, (2r12 � r13 � r14)/H6 0.9919 5.38

13–14 CH3ops (r10 � r11)/H2, (r13 � r14)/H2 0.9185 4.75

15–16 CH2ss (r15 + r16)/H2, (r17 + r18)/H2 0.9185 4.70

17–18 CH2ips (r15 � r16)/H2, (r17 � r18)/H2 0.9185 4.58

19–30 CCar R19, R20, R21, R22, R23, R24, R25, R26, R27, R28, R29, R30 0.9185 6.40

31–32 CCme R31, R32 0.9929 4.86

33–34 CC R33, R34 0.9929 4.53

35–36 CarO Q35, Q36 0.9185 5.67

37–38 OCml Q37, Q38 0.9185 5.21

39–40 COh Q39, Q40 0.9185 6.96

41–42 COdb Q41, Q42 0.8999 9.19

43–44 OH P43, P44 0.9916 4.73

45–46 HbO H45, H46 0.9933 0.41

47–48 HnbO V47, V48 – –

Bending

49–56 bCH (b49 � b50)/H2, (b51 � b52)/H2, (b53 � b54)/H2,

(b55 � b56)/H2, (b57 � b58)/H2, (b59 � b60)/H2,

(b61 � b62)/H2, (b63 � b64)/H2

0.9913 0.55

57–58 bCCme (b65 � b66)/H2, (b67 � b68)/H2 0.9913 0.90

59–60 bCarO (b69 � b70)/H2, (b71 � b72)/H2 0.9791 1.33

61–62 CH3sb (a73 + a74 + a75 � b79 � b80 � b81)/H6,

(a76 + a77 + a78 � b82 � b83 � b84)/H6

0.9913 0.60

63–64 CH3ipb (2a73 � a74 � a75)/H6, (2a76 � a77 � a78)/H6 0.9913 0.60

65–66 CH3opb (a74 � a75)/H2, (a77 � a78)/H2 0.9913 0.60

67–68 CH3ipr (2b79 � b80 � b81)/H6, (2b82 � b83 � b84)/H6 0.9913 0.72

69–70 CH3opr (b80 � b81)/H2, (b83 � b84)/H2 0.9913 0.67

71–72 CH2sci (5a85 + g87)/H26, (5a86 + g88)/H26 0.9618 0.85

73–74 OCCsci (a85 + 5g87)/H26, (a86 + 5g88)/H26 0.9791 1.61

75–76 CH2roc (b89 � b90 + b93 � b94)/2, (b91 � b92 + b95 � b96)/2 0.9618 0.79

77–78 CH2wag (b89 + b90 � b93 � b94)/2, (b91 + b92 � b95 � b96)/2 0.9618 0.73

79–80 CH2twi (b89 � b90 � b93 + b94)/2, (b91 � b92 � b95 + b96)/2 0.9618 0.67

81–82 Rtrid (d97 � d98 + d99 � d100 + d101 � d102)/H6,

(d103 � d104 + d105 � d106 + d107 � d108)/H6

0.9791 1.30

83–84 Rasyd (2d97 � d98 � d99 + 2d100 � d101 � d102)/H12,

(2d103 � d104 � d105 + 2d106 � d107 � d108)/H12

0.9791 1.37

85–86 Rasydo (d98 � d99 + d101 � d102)/2, (d104 � d105 + d107 � d108)/2 0.9791 1.28

87–88 bOCml w109, w110 0.9791 1.36

89–90 COipb (b111 � b112)/H2, (b113 � b114)/H2 0.9791 1.20

91–92 OCCb (2a115 � b111 � b112)/H6, (2a116 � b113 � b114)/H6 0.9913 1.00

93–94 OHb a117, a118 0.9618 0.91

95–96 bCOOH (b111 � a117 + n119 � n120)/2, (b113 � a118 + n121 � n122)/2 1.0309 1.17

97 b4OHOH (n123 � n124 + n125 � n126)/2 1.0309 0.15

Out-of-plane bending (wagging)

98–105 gCH v127, v128, v129, v130, v131, v132, v133, v134 0.9933 0.45

106–107 gCarO v135, v136 0.9618 0.67

108–109 gCCme v137, v138 0.9913 0.57

110–111 gCO v139, v140 0.9933 0.38

Torsion

112–113 tCH3 (t141 + t142 + t143 + t144 + t145 + t146)/H6,

(t147 + t148 + t149 + t150 + t151 + t152)/H6

0.9783 0.01

114–115 tOCml (t153 + t154)/H2, (t155 + t156)/H2 0.9783 0.05

116–117 Rpuck (t157 � t158 + t159 � t160 + t161 � t162)/H6,

(t163 � t164 + t165 � t166 + t167 � t168)/H6

0.9933 0.39

118–119 Rasyt (t157 � t159 + t160 � t162)/2, (t163 � t165 + t166 � t168)/2 0.9933 0.32

120–121 Rasyto (�t157 + 2t158 � t159 � t160 + 2t161 � t162)/H12,

(�t163 + 2t164 � t165 � t166 + 2t167 � t168)/H12

0.9933 0.35

122–123 tCH2 (t169 + t170 + t171)/H3, (t172 + t173 + t174)/H3 0.9383 0.03

124–125 tCO (t175 + t176 + t177 + t178 + t179 + t180)/H6,

(t181 + t182 + t183 + t184 + t185 + t186)/H6

0.9783 0.01


Table 6a

Vibrational assignment in the infrared spectrum of MPA dimer by normal coordinate analysis based on SQM force field calculations

Syma Frequency (cm�1) Aib Assignment with PED (%)c

Calc. Expt.

bu 3204 3443 w 4263.3 OH str (89)

3197 w

bu 3160 67.8 CH3ips (99)

bu 3077 3032 m 20.3 CHar str (99)

bu 3065 86.8

bu 3048 25.4

bu 3038 16.2

au 2980 2980 msh 45.9 CH3ss (99)

au 2966 2967 msh 28.5 CH3ops (100)

bu 2916 2919 s 34.0 CH2ips (100)

bu 2883 2875 msh 83.1 CH2ss (99)

– – 2787 m – OH str

2714 m

2588 m

bu 1744 1746 vvs 601.7 COdb str (68), COh str (10)

bu 1608 1593 m 69.1 8bCCar str (64), bCH (17)

bu 1593 20.5

bu 1523 1554 w 123.6 CH3ipb (41), bCH (28), 8aCCar str (16)

bu 1511 1531 w 31.1

bu 1503 11.4 CH3opb (92)

au 1485 1494 s 95.2 CH2sci (78)

bu 1467 81.3 bCH (51), CCar (24)

bu 1466 1454 m 60.5 CH2wag (21), CC (16), OHb (16), COh (15), bCOOH (15)

bu 1443 1421 vs 0.8 CH3sb (90)

bu 1381 1375 m 23.2 CH2wag (38), OHb (35), bCOOH (14)

bu 1320 1308 s 4.4 3bCH (46), CCar (31)

bu 1300 118.6 CCar (66), 9abCH (18)

bu 1268 1275 s 78.7 COh (29), OHb (12), CarO (13), CCar (10)

bu 1241 2.8 CH2twi (97)

au 1240 1242 vvs 959.0 CarO (20), CCar (20), COh (14), CH2wag (12), 9abCH (11)

bu 1207 1196 m 226.1 CCme (26), Rtrid (23), CCar (18), CarO (11), bCH (11)

bu 1183 1169 m 4.3 18bbCH (70), CCar (29)

bu 1138 1136 s 158.6 18abCH (30), CCar (28), OCml (14)

bu 1080 101.5 OCml (58), Rtrid (16)

bu 1072 1083 m 4.5 CH3opr (71)

au 1048 23.3 CCar (65), 18abCH (11)

au 1040 1043 m 0.5 CH2roc (82), gCO (11)

bu 1003 1020 w 8.2 CH3ipr (52), CCar (30), Rtrid (10)

au 968 0.4 17bgCH (87)

au 964 970 m 219.5 tOH (44), tCOOH (31), t4OHOH (11)

bu 929 5.1 CC (48), OCCsci (14), COh (10)

au 924 924 s 1.8 17agCH (89)

bu 851 851 m 1.0 11gCH (81)

au 841 830 w-sh 37.5 Rtrid (35), CarO (15), Rasyd (13)

bu 776 36.0 CCar (36), CCme (18), Rasydo (13)

au 761 765 vs 103.6 17agCH (72), Rpuck (13), gCarO (12)

au 720 718 w 3.4 Rpuck (60), gCarO (15), gCH (13), gCCme (10)

bu 680 678 m 63.1 COipb (36), bCOOH (17), Rasyd (11)

bu 596 3.4 Rasydo (33), Rasyd (29)

au 583 0.5 gCO (44), CH2roc (23), tCOOH (10)

Table 5 (Continued )

No. Symbol Definition Scale factorsa Force constant

126–127 tOH (t187 + t188)/H2, (t189 + t190)/H2 0.9783 0.10

128–129 tCOOH (y191 � y192 + y193 � y194)/2, (y195 � y196 + y197 � y198)/2 1.0309 0.08

130 t4OHOH (y199 � y200 + y201 � y202)/2 1.0309 0.03

131 t1butt (y203 � y204)/H2 1.0309 0.12

132 t2butt (y205 � y206)/H2 1.0309 0.12

a Incorporated only 12 different scale factor values in all for the dimer.


Table 6a (Continued )

Syma Frequency (cm�1) Aib Assignment with PED (%)c

Calc. Expt.

bu 565 566 w 8.7 OCCb (23), bCarO (22), Rasydo (11)

au 550 0.0 Rasyt (36), gCarO (22), Rpuck (15), gCH (14)

bu 499 499 w 9.1 Rasydo (21), bOCml (17), Rasyd (17)

au 453 446 m 2.1 Rasyto (55), gCCme (20), gCarO (10)

bu 438 34.5 bCCme (32), OCCb (17), HbO (13), bCarO (13)

bu 302 36.4 bCCme (33), HbO (23), OCCsci (16), bCarO (11)

au 276 0.6 gCCme (25), Rasyt (18), gCH (15), gCarO (15), Rasyto (14)

bu 248 2.7 bOCml (17), OCCsci (15), bCCme (14), Rasyd (13), CC (11)

au 180 2.0 Rasyt (49), Rasyto (16), Rpuck (10)

bu 145 16.9 HbO (27), bOCml (24), bCarO (23)

au 139 15.0 tOCml (45), tCO (20), tCH3 (11)

au 125 0.7 tCH3 (76)

au 72 1.0 t4OHOH (41), tOH (24), tCOOH (13)

au 49 0.0 tCH2 (43), tOCml (34)

bu 28 0.3 OCCsci (30), OCCb (21), HbO (19), bOCml (18)

au 15 0.4 tCO (69), tCH2 (17)

au 10 0.0 tCO (24), tOCml (23), tCH2 (19)

The notations in superscripts are as described in Varsanyi [37]. vs: very strong; s: strong; m: medium; msh: medium shoulder; w: weak; R: ring; me: methylene; ml:

methyl; ar: aromatic; db: double bond; str: stretching; b: bending; t: torsion; g: gauche; ss: symmetric stretching; ips: in plane stretching; ops: out of plane stretching;

ipb: in plane bend; opb: out of plane bend; sb: symmetric bending; wag: wagging; twi: twisting; roc: rocking; ipr: in plane rocking; opr: out of plane rocking; trid:

trigonal deformation; asyd: asymmetric deformation; asydo: out of plane asymmetric deformation; puck: puckering; butt: butterfly.a Symmetry species of C2h symmetry.b Calculated IR intensities in km mol�1.c The definitions of the internal coordinates are given in this table.


4.3.2. Methyl and methylene vibrations

The asymmetric and symmetric stretching modes of methyl

group attached the benzene ring are usually downshifted due to

electronic effects [39] and are expected near 2925 and

2865 cm�1 for asymmetric and symmetric stretching vibra-

tions. The asymmetric stretching mode is observed as a medium

intense band as out-of-plane vibration (labeled as CH3ops in

Table 6a) at 2967 cm�1 in IR. The counterpart in Raman is

missing, however, the asymmetric in-plane vibration is shifted

towards higher wavenumber (�3115 cm�1) side with weak

intensity. The reason for this increase is the change in the s

character (25.27%, sp2.95) of the hybridization state of the C11

atom arising from the hyperconjugation effect of the methyl

group, which results strengthening of the C–H bond. This is

clearly reflected in the experimental values (Table 1) of C–H

bond length by a decrease of 0.116 A, which is overestimated

by DFT calculations. The remaining normal modes of the

methyl group appear to be coupled with other modes located at

about 1554–1502 cm�1 the asymmetric deformations, at about

1421 cm�1 the symmetric deformation and in the range 1083–

1020 cm�1 the rocking modes. These characteristic frequencies

are in close agreement with those reported for the similar

compounds [40]. The vibrations corresponding to the bond

between the ring and the methyl group (labeled as CCme in

internal coordinates) are assigned in two categories. The first

one is the stretching that appears as medium intense band at

1196 cm�1 in both IR and Raman. Also the very strong Raman

band at 778 cm�1 has 18% of this stretch character because of

its association with ring C–C stretch. The second type is

bending mode, termed as gauche, which appears to be strong ca.

446 cm�1 in both IR and Raman. These bands are in agreement

with the frequencies predicated by the force field calculations.

There are series of overtone and combination bands observed in

IR in the range 2500–2000 cm�1 which is quite common in the

spectra having C–C stretching and C–H bending vibrations.

The medium band in IR observed at 2482 cm�1 is more

probably the overtone band of the ring modes ca. 1242 cm�1.

The asymmetric and symmetric CH2 stretching vibrations

normally appear strongly about 2926 and 2853 cm�1 [41]. The

NCA predicts that the strong IR band observed at 2919 cm�1 is

due to asymmetric CH2 stretching and the medium shoulder in

IR at 2875 cm�1 and strong band in Raman at 2880 cm�1 are

due to symmetric CH2 stretching.

4.3.3. Carboxylic acid vibrations

Vibrational analysis of –COOH group is significant because

the herbicidal activity of the title compound is mainly due to

either the presence of this moiety or a group that is easily

converted to it within the plant tissues [2,42]. Carboxylic acid

dimer is formed by strong hydrogen bonding in the solid and

liquid state. Vibrational analysis of carboxylic acid is made on

the basis of carbonyl group and hydroxyl group. The C O

stretch of carboxylic acids is identical to the C O stretch of

ketones, which is expected in the region 1740–1660 cm�1. The

hydrogen bonded dimer carboxylic acids possess a form that

has center of symmetry and hence the two monomers can

vibrate in-phase and out-of-phase with respect to each other.

The in-phase (symmetric) vibrations will be Raman-active only

whereas the out-of-phase vibrations will be IR-active only [41].

The asymmetrical C O stretch is observed in IR as very strong

band at 1746 cm�1 and it is inactive in Raman. But in the case

of symmetrical C O stretch, Raman is active as weak band ca.

1690 cm�1 and IR too active as strong shoulder band at

1699 cm�1, which may be due to slight loss of symmetry. The

Table 6b

Vibrational assignment of Raman bands of MPA dimer by normal coordinate analysis based on SQM force field calculations

Syma Frequency (cm�1) Ii Rb Assignment with PED (%)c

Calc. Expt.

ag 3160 3115 w 33.4 CH3ips (99)

ag 3105 241.5 OH str (89)

ag 3077 3040 s 83.5 CHar str (99)

bg 3065 78.5

ag 3048 47.6

ag 3038 33.7

bg 2980 2980 w 80.5 CH3ss (99)

bg 2966 39.8 CH3ops (100)

ag 2916 2920 s 27.1 CH2ips (100)

ag 2883 2880 s 62.0 CH2ss (99)

ag 1704 1690 w 19.1 COdb str (59), OHb str (11)

ag 1609 1600 s 87.5 8bCCar str (64), bCH (17)

ag 1593 29.9

ag 1523 18.0 CH3ipb (41) bCH (28), 8aCCar str (16)

ag 1511 13.7

ag 1503 1502 w 50.5 CH3opb (92)

ag 1487 31.7 CH2sci (78)

bg 1481 1480 w 35.8 CH2sci (16), OHb (14), bCOOH (13), CH2wag (12), COh (12)

ag 1467 1466 w 6.0 bCH (51), CCar (24)

ag 1443 1428 s 42.6 CH3sb (90)

ag 1397 1384 s 24.7 CH2wag (52), OHb (16)

ag 1320 12.3 3bCH (46), CCar (31)

ag 1300 1306 m 15.8 CCar (66), 9abCH (18)

ag 1260 1257 s 24.4 COh (21), CarO (19), CCar (17), 9abCH (12)

ag 1241 38.2 CH2twi (97)

bg 1238 1234 wsh 24.1 COh (29), CH2wag (13), CCar (13), CarO (12)

ag 1208 1196 m 6.0 CCme (26), Rtrid (23), CCar (18), CarO (11), bCH (11)

ag 1183 1170 m 36.5 18bbCH (70), CCar (29)

ag 1138 8.2 18abCH (30), CCar (28), OCml (14)

ag 1080 1090 w 6.0 OCml (58), Rtrid (16)

ag 1072 2.8 CH3opr (71)

bg 1048 1054 vs 75.5 CCar (65), 18abCH (11)

bg 1039 0.0 CH2roc (82), gCO (11)

ag 1003 1018 w 32.7 CH3ipr (52), CCar (30), Rtrid (10)

bg 968 994 m 0.6 17bgCH (87)

ag 927 936 s 64.0 CC (48), OCCsci (14), COh (10)

bg 925 5.3 17agCH (48), tOH (18), tCOOH (12)

bg 924 6.9 17agCH (44), tOH (19), tCOOH (12)

ag 851 850 m 23.5 11gCH (81)

bg 841 5.4 Rtrid (35), CarO (15), Rasyd (13)

ag 775 778 vvs 120.5 CCar (36), CCme (18), Rasydo (13)

bg 761 764 msh 11.7 17agCH (72), Rpuck (13), gCarO (12)

bg 720 716 m 1.8 Rpuck (60), gCarO (15), gCH (13), gCCme (10)

ag 666 59.5 COipb (36), bCOOH (17), Rasyd (11)

ag 595 28.7 Rasydo (33), Rasyd (29)

bg 594 13.8 gCO (44), CH2roc (23), tCOOH (10)

ag 557 560 m 36.8 bCarO (21), OCCb (19), Rasyd (13), Rasydo (12)

bg 551 1.8 Rasyt (36), gCarO (22), Rpuck (15), gCH (14)

ag 496 498 m 35.6 bOCml (20) Rasydo (20), bCarO (14), Rasyd (11)

bg 453 446 s 6.2 Rasyto (55), gCCme (20), gCarO (10)

ag 415 78.0 bCCme (34), OCCb (28)

ag 299 40.5 HbO (41), OCCsci (18)

bg 276 134.5 gCCme (25), Rasyt (18), gCH (15), gCarO (15), Rasyto (14)

ag 267 47.6 bCCme (32), bCarO (13), b4OHOH (11), OCCb (10)

bg 181 155.0 Rasyt (49), Rasyto (16), Rpuck (10),

ag 165 46.5 b4OHOH (40), OHb (19), HbO (10)

bg 150 4.5 tOCml (28), tOH (25), tCO (14), tCOOH (13)

bg 128 50.5 tCH3 (82)

ag 101 25.7 HbO (60), bOCml (14)

bg 88 354.0 tOCml (36), tOH (15), tCO (10)

ag 52 38.5 OCCsci (29), b4OHOH (17), bOCml (15), OCCb (11)

bg 38 1255.0 tOCml (39), tCO (34), tCH2 (20)

bg 24 11400 tCH2 (51), tCO (16), tOCml (13)

The notations in superscripts are as described in Varsanyi [37]. vs: very strong; s: strong; m: medium; msh: medium shoulder; w: weak; R: ring; me: methylene; ml: methyl; ar: aromatic; db: double

bond; str: stretching; b: bending; t: torsion; g: gauche; ss: symmetric stretching; ips: in plane stretching; ops: out of plane stretching; ipb: in plane bend; opb: out of plane bend; sb: symmetric bending;

wag: wagging; twi: twisting; roc: rocking; ipr: in plane rocking; opr: out of plane rocking; trid: trigonal deformation; asyd: asymmetric deformation; asydo: out of plane asymmetric deformation; puck:

puckering; butt: butterfly.a Symmetry species of C2h symmetry.b Calculated Raman intensities in arbitrary units cf. Eq. (1).c The definitions of the internal coordinates are given in this table.


Fig. 3. (a) FTIR spectra of MPA dimmer. (b) Simulated IR spectra of MPA

dimer calculated at B3LYP/6-31G(d).

Fig. 4. (a) Simulated Raman spectra of MPA dimer calculated at B3LYP/6-

31G(d). (b) FT Raman spectra of MPA dimer.


C–O stretching modes are distinctively active in IR and Raman

as strong bands at 1275 and 1257 cm�1, respectively. The

stretching frequency of C–O group attached to the phenyl ring

is significantly enhanced in both IR and Raman observed at

1242 and 1257 cm�1, respectively. This is due to the

conjugation with ring p system.

The MPA dimer contains two strong O–H� � �O hydrogen

bonds. The strength of these bonds has been well established

earlier, viz. increase of bond length, C–O–H bond angle and ED

to a value of 0.028 A, 3.88 and 0.05099e, respectively with

stabilization energies of 10.46 and 15.60 kcal mol�1. The huge

calculated IR intensity (�4263 km mol�1) obtained for the OH

stretching band at 3204 cm�1 corresponds to the observed

integrated intensity of the extremely broad but shallow OH

stretching band between 3300 and 2300 cm�1. The band width

is not reflected by the spectral simulation, however, the

intensity is much truncated in the simulated plot (Fig. 3) to

favor visual comparison. The weak line observed both in IR and

Raman at about 3440 cm�1 manifests the ‘‘unperturbed’’ OH

stretching vibration, which is in corroboration with the

monomer calculation and the reported values [16,17]. This is

quite true because the measurements were done in crystalline

state wherein four hydrogen bonds involve among the four

molecules in the unit cell [14] resulting to a complicated pattern

of OH stretch vibrations. The IR bands ca. 1699, 1454 and

1375 cm�1 and Raman bands ca. 1690 and 1384 cm�1 have

substantial O–H bending character with enhanced intensities

resulting from intermolecular hydrogen bonding interactions.

There are also considerable intensity enhancements noticed on

the torsional OH, COOH and butterfly bands observed in

Raman at 936 and 924 cm�1 in IR. The calculated low-

frequency bands below 400 cm�1 are out of range in the

measured spectra.

5. Conclusion

Geometry optimized at the B3LYP/6-31G(d) level calcula-

tion reveals that the O–H group is energetically favored to be in

cis position. The molecular structure is conformed theoretically

as near planar apart from the hydrogen atoms. Total interaction

energy �83.642 kJ mol�1 calculated after CP correction

substantiates the stabilization arising largely through the two

equivalent stable hydrogen bonded O20–H21� � �O25 and O24–

H23� � �O22 contacts. The decrease in C19–O20 bond length

shows that the bond is going from single to double bond

character due to delocalization of unpaired electron of oxygen

atom.

The NBO analysis clearly demonstrates the formation of two

strong H-bonded interactions. Hyperconjugation and rehybri-

dization act in opposite directions on the O–H bond, but the

hyperconjugative interaction is dominant and overshadows the

rehybridization effect resulting in a significant elongation in O–

H bond and a concomitant red shift in stretching frequency. The

increase of ED in the carbonyl C O antibonding orbitals s*

(C19–O22) and p* (C19–O22) causes weakening the bond and its


elongation associated with the downshift of stretching

frequency. The increase in the s character of the hybridization

state of methyl C11 atom due to hyperconjugation effect results

in strengthening of the C–H bond.

A complete vibrational analysis of MPA dimer has been

performed based on the SQM force field obtained by DFT

calculations with C2h symmetry constraints. The scaling factors

have been refined with an RMS error of 12 cm�1 between the

experimental and SQM frequencies. Possession of center of

symmetry is well evidenced by mutual exclusion between IR

and Raman spectra. The normal coordinate analysis envisages a

good agreement between the observed and calculated

frequencies. The red shifted broad and shallow O–H stretching

band offers valid spectral evidence for the existence of O–

H� � �O intermolecular hydrogen bonding.

Acknowledgements

The author CJ thanks University Grants Commission

(UGC), India for awarding Teacher Fellowship under FDP

scheme leading to Ph.D.

Appendix A. Supplementary data

Supplementary data associated with this article can be

found, in the online version, at doi:10.1016/j.vib-

spec.2008.01.006.

References

[1] J.M. Bollag, C.S. Helling, M. Alexander, Appl. Microbiol. 15 (6) (1967)

1393.

[2] R. Cremlyn, Pesticides, Wiley, New York, 1979.

[3] T. Cserhati, E. Forgacs, J. Chromatogr. B: Biomed. Sci. Appl. 717 (1/2)

(1998) 157.

[4] A.S. Crafts, Adv. Pest Control Res. 1 (1957) 39.

[5] L. Turker, Turk. J. Biol. 24 (2000) 291.

[6] R. Gurumurthy, K. Sathyanarayanan, M. Gopalakrishnan, Bull. Chem.

Soc. Jpn. 65 (1992) 1096.

[7] B. Karthikeyan, R. Gurumurthy, M. Gopalakrishnan, M. Selvaraju, Asian

Chem. Lett. 2/3 (1998) 97.

[8] C. Timchalk, Toxicology 200 (1) (2004) 1.

[9] M. Negwer, 7th ed., Organic Chemical Drugs and Their Synonyms, vol. 1,

Akademie Verlag, Berlin, 1996.

[10] H. Martin, The Scientific Principles of Crop Protection, Arnold, London,

1973.

[11] S. Sarac, C. Safak, H. Erdogan, U. Abbasoglu, Y. Gunay, Eczacilik Fak.

Derg. 11 (1) (1991) 1.

[12] J. Binoy, C. James, I. Hubert Joe, V.S. Jayakumar, J. Mol. Struct. 784

(2006) 32.

[13] C. James, A. Amal Raj, R. Reghunathan, V.S. Jayakumar, I. Hubert Joe, J.

Raman Spectrosc. 37 (2006) 1381.

[14] P.J. Cox, G. Hickey, Acta Cryst. E60 (2004) 771.

[15] V. Ashok Babu, G. Ramana Rao, Indian J. Pure Appl. Phys. 28 (1990) 1.

[16] B. Karthikeyan, B. Saravanan, Spectrochim. Acta A63 (2006) 619.

[17] N. Sundaraganesan, C. Meganathan, B. Anand, C. Lapouge, Spectrochim.

Acta A66 (2007) 773.

[18] G. Rauhut, P. Pulay, J. Phys. Chem. 99 (1995) 3093.

[19] E.M. Cabaleiro-Lago, M.A. Rios, J. Chem. Phys. 113 (21) (2000) 9523.

[20] S.F. Boys, F. Bernardi, Mol. Phys. 19 (1970) 553.

[21] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.

Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C.

Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain,

O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C.

Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala,

Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, B.

Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu,

A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J.

Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challa-

combe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C.

Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, GAUSSIAN 98,

Revision A9, Gaussian, Inc., Pittsburgh, PA, 1998.

[22] P. Pulay, G. Fogarasi, G. Pongor, J.E. Boggs, A. Vargha, J. Am. Chem. Soc.

105 (1983) 7037.

[23] J.B. Foresman, A. Frisch, Exploring Chemistry with Electronic Structure

Methods, 2nd ed., Gaussian, Inc., Pittsburgh, PA, 1996.

[24] T. Sundius, J. Mol. Struct. 218 (1990) 321.

[25] T. Sundius, Vib. Spectrosc. 29 (2002) 89.

[26] G. Keresztury, S. Holly, G. Besenyei, J. Varga, A.Y. Wang, J.R. Durig,

Spectrochim. Acta 49A (1993) 2007.

[27] G. Keresztury, in: J.M. Chalmers, P.R. Griffith (Eds.), Raman Spectro-

scopy: Theory in Handbook of Vibrational Spectroscopy, vol. 1, John

Wiley & Sons Ltd., New York, 2002, pp. 71–87.

[28] P. Hobza, R. Zahradnik, Intermolecular Complexes, Elsevier, Amsterdam,

1988.

[29] F.B. van Duijneveldt, J.G.C.M. van Duijneveldt-van de Rijdt, J.H. van

Lenthe, Chem. Rev. 94 (1994) 1873.

[30] G. Chalasinski, M.M. Szczesnaik, Chem. Rev. 94 (1994) 1723.

[31] E.D. Glendening, A.E. Reed, J.E. Carpenter, F. Weinhold, NBO Version

3.1, TCI, University of Wisconsin, Madison, 1998.

[32] J. Chocholousova, V. Vladimir Spirko, P. Hobza, Phys. Chem. Chem.

Phys. 6 (2004) 37.

[33] C.K. Nandi, M.K. Hazra, T. Chakraborty, J. Chem. Phys. 121 (16) (2004)

7562.

[34] A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88 (1988) 899.

[35] P. Pulay, G. Fogarasi, F. Pang, J.E. Boggs, J. Am. Chem. Soc. 101 (1979)

2550.

[36] G. Fogarasi, X. Zhou, P.W. Taylor, P. Pulay, J. Am. Chem. Soc. 114 (1992)

8191.

[37] G. Varsanyi, Vibrational Spectra of Benzene Derivatives, Academic

Press, New York, 1969.

[38] F.R. Dollish, W.G. Fateley, F.F. Bentley, Characteristic Raman Frequen-

cies of Organic Compounds, Wiley, New York, 1974.

[39] B. Smith, Infrared Spectral Interpretation—A Systematic Approach, CRC

Press, New York, 1999.

[40] I. Lopez Tocon, M.S. Woolley, J.C. Otero, J.I. Marcos, J. Mol. Struct. 470

(1998) 241.

[41] D. Lin-Vein, N.B. Colthup, W.G. Fateley, J.G. Grasselli, The Handbook of

Infrared an Raman Characteristic Frequencies of Organic Molecules,

Academic Press, San Diego, 1991.

[42] G.S. Gruzdyev, V.A. Zinchenko, V.A. Kalinin, R.I. Slovtsov, The Che-

mical Protection of Plants, Mir Pub., Moscow, 1983.

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