Structural and electronic contributions to hyperpolarizability

in methyl p-hydroxy benzoate

D. Sajan a, Hubert Joe a, V.S. Jayakumar a,*, Jacek Zaleski b

a Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram-695 015, Kerala, Indiab Institute of Chemistry, University of Opole, Olesksa 48 45-052 Opole, Poland

Received 14 July 2005; received in revised form 11 September 2005; accepted 22 September 2005

Available online 11 November 2005

Abstract

The single crystals of methyl p-hydroxy benzoate (MPHB) are grown from methanol solution by a low temperature solution growth technique.

The SHG efficiency is tested using Q-switched Nd:YAG laser of wavelength l at 1064 nm, which is approximately 1.2 times that of urea.

Vibrational spectral analysis using NIR-FT Raman and FT-IR spectra is carried out to understand the structural and electronic contributions to

hyperpolarizability in MPHB. The DFT computations are also performed at B3LYP/6-311G(d,p) level to derive equilibrium geometry, vibrational

wavenumbers and intensities. The results of ab initio calculations at HF/6-311G(d,p) level show that the vibrational contribution for the second-

order electro-optic coefficient in MPHB is about 19.5%. Vibrational spectral studies also provide evidence for the charge transfer interaction

between the donors and the acceptor group through the p-system. The p-electron cloud movement from donor to acceptor can make the molecule

highly polarized and the intramolecular charge transfer interaction must be responsible for the nonlinear optical properties of MPHB. The splitting

of the carbonyl mode may be attributed to the intramolecular association based on CaO/H type hydrogen bonding in the molecule. The

conjugation and influence of intermolecular hydrogen bonding (CaO/H) type network in the crystal results in lowered CaO stretching

wavenumber.

q 2005 Elsevier B.V. All rights reserved.

Keywords: Vibrational spectra; NIR-FT Raman; FT-IR; Electronic effects; Charge transfer interaction; Hypepolarizability; Density functional calculations

1. Introduction

Nonlinear optics (NLO) is at the forefront of current

research because of its importance in providing the key

functions of frequency shifting, optical modulation, optical

switching, optical logic, and optical memory for the emerging

technologies in areas such as telecommunications, signal

processing, and optical interconnections [1–3]. Organic

materials have been of particular interest because the nonlinear

optical response in this broad class of materials is microscopic

in origin, offering an opportunity to use theoretical modeling

coupled with synthetic flexibility to design and produce novel

materials [4–6]. Also, organic nonlinear optical materials are

attracting a great deal of attention, as they have large optical

susceptibilities, inherent ultrafast response times, and high

optical thresholds for laser power as compared with inorganic

materials. Organic molecules with significant nonlinear optical

0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2005.09.041

* Corresponding author. Tel.: C91 471 253 0887; fax: C91 471 253 2536.

E-mail address: vsjk@vsnl.net (V.S. Jayakumar).

activity generally consist of a p-electron conjugated moiety

substituted by an electron donor group on one end of the

conjugated structure and an electron acceptor group on the

other end, forming a ‘push–pull’ conjugated structure [7]. The

conjugated p-electron moiety provides a pathway for the entire

length of conjugation under the perturbation of an external

electric field. The donor and acceptor groups provide the

ground state charge asymmetry of the molecule, which is

required for second-order nonlinearity. The peculiar physical

properties of this interesting class of compounds are governed

by the high degree of electronic charge delocalization along the

charge transfer axis and by the low band gaps [8,9]. Due to

their potential applications in photonic devices, the nonlinear

optical properties of molecules, and their hyperpolarizabilities

have become an area of extensive research, and lot of

experimental [10,11] and theoretical efforts [12,13] are focused

on bulk NLO properties as well as their dependence on the first

hyperpolarizabilities of molecules. Vibrational spectral studies

of the molecules can provide deeper knowledge about the

relationships between molecular architecture, nonlinear

response, and hyperpolarizability and support the efforts

towards discovery of new efficient materials for technological

Journal of Molecular Structure 785 (2006) 43–53

www.elsevier.com/locate/molstruc

Fig. 2. NIR-FT Raman spectrum of MPHB.

D. Sajan et al. / Journal of Molecular Structure 785 (2006) 43–5344

applications. NIR-FT Raman spectroscopy combined with

quantum chemical computations have recently been used as

effective tools in the vibrational analysis of drug molecules

[14], biological compounds [15], natural products [16,17] and

NLO active compounds [18], since fluorescence free Raman

spectra and computed results can help unambiguous identifi-

cation of vibrational modes as well as the bonding and

structural features of complex organic molecular systems. The

present work is a vibrational spectral study on the relationships

between molecular structural features, nonlinear response and

hyperpolarizability of methyl p-hydroxy benzoate (MPHB)

with special emphasis on the role of intramolecular charge

transfer (ICT) mechanism in such organic NLO materials.

2. Experimental

2.1. Preparation

The single crystals of methyl p-hydroxy benzoate (MPHB)

were grown in a methanol solution by low temperature solution

growth technique [19]. Colourless transparent crystals of title

compound were obtained within 2 weeks. Repeated recrys-

tallisation yielded good quality crystals. The single crystals of

maximum size of around 0.6 mm were obtained.

2.2. IR and Raman measurements

The IR spectrum (Fig. 1) of MPHB was recorded using

Perkin-Elmer Spectrum One FT-IR spectrometer in the region

450–4000 cmK1 with the sample in standard KBr pellet

technique with 1 mg sample per 300 mg KBr was used. The

spectral resolution was 2 cmK1. The NIR-FT Raman spectrum

(Fig. 2) was obtained on a IFS 66v BRUKER spectropho-

tometer with the powder sample taken in a capillary tube and

the Raman spectra were recorded using Nd:YAG laser

excitation of wavelength 1064 nm with 300 mW output and

liquid nitrogen cooled Ge-diode detector. About 1000 scans

were accumulated with a total registration time of about

30 min. The spectral resolution after apodization was 4 cmK1.

A correction according to the fourth power scattering factor

was performed, but no correction to instrumental was done.

Fig. 1. FT-IR spectrum of MPHB.

The upper limit for the Raman shift is 3500 cmK1 due to the

detector sensitivity and the lower Raman shift is around

10 cmK1 due to the Rayleigh line cut off by a notch filter.

2.3. Second harmonic generation efficiency measurements

The NLO SHG efficiency of MPHB (particle size 100–

200 mm) is measured by the Kurtz-Perry powder SHG method

[20] using Q-switched Nd:YAG laser of 1064 nm wavelength.

The input laser beam was passed through the sample after

reflection from an IR reflector. The output from the sample was

filtered by an IR filter to eliminate the fundamental and the

second harmonic was detected using monochromator and

PMT. The second harmonic generation efficiency of MPHB

was evaluated to be 1.2 times that of urea.

2.4. Crystal structure

MPHB crystallizes in space group Cc. From the single

crystal XRD data [19], it is observed that the crystal belongs to

monoclinic system with the following cell dimensions: aZ13.977(1) A, bZ15.774(1) A, cZ12.684(1) A, aZgZ908,

bZ131.978. The stability of the molecular conformations is

reached with a balance between intermolecular hydrogen

bonds of the hydroxyl groups with carbonyl oxygen(CaO/H). In both the intermolecular H-bonds, one hydroxyl oxygen

atom acts as a donor to the adjacent carbonyl oxygen.

2.5. Origin of nonlinear effects

Nonlinear optical effects arise from the interactions of

electromagnetic fields in various media to produce new fields

altered in phase, frequency, amplitude or other propagation

characteristics from the incident fields. When a beam of light is

impinged on to a material, it causes the charges of the atoms to

oscillate. In the linear regime the amount of charge

displacement is proportional to the instantaneous magnitude

of the electric field. The charges oscillate at the same frequency

as the frequency of the incident light. The oscillating charges

either radiate light at that frequency or the energy is transferred

into nonradiative modes that result in material heating or other

energy transfer mechanisms. With small fields, the displace-

ment of charge from the equilibrium position, polarization

Fig. 3. Molecular structure of MPHB.

D. Sajan et al. / Journal of Molecular Structure 785 (2006) 43–53 45

(P); will be proportional to the applied field, E:

Linear polarization : P Z P0 Ccð1Þij Ej

With sufficiently intense laser radiation, the relation may not be

valid, and must be generalized to

Nonlinear polarization : P

Z P0 Ccð1Þij Ej Cc

ð2Þijk EjEk Cc

ð3ÞijklEjEkEl C/

P0: static dipole moment, c(n): nth order susceptibility.At the

molecular level nonlinear polarization p,

p Z m01 CaijEj CbijkEjEk CgijklEjEkEl C/

with m01 molecular dipole moment, aij the linear polarizability,

bijk the first hyperpolarizability and gijkl the second hyperpo-

larizability, in which, each of the i, j, k and l denotes either of

the x, y and z axes determined arbitrarily. These tensors are

defined as coefficients of the Buckingham [21] type expansion

of the total energy with respect to the applied field FZ(Fx,Fy,Fz)

E Z E0KX

i

miFiK

1

2

Xij

aijFiFjK

1

6

Xijk

bijkFiFjFk K1

24

!Xijkl

gijklFiFjFkFl C/

where E0 is the energy of the unperturbed molecules, Fi is the

field at the origin m01, aij and bijk are the components of dipole

moment, polarizability and the first hyperpolarizabilities,

respectively. The total static dipole moment m01, the mean

polarizability a0, the anisotropy of the polarizability Da and

the mean first hyperpolarizability bijk, using the x, y, z

components they are defined as

m01 Z ðm2

x Cm2y Cm2

z Þ1=2

a0 Z ðaxx Cayy CazzÞ=3

Da Z 2K1=2½ðaxx KayyÞ2 C ðayy KazzÞ

2 C ðazzKaxxÞ2 C6a2

xx�1=2

bijk Z ðb2x Cb2

y Cb2z Þ

1=2

Here

bx Z ðbxxx Cbxyy CbxzzÞ

by Z ðbyyy Cbxxy CbyzzÞ

bz Z ðbzzz Cbxxz CbyyzÞ

Fig. 4. The calculated Raman wavenumbers B3LYP/6-311G(d,p).

2.6. Computational details

The density functional theoretical (DFT) computations were

preformed at the B3LYP/6-31G (d), and B3LYP /6-311G(d,p)

levels of the theory using GAUSSIAN’98 program package [22] to

get optimized geometry (Fig. 3) and vibrational wavenumbers

of normal modes of MPHB. Molecular geometries were fully

optimized by Berny’s optimization algorithm using redundant

internal co-ordinates. All optimized structures were confirmed

to be minimum energy conformations. Harmonic vibrational

wavenumbers were calculated using analytic second deriva-

tives to confirm the convergence to minima on the potential

surface and to evaluate the zero-point vibrational energies

(ZPVE). The inclusion of ‘d’ polarization and double-zeta

function in the split valance basis set is expected to produce a

marked improvement in the calculated geometry [23]. At the

optimized structure of the examined species, no imaginary

frequency modes were obtained, proving that a true minimum

on the potential energy surface was found. The optimum

geometry was determined by minimizing the energy with

respect to all geometrical parameters without imposing

molecular symmetry constraints. As suggested by Scott and

Radom [24] for a better comparison with experiment, the

calculated wavenumbers are scaled by an empirical factor of

0.9613. This scaling factor accounts for systematic errors

caused by basis set incompleteness, neglect of electron

correlation and vibrational anharmonicity [25]. The B3LYP/

6-311G(d,p) calculated vibrational wavenumbers are presented

Fig. 5. The calculated infrared wavenumbers B3LYP/6-311G(d,p).

D. Sajan et al. / Journal of Molecular Structure 785 (2006) 43–5346

in Table 2 and Raman and infrared intensities are shown in

Figs. 4 and 5. Atomic displacements corresponding to selected

normal modes of MPHB are shown in Fig. 6.

The first hyperpolarizability (bijk) of novel molecular

system, and related properties (b, a0 and Da) of MPHB are

calculated using HF/6-311G(d,p) basis set, based on the finite-

field approach. In the presence of an applied electric field, the

energy of a system is a function of the electric field. First

hyperpolarizability is a third rank tensor that can be described

by a 3!3!3 matrix. The 27 components of the 3D matrix can

be reduced to 10 components due to the Kleinman symmetry

[26]. It can be given in the lower tetrahedral format. It is

obvious that the lower part of the 3!3!3 matrixes is a

tetrahedral. The calculated first hyperpolarizability of MPHB is

2.315!10K30 esu, which is 11.8 times the corresponding

value for urea.

Fig. 6. Atomic displacement corresponding to selected vibrational modes of

MPHB.

3. Results and discussions

3.1. Optimized geometries

The calculated structural parameters for MPHB are listed in

Table 1. The optimized geometry shows that OH and CO2CH3

groups substituted in para position of phenyl ring are co-planar

which predicts maximum conjugation of molecule with donor

and acceptor groups. The Density functional calculation also

gives shortening of angle C6–C1–C7 by 1.78 and increase of

angle C2–C1–C7 by 2.7 from 1208 at C1 position and this

asymmetry of exocyclic angles reveals the repulsion between

CO2CH3 group and the phenyl ring. The asymmetry of the

exocyclic angles O10–C4–C3 and O10–C4–C5 is more at C4

position, which gives higher repulsion of OH group with the

phenyl ring. The reduction in the angle C1–C7–O15 to 112.58

from 1208 is associated with two repulsive interactions,

repulsion between methoxy group and O14, which is evident

from the enlargement in the angle O14–C7–O15 by 2.78. Similar

repulsion between O14 and C6–H12 leads to enlargement of C1–

C7–O14 by 4.78. The computed values of above-mentioned

angles correlate well with experimental results.

3.2. Vibrational spectral analysis

The vibrational analysis of MPHB is performed based on the

characteristic vibrations of methoxy, carbonyl, phenyl groups

and the hydroxyl group. Theoretical DFT calculations were

Table 1

B3LYP/6-311G(d,p) optimized geometry of MPHB

Bond lengths (A) Bond angles (8) Torsion angles (8)

C1–C2 1.398 C1–C2–C3 120.5 C1–C2–C3–C4 0.0

C1–C6 1.402 C2–C3–C4 119.9 C2–C3–C4–C5 0.0

C1–C7 1.483 C3–C4–C5 120.0 C3–C4–C5–C6 0.0

C2–C3 1.388 C4–C5–C6 119.7 C7–C1–C2–C3 179.9

C2–H8 1.082 C2–C1–C6 119.0 H8–C2–C3–C4 180.0

C3–C4 1.398 C6–C1–C7 118.2 H9–C3–C4–C5 180.0

C3–H9 1.087 C7–C1–C2 122.8 O10–C4–C5–C6 180.0

C4–C5 1.398 H8–C2–C3 119.9 H11–C5–C6–C1 179.9

C4–O10 1.360 H9–C3–C4 120.1 H12–C6–C1–C2 180.0

C5–C6 1.384 O10–C4–C5 117.3 H13–O10–C4–C3 0.0

C5–H11 1.084 O10–C4–C3 122.7 O14–C7–C1–C6 K0.01

C6–H12 1.083 H11–C5–C6 121.6 O15–C7–C1–C2 0.0

O10–H13 1.209 H12–C6–C1 118.6 C16–O15–C7–C1 179.9

C7–O14 1.354 H13–O10–C4 109.9 H17–C16–O15–C7 180.0

C7–O15 0.964 O14–C7–C1 124.8 H18–C16–O15–C7 60.3

O15–C16 1.434 O15–C7–C1 112.4 H19–C16–O15–C7 K60.3

C16–H17 1.087 C16–O15–C7 115.5 O14–C7–O15–C16 0.0

C16–H18 1.090 H17–C16–O15 105.5 C1–C7–O15–C16 179.9

C16–H19 1.090 H18–C16–O15 110.6

H19–C16–O15 110.6

D. Sajan et al. / Journal of Molecular Structure 785 (2006) 43–53 47

performed with B3LYP/6-311G(d,p) basis set. The MPHB

molecule is almost planar and has 51 normal modes of

vibrations. The computed vibrational wavenumbers, their IR

and Raman activities, depolarization ratios, force constants and

the atomic displacements corresponding to the different normal

modes (Fig. 6) are used for identifying the vibrational modes

unambiguously. The calculated vibrational wavenumbers,

measured infrared and Raman band positions and their

assignments are given in Table 2. The wavenumbers

corresponding to normal modes of vibration of benzene

(Fig. 7) were calculated at the same level to compare with

those of MPHB are given in Table 3.

3.3. Phenyl ring vibrations

The ring stretching vibrations are interesting, as the double

bond is in conjugation with the ring. The actual positions are

determined not so much by the nature of substituents, but by

the form of substituents around the ring [27–29]. The two

doubly degenerate ring stretching modes e2g (8a,8b) and e1u

(19a,19b) of benzene are not perturbed upon substitution. The

two resulting vibrational wavenumbers in the Raman spectrum

lie close to the benzene vibrations ca. 1600 and 1500 cmK1,

respectively, resulting from skeletal stretching modes of C–C

bonds. The doubly degenerate benzene vibration e2g consist of

lateral dilation and contraction of the ring, produced mainly by

stretching and compressing of the bonds. On the removal of the

degeneracy, the components of this vibration appear separately

(8a,8b). The doubly degenerate e1u mode of benzene is

basically a ring deformation, since it involves both stretching

and bending of the carbon bonds. Under reduced symmetry, the

two components appear separately, which are active in both IR

and Raman spectra. It is usually observed that one of the

components of these two wavenumbers is lowered while the

other component retains the energy of the degenerate vibration

in benzene. For p-disubstituted benzene, it is established that

the wavenumber of mode 8b, which is less substituent

sensitive, is lower compared to 8a. The phenyl ring mode 8a

manifests as very strong bands in IR and Raman spectra at 1609

and 1613 cmK1, respectively [29,30]. Its relatively weaker

companion 8b can be found as intense band in infrared

spectrum at 1588 cmK1and as a intense band in Raman at

1594 cmK1, their intensities being dependent on the algebraic

difference of the electronic effects of the substituents [27]. This

fact is justified by the large intensity differences observed

between 8a and 8b modes in both IR and Raman spectrum.

With donor substituents, the 19a mode in p-disubstituted

benzene can be expected above 1500 cmK1 with higher

intensity and 19b appears as weak band [29] around

1400 cmK1. These vibrations are expected to interact a little

with C–H in plane bending, hydrogen and its carbon moving

oppositely but the substituents are nearly motionless [27]. The

modes corresponding to 19a can be found at 1514 cmK1 in IR

spectrum and at 1518 cmK1 in Raman spectrum. 19b can be

observed as a medium intense band at 1434 cmK1 in IR

spectrum and the corresponding band in Raman is very weak at

1434 cmK1.The computed results are in good agreement with

the assignment of ring mode 19.

The carbon–hydrogen stretching vibrations give rise to

bands in the region 3100 to 3000 cmK1 in all the aromatic

compounds [28,29]. The C–H stretching vibrations in the

benzene derivatives arises from two nondegenerate modes a1g

(3072 cmK1), b1u (3060 cmK1) and two degenerate mode e2g

(3047 cmK1), e1u (3099 cmK1), i.e. vibrations 2, 13, 7 and 20,

respectively. In p-disubstituted benzenes, four of these modes

would give rise to four C–H stretching wavenumbers which

almost retain the wavenumbers of benzene and lie in the region

of 3000–3100 cmK1,while the other two modes substituents

stretching vibrations, which depends on the mass and nature of

the substituents and decrease considerably. The most intense

polarized Raman band at 3081 cmK1 with weak IR absorption

at 3080 cmK1 is assigned to mode 2 of benzene, which

corresponds to the aromatic C–H stretching. The weak intense

infrared band at 3028 cmK1 and the corresponding weak C–H

stretching band in Raman band at 3034 cmK1 have major

contribution from 7b. The C-H stretching mode 20a observed

on the low frequency side of benzene mode 2, can be found as a

weak shoulder in Raman spectrum at 3066 cmK1.The weak

overtone vibrations overlap in the region 2670 to 1918 cmK1

and are found to be broader.

In p-disubstituted benzene, the phenyl modes 3, 9a, 18a and

18b have been reported to have C–H in plane bending character

and can be expected in the region 1300–1000 cmK1.The six C–H

in plane bending vibrations are derived from a2g (1340 cmK1),

b2u (1200 cmK1), e2g (1178 cmK1), e1u (1037 cmK1) modes of

benzene. Among these 3, 18a and 18b could be identified as

medium bands in the region of in plane bending vibrations. The

intensity enhancement of 9a is due to the difference in the

character of two substitutents as reported earlier [27–29], and is

strongly coupled with methoxy deformations. The bands

corresponding to modes 3, 9a, 18b and 18a of benzene are

found, respectively, at 1315, 1193, 1118 and 1106 cmK1 in

Table 2

Calculated vibrational wave numbers, measured infrared and Raman band positions (cmK1) assignment for MPHB

B3LYP/

6311G(d,p)

ncal

nIR nRaman IR intensity Raman

activity

Depolariz-

ation ratios

Force

constants

Assignments

Absolute Relative Absolute Relative

3609 3309 sbr 50.5 8.4 151.57 100 0.29 9.01 O–H stretch

3192 sh O–H stretch

3086 3080 vw 3081 s 5.20 0.90 90.55 59.74 0.20 6.63 2 arom. C–H stretch

3069 3066 sh 4.09 0.71 79.73 52.60 0.52 6.53 20a arom. C–H stretch

3039 3034 w 3028 vw 21.05 3.50 80.66 53.21 0.69 6.52 7 arom. C–H stretch

2948 2963 m 2969 m 27.55 3.81 64.89 42.81 0.75 6.38 CH3 asym. stretch

2856 2848 vw 2852 w 43.82 4.63 149.33 98.52 0.02 5.67 CH3 sym. stretch

2670 w

2589 w

2458 vw Combination/overtone

2132 vw

1918 vw

1711 1692 vs 275.0 46.16 97.55 64.36 0.27 21.56 CaO stretch

1680 vvs 1680 vs CaO stretch

1604 1609 vs 1613 vs 145.0 24.34 118.05 77.90 0.46 9.32 8a ring stretch

1588 1588 vs 1594 vs 62.50 10.50 21.50 14.14 0.47 9.21 8b ring stretch

1505 1514 vs 1518 w 39.05 6.55 7.00 4.61 0.53 3.83 19a ring stretch

1459 1461 sh 1461 m 9.82 1.65 14.19 9.36 0.68 1.42 CH3 umbrella mode

1429 1434 vs 1434 m 8.47 1.42 17.54 11.6 0.75 1.39 19b ring stretch

1390 1380 w 1367 vvw 18.67 3.14 3.92 2.59 0.11 1.67 CH3 umbrella mode

1355 1361 w 14.41 2.42 2.54 1.68 0.40 3.14 CH3 umbrella mode

1327 1315 s 1318 s 26.17 4.30 1.02 0.67 0.61 3.08 3 C–H i.p. bend

1286 1279 vvs 1288 vvs 22.95 3.81 2.10 1.38 0.44 1.80 C7–O15 stretch

1238 1233 s 1241 m 595.67 100 93.56 61.7 0.27 6.35 C7–O15 stretchCC–H i.

p. bend

1186 1193 s 1193 vvw 52.64 8.84 6.79 4.47 0.44 1.37 9a C–H i.p. bend

1163 1163 s 1169 s 165.10 27.72 7.53 4.96 0.48 1.22 CH3 rockingCC7–O15

stretch

1138 1127 m 187.89 31.12 13.64 9.00 0.16 0.99 CH3 twisting

1106 1118 m 1078 vw 0.72 0.11 4.42 2.91 0.75 1.05 18b C–H i.p. bend

999 1008 w 1008 vvw 4.93 0.82 0.42 0.27 0.58 1.63 C–H i.p. bend

955 956 s 964 w 11.98 2.00 4.33 2.88 0.64 4.25 12 ring i.p. bend

878 862 vs 0.30 0.001 0.56 0.37 0.75 0.72 1 ring breath

842 850 s 850sh 27.89 4.68 0.27 0.17 0.75 0.77 17a C–H o.p. bend

814 804 w 11.37 1.91 43.85 28.9 0.10 2.62 17b C–H o.p. bend

781 772 vs 772 vw 4.13 0.82 1.17 0.77 0.75 0.54 10a C–H o.p. bend

713 698 m 698 vvw 43.56 7.31 1.67 0.77 0.75 1.65 6b C–H o.p. bend

675 671 br 18.32 3.07 0.08 1.1 0.75 1.02 O–H o.p. bend

634 637 w 644 s 1.94 0.32 6.91 0.05 0.74 1.78 6a C–H o.p. bend

502 509 w 504 w 10.71 1.79 0.05 4.56 0.75 0.42 O–H torsion

486 497 w 0.75 0.13 0.64 0.03 0.75 0.79 O–H torsion

364 355 m 126.32 21.21 2.98 0.42 0.75 0.11 CH3 torsion

319 318 m 5.37 0.90 3.42 1.96 0.19 0.35 CH3 twisting

176 182 sh 2.63 0. 44 0.01 2.25 0.75 0.06 CH3 twisting

119 110 vs 0.37 0.001 0.59 0.39 0.75 0.01 CH3 twisting

85 82 sh 0.48 0.001 0.48 0.31 0.75 0.02 CH3 twisting

D. Sajan et al. / Journal of Molecular Structure 785 (2006) 43–5348

infrared spectrum, among which Raman active modes 3 and 9a

are found at 1318 and 1193 cmK1, respectively.

The absorption bands arising from C–H out of plane

bending vibrations are usually observed in the region at 1000–

675 cmK1[28,29]. The out of plane bending vibrations in the

benzene arises from b2g (995 cmK1), e2u (975 cmK1), e1g

(849 cmK1) and a2u (671 cmK1) modes of benzene. The 17a

C–H out of plane bending vibrations are observed as a strong

band at 850 cmK1 in Raman spectrum and the and 17b mode

observed in IR band at 804 cmK1. The modes correspond to

10a are observed as a intense band at 772 cmK1 in IR spectrum

and the corresponding Raman band at 772 cmK1 which are

supported by computations also. The ring breathing modes for

p-disubstituted phenyl ring with entirely different substituents

[29] have been reported to be strongly IR active with typical

bands in the interval 780–960 cmK1. This is confirmed by the

strong band in infrared spectrum at 956 cmK1, which finds

support from computed results. The phenyl ring modes 6b and

16a can be observed in IR spectrum at 637, 698 cmK1 and the

corresponding Raman bands are observed in the same region.

Fig. 7. Atomic displacement corresponding to selected vibrational modes of

benzene.

Fig. 7 (continued )

D. Sajan et al. / Journal of Molecular Structure 785 (2006) 43–53 49

3.4. Carbonyl group vibrations

Carbonyl group vibrations give rise to characteristic bands

in vibrational spectra and for this reason, such bands have

been subject of extensive studies [27,28]. The intensity of

these bands can increase due to conjugation or formation of

hydrogen bonds. The increase of conjugation, therefore, leads

the intensification of the Raman lines as well as increased

infrared band intensities. The carbonyl stretching vibrations in

Table 3

Calculated wavenumbers (cmK1) for the vibrational modes of benzene

B3LYP/6-311G(d,

p), ncal

B3LYP/6-311G(d,

p), nscaled

Calculated IR

intensity

Calculated Raman

intensity

Wilson number Symmetry species

normal co-ordinate

analysis (28)

Assignment

3212 3087 0.0 346 2 a1g Arom. C–H stretch

3201 3078 34 0.0 20b e1u Arom. C–H stretch

3200 3077 34 0.0 20a e1u Arom. C–H stretch

3186 3062 0.0 123 7b e2g Arom. C–H stretch

3185 3061 0.0 7 7a e2g Arom. C–H stretch

3175 3052 0.0 0.0 13 b1u Arom. C–H stretch

1657 1593 0.0 10 8a e2g Ring stretch

1656 1592 0.0 10 8b e2g Ring stretch

1532 1473 10 0.0 19a e1u Ring stretch

1531 1472 10 0.0 19b e1u Ring stretch

1387 1334 0.0 0.0 3 b2g Ring stretch

1357 1304 0.0 0.0 14 b2u Ring stretch

(Kekulae)

1209 1162 0.0 9 9a e2g C–H i.p. bend

1208 1161 0.0 9 9b e2g C–H i.p. bend

1186 1140 0.0 0.0 15 b2u C–H i.p. bend

1070 1028 4 0.0 18a e1u C–H i.p. bend

1069 1027 4 0.0 18b e1u C–H i.p. bend

1021 981 0.0 50 12 b1u Ring deformation

1020 980 0.0 0.0 1 a1g Ring breath

1011 972 0.0 0.0 5 b2g C–H o.p. bend

970 932 0.0 0.0 17a e2u C–H o.p. bend

969 931 0.0 0.0 17b e2u C–H o.p. bend

865 832 0.0 6 10a e1g C–H o.p. bend

864 831 0.0 6 10b e1g C–H o.p. bend

718 690 100 0.0 4 b2g C–H o.p. bend

695 668 0.0 3 11 a2u C–H o.p. bend

622 597 0.0 3 6a e2g C–C–C i.p. bend

621 599 0.0 0.0 6b e2g C–C–C i.p. bend

416 400 0.0 0.0 16a e2u C–C–C o.p. bend

415 399 0.0 0.0 16b e2u C–C–C o.p. bend

D. Sajan et al. / Journal of Molecular Structure 785 (2006) 43–5350

saturated esters are expected in the region 1750–1735 cmK1.

The stretching mode of carbonyl group of ester part may be

lowered to around 1720 cmK1 in the presence of conjugation.

In MPHB, the conjugation of C7aO14 bond with aromatic

ring, may increase its single bond character resulting in

lowered values of carbonyl stretching wave numbers. The

sharp intense band in infrared spectrum at 1680 cmK1 is

assigned to C7aO14 stretching mode. In the Raman spectra,

the carbonyl stretching mode has prominently split into

two components, 1691 and 1680 cmK1. The results of

computations give the wavenumbers of these modes to be

1711 cmK1. The splitting of the carbonyl mode might be

attributed to intramolecular association based on CaO/H

type hydrogen bonding in the molecule. The conjugation and

influence of intermolecular hydrogen bonding (CaO/H)

type network in the crystal results in lowered CaO stretching

wavenumber. When a carbonyl group is participating in

hydrogen bond and resonance [27] can occur, which puts a

partial negative charge on the oxygen atom accepting the

hydrogen bond and a positive charge on the atom donating the

hydrogen, the partial ‘transfer of allegiance’ of the proton

enhances resonance [CaO/H–XK4C–OK/H–XCa] and

lowers the CaO stretching wavenumbers. This mechanism

plays an important role in the NLO activity of MPHB. Thus in

MPHB, simultaneous infrared and Raman activation of

C7aO14 stretching modes clearly points to the charge

transfer interaction between hydroxyl group and phenyl

ring through the C7aO14 bond. The p-electron cloud

movement from donor to acceptor can make the molecule

highly polarized and the intra molecular charge transfer

interaction must be responsible for the NLO properties of

MPHB.

The strong intense bands at 1279 and 1288 cmK1 in IR and

Raman spectrum corresponding to the computed wavenumber

1286 cmK1 is assigned to C7–O15 stretching mode which is

coupled with C–H in plane bending modes. The intense band at

1256 cmK1 in infrared spectrum and the corresponding Raman

band at 1256 cmK1 have been assigned to C7–O15 stretch. But

these vibrational modes are coupled with C–H in plane mode,

so that they cannot be separated and identified in an

unambiguous manner. The intense band at 956 cmK1 in IR

spectrum and the weak band at 964 cmK1 in Raman have been

assigned to C7aO14–O15 bend which is supported by

calculation also. The strong IR band at 772 cmK1 and the

corresponding Raman band at 772 cmK1 can be attributed to

C7–O15–C16 bending modes.

D. Sajan et al. / Journal of Molecular Structure 785 (2006) 43–53 51

3.5. Methoxy group vibrations

Vibrational spectral studies on esters have shown that

asymmetric and symmetric methyl stretching bands can be

observed around 2960 and 2846 cmK1, respectively [27–30].

The asymmetric stretching and symmetric stretching modes of

methoxy group vibrations calculated to be 2948 and

2856 cmK1, respectively. The asymmetric stretching modes

of methoxy groups of the ester part MPHB are observed as a

medium shoulder in IR and Raman spectra around 2963 and

2970 cmK1, respectively, while the symmetric stretching

vibrational mode of the methoxy group MPHB are found in

the region 2848 and 2852 cmK1 in IR and Raman spectrum,

respectively. This lowering may be attributed to the electronic

effects simultaneously caused by back donation [28] and

induction due to the presence of oxygen atom [7,18,31]. For a

molecule containing methoxy group, the electronic charge is

back donated from the lone pair of oxygen atom to the s*

orbital of C–H bonds causing the weakening of C–H bonds[7].

This is followed by the increase of C–H bond distance and the

decrease of C–H force constants and can cause enhancement

of infrared band intensities of C–H stretching modes. Another

electronic effect, induction produces stronger polarization of

C–H bonds along with the increase of C–H force constant and

charge on the hydrogen atom and with the decrease of C–H

bond length and infrared C–H stretching intensity [32–36]. In

MPHB the methoxy hydrogen atoms are subjected simul-

taneously to back donation and induction which cause the

decrease of asymmetric methyl stretching wavenumbers and

the enhancement of infrared intensities as reported in

literature for similar molecular systems [7,18,33]. Thus the

C–H stretching wavenumber of the CH3 group is lowered in

IR spectrum and Raman spectrum, which may be attributed to

the electronic effects.

The asymmetric bending vibrations of methoxy group

normally appear around 1460 cmK1 [28]. The medium intense

bands at 1461 cmK1 in IR spectrum are assigned to the

methoxy group asymmetric bending modes. The umbrella

modes of methoxy group, vibrations are observed as medium

intense bands around 1380 cmK1 in infrared spectrum. The

electronic effects must be responsible for the observed

lowering of symmetric bending modes by about 10 cmK1

compared to the computed results. The rocking vibrations of

the CH3 group in MPHB are appearing as mixed vibrations.

The mixing of C–O stretch and CH3 rock, as predicted by

computations, result in the intense bands at 1163 cmK1 in IR

spectrum and the corresponding Raman bands at 1169 cmK1.

The torsional modes of methoxy group, which are strongly

coupled with some other vibrations, are observed around

182 cmK1 which is in agreement with the calculated results

also.

3.6. Hydroxyl vibrational modes

The hydroxyl stretching vibrations are generally [27–29]

observed in the region around 3500 cmK1. In MPHB, broad

band in IR spectrum at 3309 cmK1 corresponds to the O–H

stretching vibration. The calculated hydroxyl stretch is at

3609 cmK1 for the free O–H. Significant theoretical under-

estimation of the associated O–H stretching wavenumber in

the MPHB molecule about 300 cmK1 reveals a failure of the

harmonic approximation in describing vibrations of atomic

groups involved in intramolecular hydrogen bonds. This type

of interaction causes a significant anharmonicity in the

potential in which the atomic groups [37] involved in

hydrogen bonds vibrate. The strong broad band at 671 cmK1

infrared spectrum attributed to the O–H out of plane bending

mode. The band correlated with O–H torsional mode was

identified at 497 cmK1 in infrared spectra. Similar bands

positions of O–H torsional mode have been observed for other

molecules with intermolecular hydrogen bonding [37]. In the

case of compounds with free O–H bands was found below

400 cmK1. The H-bonds in molecules with interaction

between the hydroxyl group and the lone pairs are weaker

than these in typical hydrogen bonds. The experimental O–H

torsion mode was found at 509 and 504 cmK1 in infrared and

Raman spectrum, respectively. Hence, the bands position of

the O–H torsional mode may serve as a sensitive measure of

the strength of the interaction between the O–H group and the

lone pair electron of the neighboring oxygen atom.

3.7. Low wavenumbers vibrations of hydrogen bonds

The attractive interaction between the hydrogen donor

group and the acceptor moiety leads to the occurrence of new

vibrational degrees of freedom, the so-called hydrogen bond

modes [38]. Such modes are connected with elongations

changing the X/Y distance and/or the relative orientation of

the hydrogen bonded groups. Thus, they provide direct insight

into the structure of hydrogen bonds and into processes of bond

formation and cleavage. As such modes are characterized by a

high reduced mass of the oscillator and a small force constant

determined by the comparably weak attractive interaction

along the hydrogen bond, hydrogen bond modes occur at low

wavenumbers in the range between 50 and 300 cmK1. The low

wavenumber bands of the hydrogen bond vibrations are

generally found to be weak, broad and asymmetric in the

Raman spectrum. The low wavenumber degrees of freedom

such as librations as well as interaction-induced give rise to

additional absorption and Raman bands that frequently overlap

with the bands of the hydrogen bond modes. In addition, a

substantial spread of vibrational wavenumber occurs for

liquids with a multiple hydrogen bonding geometries, resulting

in a pronounced inhomogeneous broadening of the vibrational

bands. The band in Raman at 82 cmK1 has been attributed to a

translational motion of the hydrogen bonded molecules

including some bending component, the 182 cmK1 band to a

translational motion of the hydrogen bond. The lattice

vibrations of rotatory type are generally stronger in intensity

than the translatory type. The lattice modes in MPHB are found

to be very intense in Raman compared with other modes in the

high wavenumber region.

D. Sajan et al. / Journal of Molecular Structure 785 (2006) 43–5352

3.8. Vibrational contribution to hyperpolarizability

The first-order hyperpolarizability of organic molecules is

usually assumed to be of pure electronic origin, in addition to

the weak contribution from acoustic phonons. Recently, based

on the ab initio calculations of static first-order hyperpolariz-

abilities, it was realized that molecular vibrations and

associated nuclear relaxation may also contribute to the

hyperpolarizabilities of molecules. Due to the changed charge

distribution, the nuclei are displaced and then the nuclei and the

electrons relax until equilibrium is reached and the resulting

contribution to hyperpolarizability is called ‘optical phonon

contribution’ [39] or ‘(nuclear) relaxation contribution’ or the

‘(infrared) vibrational contribution’[32,40]. The vibrational

spectra (infrared and Raman) of these compounds show

common peculiar characteristics, which can be ascribed to

the presence of highly mobile and polarizable conjugated pelectrons strongly coupled with particular nuclear displace-

ments in the vibrational space. The above observation forms

the basis of the method known as effective conjugation

coordination (ECC) theory [31], developed to treat in an

analytic and compact way the vibrational problem of

conjugated organic systems. Infrared and Raman spectra can

be used not only to obtain structural information, but also to

evaluate the vibrational contribution ðbvnmpÞ to molecular

hyperpolarizabilities. In the case of second-order NLO proper-

ties, bvnmp is given by

bvnmp Z

1

4p2c2

Xk

1

n2k

vmn

vQk

� �vamp

vQk

� �C

vmm

vQk

� �vanp

vQk

� ��

Cvmp

vQk

� �vanm

vQk

� ��

where nk are vibrational frequencies, (m/(Qk and (a/(Qk are the

derivatives of the molecular dipole moment and polarizabilities

with respect to kth normal coordinate and the sum extends over

all vibrational normal modes [31]. These results show that the

clamped electro-optic response of MPHB, and most probably

also of other organic electro-optic materials, is not of pure

electronic origin, as it is usually assumed. There exists a

considerable contribution resulting from molecular vibrational

modes. The results of ab initio calculations at HF/6-311G(d,p)

level show that the vibrational contribution for the second-

order electro-optic coefficient in MPHB is about 19.5%. The

presence of such vibrational contributions can be inferred from

infrared and Raman spectra. Not every vibrational mode is both

infrared and Raman active. Though the IR and Raman spectra

for most of the modes are complimentary, the IR bands at 1607,

1588, 1513, 1279 and 1163 cmK1 and those at 1613, 1594,

1514, 1288 and 1168 cmK1 in the Raman spectrum are found

to be strongly and simultaneously active in both IR and Raman

spectra is evident from the spectra. This phenomenon is quite

unusual, since generally, even in the absence of inversion

symmetry, the infrared and Raman spectra are complimentary:

in most cases, the strongest bands in the Raman are weak in the

infrared and vice versa. These bands associated to specific

modes that are simultaneously strongly active both in IR and

Raman spectra provide evidence for the charge transfer

interaction between the donor and the acceptor group through

the p-system [31–36]. For the push pull (D-p-A) molecules,

instead the existence of one directional charge transfer from the

donor to the acceptor through the single double bond

conjugated path induces large variations of both the molecular

dipole moment and of the molecular polarizability during the

same vibrations, thus gaining at the same time strong IR and

Raman activity [32–36]. The eigenvector analysis reveals

(Fig. 6) that the corresponding vibrations 8 and 19 bands can be

mainly described as an in phase symmetric stretching of the

ring modes. This vibration spreads over the whole p-con-

jugated path with relevant vibrational amplitudes from almost

all the constituents parts of the molecule. These vibrations

favour the intramolecular charge transfer from the donor to the

acceptor and give rise to a large vibration of the dipole

moment, thus gaining a strong infrared activity. From another

point of view, these vibrations carries out the phenomenon of

the electron–phonon coupling in this conjugated material what

provokes at the same time to be very intense in the Raman

spectrum [33–35]. The p-electron cloud movement from donor

to acceptor can make the molecule highly polarized and the

intra molecular charge transfer interaction must be responsible

for the NLO properties of MPHB.

4. Conclusions

The single crystals of methyl p-hydroxy benzoate are

grown by methanol solution by a low temperature solution

growth technique. The NLO SHG efficiency is tested using

Q-switched Nd:YAG laser of wavelength l at 1064 nm, which

is approximately 1.2 times that of urea. The calculated first

hyperpolarizability of MPHB is 2.315!10K30 esu, which is

11.8 times the corresponding value for urea. The optimized

geometry shows that OH and CO2CH3 groups substituted in

para position of phenyl ring are co-planar which predicts

maximum conjugation of molecule with donor and acceptor

groups. The results of ab initio calculations at HF/6-311G(d,p)

level show that the vibrational contribution for the second

order electro-optic coefficient in MPHB is about 19.5%.

Vibrational spectral studies supported by DFT calculation

indicate that the IR bands at 1607, 1588, 1513, 1279 and

1163 cmK1 and those at 1613, 1594, 1514, 1288 and

1168 cmK1 in the Raman spectrum are found to be strongly

and simultaneously active in both IR and Raman spectra also

provide evidence for the charge transfer interaction between

the donors and the acceptor group through the p-system.

These vibrations favor the intramolecular charges transfer

from the donor to the acceptor and give rise to a large

vibration of the dipole moment, thus gaining a strong infrared

activity. From another point of view this vibrations carries out

the phenomenon of the electron–phonon coupling in this

conjugated material what provokes at the same time to be very

intense in the Raman spectrum. The p-electron cloud

movement from donor to acceptor can make the molecule

highly polarized and the intra molecular charge transfer

interaction must be responsible for the NLO properties of

D. Sajan et al. / Journal of Molecular Structure 785 (2006) 43–53 53

MPHB. The splitting and lowering of the carbonyl stretching

vibrational modes might be due to intramolecular association

based on CaO/H type hydrogen bonding in the crystal.

Acknowledgement

One of the authors(VSJ) thanks the University Grants

Commission, Govt of India, New Delhi for the award of a

minor research project.

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