Electronic absorption, vibrational spectra and nonlinear optical properties of N-(2 chlorophenyl)-1-propanamide

Solid State Sciences 13 (2011) 175e184

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Electronic absorption, vibrational spectra and nonlinear optical properties of N-(2 chlorophenyl)-1-propanamide D. Sajan a, *, N. Vijayan b a b

Department of Physics, Bishop Moore College, Mavelikara, Alappuzha 690110, Kerala, India National Physical Laboratory, New Delhi 110 012, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 October 2010 Received in revised form 1 November 2010 Accepted 4 November 2010 Available online 16 November 2010

FT-Raman, IR and UVevis spectroscopies have been applied to investigate the potential nonlinear optical (NLO) material N-(2 chlorophenyl)-1-propanamide. A detailed interpretation of the vibrational spectra was carried out with the aid of normal coordinate analysis (NCA) following the scaled quantum mechanical force field methodology. Density functional theory is applied to explore the nonlinear optical properties of the molecule. The study suggests the importance of p-conjugated systems for nonlinear optical properties and the possibility of charge transfer interactions. Good consistency is found between the calculated results and experimental data for the electronic absorption, IR and Raman spectra. The solvent effects have been calculated using time-dependent density functional theory in combination with the polarized continuum model, and the results are in good agreement with experimental measurements. Ó 2010 Elsevier Masson SAS. All rights reserved.

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

1. Introduction In recent years conjugated organic nonlinear optical (NLO) materials have been attracting attention because of their second- or third-order hyperpolorizabilities compared to those of inorganic NLO materials [1]. Many investigations are being done to synthesize new organic materials with large second-order optical nonlinearities in order to satisfy day-to-day technological requirements [2]. They have innumerable potential applications including telecommunications, optical computing, optical data storage, etc. The conjugated molecules consist of a skeleton containing conjugated p- electrons; the conjugated bridge is linked to two end groups with electron donor (D) and electron acceptor (A) character, respectively. The electron acceptor group withdraws electronic charge from the donor through the conjugated bridge: as a consequence the p -electrons of the skeleton become polarized, giving rise to a relevant molecular dipole moment which defines a charge transfer axis roughly coincident with the chain axis of the conjugated system. These molecules are known as push-pull molecules [3,4]. The basic strategy of using electron-donor and electron-acceptor substituents to polarize the p -electron system of organic materials has been illustrious for developing the NLO

* Corresponding author. Tel.: þ919495043765; fax: þ914792303230. E-mail address: [email protected] (D. Sajan). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.11.009

chromophores possessing large molecular non-linearity, good thermal stability, improved solubility and processability [5,6]. Recently, much effort is being devoted to understand the origin of non-linearity in large systems and to relate the nonlinear optical (NLO) responses to electronic structure and molecular geometry for designing and fabricating the NLO materials of large molecular hyperpolarizability [7,8]. In this series, N-(2 chlorophenyl)-1propanamide (NCP) is one of the recently discovered potential organic NLO materials. It belongs to the monoclinic system with high electro optic and nonlinear optical coefficient In the present investigation, the growth of N-(2 chlorophenyl)-1-propanamide (NCP) single crystals and the detailed vibrational spectral analysis of the molecule in the crystalline state is taken up to understand the correlation of the NLO activity, charge transfer interactions of the molecule supported by using the scaled quantum mechanical (SQM) force field technique based on density functional theory (DFT) calculations.

2. Experimental 2.1. Preparation The title compound was synthesized by adopting the procedure reported in the literature [9]. The synthesized material of NCP was purified by repeated crystallization from the methanol solution

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until optically clear crystals were obtained. Purified NCP was dissolved in methanol solvent and the temperature of the solution was raised by 2  C to get the homogeneous mixture. The mixed solution was filtered and a small portion of the filtered solution taken in a Petridish was kept for evaporation at ambient temperature. Tiny single crystals of NCP started to grow in about 10e15 days in the petridish. A good quality, transparent, macroscopic defect-free seed crystal was suspended in a round-bottomed flask containing the mother solution. The temperature of the growth solution was maintained at 35  C using a temperature controller. The growth solution was stirred continuously during the growth of the crystal to maintain the homogeneity. Optically clear single crystals of size 15 mm  3 mm  3 mm were obtained in a period of 20 days. 2.2. IR and Raman measurements FT-IR spectrum of the synthesized material was recorded in the wavenumber range 400e4000 cm1 by KBr pellet technique (PerkineElmer Spectrum One FT-IR spectrometer). The NIR-FT-Raman spectrum of NBG using 1064 nm excitation was recorded in the region 10e4000 cm1 using BRUKER IFS 66V FT-Raman Spectrometer using powder sample taken in a capillary tube and the Raman spectrum was recorded with an Nd:YAG laser at 1064 nm with an output of 200 mW used as the excitation source and with a liquid nitrogen- cooled Ge-diode detector; 1000 scans were accumulated with a total registration time of about 30 min. The spectral resolution after apodization was 2 cm1. The UVevis absorption spectrum of the sample was recorded in methanol solution using a Shimadzu UVeVis spectrophotometer in the spectral region of 200e800 nm. 2.3. Second harmonic generation (SHG) efficiency measurements The SHG efficiency of the microcrystalline powders of NCP was examined by the powder reflection technique of Kurtz and Perry [10]. Particle size (r), 150 < r < 180 mm and 90 < r < 150 mm, graded using standard sieve was used for the measurement. Laser beam from Nd:YAG pulsed laser (1064 nm, 15 ns) was used. In the experimental set-up used for the SHG, the incident beam was spilt into 95:5 ratio. The 95% fundamental was focused on the polycrystalline sample, while the 5% of intensity of the fundamental beam was used as a reference beam to normalize the fluctuations of the incident laser. The second harmonic (SH) radiation at 532 nm obtained at the output was filtered using an SH separator to remove the fundamental input radiation. SHG was detected by RCA-931A photomultiplier tube (PMT), which is connected to 100 MHz digital storage oscilloscope (DSO). The change in the intensity of the second harmonic as a function of the fundamental was experimentally measured and is shown in Fig. 1. The SHG efficiency of NCP was evaluated to be 1.2 times that of urea. 3. Computational details Density Functional Theoretical (DFT) computations were performed by using the closed-shell Becke-Lee-Yang-Parr hybrid exchange-correlation three-parameter functional (B3LYP) in combination with 6e311G (d, p) basis set to derive the complete geometry optimizations and normal mode analysis on isolated entities [11]. 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. For the frequency calculation, the B3LYP/6-311G(d, p) theory has been used, while the Harmonic vibrational wave numbers were calculated using analytic second derivatives to confirm the convergence to minima on the potential surface and to

Fig. 1. SHG variation in NCP for the particle size 150 < r < 180 mm and 90 < r < 150 mm, as compared to urea.

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. The self-consistent field equation has been solved iteratively to reach the equilibrium geometry corresponding to the saddle point on the potential energy surface (PES) and the analytic second derivative of energy leads to the vibrational frequencies. The vibrational modes were assigned on the basis of TED analysis using SQM program [12]. The calculated harmonic vibrational frequencies were uniformly scaled down, to account for systematic errors caused by basis set incompleteness, neglect of electron correlation and vibrational an harmonicity [13]. All the theoretical calculations based on the DFT theory and methods have been carried out using GAUSSIAN ’03 program package [14]. The Raman activities (Si) calculated by the Gaussian-03 program have been converted to relative Raman intensities (Ii) using the following relationship derived from the basic theory of Raman scattering [15,16].

Ii ¼

f ðno  ni Þ4 Si  i ni ni 1  exp hc kT h

(1)

where no is the exciting frequency ( in cm1 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 TD-DFT method was used to calculate energies, oscillator strengths of electronic singlet–singlet transitions and the absorption wavelengths. Solvent effects were considered using the polarizable continuum model (PCM) developed by Tomasi and co-workers [17e19]. 4. Origin of nonlinear effects Nonlinear optics deals with 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 into 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

D. Sajan, N. Vijayan / Solid State Sciences 13 (2011) 175e184

transferred into non-radiative modes that result in material heating or other energy transfer mechanisms [4,5]. With small fields, the displacement of charge from the equilibrium position, polarization (P), will be proportional to the applied field, E ð1Þ

Linear polymerization : P ¼ P0 þ cij Ej With sufficiently intense laser radiation the relation may not be valid, and must be generalized to include nonlinear contributions. Polarization include nonlinear contributions: ð1Þ

ð2Þ

ð3Þ

P ¼ P0 þ cij Ej þ cijk Ej Ek cijkl Ej Ek El þ ..: P0-static dipole moment, c(n)-nth order susceptibility. At the molecular level nonlinear polarization P,

p ¼ mi þ aij Ej þ bijk Ej Ek þ gijkl Ej Ek El þ .:: with mi molecular dipole moment, aij the linear polarizability, bijk the first hyperpolarizability and gijkl the second hyperpolarizability. For the geometries obtained we calculated the dipole moment, polarizability, static first hyperpolarizability and second hyperpolarizability tensors mi, aij, bijk and gijkl in which each of the i,j,k and l denotes either of the x,y and z axes determined arbitary. These tensors are defined as coefficients of the Buckingham type expansion [20] of the total energy with respect to the applied field F ¼ (Fx,Fy,Fz).The components of b are defined as the coefficients in the Taylor series expansion of the energy in the external electric field. When the external electric field is weak and homogeneous, this expansion becomes.

E ¼ E0 

X

mi F i 

i

1X 1X a F iFj  bijk F i F j F k 2 ij ij 6 ijk

1 X gijkl F i F j F k F l þ ..  24

177

Table 1 The first order hyperpolarizability of of NCP computed at HF/6-31G (d).

b components

HF/6-31G (d)

bxxx bxyy bxxz byyz byzz bxxy byyy bxyz bxzz bzzz btotal (e.s.u) btot(NCP)/btot(urea)

45.782 4.492 26.969 14.299 7.189 2.533 3.909 0.342 75.213 56.614 1.016  1030 3.8

To calculate the hyperpolarizability, the origin of the Cartesian coordinate system was chosen as the center of mass of the compound [21]. The calculated dipole moment mtot ¼ 5.35 D. The components of the hyperpolarizability tensor are shown in Table 1. The calculated first hyperpolarizability of NCP is 1.01644  1030 esu which is 3.8 times that of urea. 5. Results and discussions 5.1. Crystal structure NCP crystallizes in space group P21, with four molecules per unit cell (Z ¼ 4). From the single crystal XRD data [9] it is observed that the crystal belongs to monoclinic system with the following cell dimensions: a ¼ 4.853  A, b ¼ 11.040  A, c ¼ 17.255  A. In the crystal packing diagram of NCP, the hydrogen bond interactions namely NeH...O is observed.

ijkl

Where E0 is the energy of the unperturbed molecules, Fi is the field at the origin mi, aij, bijk and gijkl are the components of dipole moment, polarizability, the first hyperpolarizabilities, and second hyperpolarizabilities respectively. The total static dipole moment mi, the mean polarizability a0 , the anisotropy of the polarizability Δa and the mean first hyperpolarizability bijk, using the x, y, z components are defined as



mi ¼ m2x þ m2y þ m2z a0 ¼

1=2

 1 axx þ ayy þ azz 3 h







Da ¼ 21=2 axx  ayy 2 þ ayy  azz 2 þðazz  axx Þ2 þ6a2xx

5.2. Optimized geometry The optimized molecular structure of the monomer (Fig. 2) and dimer (Fig. 3) of NCP was calculated using Gaussian-03W program. The selected optimized geometrical parameters are given in Table 2. The available X-ray diffraction values are also given in the table for comparison. The optimized geometry shows that the benzene ring appears a little distorted with larger C1eC6 and C5eC6 bond lengths and shorter C1eC2 , C2eC3 , C3eC4 and C4eC5 , and angles slightly out of the regular hexagonal structure. It is due to the effects of substitution of chlorine and propanamide group on the benzene ring. The broad features of the effect of such substitution can be described as: (i) the bond angles at the position C1 , C4 and C5 are

i1=2

The complete equation for calculating the magnitude of first hyperpolarizability from Gaussian-03W output is given as follows:



bijk ¼ b2x þ b2y þ b2z

1=2

Here,













bx ¼ bxxx þ bxyz þ bxzz

by ¼ byyy þ bxxy þ byzz bz ¼ bzzz þ bxxz þ byyz

Fig. 2. Optimized structure of NCP calculated at B3LYP/6-311G(d,p).

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5.3. Natural bond orbital (NBO) analysis The NBO analysis was performed with the NBO 5.0 program [22] .A short outline of the NBO segments used and their structural meaning is presented below. NBO theory allows the assignment of the hybridization of atomic lone pairs and of the atoms involved in bond orbitals. These are important data in spectral interpretation since the frequency ordering is related to the bond hybrid composition. The NBO analysis allows us to estimate the energy of the molecule with the same geometry but in the absence of electronic delocalization. Moreover, only the steric and electrostatic interactions through the ELewis are taken into account. The most important interactions between ‘filled’ (donor) Lewis-type NBOs and ‘empty’ (acceptor) non-Lewis NBOs are reported in Table 2. The stabilization energy DEij associated with delocalization is estimated using the second-order perturbation theory as:

Eð2Þ ¼ ns

Fig. 3. Optimized structure of NCP dimer calculated at B3LYP/6-311G(d,p).

increases by 120.0 and (ii) the bond angles at the position C6 , C2 and C3 are decreases by 120.0 . The sum of the bond angles about N12 is 359.98 (¼360 ), indicating that this atom is sp2 hybridized. The computed values of bond length N14eC12 (1.38 Å) and C14]O15 (1.22 Å) clearly show their double bond character. The contraction of N14eC12 and C14]O15 bonds are due to the mesomeric effect, where the strong electronegative atom nitrogen is attached to a carbonyl carbon, which is justified by the X-ray crystal structure.

Fij2 hsjFjsi2 ¼ ns 3s*  3s DE

(2)

where hsjFjsi2 or F2ij is the Fock matrix (KohneSham matrix) element between the i and j NBO orbitals, 3s and 3s* are the energies of s and s* NBO’s, and ns is the population of the donor s orbital. The second-order perturbation theory analysis of Fock matrix in NBO shows strong intramolecular hyperconjugative interactions, which are presented in Table 3. The intramolecular hyperconjugative interactions are formed by the orbital overlap between p(N12), p(O15) and p*(CeO), p*(CeC)and p*(NeC) bond orbitals which results intramolecular charge transfer (ICT) causing stabilization of the system. The hyperconjugative interaction between N12/ p* (C14eO15), N12/p* (C1eC6), O15/s* (N12eC14) and O15/s* (C14eC16) antibonding orbitals are 228 kJ mol1, 160 kJ mol1, 108 kJ mol1 and 83 kJ mol1 respectively. These charge transfers interaction have higher values than the other delocalizations and increasing energies are due to strong delocalization leading to stabilization of the molecule.

Table 2 Optimized Geometry of NCP by B3LYP/6-311G (d,p) in comparison with XRD data. Bond Angle (o)

Bond length (Å)

Torsion Angle (o)

Parameter

B3LYP/ 6- 311G(d,p)

XRD

Parameter

B3LYP/ 6- 311G(d,p)

XRD

Parameter

B3LYP/ 6- 311G(d,p)

C1eC2 C2eC3 C3eC4 C4eC5 C6eC1 C6eC5 H7eC2 H8eC3 H9eC4 H10eC5 Cl11eC1 N12eC6 H13eN12 C14eN12 O15eC14 C16eC14 H17eC16 H18eC16 H19eC16 H20eC19 H21eC19 H22eC19

1.390 1.393 1.394 1.392 1.409 1.406 1.084 1.085 1.086 1.080 1.770 1.403 1.011 1.382 1.222 1.529 1.097 1.099 1.528 1.092 1.094 1.094

1.375 1.374 1.377 1.378 1.387 1.384 e e e e 1.741 1.413 e 1.350 1.219 1.505 e e 1.462 e e e

C1eC2eC3 C2eC3eC4 C3eC4eC5 C4eC5eC6 C6eC1eC2 H7eC2eC1 H8eC3eC2 H9eC4eC3 H10eC5eC4 Cl11eC1eC2 N12eC6eC1 H13eN12eC6 C14eN12eC6 O15eC14eN12 C16eC14eN12 H17eC16eC14 H18eC16eC14 H19eC16eC14 H20eC19eC16 H21eC19eC16 H22eC19eC16

119.65 119.32 121.07 121.45 121.91 119.23 119.82 120.081 121.28 118.20 119.12 114.73 128.98 124.11 113.45 110.18 107.21 112.25 110.43 111.09 110.51

119.40 119.70 120.70 120.40 121.91 e e e e 118.55 120.57 e 125.50 122.67 115.16 e e e e e e

C1eC2eC3eC4 C2eC3eC4eC5 C3eC4eC5eC6 C4eC5eC6eC1 C6eC1eC2eC3 H7eC2eC1eC6 H8eC3eC2eC1 H9eC4eC3eC2 H10eC5eC4eC3 Cl11eC1eC2eC3 N12eC6eC1eC2 H13eN12eC6eC1 C14eN12eC6eC1 O15eC14eN12eC6 C16eC14eN12eC6 H17eC16eC14eN12 H18eC16eC41eN12 H19eC16eC14eN12 H20eC19eC16eC14 H21eC19eC16eC14 H22eC19eC16eC14

0.017 0.004 0.002 0.054 0.002 179.997 179.988 179.979 179.916 179.994 179.955 0.462 179.119 1.979 176.660 32.620 82.491 156.647 54.582 64.740 175.075

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179

Table 3 Second-order perturbation theory analysis of Fock matrix in NBO basis. Donor (i)

Acceptor (j)

E(2)a (kJ mol1)

E(j)-E(i) (a.u.)

n1(Cl11) n1(Cl11) n2(Cl11) n2(Cl11) n2(Cl11) n3(Cl11) n1(N12) n1(N12) n1(O15) n1(O15) n1(O15) n2(O15) n2(O15) n2(O15)

s* (C1eC2) s* (C1eC6) s* (C1eC2) s* (C1eC6) s* (N12eH13) p* (C1eC6) p* (C1eC6) p* (C14eO15) s* (C5eH10) s* (N12eC14) s* (C14eC16) s* (C5eH10) s* (N12eC14) s* (C14eC16)

5.2 7.4 17 18 8.5 45 160 228 2.1 7.4 9 5.1 108 83

1.50 1.46 0.90 0.87 0.74 0.34 0.27 0.29 1.16 1.12 1.05 0.74 0.70 0.63

a b c

b

F(i,j) c (a.u.) 0.039 0.046 0.053 0.054 0.035 0.060 0.095 0.114 0.022 0.040 0.043 0.028 0.122 0.101

E(2) means energy of hyperconjugative interactions. energy difference between donor and acceptor i and j NBO orbitals. F(i,j) is the Fock matrix element between i and j NBO orbitals.

Fig. 4. The UVevisible absorption spectrum of NCP molecule.

5.4. Absorption spectra and solvent effects The electronic spectra of NCP were computed in the gas phase and in an acetone environment and are presented in Table 4. The solvent effect was calculated using the PCM-TD-DFT method. The observed and simulated (gas phase and acetone) UVevis spectra are shown in Figs. 4 and 5. The computed results state that the first excited state originates from the HOMO (highest occupied molecular orbital) to LUMO (lowest unoccupied molecular orbital) transition that corresponds to the lmax absorption band in the UVevis spectrum. From the calculations in the gas phase it is assigned that the frontier level HOMO has the molecular orbital number 47 with ‘A’ symmetry and the LUMO has the molecular orbital number 49 with the same ‘A’ symmetry. From Table 4, the first dipole-allowed transition is calculated at 248 nm (47A / 49A) in the gas phase with much lower oscillator strength of 0.0099. The next transitions are at 240.79 nm (46A / 49A) with a much lower oscillator strength and 234 nm (47A / 50A) with a strong oscillator strength. In the case of the methanol environment, a strong transition is observed at 237 nm (47A / 49A and 48A / 50A) with a strong oscillator strength of 0.4375. The experimental absorption spectrum shows two strong absorption peaks at 212 and 239 nm were observed owing to pp* transition. The red shift of the computed transition energies is due to the charge transfer interaction. The HOMO and LUMO of NCP are shown in Fig. 6. In the HOMO, the charge density is mainly accumulated on the aromatic pconjugated part, however in the case of the LUMO, more charge density moves to the aromatic p-conjugated part, Cl atom, NeH and CH2 fragment. The HOMO and LUMO show that a donor-to-

acceptor charge transfer (CT) interaction occurs in NCP. This agrees well with experimental observations. The broadening and intensity enhancement of the UVevis absorption spectra clearly shows the CT interaction. The conjugated molecules are characterized by a small amount of highest occupied molecular orbitalelowest unoccupied molecular orbital (HOMOeLUMO) separation, which is the result of a significant degree of ICT from the end-capping electron donor groups to the efficient electron acceptor groups through p-conjugated path. The HOMOeLUMO energy gap for AAC was computed at the B3LYP/6-31G* level. The eigen values of LUMO (470.67 kJ mol1) and HOMO (864.94 kJ mol1) and their energy gap (394.27 kJ mol1) reflect the chemical activity of the molecule. Moreover the lower in the HOMO and LUMO energy gap explains the eventual charge transfer interactions taking place within the molecule. 6. Vibrational spectra The vibrational analysis of NCP is performed on the basis of the characteristic vibrations of amine group, carbonyl group, methyl group, methylene and phenyl ring modes. The computed vibrational wave numbers and the atomic displacements corresponding to the different normal modes are used for identifying the vibrational modes unambiguously. The assignments of phenyl vibrations are made according to Wilson’s numbering convention [23]. The detailed vibrational assignments of fundamental modes along with

Table 4 Experimental and Calculated absorption wavelengths, energies and oscillator strengths of NCP using the TD-DFT method at the B3LYP/6-31G(d) level. Excitation

excited state 1 47 / 49 48 / 50 excited state 2 46 / 49 46 / 52 excited state 3 47 / 50 48 / 49

CI expansion coefficient

Wavelength (nm) Calc. Gas phase

Oscillator Strength (f)

0.40576 0.57462

248.28

0.0099

0.65866 0.21129

240.79

0.0003

0.16727 0.63194

234.20

0.3541

Excitation

excited state 1 47 / 49 48 / 50 excited state 2 47 / 50 48 / 49 excited state 3 46 / 49 46 / 52

CI expansion coefficient

Wavelength (nm) Calc. Methanol

Oscillator Strength (f)

Expt.

0.39804 0.57390

248.30

0.0152

239

0.13761 0.636780

237.47

0.4375

212

0.66239 0.20393

236.01

0.0005

180

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Fig. 5. (a) Simulated UVevis spectrum of the NCP molecule in the gas phase (b) Simulated UVevis spectrum in acetone for NCP molecule calculated with the TD-DFT/ PCM method.

the calculated IR and Raman intensities and normal mode description (characterized by PED) are reported in Table 5. For visual comparison, the observed and simulated FT-IR and FT-Raman spectra are presented in Figs. 7 and 8, respectively. 6.1. Vibrations of amine group The secondary amines exhibit bands due to the NeH stretching and deformation vibrations. The positions of the NeH stretching and deformation bands are dependent on the strength of hydrogen bond formed. The free NeH stretching modes of secondary amides generally observed in the region 3460e3300 cm1 for NeH

stretching and a weak band at 3100e3070 cm1 for overtone of NeH bend [24,25]. The strong bands at 3289 cm1 in IR spectrum and medium band at3293 cm1 in Raman spectrum correspond to the NeH stretching mode. The NeH mode appears in our calculations at 3438 cm1 with a composition of 100% NeH stretch. The lowering of NH stretching wavenumber can be attributed to the intermolecular NeH∙ ∙ ∙O interaction (Fig. 2). The red shifting is further enhanced by the reduction in the NH bond order values, occurring due to donoreacceptor interaction. The presence of strong NeH∙ ∙ ∙O intermolecular hydrogen bonding might provide the non-centrosymmetric structure for the NCP crystal. This structural organization, thus connects the molecules of forming infinite rows along the c axis and the NeH..O bonds connects the molecules in infinite rows along the b axis(Fig. 9). Together these hydrogen bonds form stabilized three-dimensional intermolecular networks in NCP crystal and are favorable in contributing to the NLO properties. These thereby create a ‘push-pull’ type motif to enhance the NLO response. The NeH in-plane deformation vibrations normally appear in the region 1630e1500 cm1 and consist of strong IR bands [26,27]. The strong band at 1527 cm1 in IR spectrum and the medium band at 1529 cm1 in Raman spectrum correspond to the NeH in-plane deformation mode, which is coupled with C]C stretch mode. The CeN stretching wavenumber was calculated to be 1155 cm1 and the corresponding medium Raman band is found at 1131 cm1. The NeH out-of-plane bending mode can be observed as a weak band in IR at 802 cm1 and in Raman at 802 cm1 where the computed value is 792 cm1. 6.2. Methyl group vibrations Three fundamental n(CH3) bands corresponding to one symmetric and two asymmetric vibrations are usually observed in the region between 2900 and 2990 cm1 [26,27]. DFT computations exhibit three vibrations at 2984 cm1, 2965 cm1 and 2906 cm1 respectively. As indicated by the PED, the first two modes involve approximately 100%. The asymmetric stretching mode is observed as a strong band at 2985 cm1 in Raman. The asymmetric in phase stretching vibration is active in IR as a medium band at 2977 cm1. The medium Raman band at 2881 cm1 and a weak band in IR at 2881 cm1 are assigned to methyl symmetric stretching mode. The mode at 2906 is 93% ns(CH3), this composition suggest that we can assign confidently this mode to the symmetric stretching of the three methyl CeH bonds. The asymmetric and symmetric bending vibrations of methyl groups normally appears in the region 1470e1440 cm1 and 1390e1370 cm1 respectively [26,27]. The intense band at 1471 cm1 in infrared spectrum and corresponding Raman band at 1465 cm1 is attributed to the CH3 asymmetric bending mode. The medium band around 1375 cm1 in IR and Raman spectrum is corresponds to the symmetric bending mode. The rocking vibrations of the CH3 group in NCP appear as mixed vibrations. These modes usually appear in the region 1070e900 cm1 [26,27]. The medium bands in IR spectrum at 992 cm1 are attributed to the CH3 rocking mode, which is coupled with aromatic CH2 rocking mode. 6.3. Methylene vibrations

Fig. 6. (a) HOMO plot of AAC at B3LYP/6-311G(d,p) and (b) LUMO plot of NCP at B3LYP/ 6-311G(d,p).

The asymmetric CeH stretching mode of methylene group is expected in the region [26,27] around 2940 cm1 and the symmetric around the region 2860 cm1. In NCP the CH2 asymmetric stretching mode is found to be strong bands in both Raman spectra at 2947 cm1. The symmetric stretching modes also observed as medium shoulder band in Raman spectra at 2845 cm1. The calculated wave numbers of the above modes are 2967 cm1

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181

Table 5 Observed and calculated wavenumbers (cm1), PED and assignment for NCP at B3LYP/6-311G (d,p) level. B3LYP/6-311G(d,p) b

IRaman

3438

11

9

3098 3061 3047 3033 2984 2965 2915 2906 2881 1693 1597 1577 1517 1469 1458 1456 1431 1425

1.9 1.4 2.9 0.8 3.9 7.2 3.6 7.1 3.7 41 22 7.8 100 2.2 4 3.1 5.7 30

10 34 27 13 9 18 16 30 27 33 93 15 14 7 13 16 9 11

1381 1345 1291 1280 1257 1228

1.5 7.5 31 2.7 0.3 4.6

3 19 50 23 12 44

1162

38

1155 1121 1071 1062 1039 1018 995 978 934 906 859 826 792 747 711 690 671 609 583 552 522 440 431

0.2 0.7 0.2 3 0.2 11 1.4 0.3 0.8 3.6 0.6 0.2 1.6 15 0 2.4 2.8 3.2 7.9 1.7 3 1.7 0.6

22 9 5 4 26 43 3 0 0 13 2 11 6 1 5 2 25 11 12 11 4 3 35

389 326 284

0.9 2.7 0.5

45 11 16

269 214 205 156 114 72 52 36

1.1 0.2 0.3 0 0.7 0.1 1.7 0

15 25 2 33 16 100 12 81

Scaled wavenumbersa

a b c d

IIR

Exp. IR

Exp. Raman

PEDd (%)

3289 vs 3195 sh 3106 w

3293 m 3194 w

NeH Stretch (100) NeH overtone 2 Aromatic CeH Stretch (98) 20a Aromatic CeH Stretch (96) 20b Aromatic CeH Stretch (99) 7 Aromatic CeH Stretch (100) CH3 asym.stretch (98) CH3 asym.stretch (98) CH2 asym.stretch (93) CH3 sym.stretch (95) CH2 sym.stretch (99) C14 ¼ O15 stretch (81) 8b Ring stretch(50)þ CeCeC in plane bend (10) 8a Ring stretch (53)þ NeH in plane bend (10) 19b Ring stretch (10)þ NeH in plane bend(36)þ CeCeH in plane bend (10) CH3 asym.bend (69)þCeCeH in plane bend(10) CeCeH in plane bend (16) CH3 asym.bend (63)þ HeCeCeC torsion (12) CH2 scissoring (69) 19a CeC stretch (30) þ HeNeC in plane bend (16) þ CH2 asym.bend (17) CH3 sym.bend (94) CH2 wagging (36) HeCeCeN torsion (27) 14 CeC stretch (39)þ CeH in plane bend (18) CeC stretch (39)þ CeN stretch (18) 3 CeC stretch (34)þ CeCeH in plane bend (22) CeCeH in plane bend (30) þ HeCeCeN torsion (44) 13 CeC stretch (12)þ CeN stretch (13)þHeNeC in plane bend (10) þCeCeH in plane bend (21) CeN stretch (35) 9a CeC stretch (10)þ CeCeH in plane bend(82) 18a CeCstretch(24)þ CeCeH in plane bend (33) CH2 twisting (29)þ CH3 rocking (25) 12 CeC stretch (29)þ CeCeH in plane bend (24) 1 CeC stretch (13)þ CeCeC in plane bend (47) CH2 rock (15)þ CH3 rocking (39) 17a HeCeCeC torsion (49)þHeCeCeN torsion (33) 10a HeCeCeC torsion (62)þHeCeCeN torsion (23) CeC stretch (24)þOeCeN o.p plane bend (19)þCeNeC o.p plane bend (13) HeCeCeC torsion (87) CeC stretch (13)þ CeN stretch (17)þ CeCeC o.p plane bend (31) HeCeCeN torsion (25) þHeCeCeC torsion (24) þOeCeNeC torsion (20) HeCeCeC torsion (86) HeCeCeC torsion (19) þ CeCeCeC torsion (31) þ NeCeCeC torsion(30) 11 CeC stretch (22)þCeCl stretch(14)þ CeCeH o.p plane bend(13) 6a CeCeC i.p plane bend (24) þ CeCl stretch (10) 6b CeCeC i.p plane bend (11) þHeNeCeC torsion (22) OeCeNeC torsion(22) CeCeC i.p plane bend (10) þ HeNeCeC torsion (52) CeCeCeC torsion (22) þ NeCeCeC torsion (14) HeNeCeC torsion (15) þ CeCeCeC torsion (15) þ NeCeCeC torsion (10) 16b CeCeCeC torsion (39) þ CleCeCeC torsion (29) þ NeCeCeC torsion (12) CeCl stretch (33)þCeCeC in plane bend (12) þ OeCeN in plane bend(11)þ NeCeC in plane bend(22) CeCl stretch (14)þ CeCeCl in plane bend(36) OeCeN in plane bend (18)þ CeCeC in plane bend (33) CeCeN in plane bend (18)þ CeCeNeC torsion (10)þ CleCeCeC torsion (10) CeNeC in plane bend (11)þ CeCeN in plane bend (13)þ CleCeCeC torsion (12) CeCeC in plane bend (12)þ CeCeCl in plane bend (32) CeCeCeN torsion (73) CeCeCeC torsion (56)þ CleCeCeC torsion (20) CeNeC in plane bend (40)þ NeCeC in plane bend (16)þ CeCeN in plane bend(20) CeNeCeC torsion (10)þ CeCeNeC torsion (52) CeNeCeC torsion (77)þ CeCeNeC torsion (10) CeCeCeN torsion (67)

c

3084 vvs 3060 w 2985 vs 2977 m 2881 w 1664 1592 1583 1527 1471 1453 1439 1430

vvs s vs vvs s vs vs m

2947 2881 2845 1670 1594

vs msh msh vs vvs

1529 m 1465 m 1450 msh

1428 m

1375 m

1375 m

1290 vs

1291 vs

1244 m 1201 s

1248 vs 1204 w

4

1162 vs 1136 w 1222 m

1131 m 1080 m

1055 m 1034 m 1022 m 992 m 947 925 858 830 802 755 718 692 671 612

w w w w w vs w s s w

1040 vs

960 wsh 933 s 853 829 802 760

s w w w

690 s 679 s

528 w 471 vs 424 vs 379 w 334 w 272 w

191 vvs 113 vvs

Obtained from the wavenumbers calculated at B3LYP/6-31IG(d,p) using scaling factors 0.974 (for wavenumbers under 1800 cm1) and 0.955 (for those over 1800 cm1). Relative absorption intensities normalized with highest peak absorption equal to 100. Relative Raman intensities calculated by Eq.(1) and normalized to 100. Only contributions  10% are listed.

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Fig. 7. (a) FTIR spectra of NCP and (b) simulated IR spectra of NCP.

(93%)and2926 cm1 (99%) respectively for methylene asymmetric and symmetric stretching modes. The CH2 scissoring vibrations appear normally in the region 1455e1435 cm1 and consist of medium intense bands [26,27]. The medium intense bands at 1430 cm1 are IR spectra correspond to the CH2 scissoring mode. As revealed by the PED, vibrational mode CH2 scissoring has only 69% of pure CH2 scissoring contribution. The methylene wagging vibrations of CH2 is found at 1345 cm1 which is justified by our PED calculation also. The wavenumbers at 1055 cm1 and 922 cm1 in IR spectrum are attributed to CH2

twisting and rocking modes, respectively which are supported by PED calculation also. 6.4. Vibrations of carbonyl group Carbonyl group vibrations give rise to characteristic bands in vibrational spectra and for this reason; such bands have been a subject of extensive studies [28]. The intensity of these bands can increase due to conjugation or formation of hydrogen bonds. The carbonyl stretching vibrations in saturated esters are expected in

Fig. 8. (a) FT-Raman spectra of NCP and (b) simulated Raman spectra of NCP.

D. Sajan, N. Vijayan / Solid State Sciences 13 (2011) 175e184

Fig. 9. Hydrogen bonding structure of NCP.

the region 1720 cm1 e1680 cm1 [29,30]. The sharp intense band in infrared spectrum at 1664 cm1 is assigned to C¼O stretching mode, which is also observed in Raman spectrum at 1670 cm1 as a strong band. The results of computations gives the wavenumber of this mode is 1693 cm1. As revealed by the PED, vibrational mode C¼O has only 100% C¼O stretching contribution. The carbonylstretching mode is simultaneously influenced by the conjugation of C¼O with amide nitrogen and the intermolecular hydrogen bonding. The deviation from the calculated wave number for this mode can be attributed to the under estimation of p -electron delocalization due to conjugation and hydrogen bonding network inside the crystal. 6.5. Phenyl ring vibrations The various normal modes of the asymmetric disubstituted benzene rings have been extensively studied by adopting Wilson’s scheme [23]. The selection rules for asymmetric disubstituted benzene derivatives allow four CeH stretching vibrations 2, 20a, 20b and 7 which are expected in the region 3100e3000 cm1 [23,26,27]. The mode 20a appears very strong in the Raman at 3084 cm1. The weak bands observed at 3106 cm1 (IR) is assigned to the 2 mode and the mode 20b is observed as a weak band in the IR spectrum at 3060 cm1. The normal modes 8a, 8b, 19a, 19b and 14 are classified as the CeC stretching vibrations. The mode 8b is found at a higher wavenumber than 8a and they appear in the range 1610e1580 cm1 [23]. According to the calculated PED, very strong band at 1594 cm1 in the Raman spectrum and its strong counterpart at 1592 cm1 in IR are assigned to mode 8, which is described as the in-phase stretching vibrations of the C]C bonds in the aromatic ring n(C1eC2) þ n(C4eC5). This mode corresponds to the so called mode 8b in the benzene ring [23]. The 8a mode appears as a very strong band in the IR at 1583 cm1. According to PED calculations, vibrational mode 8a has only 50% CeC stretching contribution and it has a 10% NeH in plane bending character. The medium band observed at 1529 cm1 in the IR is assigned to the 19b mode and the very strong counterpart in the Raman spectrum occurs at 1527 cm1. The mode 19a is observed only in the Raman spectrum as a medium band at 1428 cm1. As revealed by the calculated PED, vibrational mode 19a has only 30% CeC stretching contribution, 17% CH2 bending and it has a 16% NeHeC in plane bending character. The very strong band at 1290 cm1 in the IR and an intense band at 1291 cm1 in the Raman spectrum are attributed to 14 vibrational mode of CeC stretching. The 14 mode appears in our calculations at

183

1291 cm1 with a composition of 39% CeC stretch and 18% CeH in plane bend .The 8b, 19b and 14 modes appear simultaneously in both the IR and Raman spectra providing evidence for charge transfer interaction [29,30]. The normal vibrations 3, 9a, 18a and 15 are classified as CeH in plane bending vibrations [23]. The strong bands observed at 1244 cm1 in the IR and 1248 cm1 in the Raman spectrum are assigned to mode 3. The medium IR band appearing at 1122 cm1 is assigned to vibrational mode 9a. The mode 18b is coupled with the CeC stretching mode and is found active in the Raman spectrum at 1088 cm1 as a medium band. Strong band at 1162 cm1 in Raman corresponds to the mode 15.The in-plane ring deformation or trigonal ring breathing vibration derived from the b1u benzene vibration 12 (1010 cm1) gives rise to an intense Raman band in disubstituted benzene [23,26,27] at 1010e990 cm1. In NCP the vibrational mode 12 is identified as a very strong intense Raman band at 1040 cm1 and strong band in IR at 1034 cm1 , which is supported by computed results also. Normal vibration 1 of phenyl ring is usually referred to as a substituent sensitive vibration. For a heavy substitution these modes found in the region 1100e1000 cm1 are strongly IR active. This is confirmed by the medium bands both in IR spectrum at 1022 cm1. The CeH out-of-plane bending vibrations are expected in the region 960e780 cm1. The mode 17a is observed in the Raman at 960 cm1. The strong bands observed in the IR at 933 cm1 and in the Raman spectrum at 947 cm1 are assigned to 10a mode. The radial skeletal (6a and 6b) and the out-of-plane skeletal (16a and 16b) are listed in Table 3. 7. Conclusion The NCP crystals were grown by the slow evaporation technique. The calculated first hyperpolarizability of NCP is found to be 1.01644  1030 esu which is 3.8 times that of urea. Kurtz and Perry powder reflection studies confirm the second-order NLO properties of the molecule. The normal coordinate analysis performed on NCP reproduces its experimental geometry and harmonic vibrational wavenumbers excellently. The simultaneous occurrence of modes C]O, 8b and 19b provide evidence for the charge transfer interactions. The enhanced intensity of mode 8b clearly shows the higher degree of conjugation, which is responsible for the optical non-linearity of the crystal. The red shifting of C¼O stretching wavenumber is due to the conjugation between carbonyl groups and the aromatic ring. The lowering of HOMO and LUMO energy gap clearly explicates the charge transfer interactions taking place within the molecule. References [1] S.S. Gupte, A. Marcano, R.D. Pradhan, C.F. Desai, J. Melikechi, Appl. Phys. 89 (2001) 4939. [2] S.P. Karna (Ed.), J. Phys. Chem. A, 104, 2000. [3] D.R. Kanis, M.A. Ratner, T.S. Marks, Chem. Rev. 94 (1994) 195. [4] H.S. Nalwa, S. Miyata, Nonlinear Optical Properties of Organic Molecules and Polymers. CRC Press, Boca Raton, FL, 1996. [5] D.S. Chemla, J. Zyss, Non-linear optical properties of organic molecular crystals, vol. 1, 2, Academic Press, London, 1987. [6] Peter Gunter, Nonlinear Optical Effects and Materials. Springer-VerlagBerlin Heidelberg, New York, 2000. [7] Ch. Bosshard, R. Spreiter, L. Degiorgi, P. Gunter, Phys. Rev. B. 66 (2002) 205107. [8] P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Organic Molecules and Polymers. John-Wiley & Sons Inc, New York, 1991. [9] P. Srinivasan, T. Kanagasekaran, N. Vijayan, R. Balamurugan, A.R. Charles Sathiya Prakash, P. Kannan, R. Gopalakrishnan, P. Ramasamy, J. Cryst. Growth 297 (2006) 372. [10] S.K. Kurtz, T.T. Perry, J. Appl. Phys 39 (1968) 3798. [11] A.D. Becke, J.Chem. Phys. 98 (100) (1993) 5648. [12] M.H. Jamróz, Vibrational energy distribution analysis VEDA 4, Warsaw (2004). [13] Listing of vibrational scaling factors.http://srdata.nist.gov/cccbdb/vibscalejust. asp.

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