non-bonded interactions and its contribution to the nlo activity gsn

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Journal of Molecular Structure 877 (2008) 20–35

Non-bonded interactions and its contribution to the NLO activityof Glycine Sodium Nitrate – A vibrational approach

T. Vijayakumar a, I. Hubert Joe a, C.P. Reghunadhan Nair b, V.S. Jayakumar a,*

a Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram 695 015, Kerala, Indiab Polymers and Special Chemicals Division, Vikram Sarabhai Space Centre, Thiruvananthapuram 695 022, Kerala, India

Received 10 November 2006; received in revised form 9 July 2007; accepted 11 July 2007Available online 24 July 2007

Abstract

Vibrational spectral analysis of the novel nonlinear optical (NLO) material, Glycine Sodium Nitrate (GSN) is carried out using NIRFT-Raman and FT-IR spectroscopy, supported by Density Functional Theoretical (DFT) computations to derive equilibrium geometry,vibrational wave numbers and first hyperpolarizability. The reasonable NLO efficiency, predicted for the first time in this novel com-pound, has been confirmed by Kurtz–Perry powder SHG experiments. The influence of Twisted Intramolecular Charge Transfer (TICT)caused by the strong ionic ground state hydrogen bonding between charged species making GSN crystal to have the non-centrosymmet-ric structure has been discussed. The shortening of CAH bond lengths, blue-shifting of the stretching frequencies and intensity variationindicating the existence of ‘blue-shift or improper’ CAH� � �O hydrogen bonding. The intense low wavenumber H-bond Raman vibrationsdue to electron–phonon coupling and non-bonded interactions in making the molecule NLO active have been analyzed based on thevibrational spectral features. The Natural Bond Orbital (NBO) analysis confirms the occurrence of a strong intra- and intermolecularNAH� � �O and CAH� � �O hydrogen bonds.� 2007 Elsevier B.V. All rights reserved.

Keywords: NIR FT-Raman; FT-IR; Ionic hydrogen bonds; Nonlinear optics; SHG; First hyperpolarizability; DFT; Ab initio computations; TwistedIntramolecular Charge Transfer (TICT); Glycine conformers; Natural Bond Orbital analysis

1. Introduction

Nonlinear optical (NLO) materials are active elementsfor optical communications, optical switching data storagetechnology, optical mixing and electro-optic application[1–4]. The development of photonic and optoelectronictechnologies rely heavily on the growth of NLO materialswith high nonlinear optical responses and the developmentof novel and more efficient materials [5]. To design and fab-ricate the NLO materials, much effort is being devoted tounderstand the origin of nonlinearity in large systems andto relate NLO responses to electronic structure and molec-ular geometry. The molecular engineering approach has ledto better understanding of the relationship between the

0022-2860/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2007.07.021

* Corresponding author. Tel.: +91 471 2530887.E-mail address: [email protected] (V.S. Jayakumar).

crystal structure and its optical nonlinearities [6–8]. Chiral-ity [9] and hydrogen bonding [10] are important factors inthe design of NLO chromophores since the measurementof bulk NLO properties in the solid state is dependent ona non-centrosymmetric packing environment within thecrystal lattice. Molecules in pure organic crystals are oftencoupled by relatively weak van der Waals forces or hydro-gen bonding, resulting in rather poor mechanical proper-ties. In such organic crystals, two requirements to besatisfied are: (i) they are made of highly polarizable mole-cules, the so-called conjugated molecules, where the asym-metric p electron system of aromatic molecules can easilymove between a donor and an acceptor substituent groupsthat induce a molecular charge transfer and (ii) the mole-cules are adequately packed to build up a non-centrosym-metric crystal structure that provides non-vanishingsecond order nonlinear coefficients [11,12]. Metal-organic

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Fig. 1. Optimized molecular structure of GSN.

T. Vijayakumar et al. / Journal of Molecular Structure 877 (2008) 20–35 21

hybrids also offer interesting advantages with respect toboth pure organics and pure inorganic materials in thatthey may be reliably designed by integrating highly predict-able structural features, such as hydrogen bonds and coor-dination bonds, in each of these cases which are usedjointly to achieve spatial and dimensional control inorganic–inorganic hybrid [13].

Presently, semi organics are being explored, which canshare the properties of both organic and inorganic materi-als, such as high laser damage threshold, optical transpa-renc and high efficiency. Complexes of amino acids withinorganic salts have been of interest as materials for opticalsecond harmonic generation (SHG), and all amino acidsexcept glycine contain chiral carbon atoms and perhapscrystallize in the non-centrosymmetric space group [8].Dipolar molecules possessing an electron donor groupand an electron acceptor group contribute to large secondorder optical nonlinearity arising from the intramolecularcharge transfer between the two groups opposite nature.Due to this dipolar nature, amino acids have been consid-ered potential candidates for NLO applications [6,8].Although the salts of amino acids like L-Arginine [14], L-Histidine [15] and L-Proline [7,16] are reported to haveNLO properties; the complexes of glycine with inorganicsalts are not explored for optical SHG so far, since glycine,the simplest amino acid, does not possess the asymmetriccarbon, it is NLO inactive. Out of the number of semiorganic single crystals of glycine that have been alreadyreported, most of them are not NLO active [17–19].

Glycine has three polymorphic crystalline forms a, b, c[20,21]. Both a and b forms crystallize in centrosymmetricspace groups ruling out the possibility of optical secondharmonic generation. But c-glycine crystallizes in non-cen-trosymmetric space groups P31 making it a possiblecandidate for NLO applications and it is difficult to growthe c-glycine crystals [22,23]. The thermodynamic stabili-ties of the three polymorphs of glycine at room tempera-ture are in the order c > a > b [24]. It has recently beenreported that complexes of the c-glycine can be efficientin optical SHG with inorganic salt sodium nitrate [8].Due to their potential applications in photonic devices,bulk NLO properties of materials as well as their depen-dence on the first hyperpolarizabilities of molecules haveevoked a lot of experimental efforts [25–27] and theoreticalresearch [28–30]. The Natural Bond Orbital (NBO) analy-sis can be employed to identify and substantiate the possi-ble intra- and intermolecular interactions between the unitsthat would form the H-bonded network [31]. Vibrationalspectral studies of the molecules can be used to providedeeper knowledge about the relationships between molecu-lar architecture, nonlinear response and hyperpolarizabil-ity. NIR FT-Raman spectra combined with quantumchemical computations have recently been effectivelyapplied in the vibrational analysis of drug molecules [32],biological compounds [33,34], natural products [35,36]and NLO active compounds [7,37–39], since fluorescencefree Raman spectra and computed results help unambigu-

ous identification of vibrational modes and provide deeperinsight into the bonding and structural features of complexorganic molecular systems. Glycine Sodium Nitrate (GSN)crystals, being the first complex of glycine reported withNLO property, the vibrational spectral studies of this novelNLO system is taken up, based on NIR FT-Raman and IRspectra along with DFT and MP2 theoretical support toelucidate the relationship between the molecular structuralfeatures and NLO properties.

2. Experimental

2.1. Preparation

Glycine Sodium Nitrate (GSN) crystals grown by slowevaporation [40] were subjected to repeated recrystalliza-tion and good quality single crystals with size of around0.6 mm were obtained.

2.2. Crystal structure

GSN (Fig. 1) crystallizes in monoclinic space group Ccwith four formula units in unit cell (Z = 4) [40]. The celldimensions are: a = 14.329(3) A, b = 5.2662(11) A,c = 9.1129(18) A, b = 119.10(3)�. The glycine moleculesare seen ‘sandwiched’ between layers of Na (NO3) asshown in Fig. 2. Both carboxyl O atoms participate inthe hydrogen bonds as acceptors forming head-to-tailhydrogen bonds. Almost linear OANaAO chains involvingcarbonyl O atoms run along the (20 �2) plane in the [101]direction. X-ray powder diffraction was used for the identi-fication of the grown crystals of GSN. Usually, the charac-teristic strong peaks of a-glycine and c-glycine are expectednear 2h values of 29� and 24�, respectively [41].

2.3. Raman and IR measurements

The NIR FT-Raman spectrum (Fig. 3) of GSN wasobtained on a Bruker RFS 100/S FT-Raman Spectrometer

Fig. 2. Intermolecular interactions (shown by dotted lines) in GSNcrystal.

22 T. Vijayakumar et al. / Journal of Molecular Structure 877 (2008) 20–35

with a liquid nitrogen-cooled Ge-diode detector using Nd:YAG laser at 1064 nm of 300 mw output as the excitationsource with the powder sample in a capillary tube. Thou-sand scans were accumulated with a total registration timeof about 30 min. The spectral resolution after apodizationwas 4 cm�1. A correction according to the fourth-powerscattering factor was performed, but no instrumental cor-rection was made. The upper limit for the wave numbersis 3500 cm�1 owing to the detector sensitivity and the lowerlimit is around 50 cm�1 owing to the Rayleigh line cut-offby the notch filter. The IR spectrum of GSN in the region600–4000 cm�1 (Fig. 4) was recorded using a TENSOR-27MICRO ATR accessory MIRACLE, Pike; ZnSe crystalFT-IR spectrometer with the sample in KBr matrix. Thelower region available with the spectrum is up to600 cm�1.The resolution is about 2 cm�1 and 300 scanswere used.

Fig. 3. NIR FT-Raman

2.4. Second harmonic generation (SHG) efficiency

measurements

Second harmonic generation from microcrystalline pow-ders of GSN was examined using Kurtz–Perry powderSHG method [42]. Particle sizes, ranging from 100 to300 lm, graded using standard sieves were used for thestudy. Samples were loaded in glass capillaries having aninner diameter of 600 lm. The fundamental beam(1064 nm) of a Q-switched ns-pulsed (6 ns, 10 Hz) Nd:YAG laser (Spectra Physics model INDI-40) was used.The second harmonic signal was collected using appropri-ate optics and detected using a monochromator, PhotoMultiplier Tube (PMT) with a boxcar integrator and oscil-loscope (Tektronix model TDS 210, 60 MHz). Filters wereused to bring the signals for all the sample size in the samerange. Urea with particle size of P150 lm was used as thereference and the calibration measurements were carriedout using N-(4-Nitrophenyl)-(L)-Prolinol (NPP). TheSHG efficiency of GSN was evaluated (Table 1) to be anaverage of 0.3 times that of urea and the Fig. 5 describesthat GSN crystal is reasonably phase matchable.

3. Computational

The complete geometry optimizations and normal-mode analysis were performed employing the Becke–Lee–Yang–Parr hybrid exchange-correlation three-parame-ter functional (B3LYP) [43], Moller–Plesset second orderperturbation (MP2) and ab initio computations to com-pare the accuracy of different kinds of computationalmethods. Molecular geometries were fully optimized byBerny’s optimization algorithm using redundant internalcoordinates and confirmed to be minimum energy con-formations. The special basis set LANL2DZ has been

spectrum of GSN.

Fig. 4. FT-IR spectrum of GSN.

Table 1Dependence of SHG efficiencies with particle size

S.No Average particle size (lm) SHG efficiency (·Urea)

1 125 0.352 175 0.363 225 0.254 275 0.29


adopted with Hatree–Fock (HF), Density FunctionalTheory (DFT) and Moller–Plesset second order perturba-tion (MP2) methods due to the existence of metal atomNa in GSN. The self-consistent field equation has beensolved iteratively to reach the equilibrium geometry cor-responding to the saddle point on the potential energy

120 140 160 180

0.24

0.26

0.28

0.30

0.32

0.34

0.36

0.38

SH

G In

tens

ity (

X U

rea)

Particle s

Fig. 5. Dependence of the second harm

surface (PES) and the analytic second derivative ofenergy leads to the vibrational frequencies. The calcu-lated harmonic vibrational frequencies were uniformlyscaled down [44], to account for systematic errors causedby basis set incompleteness, neglect of electron correla-tion and vibrational anharmonicity. All theoretical calcu-lations were carried out using GAUSSIAN ’98 programpackage [45] and the theoretical Raman and IR spectraare shown in Figs. 6 and 7.

The first hyperpolarizability (b0) of this of novel molec-ular system and related properties (b, l) of GSN are calcu-lated using standard basis set. In the presence of an appliedelectric field, the energy of a system is a function of the elec-tric field. First hyperpolarizability is a third rank tensor

200 220 240 260 280

ize (μm)

onic intensity on the particle size.

Fig. 6. Theoretical Raman spectrum from HF/LANL2DZ.

Fig. 7. Theoretical IR spectrum from HF/LANL2DZ.


that can be described by a 3 · 3 · 3 matrix. The 27 compo-nents of the 3D matrix can be reduced to 10 componentsdue to the Kleinman symmetry [46]. The components ofb are defined as the coefficients in the Taylor series expan-sion of the energy in the external electric field. When theexternal electric field is weak and homogeneous, thisexpansion becomes.

E ¼ Eo � liF i � 1=2aijF iF j � 1=6bijkF iF jF k

� 1=24cijklF iF jF kF l þ . . . :

where Eo is the energy of the unperturbed molecules, Fi isthe field at the origin li,aij, bijk and cijkl are the componentsof dipole moment, polarizability, the first hyperpolarizabil-ities and the second hyperpolarizabilities, respectively.


4. Optimized Geometries

The optimized structural parameters of GSN for differ-ent schemes of computations are given in Tables 2–4. Thecorresponding values from X-ray diffraction are also givenfor comparison. The optimized molecular structure of thecompound with atom numbering scheme adopted in thecomputations is shown in Fig. 1. From Tables 2–4, it canbe seen that there are some deviation in the computed geo-metric parameters from the corresponding XRD data [40]and these differences are probably due to the intermolecu-lar interactions in the crystalline state. All the bond lengthsof H atoms determined experimentally by the XRDmethod are 0.94 A. The correct value determined by theneutron diffraction method would be 1.08 A [47].

The bond lengths N1AC2 and C2AC3 corresponding tothe a- and c-glycine are measured to be 1.474, 1.523 Aand 1.460, 1.53 A, respectively [22]. DFT computations

Table 2Optimized bond lengths (A) in GSN

Bond Experimental B3LYP/6-311G(d,p) BLYP/6-31G(d)

N1AC2 1.480 1.502 1.520N1AC5 0.890 1.036 1.050N1AH6 0.890 1.017 1.030N1AH7 0.890 1.060 1.082C2AC3 1.520 1.556 1.572C2AH8 0.970 1.087 1.096C2AH9 0.970 1.091 1.100C3AO4 1.242 1.241 1.260C3AO10 1.247 1.261 1.283N5AO11 2.062 1.664 1.659O11AN12 1.241 1.301 1.336N12AO13 1.235 1.217 1.242N12AO14 1.247 1.256 1.280O14ANa15 2.615 2.355 2.370

Table 3Optimized bond angles (�) in GSN

Bond angle Experimental B3LYP/6-311G(d,p) BLYP/6-31G(d)

N1AC2AC3 111.92 104.28 103.57H5AN1AH6 109.41 112.94 113.32H5AN1AH7 109.43 103.92 103.35H6AN1AH7 109.46 112.47 112.70H5AN1AC2 109.55 103.32 102.61H6AN1AC2 109.49 115.31 115.49H7AN1AC2 109.48 107.87 108.22C2AC3AO4 117.76 117.62 117.76C2AC3AO10 116.19 113.70 113.653H8AC2AC3 109.21 111.26 111.78H8AC2AN1 109.18 109.74 109.45H9AC2AC3 109.25 111.66 112.57H9AC2AN1 109.22 109.11 108.61O4AC3AO10 126.03 128.30 128.13N1AH7AO11 154.90 146.35 149.59H7AO11AN12 131.55 132.52 119.41O11AN12AO13 128.46 120.41 120.07O11AN12AO14 119.07 115.94 116.06O13AN12AO14 120.47 123.65 123.86N12AO14ANa15 97.07 94.65 92.348

with LANL2DZ basis set show the corresponding bondlengths are 1.518 and 1.564 A whereas with 6-311G (d, p)basis set, the bond lengths are 1.502 and 1.556 A. InGSN, the bond lengths N1AC2 and C2AC3 are measuredto be 1.480 and 1.52 A. In a-glycine, c-glycine and GSN,the experimental bond lengths are rather lower than thedata measured by the different theoretical methods includ-ing the special basis set LANL2DZ. All experimental andtheoretical bond lengths and bond angles of the GSN crys-tal are reasonably comparable with the a-glycine ratherthan c-glycine. All dimensions of the c-glycine are veryclose to that of other forms, a and b, with the exceptionof the rather short C3AO4 bond lengths of 1.237 A. Thismay be interpreted partly due to the shortening causedby the angular oscillations of the oxygen atom around acentre near the C3 atom [23]. The C3AO4 and C3AO10

bond lengths are measured to be 1.242 and 1.247 A inGSN and the corresponding bond angles for the a-glycine

CBS-4 HF/LANL2DZ B3LYP/LANL2DZ MP2/LANL2DZ

1.520 1.510 1.518 1.5431.016 1.008 1.055 1.0421.010 1.006 1.022 1.0271.052 1.027 1.053 1.0621.563 1.547 1.564 1.5681.079 1.078 1.091 1.0981.078 1.078 1.094 1.1011.215 1.232 1.258 1.2961.281 1.282 1.318 1.3201.621 1.713 1.677 1.6771.345 1.306 1.347 1.3681.220 1.225 1.266 1.2971.288 1.283 1.314 1.3262.193 2.334 2.391 2.450

CBS-4 HF/LANL2DZ B3LYP/LANL2DZ MP2/LANL2DZ

107.88 109.85 105.85 104.06111.76 109.94 114.49 112.68103.88 104.18 104.09 105.33110.63 108.77 112.22 112.00106.36 109.24 101.24 104.71113.32 112.28 114.63 114.00110.41 112.11 109.15 107.47116.27 116.19 118.67 118.57112.83 113.60 112.55 114.06110.52 109.88 110.16 110.96108.64 108.60 110.39 109.36110.14 110.12 110.25 112.08109.37 109.19 110.30 108.69130.89 130.19 128.75 126.96164.94 155.83 143.73 145.64115.74 148.81 147.65 142.39120.25 121.12 121.21 120.80115.99 115.75 114.96 115.58123.75 123.14 123.84 123.62693.07 98.55 95.68 94.685

Table 4Optimized torsion angles (�) in GSN

Dihedral angle Experimental B3LYP/6-311 G(d,p) BLYP/6-31G(d) CBS-4 HF/LAN L2DZ B3LYP/LANL2DZ MP2/LANL2DZ

N1AC2AC3AO4 6.64 �31.83 �32.663 6.661 8.300 �15.427 �33.270N1AC2AC3AO10 �175.23 141.71 140.17 �174.01 �172.61 162.56 139.91H5AN1AC2AC3 60.03 40.62 41.799 38.845 51.957 28.482 44.027H6AN1AC2AC3 �179.97 164.33 165.585 162.03 174.19 152.26 167.61H7AN1AC2AC3 �59.98 �69.03 �67.033 �73.235 �63.009 �80.894 �67.636H8AC2AC3AO4 �114.38 �150.05 �150.40 �111.98 �111.11 �134.76 �150.78H8AC2AC3AO10 68.75 23.478 22.438 67.350 67.981 43.223 22.405H9AC2AC3AO4 127.73 85.855 84.461 125.98 128.60 103.85 83.995H9AC2AC3AO10 �54.13 �100.61 �102.71 �54.694 �52.313 �78.159 �102.82H8AC2AN1AH7 61.06 50.234 52.307 46.596 57.180 38.296 50.973H9AC2AN1AH7 178.91 171.55 173.10 166.96 176.14 159.86 172.78C2AN1AH7AO11 �32.29 60.784 65.338 96.348 138.46 78.237 57.552N1AH7AO11AN12 4.42 �135.12 �125.51 �126.55 175.24 �151.49 �152.44H7AO11AN12AO13 �76.80 �37.407 �42.322 �65.838 �74.083 �32.521 �22.291H7AO11AN12AO14 103.93 142.44 137.22 113.12 106.00 147.23 157.90O11AN12AO14ANa15 �4.03 �13.095 �21.009 �16.278 �5.640 �5.128 �6.897O13AN12AO14ANa15 176.69 166.75 158.51 162.64 174.45 174.61 173.30


and c-glycine are reported to be 1.252, 1.255 A and 1.254,1.237 A, respectively. Furthermore, the bond angle corre-sponding to the glycine skeleton molecule in a-form andc-form is found to be 111.8� for and 107�, respectively,and it is evident that the twist about CAN bond in c-gly-cine makes the c-form asymmetric. It is interesting to notethat the experimental and theoretical bond lengths andbond angles of the glycine molecules in GSN are quitecomparable to that of the a-glycine than c -glycine, thoughthe c-form preferably crystallizes in non-centrosymmetricstructure resulting to be NLO active [41].

The GSN molecule was primarily optimized using stan-dard B3LYP, BLYP and HF levels at different split valencebasis sets 6-31G(d, p), 6-31G(d). The dihedral anglesN1AC2AC3AO4 and N1AC2AC3AO10 of the glycine skele-ton in GSN are calculated to be �32.6� and 140.3� for theDFT (B3LYP and BLYP) calculations with 6-31G(d) basisset and 6.6� and �174.01� for the CBS-4 calculation. Withthe special basis set LANL2DZ, the same dihedral anglesare found to be 8.3� and �172.6�, �15.4� and 162.6�,�32.2� and 139.9� corresponding to the HF, B3LYP andMP2 computations, respectively, while the experimentallymeasured values are 6.6� and �175.2�. It is also inferredthat the glycine skeleton in GSN is reasonably twisted byaround 6.6� than the glycine zwitterion where the samedihedral angle is found to be 0.4� that reveals the glycinemolecule in GSN is twisted. The optimized structures ofGSN based on HF/LANL2DZ and CBS-4 calculationsare observed to be quite comparable with the experimentalmeasured values than other DFT and MP2 calculations.Although the dihedral angles of the GSN molecule inHF/LANL2DZ calculation are more similar than theCBS-4 calculation with the experimental values, slight devi-ations of few dihedral angles are found mainly between theglycine cation and nitrate anion. The dihedral angleC2AN1AH7AO11 between glycine cation and nitrate anionare measured to be �26.33� whereas the same torsional

angles are computed to be 138.46� and 96.35� from HF/LANL2DZ and CBS-4 calculations. It has been observedfrom the computed and measured dihedral angles thatthe deviations of few dihedral angles are mainly due tothe influence of metal atom Na coordination with oxygenatoms of both nitrate and carbonyl groups which distortedor twisted the geometry of GSN molecule in the crystal.

5. Natural Bond Orbital analysis

NBO analysis is proved to be an effective tool for chem-ical interpretation of hyperconjugative interaction and elec-tron density transfer (EDT) from filled lone electron pairsof the n (Y) of the ‘‘Lewis base’’ Y into the unfilled anti-bond r* (XAH) of the ‘‘Lewis acid’’ XAH in XAH� � �Yhydrogen bonding systems [48]. To elucidate intermolecu-lar hydrogen bonding, intermolecular charge transfer(ICT), rehybridization, delocalization of electron densityand cooperative effect due to n (O) fi r* (NAH), theNBO analysis has been performed on GSN and glycine,and the corresponding results are presented in Tables 5and 6. The intermolecular NAH� � �O hydrogen bondingis formed by the orbital overlap between the n (O) and r*

(NAH) which results ICT causing stabilization of the H-bonded systems. Hence hydrogen bonding interaction leadsto an increase in electron density (ED) of NAH anti-bond-ing orbital. The increase of population in NAH anti-bond-ing orbital weakens the NAH bond. Thus the nature andstrength of the intra- and intermolecular hydrogen bondingcan be explored by studying the changes in electron densi-ties in vicinity of N� � �H hydrogen bonds. The NBO analy-sis clearly shows the existence of strong NAH� � �Ointermolecular hydrogen bonding in GSN. This investiga-tion obviously clarifies the formation of H-bonded interac-tion between n (O11) and r* (N1AH7) anti-bondingorbitals. The difference in stabilization energy E(2) associ-ated with the hyperconjugative interaction n1 (O11) fi r*

Table 5Second order perturbation theory analysis of Fock matrix in NBO basis

Donor NBO(i)

AcceptorNBO (j)

E(2) kcal mol�1 E(j) � E(i)a.u.

F(i, j)a.u.

LP1O11 r*N1AH7 16.31 1.19 0.125LP2O11 r*N1AH7 1.64 0.69 0.030LP3O11 r*N1AH7 7.24 0.68 0.067LP2O10

(GSN)r*N1AH6 0.88 0.63 0.022

LP2O10

(Glycine)r*N1AH6 1.14 0.56 0.023

E(2), energy of hyperconjugative interactions (stabilization energy);E(j) � E(i), energy difference between donor i and acceptor j NBO orbi-tals; F(i, j), Fock matrix element between i and j NBO orbitals.


(N1AH7), n2 (O11) fi r* (N1AH7) and n3(O11) fi r*

(N1AH7) are 16.31, 1.64 and 7.24 kcal mol�1, respectively,which is due to the accumulation of electron density in theNAH bond drawn not only from n (O) of the hydrogenacceptor but from the entire molecule leading to its elonga-tion and concomitant red shift of the NAH stretchingwavenumber [49].

6. Vibrational Spectral Analysis

The vibrational spectral analysis is performed based onthe characteristic vibrations of the glycine molecule andinorganic nitrate separately. The bands observed between3500–300 cm�1 arise from the internal modes of the glycinemolecules and the internal modes of inorganic nitrateswhile the bands below 300 cm�1 occur due to the externalmodes of glycine molecule, the liberational and transla-tional modes of Na and the vibrations of the low frequencyhydrogen bonds. The computed vibrational wave numbers,their IR and Raman activities and the atomic displace-ments corresponding to the different normal modes areused to identify the vibrational modes unambiguously.The calculated vibrational wave numbers, measured infra-red and Raman band positions and their tentative assign-ments are presented in Table 7.

6.1. Glycine vibrations

The internal vibrations of the glycine molecule are dis-cussed based on the vibrations of amino, methylene, car-

Table 6NBO results showing the formation of Lewis and non-Lewis orbitals

Bond (AAB) ED/Energy (a.u.) EDA% EDB%

rN1AH7 1.98985 76.92 23.08rN1AH6 1.99393 72.34 27.66rN1AH5 1.99327 73.88 26.62r*N1AH7 0.06090 23.08 76.92r*N1AH6 0.01209 27.66 72.34r*N1AH5 0.01288 26.62 73.38

LP2O35 1.94784LP2O35 1.92749LP2O35 1.70094

boxylate groups, skeletal modes, and low frequencyhydrogen bonds.

6.1.1. Amino vibrations

In saturated amines, the asymmetric NH2 stretch andtheir symmetric counterpart are usually expected in theregion 3380–3350 cm�1 and 3310–3280 cm�1, respectively[50]. However, the protonation of NH2 group can shift inband position towards the range 3300–3100 cm�1 and3100–2600 cm�1 for asymmetric and symmetric stretchingmodes, respectively, as observed in glycine derivatives[51]. The NHþ3 stretching bands are broader and weakerin IR than those arising from the uncharged NH2 groups.The asymmetric stretching mode of the NHþ3 groupappears at 3241 and 3244 cm�1, respectively, in the IRand Raman spectra as weak bands. Furthermore, the posi-tion and broadness of this mode NHþ3 asymmetric stretch-ing frequency indicates the formation of both intra- andintermolecular strong NAH� � �O hydrogen bonding of theNHþ3 group with oxygen of both the carbonyl group andthe inorganic nitrates. The presence of strong NAH� � �Ointra- and intermolecular hydrogen bonding is also evidentfrom the lowering of the NHþ3 symmetric stretching fre-quency to 2886 cm�1 in IR spectrum. Table 8 explainsthe hydrogen bonding geometry of the GSN crystal whichclearly explains the existence of strong intra- and intermo-lecular NAH� � �O hydrogen bonding. The bond distanceand bond angle between the N1 atom of the glycine mole-cule and the O atoms of nitrate group and of carboxylategroup are around 2.724, 3.003 A and 146�, 113�, respec-tively, resulting in strong NAH� � �O intramolecular hydro-gen bonding between the ionic spices, which might providethe non-centrosymmetric structure for the GSN crystal.

The crystal structure [40] and optimized geometries ofGSN show that the organic molecular units are locatedbetween layers of Na (NO3) chains and linked to sodiumnitrate by strong intramolecular hydrogen bonds of theN+AH� � �O type. This structural organization of infinitechains of highly polarizable entities connected in a head-to-tail arrangement in GSN is favourable in contributingto the NLO properties of the crystal. Another structuralfeature of interest in this material is the role of Sodiumatom in the inorganic sodium nitrate to make GSN crystalNLO active, though the inorganic salts are not forming

NBO S% P%

0.8770(sp2.75) N + 0.4804 (s) H 26.66 73.270.8505(sp3.38) N + 0.5259 (s) H 22.79 77.110.8566(sp3.48) N + 0.5160 (s) H 22.32 77.590.4804(sp2.75) N � 0.8770 (s) H 26.66 73.270.5259(sp3.38) N � 0.8505 (s) H 22.79 77.1105160(sp3.48) N � 0.8566 (s) H 22.32 77.59

sp0.27 78.43 21.56sp99.99 0.68 99.25sp99.99 0.27 99.63

Table 7Calculated vibrational wavenumbers, measured infrared and Raman band positions (cm�1) and assignments for GSN

HF/LANL2DZmcal/cm�1

mIR/cm�1 mRaman/cm�1 IR intensity(Absolute)

Raman activity(Absolute)

Force constant(mdyne/A)

Depol.ratio

Assignment

3392 3241 wbr 3244 vwbr 48.12 129.98 7.92 0.31 NHþ3 asym stretch3016 3026 wsh 3023 w 124.8 43.890 6.64 0.12 CH2 asym stretch2947 2959 wbr 2969 m 4.460 103.66 5.96 0.13 CH2 sym stretch2810 2880 wbr 2886 vw 1110.8 154.06 5.19 0.27 NAH� � �O sym stretch

2802 wsh Overtones/combinations2714 vw 2723 vw2625 vw 2615 vw2440 vw2405 vw2009 vwbr Combination of NHþ3 asym

bend and torsion1768 vw Combination m1 and m4 of NO�3

1631 1615 m 1616 vw 322.82 1.83 7.56 0.72 NHþ3 asym bend1623 1579 s 199.62 3.07 2.08 0.75 COO� asym stretch1517 1506 m 1510 vw 137.87 4.57 2.02 0.72 NHþ3 sym bend1460 1449 m 1449 w 120.73 7.60 1.43 0.68 CH2 scissoring1433 1414 ssh 1408 wsh 143.53 8.74 1.56 0.74 COO� sym stretch1389 1353 vvs 287.63 9.69 5.10 0.72 NO�3 asym stretch1320 1328 w 146.48 3.61 5.40 0.42 CH2 wagging1310 1307 vs sh 1309 vw 146.48 3.61 5.40 0.42 CH2 wagging1151 1137 m 1143 vw 263.59 4.97 2.18 0.75 CH2 twisting1120 1116 s 1118 vw 46.710 4.44 1.09 0.50 NHþ3 rocking1084 1068 vw 4.840 3.31 0.91 0.67 CH2 rocking1072 1052 vvs 114.34 23.11 9.13 0.10 NO�3 sym stretch1016 1039 m 1039 wsh 13.74 3.64 1.68 0.63 CAN stretch940 935 vs 935 vw 29.96 1.62 0.80 0.46 CH2 rocking902 890 s 895 m 44.45 6.92 2.69 0.05 CAC stretch829 829 s 16.03 0.27 5.66 0.60 cNO�3 out of plane deform741 721 m 4.61 1.60 4.57 0.68 COO� deform686 676 s 677 vw 3.641 1.55 2.03 0.69 dNO�3 in-plane deform598 586 vw 16.161 2.40 1.69 0.70 COO� deform515 509 w 34.651 2.42 0.43 0.40 COO� rocking418 399 vw 19.291 0.35 0.18 0.45 NHþ3 torsion323 331 vw 48.151 1.22 0.12 0.22 CCN bending213 178 msh 42.011 0.69 0.43 0.24 Na+ translation143 138 ssh 5.451 0.65 0.16 0.72 COO� torsion107 109 vs 3.261 0.58 0.10 0.69 N � � �O vibrations=

NO�3 torsion

Table 8Hydrogen bonding geometry

DAH� � �A DAH H� � �A D� � �A DAH� � �AN1AH5� � �O10 1.036 1.967 3.003 112.7N1AH7� � �O11 1.060 1.664 2.724 146.3N1AH7� � �O11

a 0.890 2.062 2.952 154.9N1AH5� � �O4

a 0.890 2.030 2.789 142.0N1AH6� � �O10

a 0.890 1.970 2.780 151.0C3AH14� � �O16

a 0.970 2.540 3.256 131.0

a Indicates intermolecular hydrogen bonding taken from ref. [40].


non-centrosymmetric crystals with glycine. The aminogroup of the glycine can be act as a donor and both theoxygen atoms of the carboxylate group and the nitrategroup can be act as acceptors, which are connected bythe strong intramolecular ionic hydrogen bonds, are clearlysubstantiated by the DFT computations. The presence ofstrong NAH� � �O intra- and intermolecular hydrogenbonding is also substantiated by the NBO analysis.

It has been recently explored that the ground statehydrogen bonding between the electron donor and the elec-tron acceptor as the major cause of the Twisted Intramo-lecular Charge Transfer (TICT) state formation by meansof ‘promotion effect’ [52]. The analogues of the materialcontain an electron donor in the form of amino groups,and therefore, already in their ground states they are aptin the form of hydrogen bond with excellent electron accep-tors. In the excited ICT state, the H-bond to the positivelycharged amino group is expected to break, and new H-bonds are expected to form at the sites of high electrondensity, on breaking the H-bond in the excited state, a pre-condition for the twist [52]. As discussed earlier in the pre-vious section that the twist about CAN bond in c-glycinemakes the c-form asymmetric unlike the a-form of the gly-cine which normally does not have any twist about theskeleton of glycine molecule. XRD and DFT investigationsare quite favourable to the existence of the a-form ratherthan c-form in GSN. However, the twist of the glycine skel-


eton of a-form, which is pre-requisite to make the glycinemolecule NLO active, may be introduced by the TwistedIntramolecular Charge Transfer (TICT) due to the pres-ence of strong ionic intra- and intermolecular N+AH� � �Ohydrogen bonding.

The NHþ3 asymmetric and symmetric bending vibrationsare generally expected near 1660–1610 cm�1 and 1550–1485 cm�1, respectively [51]. The NHþ3 asymmetric defor-mation mode identified in IR as medium band at1615 cm�1 and as weak Raman band at 1616 cm�1, unam-biguously. Further, the medium IR band at 1506 cm�1 andweak Raman band at 1510 cm�1 correspond to the NHþ3symmetric deformation mode. Generally, in the aminoacids, multiple combination bands of weak intensity occurin the 2222–2000 cm�1 region, the most prominent beingthe band near 2000 cm�1. In GSN, an additional band isidentified at 2009 cm�1 can be unambiguously correlatedto the combination of the torsional oscillation and degen-erate deformation of the NHþ3 groups, which can be useda ‘finger print’ for the identification of NHþ3 group inGSN crystal. This band is the characteristic marker ofthe NHþ3 groups present in the organic compounds partic-ularly in the amino acids, as this mode disappears in thespectrum of the deuterated derivatives [53]. The rockingand torsion vibrations of NHþ3 group has been identifiedand assigned with the aid of DFT calculations.

6.1.2. Methylene vibrations

The internal vibrational modes of the CH2 group arewell separated from the vibrations of the other part ofthe glycine molecule. This is particularly expected for thestretching CH2 vibrations. Absorption arising from CAHstretching in the alkanes occurs in the region 3000–2840 cm�1. The qualitative interpretation of intensitiesmust rely upon the understanding of some basic aspectsof intramolecular charge distribution and on their effectson infrared intensities [54]. The molecules that show well-recognized effects of induction from electronegative atomsin the surrounding or of backdonation of negative chargefrom lone pairs in the same molecules or of hyperconjuga-tion with systems of an aromatic rings or multiple bondscan give rise to specific signals in the spectrum, especiallyin intensity. These signals are so strong that they can leadto a quantitative diagnosis of charge distribution directlyfrom a careful analysis of the vibrational spectrum. Differ-ent theoretical methods have been proposed for infraredintensities, which can reveal a depiction of the charge dis-tribution and charge mobility in molecules [55,56]. Theparameters charge (q0

H) and charge flux (oqH/orCH) are veryimportant markers of the charge distribution inside themolecule and are directly related to the molecular struc-ture, the vibrational potential and the molecular confirma-tion [57]. Particularly, a large charge in general implies astrong bond, with a short interatomic distance r0

CH and alarge stretching force constant kCH. It is important to beaware that the infrared intensity in the CAH stretchingregion is a genuine marker of the charge distribution

[58,59]. It has been shown that ECCF from infrared spec-trum account quantitatively for several intramolecular elec-tronic effects including induction and backdonation. Forthe molecules in which induction produces stronger polar-ization of CAH bonds, along with the increase of bothCAH force constant and charge of the hydrogen atomand decrease of CAH stretching intensity and CAH bondlength, it can cause the enhancement of vibrational wave-number of CAH stretching modes. The methylene grouphydrogen atoms in GSN are subjected to the electroniceffect induction leading to the enhancement of stretchingwavenumbers and decrease of infrared intensities.

6.1.2.1. Improper, blue-shifting hydrogen bonding. Hydro-gen bonding (H-bond) originates from an attractive inter-action between the electron-deficient hydrogen donorgroup (AAH) and a region of high electron density accep-tor atom (B), leading to the variation of H� � �B distancethan the van der Waals radii of the isolated H and B atoms.The attractive interaction between the hydrogen donorgroup and the acceptor atom modifies the molecular poten-tial energy surface and has dramatic consequences for thevibrational spectra. Most frequently, an H-bond is of theXAH� � �Y type, where X and Y are electronegative ele-ments and Y possesses one or two electron lone pairs.Although the interaction energy of a CAH� � �O hydrogenbond is less than those of typical NAH� � �O and OAH� � �Otype bonds, the CAH type hydrogen bond plays an impor-tant role in determining higher order structure in proteins,molecular structure and conformation and crystal packing.In CAH� � �O type hydrogen bond where the proton donoris sp3-hybridized, the hydrogen bonded CAH undergoescontraction due to interaction with a proton acceptor.The shortening of CAH bond contrasts sharply with theelongation of OAH and NAH in OAH� � �O and NAH� � �Otype hydrogen bonds. It has been concluded that the differ-ences in charge transfer exist between CAH� � �O type andtypical OAH� � �O, NAH� � �O type hydrogen bonds [60,61]and also the mechanism of OAH� � �O and NAH� � �O typehydrogen bond formations is a direct process where the pri-mary effect is the charge transfer from the proton acceptorto the OAH and NAH anti-bonding orbital of the protondonor, and thus an increase in electron density in this orbi-tal leads to weakening of the OAH and NAH bonds,accompanied by their elongation. In CAH� � �O hydrogenbond, alternatively, a charge transfer from the lone pairsof the electron donor is directed mainly to the anti-bondingorbitals in the remote part of the complex, thus causingelongation in that part of a complex. This primary effectof elongation is accompanied by a secondary effect of struc-tural reorganization of the proton donor, leading to con-traction of the CAH bond. Therefore, a CAH� � �Ohydrogen bond is considered an ‘improper hydrogen bondor blue-shifting H-bond’ [62]. Blue- shifting hydrogenbonds are characterized by a contraction of the CAH dis-tance, a blue shift of the CAH stretching vibrational mode,and a reduction of its infrared intensity, features which are


in sharp contrast to those rooted to the conventionalhydrogen bonds [63].

The asymmetric CH2 stretching (mas CH2) mode isexpected in the region near 2926 cm�1 and the symmetricmode (ms CH2) around 2853 cm�1 [51]. But, for aminoacids, mas CH2 vibrations generally appear in the region3100–3000 cm�1 while ms CH2 vibrations appear around3000–2900 cm�1 [33]. The CH2 asymmetric stretchingappears at 3026 cm�1 in IR and in the Raman spectrumat 3023 cm�1 as weak bands. The corresponding stretchingmode is computed at 3016 cm�1 by the DFT calculations,which explains that the observed vibrational wavenumbersare larger by around 10 cm�1 than the computed valuefrom their normal coordinates, as shown in Fig. 8. Thesymmetric CH2 stretching mode band is observed at2959 cm�1 as weak IR band and in the Raman spectrumat 2969 cm�1 as medium band, which is calculated at2947 cm�1. As observed in the asymmetric stretchingmode, the calculated symmetric stretching vibration is low-ered from the observed wavenumbers by around 20 cm�1.The bond lengths of C2AH9 and C2AH8 bonds are calcu-lated to be 1.09 and 1.10 A from the distance matrix ofthe optimized geometry of GSN while the experimentallyobserved bond lengths for the corresponding bonds arefound to be 0.97 and 0.97 A, respectively. The experimentalCAH bond lengths are shortened about 0.12 and 0.13 Aover the calculated values, which can be attributed to theincrease in wavenumbers of the stretching modes of theCH2 group in GSN. Shifting of the stretching frequenciestowards higher wavenumbers, intensity variation and theCAH bonds contraction indicating the existence ofCAH� � �O hydrogen bonding throughout the GSN crystal.This fact is further substantiated by XRD analysis of GSNcrystal that explains the CAH� � �O bond distance and thecorresponding bond angle between molecules of GSN areto be 3.2 A and 131�, respectively.

The bending vibrations of the CAH bonds in the meth-ylene group are identified in their respective positions. Thescissoring mode of the CH2 group gives rise to a character-istic band near 1465 cm�1 in IR and Raman spectra. Thismode appears as intense Raman band at 1449 cm�1, whichis calculated at 1421 cm�1. The twisting, wagging and rock-ing vibrations appear in the region of 1422–719 cm�1. Thefrequencies corresponding to IR bands at 1307 cm�1 (verystrong), 1137 cm�1 (medium) and 935 cm�1 (very strong)are correlated to the CH2 wagging, twisting and rockingmodes, respectively, which are supported by computationalwave numbers.

6.1.3. Carboxylate vibrations

The carboxylate ion gives rise to two modes; asymmetricstretching near 1650–1550 cm�1 and symmetric stretchingmode near 1400 cm�1 while the C@O stretching wave num-bers of the un-ionized carboxylic group of the glycine mol-ecule is usually found near 1740 cm�1 as an intense IRband [51,64]. The asymmetric stretching mode of COO�

vibration appears in IR as intense band at 1579 cm�1 (cal-

culated at 1623 cm�1) and the COO� symmetric stretchingmode observed at 1414 cm�1 (calculated at 1433 cm�1) inthe Raman spectrum. The lowering of this mode fromthe computed values can be due to the interaction of thelone pair oxygen atoms with the nucleophilic atoms ofGSN molecules through the strong intra- and intermolecu-lar NAH� � �O and CAH� � �O hydrogen bonding. Further-more, as discussed in the preceding section optimizedgeometry, the considerable lowering of both asymmetricand symmetric carboxylate wavenumbers by 44 and19 cm�1, respectively, in GSN is mainly due to the intermo-lecular non-bonded interaction of metal atom Na witheight neighbouring oxygen atoms of the carboxylate andnitrate groups. The COO� deformation, wagging and rock-ing modes have been identified and assigned, which con-firms that the glycine molecule in GSN exists inzwitterionic form with deprotonated carbonyl groups andprotonated amino groups.

6.2. Sodium nitrate vibrations

The nitrate anion in sodium nitrate is a potential acceptorof six hydrogen atoms and thus six NAH� � �O hydrogenbonds may be formed. The crystal structure [40] shows thatthere are four such hydrogen bonds in a two dimensionalnetwork. As such, one O atom is an acceptor of two protonsand the other two O atoms accept a proton each. The nitro-gen/oxygen stretch of inorganic nitrate appears as an intenseband between 1400 and 1340 cm�1. Planar XY3 moleculeshave D3h symmetry and their normal vibrations m1 mode(symmetric stretching), m2 mode (out-of-plane deformation),m3 mode (asymmetric stretching) and m4 mode (out-of-planedeformation) are concurrently active, except one mode, inIR and Raman spectra. The inorganic nitrate anion, NO�3 ,is planar and their m1 mode, m sym NO�3 , is IR inactive andthe m2 mode, cNO�3 , is Raman inactive. Thus, the IR activemodes are m2 through m4 and all but m2 are Raman active.Generally, the m1 mode appears in the region 1034–1067 cm�1 while the m2 mode is expected in the region 801–834 cm�1. Moreover, the m3 (m asym NO�3 ), and m4 (dNO�3 ),modes are doubly degenerate, which usually occur in theregion 1300–1550 cm�1 and 700–751 cm�1, respectively [65].

The most intense Raman band of the GSN crystalobserved at 1052 cm�1 can be correlated to the symmetricstretching m1 mode of the nitrate anion and the nitrateasymmetric stretching m3 mode appears at 1353 cm�1 asmost intense doublet IR bands, which is overlapping withthe wagging mode of the methylene group. The NO3 asym-metric and symmetric stretching vibrations are calculatedat 1389 and 1072 cm�1, respectively, and observed frequen-cies are lowered from the computed values by around25 cm�1 which reveals the non-bonded interactions of oxy-gen atoms of the nitrate groups with neighbouring metalatom Na and amino groups of the glycine molecules. Fur-thermore, these modes are broader and stronger than thoseof all other modes in the GSN crystal suggesting that beinga strong acceptor these nitrate groups are actively partici-

Fig. 8. Atomic displacements of selected modes.


pated in both intra- and intermolecular NAH� � �O ionichydrogen bonding with the other electron-deficient atoms.The m2 and m4 modes of the inorganic nitrate anion vibra-tions have been identified and assigned.

Inorganic nitrates exhibit one or two weak IR bands inthe region 1734–1790 cm�1, which are correlated to the com-bination tone m1 + m4. This combination band appears at

1768 cm�1 in IR spectrum, as expected, as an additionalweak band, which is not computed in the DFT calculations.

6.3. Skeletal modes

The absorption bands arising from CAN and CACstretching vibrations are generally measured in the region

Fig. 8 (continued)


1150–850 cm�1 [49,66]. The C2AC3 stretching modeappears as medium IR band at 895 cm�1 and in Ramanspectrum at 890 cm�1 (medium). Likewise, the C2AN1

stretching mode observed in IR at 1039 cm�1 (medium)and at 1039 cm�1 in Raman spectrum. The very weakRaman band at 331 cm�1 is attributed to the C3AC2AN1

deformation mode. The translation mode of the Na+ ion

appears in Raman spectrum at 178 cm�1 as mediumband.

6.4. Low wavenumber hydrogen bond vibrations

The attractive interaction between the hydrogen donorgroup and the acceptor moiety leads to the occurrence of


new vibrational degrees of freedom, the so-called hydrogenbond modes. Such modes are connected with elongationschanging the A� � �B distance and/or the relative orientationof the hydrogen bonded groups. Thus, they provide directinsight into the structure of hydrogen bonds and into pro-cesses of bond formation and cleavage. As such modes arecharacterized by a high reduced mass of the oscillator and asmall force constant determined by the comparably weakattractive interaction along the hydrogen bond, hydrogenbond modes occur at low frequencies in the range betweenabout 50 and 300 cm�1 [67]. An interesting feature of thesevibrations is the occurrence of the intense Raman bandobserved at 109 cm�1 is correlated to the N� � �O stretchingH-bonds vibrations [7,68]. Such Raman bands areobserved in the amino acid based NLO active crystals thatstabilize the crystal structure through the strong hydrogenbonded networks. However, these bands appear in certainNLO inactive crystals like glycine lithium nitrate (GLiN) inwhich two Raman bands appear at 132 and 104 cm�1 withthe same intensity as the Raman band 109 cm�1 of GSNcrystal [69]. It is interesting to note that, in GLiN, the highwavenumber band 132 cm�1 disappears both on deutera-tion or when the lithium atom is replaced by a sodiumatom. These spectral features in such crystals are significantarea of research to be explored. The analysis of the eigen-vectors reveals that the vibration corresponding to the119 cm�1 band is associated with the in-phase vibrationsof atoms of the molecule connected through the ionicNAH� � �O hydrogen bonding. The twisted intramolecularcharge transfer from the donor to the acceptor throughthe H-bond results in electron–phonon coupling in thishydrogen bonded system, which provokes the band to bevery intense in the Raman spectrum. This reveals the vibra-tional contribution to the hyperpolarizability of such non-linear optical crystals [12,70] that provide the non-centrosymmetric structure contributing to render GSNcrystal NLO active.

6.5. Effect of ionic hydrogen bonds on NLO properties

It is generally recognized that the first electronic hyper-polarizability value (b) is enhanced when there is a low-lying ground-to-excited state transition with large changein the dipole moment [71,72]. However, much less progresshas been made in understanding the microscopic opticalnonlinearity in inorganic materials. Arising from the com-plexation of organic molecules based on acid–base interac-

Table 9Comparison of static first hyperpolarizability, dipole moment, HOMO–LUM

Compounds Dipole moment lg (Debye) Hyperpola

Glycine (Molecular) 6.2602 0.1536Glycine (Zwitterion) 10.945 12.537Glycine Sodium Nitrate 6.1877 14.543Sodium nitrate 8.7763 0.2365

tions, highly polarizable cations, responsible for NLOproperties, are linked to anions through hydrogen bondnetworks, which generate a non-centrosymmetric struc-tural organization. In the case of molecules with lowersymmetry, it is interesting to note that an alternated stackwould necessarily result in a non-centrosymmetric chain.Recently the method derived from the dielectric theory ofcomplete crystals and the Levine bond charge model tothe hydrogen bonded solids has been successfully appliedand the results obtained show that the hydrogen bond con-tributes to the second order NLO tensor coefficient (dijk) ofthe crystals [73]. Certainly hydrogen bonds create and sta-bilize the crystal structures but more evidently that theyalso contribute considerably to the enhancement of hyper-polarizability of hydrogen bonded molecular systems [74]or to the enhancement of the second order susceptibilityof the crystals [75].

The low wavenumber wave packet motions have beenrecently observed along moderately strong intramolecularhydrogen bonds with well-defined geometries by thepump-probe spectroscopy [67]. It would be useful to men-tion that with the extent of charge transfer is expected toprovide stronger H-bonding interaction between the accep-tor group and the H atom of the donor group [76]. The cal-culated the first hyperpolarizability (btot) and the groundstate dipole moment (lg) is 14.5487 · 10�31 esu and 6.18Debye, respectively. Table 9 shows that btot has the largestcalculated value for GSN crystal and the zwitterionic gly-cine molecule while btot is considerably decreased for theconstituents of the GSN crystal. As can be seen that thecompounds having the higher dipole moment results inthe higher btot value and the corresponding HOMO–LUMO energy gap is quite low. This clearly indicates thatthe strong hydrogen bonding between the charged speciesreduces the energy gap considerably with the formationof the charge transfer axis [77]. In acid–base hybrid crystalshydrogen bonds play an important role not only in the cre-ation of crystal structure and its stability, but also in theenhancement of second order susceptibility of the crystaldue to the perturbation of the electronic structure of theorganic partner and also due to the strong electron–pho-non coupling [72,78].

7. Conclusions

Single crystals of GSN grown by slow evaporation tech-nique and the second harmonic generation efficiency was

O energy gap and SCF energy for the constituents of GSN

rizability btot (·10�31 esu) Energy gap a.u. SCF energy a.u.

0.6038 �282.8250.5243 �282.7860.2216 �727.2930.3943 �440.765


measured by Kurtz–Perry powder SHG experiments,which is around 0.3 times that of urea. The calculated firsthyperpolarizability of GSN is found to be 14.54 · 10�31 esu,which is 7 times that of urea. The equilibrium geometryoptimizations of GSN were carried out using DFT, MP2and ab initio computations at different special basis setincluding LANL2DZ and analyzed. In a-glycine, c-glycineand GSN, the experimentally measured bond lengths arerather lower than the data measured by the different theo-retical methods including the special basis set LANL2DZ.In GSN, XRD shows the same angle is 111.9� and it isinteresting to note that the experimental and theoreticalbond lengths and bond angles of the glycine molecules inGSN are quite comparable to that of the a-glycine thanc-glycine. However, the twist of the glycine skeleton ofa-form, which is pre-requisite to make the glycine moleculeNLO active, may be introduced by the Twisted Intramolec-ular Charge Transfer (TICT) due to the presence of strongionic intra- and intermolecular N+AH� � �O hydrogenbonding.

The presence of strong NAH� � �O intra- and intermolec-ular hydrogen bonding is also evident from the lowering ofthe NHþ3 symmetric stretching frequency to 2886 cm�1 inIR spectrum, which is further substantiated by the NBOanalysis and computations. The experimental CAH bondlengths are shortened about 0.12 and 0.13 A over the calcu-lated values, which can be attributed to the increase inwavenumbers of the stretching modes of the CH2 groupin GSN. Shifting of the stretching frequencies towardshigher wavenumbers, intensity variation and the CAHbonds contraction indicating the existence of ‘blue-shiftor improper’ CAH� � �O hydrogen bonding throughoutthe GSN crystal. The occurrence of the intense Ramanband observed at 109 cm�1 is correlated to the N� � �Ostretching H-bonds vibrations. This vibration favours thetwisted intramolecular charge transfer from the donor tothe acceptor and carries out the phenomenon of the elec-tron–phonon coupling in this hydrogen bonded materialwhat provokes to be very intense in the Raman spectrum.These vibrations provide the non-centrosymmetry struc-ture may be contributing to make the GSN crystal NLOactive, which may be considered as a diagnostics tool foridentifying the crystals to be NLO active. In acid–basehybrid crystals such ionic hydrogen bonds play an impor-tant role not only in the creation of crystal structure andits stability, but also in the enhancement of second ordersusceptibility of the crystal due to the perturbation of theelectronic structure of the organic partner and also dueto the strong electron–phonon coupling.

Acknowledgements

The authors are grateful to the Department of Space,Government of India, and Vikram Sarabhai Space Centre,Trivandrum, for financial support through the RESPONDproject. The authors thank Professor T.P.Radhakrishnanand Dr. Philip Anthony of the School of Chemistry,

University of Hyderabad for their support in NLO mea-surements. The assistance from BRUKER OPTICS inrecording the NIR FT-Raman and FT-IR spectra is grate-fully acknowledged.

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non-bonded interactions and its contribution to the nlo activity gsn

Documents

nlo responses

growth of nlo materials

noncentrosymmetric structure

optical communications

optical mixing

optical transparenc

optical nonlinearities

order optical nonlinearity