Electronic structure, vibrational spectral and intervening orbital interactions studies of NLO material: Guanidinium 4-nitrobenzoate

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Electronic structure, vibrational spectral and intervening orbital interactions studies of NLO material: Guanidinium 4-nitrobenzoate V. Sasikala a, D. Sajan a,⇑, K. Job Sabu a, T. Arumanayagam b, P. Murugakoothan b a b

Post Graduate & Research Department of Physics, Bishop Moore College, Mavelikara, Kerala 690 110, India Post Graduate & Research Department of Physics, Pachaiyappa’s College, Chennai 600 030, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Symmetrically perturbed NH2 groups

are the characteristic of GPNB molecule.  Moderate and weak intramolecular NAH  O hydrogen bonds have been reported.  Intramolecular hydrogen bonds and n ? r⁄ and r ? r⁄ interactions monitored the planar molecular structure.  Intervening orbital interactions promoting charge transfers for NLO system is investigated.  Global reactivity descriptors for GPNB are calculated.

a r t i c l e

i n f o

Article history: Received 20 October 2014 Received in revised form 3 December 2014 Accepted 10 December 2014 Available online xxxx Keywords: Guanidinium 4-nitrobenzoate NLO NBO FT-IR FT-Raman Soft molecule

a b s t r a c t Single crystals of guanidinium 4-nitrobenzoate (GPNB) have been grown by slow evaporation method. Grown crystals were characterized by FT-IR, FT-Raman, UV–Vis absorption and UV–Vis transmission spectroscopies. Crystal defects and surface morphology were studied by etching method. Dielectric properties of the crystal such as dielectric constant, dielectric loss and AC electrical conductivity as function of frequency (50 Hz–5 MHz) at two temperatures (35 °C and 100 °C) were measured. The frequency and temperature dependence of dielectric behaviour were investigated. The equilibrium geometry, vibrational spectral analysis, intramolecular charge transfer interactions using NBO method, first order hyperpolarizability, molecular electrostatic potential and frontier molecular orbital analysis for GPNB have been studied using density functional theory at B3LYP/cc-pVTZ level. Vibrational spectral study reveals the presence of moderate and weak NAH  O bonds in GPNB. NBO analysis also confirms the presence of intramolecular NAH  O hydrogen bonding and investigates the stability as well as the intervening orbital interactions. The electronic absorption spectrum of the gas and water phases of GPNB were simulated using time dependent density functional theory and NBO transitions for the three lowest excited states were assigned and studied. Ó 2014 Elsevier B.V. All rights reserved.

Introduction

⇑ Corresponding author. Tel.: +91 9495043765; fax: +91 479 2303230.

Organic derivatives possessing polarizable electrons spread over a large distance with various combinations of terminal electron donor and/or acceptor groups have been the objective of

E-mail addresses: [email protected], [email protected] (D. Sajan). http://dx.doi.org/10.1016/j.saa.2014.12.013 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: V. Sasikala et al., Electronic structure, vibrational spectral and intervening orbital interactions studies of NLO material: Guanidinium 4-nitrobenzoate, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/10.1016/j.saa.2014.12.013

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V. Sasikala et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx

recent research, particularly in view of their large molecular hyperpolarizabilities and good crystallizability [1–5]. The large non-linear optical response [6] in such system made them as potential candidates for wide range of NLO applications [7,8]. Organic molecules which comprise of cationic and anionic entities with directional hydrogen bonding interactions possess strong hyperpolarizability are known to be exhibit nonlinear optical properties [9–13] and such molecules expected with large values of molecular hyperpolarizability were analyzed by means of vibrational spectroscopy [14]. Density functional theory calculations are used to provide accurate molecular structure and excellent vibrational frequencies of organic compounds if the calculated frequencies are scaled to compensate for the approximate treatment of electron correlation, for basis set deficiencies and vibrational anharmonicity [15–18]. The computed vibrational frequencies can provide valuable feedback for the interpretation of experimental data. The spectroscopic and theoretical investigations on the structure and fundamental vibrations of guanidinium and its derivatives are still being carried out increasingly [19–30]. Many of the guanidinium family complexes [28,29,31–34] are potential materials for non-linear optics. This is because of the ability of guanidinium cation has to form extensive family of crystals having different hydrogen-bonded configurations. Para-substituted benzene derivatives give a special attention among NLO materials as they generally have larger dipole moments than that of monosubstituted benzene compounds. An understanding of the relationship between molecular structure, dipole moment, polarizability and first hyperpolarizability can offer a useful insight for the design of new NLO materials. So a complete spectroscopic characterization is required to obtain information about intra molecular interactions, electronic properties as well as non-linear optical mechanism at the molecular level. Quantum chemical computations especially, based on density functional theory (DFT) calculations have been employed extensively to study the relationship between the electronic structure and NLO response of the molecular systems. The computational approach allows the determination of molecular NLO properties as an inexpensive way to design molecules by analyzing their potential before synthesis and to determine the higher order hyperpolarizability tensors of molecules. The present work aims to investigate the vibrational spectral characterization, molecular structure, electronic and non-linear optical properties of guanidinium 4-nitrobenzoate (GPNB) using DFT computational method. Guanidinium 4-nitrobenzoate is an organic NLO crystal which crystallizes in the monoclinic system with lattice parameters a = 6.932Å, b = 6.620 Å, c = 11.131 Å, b = 107.966° and belongs to the noncentrosymmetric space group P121/m1 [35] and its SHG efficiency is 3.2 times that of standard NLO material, pottassium dihydrogen phosphate (KDP) [36]. Natural bond orbital interactions, intramolecular hydrogen bonding, frontier molecular orbital study, electronic absorption mechanism, first hyperpolarizability and molecular electrostatic potential for GPNB have been investigated. The sample crystal was grown for the study and it’s FT-IR, FT-Raman and UV–Vis transmission and absorption, dielectric and chemical etching studies have been carried out and experimental and simulated results are discussed in detail.

Experimental Preparation of the crystal The title crystal was prepared by adding high purity guanidine carbonate and 4-nitrobenzoic acid (Merck, AR grade) in the molar ratio 0.5:1.0 in aqueous solution. The filtered solution was allowed

to dry at room temperature. The purity of the synthesized salt was further purified by successive recrystallization in de-ionized water. The saturated solution at 40 °C temperature was prepared using recrystallized GPNB crystalline material in accordance with the solubility data reported in the literature [37]. The filtered solution was kept in the temperature bath and the solution was allowed to cool at the rate of 0.03 °C/day till the room temperature was reached. Optically transparent pale yellow colour single crystals of GPNB are harvested after a period of 23 days. Spectroscopic measurements FT-IR spectrum of the crystal has been recorded in the range 4000–400 cm1 by Perkin Elmer spectrometer using the KBr pellet technique. Perkin Elmer GX 2000 FT-Raman spectrometer was used to record the Raman spectrum in the range 4000–50 cm1. The UV–Vis transmission and absorption measurements (200–800 nm) were taken using Shimadzu Model 1601 spectrophotometer. Dielectric measurement The instrument, HIOKI IM3570 Impedance Analyzer was used to measure the dielectric parameters of the title crystal. Etching measurement Etching was carried out by dipping the cut crystal plate in etchants for the required period of time at room temperature. Then the crystal was wiped with dry filter paper and the observed etch patterns were photographed under the optical microscope OLYMPUS U-TV0.5XC-3. Computational procedures and theory The molecular structure of GPNB was optimized using density functional theoretical calculations with a hybrid exchange functional B3LYP (Becke’s three parameter hybrid functional using the Lee–Yang–Parr correlation functional) [38] at cc-pVTZ [39] basis set by the Berny method were performed with Gaussian 09W software package [40]. The self- consistent field equation has been solved iteratively to reach the equilibrium geometry corresponding to the saddle point on the potential energy surface. The harmonic vibrational wavenumbers were analytically calculated by taking the second derivative of energy at the same level of theory for the optimized structure and the obtained wavenumbers were scaled by 0.9682 [41]. Assignments have been made using the GaussView 5.0 molecular visualization program [42] and also on the basis of relative intensities and magnitudes of wavenumbers. The Raman activities (Si) calculated with the Gaussian 09W program was converted to relative Raman intensities (Ii) using the following relationship derived from the basic theory of Raman scattering [43,44]:

Ii ¼

f ðt0  ti Þ4 Si ti ½1  ehcti =kT 

ð1Þ

where t0 is the exciting wavenumber, ti is the vibrational wavenumber of the ith normal mode, h and k are universal constants and f is the suitably chosen common scaling factor for all the peak intensities. The natural bond orbital interaction analysis at DFT/B3LYP/ cc-pVTZ level has been performed by using the NBO 3.1 program [45] implemented in Gaussian 09W software package. NBO analysis is used to investigate the extent of hyperconjugation or delocalization of various second order interactions between filled

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V. Sasikala et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx

orbitals of one subsystem to vacant orbitals of another subsystem [46]. The second-order Fock matrix was used to evaluate the donor–acceptor interactions [47]. The hyperconjugative interaction energy, E(2) associated with the electron delocalization between donor and acceptor can be estimated from the secondorder perturbation approach [48] as

Eð2Þ ¼ nr

F 2ij < rjFjr2 > ¼ nr er  er DE

ð2Þ

where or Fij is the Fock matrix element between the donor (i) and acceptor (j) NBOs, er and er are the energies of r and r⁄ NBOs and nr is the population of the donor r orbital [46]. The electron densities of donor and acceptor NBOs, energy of hyperconjugative interactions (stabilization energy) and energy difference between donor and acceptor NBOs have been computed at the DFT level. The possible intramolecular and hydrogen bonded interactions and natural population analysis were investigated. The first hyperpolarizability (b), the mean polarizability (a), the anisotropy of the polarizabilty (Da) and the total static dipole moment (l) of GPNB molecule have been evaluated on the optimized geometry with DFT/B3LYP/cc-pVTZ method. The first hyperpolarizability is a third rank tensor that can be described by a 3  3  3 matrix. The 27 components of the 3D matrix can be reduced to 10 components due to the Kleinman symmetry [49]. The components of b are defined as the coefficients in the Taylor series expansion of the energy in the external electric field. If the external electric field is weak and homogenous, the expansion becomes:

E ¼ E0 

X i

li F 

1X 1X aij F i F j  b FiFjFk     2 ij 6 ijk ijk

ð3Þ

where E is the energy of the molecule under the electric field F, E0, is the unperturbed energy of a free molecule, Fi is the vector component of the electric field in the i direction, li is the dipole moment, aij is the linear polarizability and bijk is the first hyperpolarizability. Each of the subscripts i, j, k and l denotes the index of the Cartesian axes x, y, z, and a repeated subscript means a summation over the Cartesian indices x, y and z. The output from Gaussian 09 provides 10 components of b as bxxx, byxx, bxyy, byyy, bzxx, bxyz, bzyy, bxzz, byzz and bzzz and 6 components of a as axx, axy, ayy, axz, ayz, and azz. The total first hyperpolarizability using the x, y and z components are defined as

btotal ¼ ðb2x þ b2y þ b2z Þ

1=2

ð4Þ

where,

bx ¼ bxxx þ bxyy þ bxzz by ¼ byyy þ byzz þ byxx bz ¼ bzzz þ bzxx þ bzyy The equations for calculating the magnitude of mean polarizability, anisotropy of the polarizability and total static dipole moment are defined as follows:

1 3

a ¼ ðaxx þ ayy þ azz Þ

ð5Þ

1 1 Da ¼ pffiffiffi ½ðaxx  ayy Þ2 þ ðayy  azz Þ2 þ ðazz  axx Þ2 þ 6a2xy þ 6a2yz þ 6a2xz 2 2

ð6Þ

l ¼ ðl2x þ l2y þ l2z Þ

1=2

ð7Þ

The time dependent density functional theory (TD-DFT) with the B3LYP/cc-pVTZ level has been used to study the electronic absorptions in gas and water phases of GPNB.

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Results and discussion Optimized geometry DFT/B3LYP/cc-pVTZ level calculations of geometry optimization locate the minimum energy molecular structure which agrees closely with the specified starting structure. The molecular structure of GPNB with symbols and atom numbering scheme adopted in the computations is shown in Fig. 1. The optimized structural parameters of GPNB are listed in Table 1. Since the crystalline structure information of the title crystal is not available till now, the optimized structure can be compared with available XRD values of similar system such as guanidinium 3-nitrobenzoate [50]. The optimized geometric parameters of GPNB show good coincidence with the crystal parameters of guanidinium 3-nitrobenzoate except for the intramolecular parameters between cation and anion. The monomer is planar in the optimized geometry which consist of guanidinium and 4-nitrobenzoate groups that are linked through NH  O hydrogen bonds. Hydrogen bonding geometry in GPNB molecule is listed in Table 2. Guanidinium have three NH2 groups; two of these are semideprotonated because of the involvement in moderate intramolecular hydrogen bond formation. One NH bond from each of the two NH2 groups were elongated and attains large single bond length and induces partial double bond character to N@C bonds. The NH bonds which took part in moderate hydrogen bonds were N17H18 and N21H22 and whose bond length values were 1.07335 Å and 1.07340 Å, respectively, and these values were much greater [20,21] than the reported values of other guanidinium based complexes in the gas phase. But some other guanidinium compounds [22] in crystalline and aqueous phases found to have 1.17 Å and 1.114 Å as well as 1.08 Å as their NAH bond lengths, respectively. The bond lengths calculated for N18AH23 and N21AH24 are 1.00406 Å and 1.00407 Å, respectively, and these bonds form weak hydrogen bonds with carboxylate oxygen atoms. However, in the case of unperturbed NH2 group, the bond lengths were calculated to be of the order of 1.00503 Å for both the N20AH25 and N20AH26 bonds. DFT predicted the existence of moderate intramolecular NAH  O hydrogen bonds for GPNB as it is evident from the straight line geometry of NAH  O bond angle (179°), N  O distance of 2.625 Å and the H  O bond lengths of 1.55 Å. The NAH bonds which involved in moderate intramolecular hydrogen bonding are lengthened by 0.07 Å over the bond lengths of other NAH bonds. The central carbon atom of guanidinium group is bonded to the three nitrogen atoms in a trigonal planar geometry and the positive charge is delocalized in the C19N18,20,21 plane. The presence of hydrogen bonds shortens the bond lengths of N18AC19 (1.32 Å) and C19AN21 (1.32 Å) compared to bond length of C19AN20 (1.37 Å). The C4AN10 bond length value calculated for the monomer is 1.48 Å which shows its single bond nature. The optimized CANAH bond angles of guanidinium shows three different sets angle values. The bonds which involved in moderate NAH  O interactions gives the calculated angle values about 120.2916° for C19AN18AH17 and 120.2909° for C19AN21AH22. The NAH bonds which depend on weak hydrogen bond interaction show associated bond angle values about 119.3969° for C19AN18AH21 and 119.3918° for C19AN21AH24. The bond angles through free NAH are calculated to be 117.9960° (C19AN20AH25) and 117.9995° (C19AN20AH26). The bond angle of free NH2 group (H25AN20AH26; 114.8072°) is much smaller than that of perturbed NH2 groups (120°). Due to hydrogen bonding, the N18AC19AN21 (120.10°) bond angle is slightly expanded about 0.16° over the other two NACAN angles of guanidinium cation. The angles, N18AC19AN20 and N20AC19AN21 are identical and have equal bond angles of

Please cite this article in press as: V. Sasikala et al., Electronic structure, vibrational spectral and intervening orbital interactions studies of NLO material: Guanidinium 4-nitrobenzoate, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/10.1016/j.saa.2014.12.013

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V. Sasikala et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx

Fig. 1. Optimized molecular structure of guanidinium 4-nitrobenzoate.

Table 1 Optimized geometric parameters of guanidinium 4-nitrobenzoate at B3LYP/cc-pVTZ level and given the available XRD* values of guanidinium 3-nitrobenzoate for comparison.

*

Bond lengths

B3LYP (Å)

XRD (Å)

Bond angles

B3LYP (°)

XRD (°)

Dihedral angles

B3LYP (°)

XRD (°)

C1AC2 C2AC3 C3AC4 C4AC5 C5AC6 C1AC7 C6AH8 C5A H9 C4AN10 C3AH11 C2AH12 N10AO13 N10AO14 C7AO15 C7AO16 O16  H17 H17AN18 N18AC19 C19AN20 C19AN21 O15  H22 N18AH23 N21AH24 N20AH25 N20AH26

1.395 1.386 1.388 1.388 1.386 1.515 1.080 1.079 1.475 1.079 1.080 1.223 1.223 1.259 1.259 1.552 1.073 1.321 1.365 1.321 1.552 1.004 1.004 1.005 1.005

1.400 1.389 1.381 1.380 1.374 1.510 0.930 0.930 1.475 0.930 0.930 1.210 1.221 1.251 1.254 4.148 0.818 1.332 1.320 1.320 1.904 0.829 0.855 0.855 0.910

C1AC2AC3 C2AC3AC4 C3AC4AC5 C4AC5AC6 C2AC1AC7 C1AC6AH8 C4AC5AH9 C3AC4AN10 C4AC3AH11 C1AC2AH12 C4AN10AO13 C4AN10AO14 C1AC7AO15 C1AC7AO16 C7AO16  H17 O16  H17AN18 H17AN18AC19 N18AC19AN20 N18AC19AN21 C7AO15  H22 H17AN18AH23 H22AN21AH24 C19AN20AH25 C19AN20AH26

120.6 118.6 122.0 118.6 120.2 118.5 119.6 119.0 120.0 119.0 118.0 118.0 117.0 117.0 118.0 179.0 120.3 119.9 120.1 118.0 120.0 120.0 118.0 118.0

119.0 122.2 118.4 121.0 120.3 120.0 120.0 119.3 121.0 121.0 119.0 119.0 118.0 118.0 52.2 167.3 116.2 120.2 120.0 131.0 128.2 126.0 119.0 123.0

C1AC2AC3AC4 C2AC3AC4AC5 C3AC4AC5AC6 C3AC2AC1AC7 C2AC1AC6AH8 C3AC4AC5AH9 C2AC3AC4AN10 N10AC4AC3AH11 C7AC1AC2AH12 C3AC4AN10AO13 C5AC4AN10AO14 C6AC1AC7AO15 C2AC1AC7AO16 C1AC7AO16  H17 C7AO16  H17AN18 O16  H17AN18AC19 H17AN18AC19AN20 H17AN18AC19AN21 C1AC7AO15  H22 O16  H17AN18AH23 O15  H22AN21AH24 N18AC19AN20AH25 N21AC19AN20AH26

0.0 0.0 0.0 180.0 180.0 180.0 180.0 0.00 0.0 0.0 0.0 180.0 180.0 179.8 7.1 5.2 174.8 3.7 179.8 168.0 168.0 163.3 163.3

0.2 0.4 0.2 179.5 179.0 180. 0 180.0 0.3 0.5 5.8 4.9 162.0 160.0 153.9 0.3 2.1 179.5 0.1 27.8 178.9 144.3 178.6 178.5

Ref. [50].

Table 2 Hydrogen bonding geometry. XAH  Y

XAH length (Å)

H  Y length (Å)

X  Y length (Å)

XAH  Y angle (°)

N21AH22  O15 N21AH24  O15 N18AH17  O16 N18AH23  O16

1.07340 1.00407 1.07335 1.00406

1.5519 3.2557 1.5521 3.2559

2.6251 2.6251 2.6252 2.6252

178.5 43.8 178.5 43.8

119.94°. Three set bond length values were calculated for CAC bonds of benzene ring. The C1AC2 and C1AC6 have an equal bond length values of 1.395 Å, C2AC3 and C5AC6 bonds have same bond length values of 1.386 Å whereas C3AC4 and C4AC5 bonds have 1.388 Å for each bonds. The differences in CAC bond lengths are clearly due to the substituent effects which were of inductive electron withdrawing and donating origins attached in the benzene ring. Sigma conjugative interactions of aromatic CAC bonds induce a slight increase in the bond lengths of CAH bonds. The p conjugative interactions between N10AO14 and C4AC5 bond orbital causes a slight expansion of C3AC4AC5 angle over the other CCC angles of benzene ring. Both the N10AO13 and N10AO14 bonds of nitro group have equal bond lengths of 1.223 Å which characterize their partial double bond nature. The CAO bonds of

the carboxylate group are of equal lengths (1.26 Å) in the hybrid structure and the predicted bond length values lie intermediate between the double (1.20 Å) and single bond (1.30 Å) values which explain the partial double or rather elongated behaviour of the C@O bonds in the monomer structure. The predicted dihedral angles for CACANAO (0°) and CACACAO (180°) show the coplanarity of nitro and carboxylate groups with the benzene ring. Vibrational analysis The computed wavenumbers, IR intensity and Raman activity corresponding to different modes of vibration and their assignments are listed in Table 3. The predicted positive wavenumbers are an evidence for the stable molecular structure existence. The

Please cite this article in press as: V. Sasikala et al., Electronic structure, vibrational spectral and intervening orbital interactions studies of NLO material: Guanidinium 4-nitrobenzoate, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/10.1016/j.saa.2014.12.013

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V. Sasikala et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx Table 3 Observed FT-IR, FT-Raman and computed wavenumbers (in cm1), IR intensities, Raman activities, vibrational assignments of guanidinium 4-nitrobenzoate. Theoretical unscaleda

Theoretical scaledb

3702 3662 3660

3584 3546 3544

3596 3226 3225 3208 3207 2575

3482 3123 3122 3106 3105 2493

2403

2327

1756 1742

1700 1687

1671 1659 1638 1637 1581 1580

1618 1606 1586 1584 1531 1530

1530 1520 1432 1412 1370

1481 1472 1386 1367 1326

1357 1318 1191 1177 1168 1154 1119

1314 1276 1153 1140 1131 1117 1083

1118

082

1096 1056 1038

1061 1022 1005

1016 1014 1011 1008 913 879 864 839 815

984 982 979 976 884 851 837 812 789

747 734 726 690 644 594 560

723 710 703 668 624 575 542

527 519 517

510 502 500

501 467 454 425 404 354

485 452 440 412 391 343

Mode

FT-IR

2 20a 20b 7a

IIRc (km/mol)

Experimental

3486 ms 3383 s 3105 s 3084s 3061s 3051 s 2439 vw 2263 vw

FT-Raman 0.006 0.004 0.012

2.74 6.96 1.86

3347 3120 3098 3084 3060 2468

0.007 0 0 0 0 1.000

8.72 12.29 1.13 3.57 2.06 42.97

ts(H25N20H26) t(CH) t(CH) t(CH) t(CH) t(N18H17  O16), t(N21H22  O22)

vw vw vw vw vw vw

0

0.51

t(N18H17  O16), t(N21H22  O22)

1737 vw 1648 vw

0.056 0.015

0.25 4.23

c(NH2), tas(N18C19 N21), b(NH  O), tas(OCO), b(CNH) c(NH2)

1613 vw

1595 ms 1533 vw 1501 w

0.019 0.006 0.025 0 0.026 0.005

4.01 0.59 0.32 88.6 2.26 2.30

1483 1472 1423 1378 1336

vw vw vw vs vs

0.005 0 0 0.078 0.098

1.04 0.22 0.11 82.97 100

1294 1276 1168 1157 1127

vw vw vw vw vw

1102 ms

0.005 0 0 0.008 0.009 0 0.007

0.03 0.06 1.32 0.17 1.58 9.80 58.49

1076 vw

0.001

0.05

b(CCH)

1062 vw 1040 vw 1009 ms

0 0.013 0.002

0.92 1.14 1.60

q(NH2), b(NH  O), b(CNH)

970 vw 893 vw 865 ms 830 vw 803 vw 784 vw

0 0 0.012 0 0.003 0.002 0 0.012 0

0.71 0.05 19.28 0.10 0.00 30.88 0.81 3.72 3.70

724 711 687 673 633 606 586

vw vw vw vw w vw vw

0.008 0 0.001 0 0 0.017 0

0.07 0.71 0.71 0.02 10.97 3.65 0.57

507 w

550 vw 535 vw 504 vw

0.011 0.041 0

0.41 4.68 4.91

477 vw 470 vw

487 474 428 407 391 350

0 0 0 0 0.023 0.014

1.27 0.04 2.89 0.00 3.03 0.49

1554 s 1512 vs 19a 19b 1369 s 1343 vs 1320 s 1168 w 1159 w 1127 w 1101 ms 1076 vw

1009 vw 5 17a

17b

888 vw

10a 803 ms

11

725 s

6b

670 w 630 w 551 w

16b 16a

tas(H25N20H26) t(N18H23), t(N21H24) t(N18H23), t(N21H24)

3592 vw 3550 vw 3376 vw

8a 8b

18b

Assignments

2297 vw

1664 vs

14 3 9a

IRamand (a.u)

vw vw vw vw vw vw

c(NH2), t(C19N20), tas(CN18,20,21) tr(CC), b(CCH), tas(ONO), c(NH2), tas(OCO), b(NH  O), b(CNH) tr(CC), b(CCH), tas(ONO), tas(OCO), c(NH2), b(NH  O) b(CCH), tr(CC), t(C1C7) c(NH2), tr(CC), b(NH  O), tas(ONO), b(CNH) b(NH  O), c(NH2), tas(CN18,20,21), b(CCH), b(CNH) c(NH2), b(NH  O), tas(OCO), b(CNH) tr(CC), b(CCH), t(C1C7) tr(CC), b(CCH), b(NH  O) b(CCH), t(C4AN10), b(NH  O),t(C1C7), ts(OCO), ts(ONO) b(CCH), t(C4AN10), t(C1C7), ts(ONO), ts(OCO), b(NH  O) tr(CC), b(CCH) b(CCH) b(CCH) q(NH2), b(NH  O), b(CNH) b(CCH), q(NH2), b(NH  O), b(CNH) b(CCH), ts(OCO), q(NH2), t(C1C7), b(NHO), b(CNH) b(CCH), t(C4AN10)

d(NH  O) b(CCH), d(CCC) d(CCH) d(CCH), d(NH  O) ts(CN18,20,21), d(NH  O) C(NH2), d(NH  O) d(CCH), d(CCN), d(C7CC) C(NH2), d(CCC), d(NH  O), t(C4AN10), c(NO2), c(OCO) d(CCH) C(NH2), x(NO2), d(CCC), t(C4AN10), d(CCN), d(NH  O), t(C1C7), c(OCO) x(NO2), d(CCH), d(NH  O), Rbr, d(CCN), d(CNO), x(OCO) d(CC7O), d(C7CC) Rbr, x(NO2), d(CCH), d(CC7O), d(CCN), d(NH  O), d(CNO), x(OCO) d(CN18,20,21) x(OCO), t(C4AN10) C(NH2), x(NO2), d(CCC) d(CCH), x(OCO), x(NO2), C(NH2), d(NH  O), d(CCN), d(CNO), d(C7CC) d(CCH), d(CCC) q(OCO), d(NH  O), x(NH2), d(CN18,20,21) q(OCO), q(NO2), d(CCN), d(CNO) d(NH  O), d(CC7O), d(C7CC), x(NH2), d(CN18,20,21) d(NH  O), q(OCO), q(NO2), d(CN18,20,21) x(NH2), d(CN18,20,21) q(OCO), d(CCN), d(CNO), d(CC7O), q(NO2), d(CN18,20,21), x(NH2), d(NH  O) q(OCO), q(NO2), d(NH  O), x(NH2), d(CN18,20,21) d(CCH), d(OCO), d(CCN), s(CCCC), d(C7CC) x(NH2) d(CCH), s(CCCC) x(NH2) s(NH2), d(NH  O), (continued on next page)

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V. Sasikala et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx

Table 3 (continued) Theoretical unscaleda 309 256 204 182 162 119 105 75 73 64 51 29 23

Theoretical scaledb 299 248 198 176 157 115 102 73 71 62 49 28 22

Mode

Experimental FT-IR

IIRc (km/mol)

IRamand (a.u)

Assignments

FT-Raman 286 vw 255 vw 215 vw 186 w 149 ms 118 s 93 ms 75 s

0 0 0 0.007 0 0.002 0 0.002 0 0 0 0 0

s(NO2), d(CNO), d(CCN), s(OCO), s(NH2), d(CC7O), d(C7CC), d(NH  O) d(CCH), d(CCN), s(OCO), d(NH  O) s(OCO), d(CCN), d(CNO), s(NO2), s(NH2), d(NH  O) s(NH2), s(NO2), d(NH  O) s(OCO), s(NH2), d(NH  O) s(OCO), s(NH2), d(NH  O) s(OCO), s(NO2), s(NH2), d(CCN) d(NH  O) s(OCO), s(NO2), s(NH2) d(CCN), d(NH  O) s(OCO), s(NO2), s(NH2) d(NH  O) s(OCO), d(CCN), s(NO2), s(NH2), d(NH  O) s(OCO), s(NO2), s(NH2), d(NH  O) s(OCO), s(NH2) s(NO2), d(NH  O) d(NH  O), s(OCO), s(NO2), s(NH2)

3.61 9.04 1.38 10.9 3.76 4.26 6.20 30.85 14.13 0.12 0.06 1.12 7.25

m: Stretching, c: scissoring, x: wagging, s: torsion, q: rocking, C: twisting, b: in-plane bending, d: out- of plane bending, Rbr: ring breathing, s: symmetric, as: antisymmetric, vs: very strong, s: strong, ms: medium strong, vw: very weak, w: weak. a Obtained from the wavenumbers calculated at B3LYP/cc-pVTZ. b Obtained from the wavenumbers calculated at B3LYP/cc-pVTZ using scaling factor 0.9682. c Relative IR absorption intensities normalized with the highest peak absorption equal to 1. d Relative Raman activities calculated by Eq. (1) and normalized to 100.

observed and simulated FT-IR and FT-Raman spectra are shown in Figs. 2 and 3 for visual comparison. Guanidinium group vibrations NH2 vibrations. The NH stretching vibrations of guanidinium is usually observed in the region 3500–3100 cm1. The asymmetric NH2 stretching vibration is observed as very weak band in the Raman spectrum at 3592 cm1 which corresponds to the scaled mode at 3584 cm1. Strong peak at 3383 cm1 in IR and very weak peak at 3347 cm1 in Raman are attributed to the symmetric stretching modes of NH2 vibration. The theoretical calculation predicted the NH2 symmetric stretching mode at 3482 cm1. The semideprotonated NH stretching band of NH2 splits into quadruplets. Of these, two of the higher frequency components constitutes weak intramolecular NH  O hydrogen bonds with neighbouring oxygen atoms and the other two low frequency components belongs to t(NH) modes form moderate NH  O bonds within the molecule. The very weak bands observed at 3550 cm1 and at 3376 cm1 in Raman are attributed to the NH stretching vibrations while in IR spectrum this mode is observed as a medium strong band at 3486 cm1 and the scaled wavenumbers correspond to these modes are at 3546 cm1 and at 3544 cm1. Similar NH2 stretching wavenumber region were noticed for guanidinium selenate and guanidinium sulphate monomers [21]. The very weak broad bands observed in IR at 2439 cm1 (2468 cm1; Raman) and 2263 cm1(2297 cm1; Raman) are attributed to the moderate NH  O stretching modes. The excess electron density distribution in neighbouring carboxylate oxygen atoms perturbs the NH bond vibrations and resulting in a large red shift of stretching wavenumbers. The lowering of observed stretching wavenumbers and an increment in IR intensity value explains the involvement of t(NH) modes in hydrogen bonding interactions which induces strong double bond character to N18AC19 and C19AN21 bonds and also the up-shift in calculated wavenumbers associated with these modes clearly explains the strengthening of N18AH23 and N21AH24 bonds in GPNB molecule. The in-plane deformation vibrations of the NH2 group are expected in the region 1650–1550 cm1 [23]. The wavenumbers for scissoring deformation of NH2 group is usually observed around 1630 cm1. The bands observed in the theoretical spectra at 1700 cm1, 1687 cm1, 1618 cm1, 1606 cm1, 1586 cm1, 1531 cm1, 1530 cm1 and 1481 cm1 are assigned to the NH2 scissoring vibrations. These modes of vibrations are observed as very weak intensity peaks in the Raman spectrum at 1648 cm1

and 1613 cm1. The NH2 rocking modes of vibrations are expected in the region 1150–1060 cm1 [23]. The Raman spectrum of GPNB shows the peaks corresponds to these modes of vibrations as very weak bands at 1157 cm1, 1127 cm1 and at 1062 cm1 and weak bands in the IR spectrum at 1159 cm1 and at 1127 cm1. The theoretical calculation predicts the wavenumbers for these modes were at 1140 cm1, 1131 cm1, 1117 cm1 and at 1061 cm1. The peaks observed at 970 cm1 (Scaled 976 cm1), 865 cm1 (scaled 851 cm1), 803 cm1 (scaled 812 cm1), 687 cm1 (scaled 703 cm1) and 673 cm1 (scaled 668 cm1) in the Raman spectrum and medium strong peak at 803 cm1 and weak peak at 670 cm1 in IR are attributed to NH2 twisting modes. In IR spectrum, the peaks corresponds to the wagging vibrations of NH2 are appeared as weak bands at 551 cm1 and at 477 cm1 while in Raman spectrum these modes appeared as very weak bands at 606 cm1, 586 cm1, 535 cm1, 504 cm1, 487 cm1, 428 cm1 and at 391 cm1. The scaled wavenumbers associated with NH2 wagging band are predicted at 710 cm1, 575 cm1, 542 cm1, 502 cm1, 500 cm1, 485 cm1, 440 cm1 and at 391 cm1. The torsional modes of amino groups are observed as strong to very weak bands in the Raman spectral region 350–75 cm1 and computed wavenumber for these modes found in the region 343–22 cm1. CAN vibrations. C@N stretching absorptions of guanidinium is expected in the region 1685–1580 cm1 [51–53]. The asymmetric stretching vibration of N18C19N21 is IR inactive but Raman active which is appeared as very weak intense band at 1737 cm1 and scaled wavenumber at 1700 cm1. The experimental and theoretical results shows an up-shift in wavenumber for t(N18C19N21) mode, which is clearly due to the strong coupling of this mode with intramolecular NAH  O vibrations. The asymmetric stretching vibrations of CN18,20,21 group for GPNB are found in Raman spectra at 1613 cm1 and at 1501 cm1 (1512 cm1; IR). The theoretical calculation proposed the wavenumbers of these modes of vibrations are at 1618 cm1 and 1530 cm1, respectively. The peaks corresponds to the CN18,20,21 symmetric stretching vibrations of guanidinium group are inactive in both IR and Raman spectra and the theoretical calculation contributes this mode at 979 cm1. The peaks corresponds to the in- plane bending vibrations of CN18,20,21 is observed as very weak Raman band at 711 cm1 and scaled wavenumber at 710 cm1. According to the theoretical calculations, the very weak bands observed in the Raman spectra at 606 cm1, 586 cm1, 550 cm1, 535 cm1, 504 cm1 and at 487 cm1 were assigned to the CN18,20,21

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out-of-plane bending modes. The CANAH bending vibrations are generally expected in the region 1689–1500 cm1 [52,53]. The calculated wavenumbers predicted for these mode were at 1700 cm1, 1606 cm1, 1531 cm1, 1530 cm1, 1481 cm1, 1140 cm1, 1131 cm1, 1117 cm1and 1061 cm1.

Phenyl ring vibrations CAH vibrations. Different normal vibrations of para-di-lightsubstituted phenyl ring are assigned the Wilson’s numbering and are compared with the calculated wavenumbers [54]. Aromatic CAH stretching vibrations are usually observed around 3100– 3000 cm1 [55]. It is the consequence of aromatic ring CH vibrations that results in strong IR absorption bands but only weak Raman bands. In GPNB, the peaks corresponds to these modes appeared as strong as well as broad bands in IR region 3105– 3051 cm1 and weak bands in Raman region 3120–3060 cm1. The theoretically simulated spectra shows the multiplicity of the weak to moderate intense bands at 3123 cm1, 3122 cm1, 3106 cm1 and 3105 cm1 associated with phenyl ring CAH stretching modes 2, 20a, 20b and 7a, respectively. The weak bands are due to the decrease of dipole moment caused by the electron withdrawal on the carbon atom by the substituent because of the decrease of inductive effect. In substituted benzene, the in-plane CAH bending vibrations occur in the range 1300– 1000 cm1 [56] and out-of-plane bending vibrations appear in the range 1000–675 cm1 [57]. For p-substituted phenyl ring, the CAH in-plane bending mode 3 is normally appears as very weak band in the region 1292–1272 cm1 [58]. Both the Raman and simulated spectra shows very weak band at 1276 cm1 which is due to the CAH in-plane bending mode of the phenyl ring. Para-di-light substituted phenyl compounds show CAH in-plane bending modes 9a and 18b in the regions 1190–1140 cm1 and 1130–1085 cm1, respectively. For GPNB, mode 9a appears as weak IR and as very weak Raman bands at 1168 cm1 whereas mode 18b appears as very weak and broad bands in both IR and Raman spectra at 1076 cm1. The CAH out-of-plane bending modes 5 and 17a are active in the simulated spectra for GPNB which were predicted at 984 cm1 and 982 cm1, respectively. Due to the attachment of the strong electron withdrawing group to the phenyl ring, the 17b mode is slightly up-shifted and appears as very weak bands in both IR and Raman spectra at 888 cm1 and 893 cm1, respectively. The very weak peak observed in the Raman spectra at 830 cm1 and corresponding scaled wavenumber at 837 cm1 are attributed to the 10a mode of CAH out-of plane bending vibrations. The band of CAH out-of-plane bending mode 11 of GPNB is strong in IR spectrum which observed at 725 cm1 and as very weak band in Raman spectrum at 724 cm1 (scaled at 723 cm1). Also, the Raman bands which appear as very weak bands at 784 cm1 (IR inactive) and at 673 cm1 (IR at 670 cm1) and corresponding predicted wavenumbers at 789 cm1 and at 668 cm1, respectively, are contributed to the out-of-plane bending vibrations of CACAH bonds. Mode 6b is observed in IR spectra at 630 cm1 and in Raman spectra at 633 cm1 as weak intensities. The CAH out-plane bending modes 16b and 16a were observed to be very weak intensity peaks in Raman spectrum at 474 cm1and at 407 cm1, respectively. The simultaneous occurrence of modes 11 and 6b in both IR and Raman spectra are due to the existences of intramolecular charge transfer interactions through sigmadouble bond conjugated path and also the presence of inductive as well as resonance electron withdrawing and inductive electron donating substituents to the benzene ring which were stimulate an appreciable change in molecular dipole moment as well as in molecular hyperpolarizability and thereby results a lowered symmetry to the title molecule which facilitates the GPNB to be an efficient NLO system.

7

CAC vibrations. For the 1,4-substituted phenyl ring, the allowed ring carbon–carbon stretching modes are 8a, 8b, 19a, 19b and 14 [51,59] which are expected in the region 1620–1280 cm1 [56]. In p-di-light substituted benzene derivatives, the degenerate modes 8a and 8b are expected in the regions 1570–1628 cm1 and 1552–1605 cm1, respectively [54] and these modes were absent in observed spectra of the title crystal. DFT calculation predicts mode 8a at 1606 cm1 and mode 8b at 1586 cm1 for monomer. The very weak peaks observed in the Raman spectra at 1472 cm1 and 1423 cm1 are attributed to CAC ring stretching modes 19a and 19b, respectively and the corresponding scaled wavenumbers were found to be at 1472 cm1 and 1386 cm1 as expected [54]. These degenerate modes were coupled with CAH in-plane bending modes. Mode 14 of phenyl ring is observed as strong intense peak in IR at 1320 cm1 and as very weak peak in Raman at 1294 cm1. The peaks observed at 1595 cm1 (Raman), 1554 cm1 (IR) and 1533 cm1 (Raman) are due to CAC ring stretching absorptions of the title crystal. The theoretically scaled wavenumbers corresponds to these modes show good agreement with the observed wavenumbers. Completely different substituent attached to the 1,4 positions of benzene ring [60] shows strong peaks corresponds to the ring breathing vibrations in the IR region 840–740 cm1. The strong band observed in IR spectra at 725 cm1 and very weak Raman band at 724 cm1 are due to the ring breathing modes for the title crystal. This is supported by the GaussView molecular visualization program which displays the DFT results of these modes at 789 cm1 (Raman band at 784 cm1) and 723 cm1. The CCCC in-plane deformation mode was observed as a weak intensity band at 474 cm1 in Raman spectrum. The out-of-plane deformation mode was observed as weak intensity peak at 407 cm1 in Raman spectrum. NO2 vibrations Aromatic NO2 asymmetric and symmetric stretching absorptions are expected in the regions 1570–1485 cm1 and 1370– 1320 cm1, respectively [61]. In GPNB, the asymmetric stretching modes are observed as medium intense strong sharp peak at 1554 cm1 in IR and very weak band at 1533 cm1 in Raman spectra and which correspond to the scaled wavenumber at 1531 cm1. Also, the DFT calculation predicted two low intense asymmetric stretching bands for GPNB molecule at 1606 cm1 and 1586 cm1 and its experimental counterpart for crystalline state was absent in both FT-IR and FT-Raman spectra. The up-shift in predicted wavenumber for asymmetric mode is caused by the electron withdrawing property of nitro group. The most intense and very strong sharp Raman band observed at 1336 cm1 and strong intense and very strong IR peak observed at 1343 cm1 are attributed to the symmetric stretching modes of NO2 group. DFT calculations give the corresponding mode at 1326 cm1. The sharp peaks for symmetric mode clearly illustrate the involvement of NO2 group in non-hydrogen bonded interactions within GPNB molecule. The strong conjugative interactions between NO2 group and the substituent induce an increment in intensity and strength of the peaks associated with NO2 symmetric stretching mode compared to that of asymmetric mode. The scissoring, wagging and rocking modes of aromatic nitro compounds are active in the region 850 ± 60 cm1, 740 ± 50 cm1 and 540 ± 70 cm1, respectively [56]. For GPNB, the scissoring mode of vibration appears as medium strong peak in the Raman spectrum at 865 cm1 (scaled at 851 cm1) and the corresponding counterpart was inactive in IR. The peaks at 803 cm1 which appears as medium strong in IR and as very weak band in Raman, at 784 cm1, 724 cm1 (725 cm1, IR), 687 cm1 and 673 cm1 as very weak Raman bands are attributed to the wagging mode of NO2. The very weak peaks observed in the Raman spectrum at 586 cm1, 550 cm1, 504 cm1 and 487 cm1 and corresponding DFT calculated wavenumbers at 542 cm1,

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Fig. 2. (a) FT-IR spectra of GPNB. (b) Simulated IR spectra of GPNB.

Fig. 3. (a) FT-Raman spectra of GPNB. (b) Simulated Raman spectra of GPNB.

510 cm1, 500 cm1 and 485 cm1, respectively, are associated with the NO2 rocking modes. The torsional modes of nitro group are observed in the Raman region 286–75 cm1 and theoretically predicted wavenumbers assigned to these modes were to be between 299 cm1 and 22 cm1 for GPNB. CANO13,14 stretching absorption is usually expected in the region 1180–865 cm1 [51,62–64]. Clarkson et al. reported the coupling of t(CANO13,14) mode with ring vibrations at 1108 and 392 cm1 for nitrobenzene [65]. The para positioned NO2 group of phenyl ring increase the CringANO13,14 stretching wavenumbers for GPNB. The CAN stretching mode was strongly coupled with symmetric stretching modes of NO2. The very strong peaks observed in IR spectrum at 1369 cm1 and in Raman spectrum at 1378 cm1, 1343 cm1 (IR) and 1336 cm1 (Raman), 1101 cm1 (IR) and 1102 (Raman), medium strong band at 865 cm1 in Raman, 803 cm1 (IR, Raman) and very weak peak at 687 cm1 in Raman spectrum are due to C4–N10 stretching absorptions. DFT calculation predicted the corresponding modes at 1367 cm1, 1326 cm1, 1083 cm1, 851 cm1, 812 cm1 and 703 cm1. DFT results predicted the CACAN bending vibrations were at 884 cm1, 812 cm1, 789 cm1, 668 cm1, 542 cm1, 500 cm1, 452 cm1, 299 cm1, 248 cm1, 198 cm1, 102 cm1, 73 cm1 and 62 cm1. The CANAO bending vibrations of aromatic nitro compounds are usually observed around 610 cm1 [66]. In GPNB, the aforementioned mode was overlapped with CACAN bending vibrations.

stretching modes of carboxylate contribute equally to symmetric and asymmetric vibrational modes and produce strong and unique infrared absorbance bands. The carboxylate group gives the asymmetric stretching absorption bands in the region 1600– 1540 cm1 and symmetric stretch in the region 1300–1100 cm1. In FT-Raman spectrum of GPNB crystal, very weak peaks observed peaks at 1737 cm1 and 1483 cm1 are attributed to the asymmetric stretching absorptions and the two very strong and maximum intense peaks at 1378 cm1 and at 1336 cm1 have been assigned to the symmetric OCO stretching modes. In IR spectrum, the asymmetric stretching mode is inactive and the symmetric counterpart is appeared as highly intense and very strong peaks at 1369 cm1 and at 1343 cm1. The high intensity of these bands elucidates the effects of conjugation among carboxylate group due to the large dipole moment of the carbon–oxygen bonds. The scaled wavenumbers assigned to these modes are at 1700 cm1, 1606 cm1, 1586 cm1, 1481 cm1, 1367 cm1, 1326 cm1and at 1117 cm1. The conjugative and partial carbon–oxygen double bond character as well as moderate intra and may be strong inter molecular hydrogen bonding interactions in the crystal causes the down shifting of these stretching wavenumbers. The scissoring, wagging, rocking and torsional in-plane and out-of-plane deformation modes of carboxylate group are expected in the region 795 ± 65 cm1, 670 ± 30 cm1, 515 ± 65 cm1 and 195 ± 50 cm1, respectively [56]. For the title crystal, the scissoring deformation mode is identified as medium strong intense peaks in Raman spectrum at 865 cm1 (scaled at 851 cm1) and in IR at 803 cm1 (very weak peak at 803 cm1 in Raman). The wagging mode appears at 725 cm1, 670 cm1 in IR spectrum 784 cm1, 724 cm1, 687 cm1, 673 cm1 in Raman spectrum and 789 cm1, 723 cm1, 703 cm1, 668 cm1 in simulated spectrum. The rocking and torsional modes were spread over the region 606–487 cm1

Carboxylate vibrations The vibrational frequencies of carboxylate group at the molecular level of the title crystal are much more influenced by the neighbouring hydrogen bond inducing functional groups. The CAO

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V. Sasikala et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx Table 4 Second order perturbation theory analysis of Fock matrix in NBO basis including the stabilization energies using DFT at B3LYP/cc-pVTZ level. Donor (i)

ED (i) (e)

Acceptor (j)

ED (j) (e)

E(2)a (kJ/mol)

E(j)–E(i)b (kJ/mol)

F(i,j)c (kJ/mol)

1.9788

r⁄(C2AH12) r⁄(C3AH11) r⁄(C6AH8) r⁄(C7AO16) r⁄(C2AH12) r⁄(C5AH9) r⁄(C6AH8) r⁄(C7AO15) p⁄(C2AC3) p⁄(C4AC5) r⁄(C2AH12) r⁄(C3AH11) r⁄(C4AN10) p⁄(C1AC6) p⁄(C4AC5) r⁄(C1AC2) r⁄(C1AC6) r⁄(C3AC4) r⁄(C2AH12) r⁄(C3AH11) r⁄(C5AH9) r⁄(N10AO14) r⁄(C1AC2) r⁄(C2AC3) r⁄(C4AC5) r⁄(C4AN10) r⁄(C3AH11) r⁄(C5AH9) r⁄(C6AH8) r⁄(N10AO13) p⁄(C1AC6) p⁄(C2AC3) p⁄(N10AO14) r⁄(C2AC3) r⁄(C5AC6) r⁄(C4AN10) r⁄(C5AH9) r⁄(C6AH8) r⁄(C1AC6) r⁄(C3AC4) r⁄(C5AC6) r⁄(C1AC2) r⁄(C1AC6) r⁄(C4AC5) r⁄(C1AC6) r⁄(C1AC7) r⁄(C7AO16) r⁄(C1AC2) r⁄(C1AC7) r⁄(C7AO15) r⁄(C4AC5) r⁄(C4AN10) r⁄(C3AC4) r⁄(C4AN10) p⁄(C4AC5) p⁄(N10AO14) r⁄(C4AN10) r⁄(N10AO14) r⁄(C2AC3) r⁄(C4AC5) r⁄(C4AN10) r⁄(N10AO14) p⁄(N10AO14) r⁄(C4AN10) r⁄(N10AO13) r⁄(C3AC4) r⁄(C4AN10) r⁄(C5AC6) r⁄(N10AO13) r⁄(C7AO16) r⁄(C1AC7) r⁄(C7AO16) r⁄(C7AO15)

0.0159 0.0147 0.0159 0.0465 0.0159 0.0147 0.0159 0.0465 0.2839 0.3880 0.0159 0.0147 0.1166 0.3295 0.3880 0.0213 0.0213 0.0227 0.0159 0.0147 0.0147 0.0577 0.0213 0.0146 0.0227 0.1166 0.0147 0.0147 0.0159 0.0577 0.3295 0.2838 0.6217 0.0146 0.0146 0.1166 0.0147 0.0159 0.0213 0.0227 0.0146 0.0213 0.0213 0.0227 0.0213 0.0885 0.0465 0.0213 0.0885 0.0465 0.0227 0.1166 0.0227 0.1166 0.3880 0.6217 0.1166 0.0577 0.0146 0.0227 0.1166 0.0577 0.6217 0.1166 0.0577 0.0227 0.1166 0.0146 0.0577 0.0465 0.0885 0.0465 0.0465

4.27 11.39 11.01 6.28 11.01 11.39 4.27 6.28 81.48 99.60 3.31 4.19 20.31 85.79 91.02 2.18 22.36 19.85 11.10 4.10 9.50 9.63 19.13 2.43 22.40 3.01 9.50 4.10 11.10 9.63 74.02 80.14 104.54 7.83 7.83 20.31 4.19 3.31 19.13 22.40 2.43 22.36 2.18 19.85 6.20 2.51 3.56 6.20 2.51 3.56 4.23 2.14 4.23 2.14 12.77 31.07 19.59 11.97 2.72 3.48 63.14 81.31 689.98 19.59 11.97 3.48 63.14 2.72 81.31 35.25 74.11 38.69 35.25

2993.1 2940.6 2993.1 3281.9 2993.1 2940.6 2993.1 3281.9 735.1 682.6 3019.3 2993.1 2546.7 761.4 708.9 2756.8 2756.8 2730.5 3045.6 2993.1 2993.1 2993.1 2809.3 2809.3 2756.8 2047.9 2993.1 2993.1 3045.6 2993.1 787.7 787.6 367.6 3518.2 3518.2 2546.7 2993.1 3019.3 2809.3 2756.8 2809.3 2756.8 2756.8 2730.6 3964.5 3570.7 3990.8 3964.5 3570.7 3990.8 4227.1 3518.2 4227.1 3518.2 1207.7 840.2 2756.8 3203.1 2205.4 2152.9 794.2 1864.1 393.8 2756.7 3203.1 2152.9 794.2 2205.4 1864.1 3098.1 1890.4 2336.7 3098.1

78.8 128.6 128.6 102.4 128.6 128.6 78.8 102.4 175.9 186.4 70.9 78.8 165.4 181.2 181.2 55.1 175.9 165.4 131.3 78.8 120.8 120.8 162.8 57.8 175.9 57.8 120.8 78.8 131.3 120.8 173.3 181.2 152.3 118.1 118.1 165.4 78.8 70.9 162.8 175.9 57.8 175.9 55.1 165.4 110.3 68.3 84.0 110.3 68.3 84.0 94.5 63.0 94.5 63.0 97.1 136.5 168.0 139.2 55.1 63.0 212.7 278.3 370.2 168.0 139.2 63.0 212.7 55.1 278.3 233.7 270.4 217.9 233.7

Within unit 1

r(C1AC2) r(C1AC2) r(C1AC2) r(C1AC2) r(C1AC6) r(C1AC6) r(C1AC6) r(C1AC6) p(C1AC6) p(C1AC6) r(C2AC3) r(C2AC3) r(C2AC3) p(C2AC3) p(C2AC3) r(C2AH12) r(C2AH12) r(C2AH12) r(C3AC4) r(C3AC4) r(C3AC4) r(C3AC4) r(C3AH11) r(C3AH11) r(C3AH11) r(C3AH11) r(C4AC5) r(C4AC5) r(C4AC5) r(C4AC5) p(C4AC5) p(C4AC5) p(C4AC5) r(C4AN10) r(C4AN10) r(C5AC6) r(C5AC6) r(C5AC6) r(C5AH9) r(C5AH9) r(C5AH9) r(C6AH8) r(C6AH8) r(C6AH8) r(C7AO15) r(C7AO15) r(C7AO15) r(C7AO16) r(C7AO16) r(C7AO16) r(N10AO13) r(N10AO13) r(N10AO14) r(N10AO14) p(N10AO14) p(N10AO14) n1(O13) n1(O13) n2(O13) n2(O13) n2(O13) n2(O13) n3(O13) n1(O14) n1(O14) n2(O14) n2(O14) n2(O14) n2(O14) n1(O15) n2(O15) n2(O15) n1(O16)

1.6383 1.9742

1.9736

1.6464

1.9889

1.9736

1.9742

1.9945

1.9945

1.9956 1.9956

1.9797 1.8963

1.4505 1.9797 1.8963

1.9591 1.8352 1.9591

(continued on next page)

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Table 4 (continued) Donor (i) n2(O16) n2(O16)

E(j)–E(i)b (kJ/mol)

F(i,j)c (kJ/mol)

74.11 38.69

1890.4 2336.7

270.4 217.9

0.1035 0.1035 0.1035 0.1035 0.0205 0.1035 0.0205 0.1035 0.0063 0.1035 0.0205 0.1035 0.0205 0.0063

1.80 1.80 1.72 1.72 0.33 31.90 0.46 173.21 0.33 31.90 0.33 173.08 0.46 0.33

2599.2 2599.2 3518.2 3518.2 2861.8 2599.2 2100.4 1837.8 1864.1 2599.2 2861.8 1837.8 2100.4 1864.1

49.9 49.9 55.1 55.1 21.0 207.4 23.6 407.0 18.4 207.4 21.0 407.0 23.6 18.4

r⁄(C7AO16) r⁄(C7AO15)

0.0465 0.0465

1.76 1.76

3203.1 3203.1

52.5 52.5

r⁄(C19AN20) r⁄(C19AN20) r⁄(C19AN21) r⁄(N20AH25) r⁄(N21AH24) r⁄(C19AN21) r⁄(H17AN18) r⁄(N18AC19) r⁄(C19AN21) r⁄(N21AH22) r⁄(N18AC19) r⁄(N18AH23) r⁄(C19AN20) r⁄(N20AH26) r⁄(N18AC19) r⁄(C19AN21) r⁄(C19AN20) r⁄(N18AC19)

0.0282 0.0282 0.0205 0.0060 0.0063 0.0205 0.1035 0.0205 0.0205 0.1035 0.0205 0.0063 0.0282 0.0060 0.0205 0.0205 0.0282 0.0205

28.18 3.27 5.95 6.66 8.42 24.33 7.58 2.81 2.81 7.58 5.95 8.42 3.27 6.66 20.14 20.18 28.18 24.33

2730.5 3386.9 3596.9 3308.1 3386.9 3045.6 3334.4 3570.7 3570.7 3334.4 3596.9 3386.9 3386.9 3308.1 3124.3 3124.3 2730.5 3045.6

196.9 76.1 105.0 105.0 118.1 194.3 115.5 70.9 70.9 115.5 105.0 118.1 76.1 105.0 178.5 178.5 196.9 194.3

ED (i) (e)

Acceptor (j)

ED (j) (e)

1.8353

r (C1AC7) r⁄(C7AO15)

0.0885 0.0465

1.9727

r⁄(H17AN18) r⁄(N21AH22) r⁄(N21AH22) r⁄(H17AN18) r⁄(C19AN21) r⁄(N21AH22) r⁄(C19AN21) r⁄(N21AH22) r⁄(N21AH24) r⁄(H17AN18) r⁄(N18AC19) r⁄(H17AN18) r⁄(N18AC19) r⁄(N18AH23)

1.9815 1.9814 1.9815 1.9902

⁄

E(2)a (kJ/mol)

From unit 1 to unit 2

r(C1AC7) r(C1AC7) r(C7AO15) r(C7AO16) n1(O15) n1(O15) n2(O15) n2(O15) n2(O15) n1(O16) n1(O16) n2(O16) n2(O16) n2(O16)

1.9945 1.9945 1.9591 1.8352

1.9591 1.8353

From unit 2 to unit 1

r(H17AN18) r(N21AH22) Within unit 2

r(H17AN18) r(N18AC19) r(N18AC19) r(N18AC19) r(N18AC19) r(N18AH23) r(C19AN20) r(C19AN20) r(C19AN20) r(C19AN20) r(C19AN21) r(C19AN21) r(C19AN21) r(C19AN21) r(N20AH25) r(N20AH26) r(N21AH22) r(N21AH24) a b c

1.9851 1.9921

1.9902

1.9878 1.9878 1.9814 1.9851

E(2) means energy of hyperconjugative interactions (stabilization energy). Energy difference between donor i and acceptor j NBO orbitals. Fock matrix element between i and j NBO orbitals.

and 286–75 cm1, respectively in Raman and DFT calculations predicted these modes were to be in the range 575–485 cm1 and 299–22 cm1, respectively. According to the predicted results from DFT calculations, the Cring–Ccarboxylate stretching vibrations of GPNB are coupled with phenyl ring bending vibrations and the peaks corresponds to such modes were observed as medium to strong intense peaks in the Raman region 1595–803 cm1. The CCC and CCO bending modes due to the vibrations of phenyl ring and carboxylate groups were predicted to be at 884 cm1, 789 cm1, 668 cm1, 542 cm1, 452 cm1, 299 cm1 and at 789 cm1, 723 cm1, 542 cm1, 500 cm1, 299 cm1, respectively. The observed Raman band positions correspond to these wavenumbers appears with very weak intensity. NAH  O vibrations According to the DFT calculations, the highest intense and very weak broad band occurred at 2439 cm1 in IR and very weak peak with medium intensity observed in the Raman spectrum at 2468 cm1 are assigned to the stretching vibrations of NAH  O bonds. Also the calculated asymmetrical stretching vibrations of NHO bonds are observed as very low and very weak intense bands compared to the corresponding symmetric stretching modes. The lowering of NHO stretching wavenumbers and the weakening of stretching intensities confirm the influence of large r conjugative

and homoanomeric interactions between the NH bonds of guanidinium group and the carboxylate group of the disubstituted benzene ring system. The lowering of NH stretching wavenumber is an evidence for the existence of moderate hydrogen bonds. The NH  O bending vibrations were strongly coupled with carboxylate and NH2 bending vibrations. NBO analysis Orbitals involved in intramolecular interactions and the energetic contribution to the stabilization of the molecule through donor–acceptor interactions for GPNB have been investigated using natural bond orbital (NBO) method. In the NBO analysis the electronic wavefunction is interpreted in terms of a set of occupied Lewis and a set of unoccupied non-Lewis localized orbitals. Delocalization effects can be identified from the presence of off-diagonal elements of the Fock matrix in the NBO basis. The results obtained from the second order perturbation theory analysis of Fock matrix in NBO basis is presented in Table 4. NBO analysis identified the cyclic pattern of p ? p⁄ conjugative strong stabilizing delocalizations and resonance structure in the benzene ring. The benzene ring is strengthened by the inherently cooperative donor–acceptor interactions which allow each localized p bond to delocalize into two adjacent p⁄ antibonds in counter rotating triple cycles such as p(C1AC6) ? p⁄(C2AC3),

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p(C2AC3) ? p⁄(C4AC5), p(C4AC5) ? p⁄(C1AC6) and p (C1AC6) ? p⁄(C4AC5), p (C4AC5) ? p⁄(C2AC3), p(C2AC3) ? p⁄(C1AC6). These conjugative delocalizations that stabilizes the benzene ring with stabilization energies of 81.48 kJ/mol, 91.02 kJ/mol, 74.02 kJ/mol, 99.60 kJ/mol, 80.14 kJ/mol and 85.79 kJ/mol, respectively. The presence of carboxylate and para positioned nitro groups induces strong cyclic delocalization of p electrons in the double bond mostly through the hyperconjugation with r bonds and least but strongly by p conjugation on the benzene ring. The p orbitals of one of the N@O bonds (N10AO14) of nitro group is strongly in cross conjugation with one of the C@C bonds (C4AC5) of phenyl ring system. The interaction between p (C4AC5) donor and p⁄ (N10AO14) acceptor provides stabilization of 104.54 kJ/mol with the highest occupancy of 0.6217e whereas p (N10AO14) ? p⁄ (C4AC5) interaction gives stabilization of 12.77 kJ/mol with occupancy of 0.3880e to the antibond. Even though both of these interactions cause an increment in electron population of their respective antibonds, the p (C4AC5) ? p⁄ (N10AO14) interaction is dominant. Also, the weak sigma conjugation between carboxylate group and electron abundant benzene ring due to C7AO15 M C1AC6 and C7AO16 M C1AC2 interactions as well as between nitro group and benzene ring give arise to N10AO13 M C4AC5 and N10AO14 M C3AC4 interactions results in the delocalization of the electrons of both the groups and the benzene ring and reduce the double bond character of both the interacting bonds and thus giving stabilization energy in the range 6.20A6.28 kJ/mol and 4.23A9.63 kJ/mol, respectively with carboxylate and nitro groups. The hyperconjugative interactions n2(O13) ? r⁄(C2AC3), n2 (O13) ? r⁄(C4AC5), n2(O14) ? r⁄(C3AC4) and n2(O14) ? r⁄(C5AC6) stabilizes the respective antibonds with stabilization energy in the range 2.72A3.48 kJ/mol. The most dominant charge transfer interactions in GPNB was the hyperconjugative interactions between n3(O13) and p⁄(N10AO14) with the highest delocalization energy of 689.98 kJ/ mol. Through this charge delocalizing interaction, the antibonding orbital gains the highest occupancy of 0.6217e in the system and stabilize the O'N'O resonance structure. The self conjugative interactions of the bonding and anti-bonding orbitals of N10AO14, hyperconjugative interactions of n1 (O13) ? r⁄(N10AO14) and n2 (O13) ? r⁄(N10AO14) as well as the conjugation of p(C4AC5) ? p⁄(N10AO14) are also responsible for the strong p electron delocalization in N10AO14 bond and which cause respective stabilizations of 31.07 kJ/mol, 11.97 kJ/mol, 81.31 kJ/mol and 104.54 kJ/mol. The antibonding r⁄(N10AO13) orbitals have acquire an occupancy of 0.0577e by the hyperconjugative interactions of the first and second occupied orbitals of lone pairs of oxygen atom (O14) causing stabilization of 11.97 kJ/mol and 81.31 kJ/mol, respectively. When bonding electrons delocalize, they can lead to alternative bonding patterns. According to the NBO calculations of GPNB, the electron delocalization associated with r(C1AC7) ? r⁄(H17AN18) and r(C1AC7) ? r⁄(N21AH22) interactions correspond to partial double bond resonance structure to O'C'O bond. These interactions resides negative charge on the carbon (C1) atom and positive charges on the hydrogen (H17, H22) atoms. In addition, these interactions are also responsible for inducing elongation of bonds and the large bond length values for C1AC7 (1.52 Å), H22  O15 (1.5519 Å) and for H17  O16 (1.5521 Å) were in accordance with the above result. However, these interactions are weak and certainly rewarded with low stabilization energies of 1.80 kJ/mol to the monomer. Furthermore, the weak hyperconjugative interaction associated with carboxylate group such as n (O15) ? r⁄(C7AO16) and n (O16) ? r⁄(C7AO15) stabilize the CAO bonds by symmetric delocalization of charge with low occupancies of 0.0465e estimated at 35.25 kJ/mol and at 38.69 kJ/mol by the second order perturbation theory. Also, the r conjugations between the bonding and the antibonding orbitals for C7AO15 M C7AO16 give weak

11

contribution towards the redistribution of electron density within the carboxylate group and stabilize the bonds with small acceptor occupancy of 0.0465e with stabilization of 3.56 kJ/mol. Thus in GPNB, the carboxylate bond are formed from the hyperconjugative resonance stabilization for the electrons of r-type single bonds. Moreover, the carbon atom of carboxylate group forms single bonds between the two oxygen atoms within the group. It is obtained from the NBO analysis that both the r (C7AO15) and r (C7AO16) bonds are formed from the interactions between sp2.02 hybrids centered on the carbon atoms and sp1.77 hybrids centered on the oxygen atoms and the hybrids were composed of 33.12%s, 66.81%p, 0.06%d, 0.01%f atomic orbitals and of 35.98%s, 63.54%p, 0.46%d and 0.02%f atomic orbitals, respectively. The CAO bonds were occupied by 1.9945 electrons and the polarization of both the bonds directed to the oxygen atoms with 64.96%. But the high contributions of pr make partial double bond character to both the CAO bonds and also the predicted bond length value of 1.26 Å suggests its partial double bond nature. The most prominent orbital interactions between the occupied orbitals n (O) of the lone pair electrons and the adjacent empty anti-bonding r⁄(NAH) orbitals can be used to study the strength and weakness of NAH  O hydrogen bonds in the monomer. The interaction between n2 (O15) and r⁄(N21AH22) and between n2 (O16) and r⁄(H17AN18) stabilizes with energies of 173.21 kJ/mol and 173.08 kJ/mol, respectively. Accumulation of high occupancies of 0.1035e to the r⁄(N21AH22) and to the r⁄(H17AN18) bond causes weakening of the respective bonds. It can be seen from the Table 4 that the interactions n1 (O15) ? r⁄(N21AH22), n1 (O16) ? r⁄(H17 AN18) and n2 (O15) ? r⁄(N21AH24), n2 (O16) ? r⁄(N18AH23) stabilize with equal charge transfer stabilization energies of 31.90 kJ/mol and 0.33 kJ/mol, respectively. Thereby these high and low stabilization energy interactions respectively constitute moderate and weak donor–acceptor interactions. It is worthwhile to mention that all of the donor–acceptor N  O distances have the shortest values nearly of 2.625 Å which explains the higher stabilization energies for n2(O) ? r⁄(NAH) due to the electrostatic repulsion between the oxygen and the nitrogen atoms and thereby forming moderate intramolecular hydrogen bonds. This may accounts for the higher stability of the monomer. The electrostatic nature of d+ d+ C7AOd H22AN21 and C7AOd H18AN17 hydrogen bonds 15  16  are explained by the equal contributions of 1.72 kJ/mol and increased occupancies of 0.1035e for the stabilizing interactions r(C7AO15) ? r⁄(N21AH22) and r(C7AO16) ? r⁄(N17AH18), respectively. The interactions r (H17AN18) ? r⁄(C7AO16) and r (N21AH22) ? r⁄(C7AO15) destabilize the respective antibonds with less occupancies of 0.0465e. However, the formation of Cd+AOdd+HANd bonds are more pronounced when the effect of nitrogen atoms of cation are transmitted through r bonds, but the influence of Cd+AOd dipole operates through space by an electrostatic interactions through-space or through hydrogen bond formation. Consequently the hydrogen bonding between carboxylate oxygen atoms and nearby hydrogen atoms of guanidinium were of the combined effects of through-space and through-bond inductive effect. Moderate and weak hydrogen bonds formation, hyperconjugative n(O) ? r⁄(CAN) and sigma conjugative NAH M CAN interactions causes the acyclic charge delocalization in the cation. The charge delocalizing interactions between electron abundant NAH and electron deficient C19AN20 components increases the charge transfer of r(N21AH22) and of r(H17AN18) to r⁄(C19AN20) orbital in 28.18 kJ/mol is a favourable donor–acceptor interaction which stabilizes the antibond by occupancy of 0.0282e. As a result of these interactions, the C19AN20 is slightly elongated by 0.05 Å over the other two CAN bonds of the cation moiety. However, in the case of the other two CAN bonds, the hyperconjugative interactions n(O15) ? r⁄(C19AN21) and n(O16) ? r⁄(N18AC19) induces partial p character to the respective antibonds and thereby a reduction

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in the bond length values. In addition, the interactions r(N18 AH23) ? r⁄(C19AN21), r(N20AH26) ? r⁄(C19AN21), r(N20AH25) ? r⁄(N18AC19) and r(N21AH24) ? r⁄(N18AC19) stabilize the C19AN21 and C19AN18 bonds with stabilization energies of 24.33 kJ/mol, 20.18 kJ/mol, 20.14 kJ/mol and 24.33 kJ/mol, respectively and with less occupancies of 0.0205e compared to the antibonding r⁄(C19AN20). The predicted electron population in r⁄ antibonding orbitals of C19AN20, C19AN21 and C19AN18 bonds clearly explains the differences in the respective bond length values. Also, the semideprotonated behaviour of NH2 groups in gas phase GPNB is evident from the double bond character of C19AN18 (1.321 Å) and C19AN21 (1.321Å) bonds, highly elongated nature of N18AH17 (1.073 Å) and N21AH22 (1.073 Å) bonds as well as C19AN20 (1.365 Å) bond. This is because of the sigma conjugation between r(N21AH22), r (H17AN18) and r⁄(C19AN20) orbitals which distorts the NH bonds and thereby elongation in the bond, resulting semideprotonated nature to intramolecular hydrogen bonded NH2 groups. The stabilization energies for the hyperconjugative interaction between the first occupied orbital, n(O13) and second occupied orbital, n(O14) of the lone pair electrons of oxygen atoms and the adjacent anti-bonding r⁄(C4AN10) orbitals are 19.59 kJ/mol and 63.14 kJ/mol, respectively. Since the small value of the electron population of 0.1166e of the antibonding C4AN10 orbital, the bond strengthens and may leads to an up-shift in corresponding stretching frequency. The C4AN10 bond is also strengthened by the sigma conjugation of electron donors of C2AC3 and C5AC6 bonds and resulting stabilization of 20.31 kJ/mol. The interactions n (O15) ? r⁄(C1AC7) and n(O16) ? r⁄(C1AC7) increases the electron population of the antibonds by 0.0885e and giving stabilization of 74.11 kJ/mol. The high electron density in the r⁄(C1AC7) orbital elongates the C1AC7 bond. The sigma conjugative charge transfer interactions in either direction such as CAN M NAH within the cationic moiety, CAO M NAH among cation and anion, and CAC M CAH within the anionic moiety resulted in lengthening of NAH and CAH bonds in GPNB. The increase of electron densities in the antibonding orbitals of r⁄(NAH) and r⁄(CAH) leads to up-shifting in the respective bond stretching frequencies at the molecular level. Hence the stability of the monomer is mostly devoted to the sigma conjugative and hyperconjugative interactions which causes a highly polarized nature to the molecule and thereby induces strong NLO activity to GPNB. The molecular charge distribution in terms of natural population analysis is listed in Table 5. In GPNB, the carbon atoms of carboxylate and of guanidinium have the most and the next highest positive charges, respectively. All the carbon atoms of phenyl ring, except the one carbon atom which attached with nitro substituent, possess negative charges. The nitro substituent reduces the electron density on the carbon atom (C4) in the benzene ring. This is due to the combined effect of the three electronegative atoms in NO2 and the high electron deficiency on nitrogen in this group enables it to pull electrons toward itself and positively polarizes the carbon to which it is bonded. Since the electron withdrawing property through large inductive and resonance effect of NO2 group its nitrogen atom has positive charge whereas nitrogen atoms of cation group have negative charges. Of the three nitrogen atoms of guanidinium cation, N20 atom has more negative charge than the other two nitrogen atoms involved in intramolecular hydrogen bonding interactions. Charges on the carboxylate oxygen atoms are more negative than those on the oxygen atoms of nitro group. It is found from the Table 5 that cationic hydrogen atoms have more positive charge than the charge on the anionic hydrogen atoms.

Table 5 Natural population analysis of GPNB calculated by DFT/B3LYP/cc-pVTZ method. Atoms

Natural charge

C1 C2 C3 C4 C5 C6 C7 H8 H9 N10 H11 H12 O13 O14 O15 O16 H17 N18 C19 N20 N21 H22 H23 H24 H25 H26 Total

0.1007 0.1665 0.1883 0.0376 0.1883 0.1665 0.7819 0.2289 0.2345 0.5207 0.2345 0.2289 0.3817 0.3817 0.7394 0.7394 0.4461 0.7558 0.6468 0.7770 0.7558 0.4461 0.3799 0.3799 0.3876 0.3876 0.0000

Molecular electrostatic potential Molecular electrostatic potential (MEP) surface for GPNB was generated by mapping B3LYP/cc-pVTZ electrostatic potential onto the molecular electron density surface. MEP plot of GPNB is shown in Fig. 4. MEP surface plot is a useful descriptor for the qualitative interpretation of the sites for electrophilic and nucleophilic attacks as well as hydrogen bonding interactions [67–69]. MEP map displays molecule’s positive, negative and neutral electrostatic potential regions in terms of colour grading. In the MEP plot of GPNB, the isosurface = 0.03501 a.u representing the regions that readily donate electrons and the isosurface = +0.03501 a.u representing the regions that readily accept electrons i.e., the nucleophilic centres are indicated by deep red colour and the deep blue colour region represents the electrophilic centres. The electronegative regions in GPNB molecule were localized on the oxygen atoms of nitro and carboxylate groups and this region acts as nucleophilic centres and undergoes nucleophilic attack. The positive charges were delocalized over the guanidinium group, implying its reactivity towards nucleophilic attack. The transfer of charge density from the electron rich carboxylate bonds to the two O  H bonds specify the yellow region over the O  H bonds. This charge delocalization may cause a slight increment in C@O bond lengths but this effect may dominated by the moderate hyperconjugative hydrogen bond interactions between oxygen lone pairs and NAH bonds ensuring partial double bond character to the carbonyl bonds. The neutral green region over the NAH bonds of intramolecular NAH  O, hydrogen atoms of phenyl ring and C4AN10 bond reflects the less reactive behaviour of these sites. The yellow region of phenyl ring indicates its electron richness. Frontier molecular orbital analysis The frontier molecular orbitals (HOMO and LUMO) have been generated at DFT/B3LYP/cc-pVTZ level. The HOMO–LUMO analysis

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Fig. 4. MEP plot of guanidinium 4-nitrobenzoate.

has been carried out to explain the electronic and optical properties, UV–Vis spectra [70], kinetic stability and chemical reactivity of the molecule [71]. The spatial distributions of frontier molecular orbitals (HOMO and LUMO) are used to analyze the electron transport mechanism through the molecule. Fig. 5 illustrates the frontier molecular orbitals of HOMO to LUMO transitions computed at the B3LYP/cc-pVTZ level of theory for GPNB. HOMO is concentrated over the oxygen atoms of nitro group, C1AC2, C2AC3, C5AC6, C1AC6, C6AH8 and C2AH12 bonds of phenyl ring, carboxylate group, NAH  O bonds as well as the NH2 groups which involved in intramolecular hydrogen bond formation. Highly delocalized nature of HOMO facilitates the ease intramolecular charge transfer. The electron density of LUMO is delocalized over the oxygen atoms of nitro and carboxylate groups, C4AN10, C1AC7, C2AC3AH11 and C6AC5AH9 bonds of nitrobenzoate group. Both the HOMO and LUMO orbitals are primarily localized on the nitrobenzoate moiety. The HOMO and the LUMO energies and energy gaps are quantitatively relative to optical and electronic

properties. The energy difference between the molecular orbitals is given below: HOMO energy, EHOMO = 6.9128 eV LUMO energy, ELUMO = 2.2659 eV HOMO–LUMO energy gap [72–75], DEGAP = ELUMO  EHOMO = 4.6469 eV. The chemical reactivity and site selectivity of the molecular systems have been determined by the conceptual density functional theory highly successful in predicting global reactivity trends [76]. The important quantum chemical molecular properties i.e., global reactivity descriptors such as ionization potential (IP), electron affinity (EA), electronegativity (v), electrophilicity index (x), chemical hardness (g), global softness (S), chemical potential (l), total energy change (DET) and overall energy balance (DE) for GPNB have been calculated on the basis of Koopman’s theorem [77] using HOMO and LUMO energies. The energy of the HOMO is directly related to the ionization potential and characterizes the susceptibility of the molecule toward attack by electrophiles.

Fig. 5. HOMO–LUMO plot of GPNB.

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The energy of the LUMO is directly related to the electron affinity and characterizes the susceptibility of the molecule toward attack by nucleophiles. The qualitative definition of hardness is closely related to the polarizability, since a decrease of the energy gap usually leads to easier polarization of the molecule [78]. According to the Koopman’s theorem [77], the HOMO energy (EHOMO) can be used to relate the ionization potential (IP) and the LUMO energy (ELUMO) can be used to estimate the electron affinity (EA) [79]. If EHOMO  IP and ELUMO  EA, then the electronegativity (v) [80] can be defined as, v = (IP + EA)/2. The HOMO–LUMO energy gap is related to the chemical hardness (g) [78,81–83] by the relation, g = (IP  EA)/2. The hardness is the ability of chemical system to resist the deformation of electron cloud under small perturbation encountered during the chemical process. The extent of chemical reactivity can be expressed by the term global softness [84] which can be related to hardness as, S = 1/2g. A large value of HOMO– LUMO energy gap indicates the chemical hardness, stability and chemically unreactive properties of molecule and small HOMO– LUMO energy gap suggests a soft molecule. The chemical potential can be defined as; l = (IP + EA)/2. Parr et al. [85] proposed a new global reactivity descriptor of molecule which measures the energy lowering due to maximal electron flow between donor and acceptor and defined as electrophilicity index, x = l2/2g. The total energy change is defined as, DET = g/4 [86,87]. The overall energy balance (DE) which determines the energy gain or lost, in an electron donor–acceptor transfer is given as, DE = EA  IP. The calculated global reactivity descriptors for GPNB are listed in Table 6. The calculated small HOMO–LUMO gap (4.6469 eV) which arose due to the effect of the utmost stabilization of LUMO, explains the intramolecular charge transfer interactions in GPNB. The small energy gap between the HOMO of electron donors and the LUMO of electron acceptors promotes the interactions and stabilizes the molecule and thereby NLO activity. The energy gap between the HOMO and the LUMO reflects the chemical reactivity and softness of GPNB. The predicted chemical potential value is negative (4.5894 eV) and is smaller than the calculated value of electrophilicity index (4.5324 eV) for GPNB, implying the stability of the molecule.

First order hyperpolarizability The direct information about the relationship between molecular structure and NLO property is obtained by analyzing the dipole moment, polarizability and hyperpolarizability values. The molecular first hyperpolarizability is a first order non-linear response function which depends on the strength of the donor and acceptor groups and the nature and length of p-bridge. A molecular system with non-zero value of first hyperpolarizability should be lacking

Table 6 Calculated quantum chemical molecular orbital properties for GPNB at DFT/B3LYP/ccpVTZ method. Parameters

B3LYP/cc-pVTZ

HOMO energy, EHOMO (eV) LUMO energy, ELUMO (eV) HOMO–LUMO energy gap, DEGAP (eV) Ionisation potential, IP (eV) Electron affinity, EA (eV) Electronegativity, v (eV) Chemical hardness, g (eV) Global softness, S (eV)1 Chemical potential, l (eV) Electrophilicity index, x (eV) Total energy change, DET (eV) Overall energy balance, DE (eV) SCF energy (eV)

6.9128 eV 2.2659 eV 4.6469 eV 6.9128 eV 2.2659 eV 4.5894 2.3235 0.2152 4.5894 4.5324 0.5809 4.6469 22614.0458

the centre of symmetry. The results of the B3LYP/cc-pVTZ calculations for the first hyperpolarizability, polarizability, anisotropy of the polarizability and total static dipole moment are presented in Table 7. The calculated values of hyperpolarizability, mean polarizability, anisotropy of the polarizability and dipole moment for GPNB are compared with the respective values calculated at DFT/ B3LYP/cc-pVTZ level for the molecule of standard organic NLO crystal, urea. The predicted value of first hyperpolarizability for GPNB is 302.43  1033esu which is 0.46 times that of urea (660.85  1033 esu). Also, the calculated first hyperpolarizability of GPNB is much small value compared to the reported values of guanidinium complexes [28,30,31]. In addition to the small p-conjugation length in the molecule, the inhibition of rotation of carboxylate and cationic moieties in GPNB as is evident from the partial double bond character of C19AN18 and C19AN21 as well as the two centered hydrogen bonding interactions in the monomer, and also the existence of electron withdrawing and inductive electron donating groups might be causes a large decrement in the total hyperpolarizability value and the predicted btotal value was the maximum for the planar structure. All the b tensor components show significant hyperpolarizability values in their direction due to asymmetric spatial orientation of the molecule and the magnitudes of these values are strongly influenced by the transition dipole moments of the HOMO–LUMO transitions as well as by the strong electron withdrawing group in the molecule and the presence of which induce more negative values for certain b components. The dipoles may oppose or enhance one another or, at least, bring the dipoles into or out of the required net alignment necessary for enhancing and decreasing the btotal values [88]. The DFT calculation predicted a large gas phase dipole moment value for GPNB (14.1218 Debye), which is 3.8 times greater than that of urea (3.7321 Debye). The calculated dipole moment for GPNB is nearly equal to the reported values of other guanidinium based NLO system such as for guanidinium maleic acid molecule [28],

Table 7 The predicted values of first hyperpolarizability (b), mean polarizability (a), anisotropy of the polarizability (Da), and total static dipole moment of GPNB by DFT/B3LYP/cc-pVTZ method. b Components

esu(1033)

bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz First hyperpolarizability, btotal

787.65 0.0458 911.26 0.1085 164.40 0.5188 1.7959 121.30 0.1318 14.824 302.43

a Components

esu(1024)

axx axy ayy axz ayz azz

33.000 4.446  107 21.241 0.0206 5.418  105 9.911 21.384 19.997

Mean polarizability, a Anisotropy of polarizability, Da

l Components

Debye

lx ly lz

14.1161 0.0004 0.4015 14.1218

Total dipole moment, ltotal

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the time dependent Hartree Fock (TD-HF) method calculations predicted the dipole moment value of 14.909 Debye. The predicted value for GPNB is much greater than the reported dipole moments for guanidine acrylic acid molecule (7.78 Debye) [31], diguanidinium arsenate monohydrate (7.23 Debye) and diguanidinium phosphate monohydrate (2.81 Debye) by TD-HF method [30]. The calculated values of mean polarizability (a) and anisotropy of the polarizability (Da) are found to be 21.384  1024 esu and 19.997  1024 esu, respectively. The predicted mean polarizability and anisotropy of the polarizability values are 4.7 times and 8.6 times greater than those of urea (a = 4.574  1024 esu and Da = 2.333  1024 esu), respectively. The high value of polarizability signifies the extent of charge delocalization within the monomer and the highly polarizable nature may lead it to soft molecule category. Such a high dipole moment and mean polarizability values reflects the considerable intramolecular charge transfer interactions and polarity of GPNB.

UV–Vis spectral studies Fig. 7. UV–Vis transmission spectrum of GPNB.

The observed and theoretically modeled electronic absorption spectra and transmission spectrum of GPNB are shown in Figs. 6 and 7. The cut-off wavelength is observed at 380 nm. The strong orbital electron transfer interactions in GPNB make very poor transmission in the region 200–435 nm for the sample. The spectrum shows the good transparency in the entire visible region 435–800 nm due to the presence of NO2, COO and NH2 groups and limited conjugation length as well as the strong resonance in the crystalline state of GPNB. The absorption spectrum of crystalline phase shows two absorptions for GPNB which are an

intense primary absorption at 202.91 nm and the secondary absorption observed around 311.67 nm. The former can be due to the n ? r⁄ transition and the latter due to forbidden n ? p⁄ transition. The electron withdrawing nitro group stabilize the lone pair and causes red shift in wavelength of n ? p⁄ transition. The peak corresponds to the longest wavelength can be considered as the result of n(O) ? p⁄(NAO) transition and the peak due to the shortest wavelength may be originated from n(O) ? r⁄(CAN) transition. The energies for the longest wavelength transition of electron from the HOMO to the LUMO, oscillator strengths, maximum absorption wavelength and NBO transitions for the three lowest excited states of the gas phase and water phase GPNB calculated by the TD-DFT/B3LYP/cc-pVTZ level are listed in Table 8. The absorption maxima calculated for gas phase was at 296.54 nm and for water phase at 316.61 nm and were assigned for the forbidden n ? p⁄ transitions. The vacant p⁄ orbital is mostly originated from the LUMO of 4-nitrobenzoate group and probably the transitions are arose due to nitro group. The null value of oscillator strengths (f = 0.0000) for HOMO ? LUMO, HOMO-5 ? LUMO and HOMO-7 ? LUMO may be reflect the mutually perpendicular nature of the respective ground and excited state orbitals and there are no significant contributions from these excited state transitions to GPNB’s electronic absorption cross-section. Also, the small value of oscillator strength (f = 0.0001) is apparently due to the contribution from the forbidden electronic transitions between HOMO-2 and LUMO and are responsible for the electronic absorption maxima for gas and water phases of GPNB. Furthermore, the low oscillator strength value indicates the low first order hyperpolarizability value for the molecule. The red shifting of absorption maximum manifests the small HOMO–LUMO gap in GPNB. TD-DFT calculation predicted the electronic excitation energy of 4 eV for the maximum wavelength transitions. Dielectric studies

Fig. 6. (a) UV–Vis absorption spectrum of GPNB crystal. (b) Simulated UV–Vis absorption spectrum of GPNB in water phase. (c) Simulated UV–Vis absorption spectrum of GPNB in gas phase.

Good quality single crystal sample with dimensions 3.19 mm  3 mm  1.36 mm was used for dielectric measurements such as capacitance, dielectric loss and AC conductivity as function of frequency (50 Hz–5 MHz) and at temperatures 35 °C and 100 °C. The frequency dependence of dielectric constant is shown in Fig. 8. It is observed from the figure that the dielectric constants at both the temperatures decrease with the increase of frequencies and the value becomes independent at high

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Table 8 Calculated absorption wavelengths, electron excitation energies and oscillator strengths of GPNB by TD-DFT/B3LYP/cc-pVTZ method. Phase

Excited state

Wavelength (nm)

Excitation energies (eV)

Oscillator strengths (f)

Assignment

Gas phase

S1(H-5 ? L) S2(H ? L) S3(H-2 ? L)

326.39 313.14 296.54

3.7986 3.9594 4.1810

0.0000 0.0000 0.0001

n ? p⁄ n ? p⁄ n ? p⁄

Water phase

S1(H ? L) S2(H-2 ? L) S3(H-7 ? L)

330.47 316.61 315.44

3.7518 3.9160 3.9306

0.0000 0.0001 0.0000

n ? p⁄ n ? p⁄ n ? p⁄

Fig. 10. Variation of ac electrical conductivity with frequency. Fig. 8. Frequency dependence of dielectric constant.

Fig. 9. Variation of dielectric loss with frequency.

frequencies. Also the value of dielectric constant is found to decrease with the increase of temperature. The high value of er at low frequency is due to the presence of electronic, ionic, orientation and space charge polarizations. The low values of er at high frequencies are due to the significant loss of all polarizations gradually [89] which is important for materials having photonic and NLO device applications. Fig. 9 shows the variation of dielectric loss with frequency. It is seen that the value of dielectric loss is increase with temperature and becomes independent at high frequencies. Also the value of

dielectric loss decreases with the increase of frequency at both temperatures and appears to achieve saturation beyond 100 kHz frequency region. The high value of dielectric loss occurs at high temperature and lower frequency is due to the macroscopic distortion in the charges [90]. The lower value of the dielectric loss at higher frequency suggests the minimum density of defect and good optical quality of the grown crystal. Dielectric loss constant has special attention for NLO materials in their applications [91]. The variation of ac electrical conductivity with frequency at 35 °C and at 100 °C is shown in Fig. 10. It can be seen from the figure that the value ac conductivity increases with frequency. The increase in conductivity at higher frequency can be attributed to the effect of decrement in space charge polarization [92]. Also, it is interpreted for title crystal that the effect of impurity has no appreciable influence on electrical conduction at both the temperatures. The crystal expansion and electronic and ionic polarizations causes significant variation of these dielectric parameters at low temperature whereas the thermally generated charge carriers and impurity dipoles are responsible for the variation at high temperature. The large values of dielectric constant and dielectric loss at low frequency arises due to the presence of space charge polarization near the grain boundary interfaces which depends on the purity and perfection of the sample [89,93]. Etching study The growth mechanism and surface features of the title crystal was investigated using chemical etching study with water–ethanol (1:1) as the etchant. Fig. 11 depicts the variation of etch pits with different etching times 2 s, 15 s and 30 s, respectively. Round shape etch pits were observed for 2 s etching conditions. The projected etch features in figure for 15 s may be correlated

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Fig. 11. Etch pits observed for (a) 2 s, (b) 15 s and (c) 30 s.

with the density of dislocations in the crystal and suggesting that the formation of these etch pits should be on the dislocations. Triangular shaped etch pits are observed when GPNB crystal etched for 30 s. These triangular pits may be associated with the growth disturbances which arose due to inclusions. It is found that the mixture of water and ethanol in 1:1 proportion is being a good etchant for the surface morphological study of GPNB. It can be concluded from the study that the size, shape and distribution of etch pits were observed in the examining area on the sample vary with different etching duration indicating the variation of dislocation patterns which influence the different phases of growth for the sample crystal.

Conclusion Single crystals of guanidinium 4-nitrobenzoate has been grown by slow evaporation method. The existence of dislocations in the grown crystal was confirmed by chemical etching study. Dielectric study substantiates the possibility of using the crystal in photonic and NLO device applications. Vibrational spectral analysis has been carried out using FT-IR, FT-Raman and DFT methods. The red shifting of NH stretching wavenumbers in both the observed and predicted wavenumbers confirm the existence of NAH  O type bonds in GPNB. DFT computation predicted a stable planar structure to the molecule of GPNB. The NAH  O bond angles have a lot closer to the maximum of 178.5° angle, suggesting the strength of hydrogen bonds involved in neutral monomer formation. The molecule is stabilized through moderate and weak NH  O interactions and is observed to exhibit two two-centered hydrogen bonds. The low frequency modes depends up on the NH groups form moderate hydrogen bonds while the weak hydrogen bonds are associated with the high frequency stretching components. The differences in stretching frequencies and in NH bond lengths clearly explain the partially deprotonated behaviour of NH2 groups. NBO analysis predicted the responsibility for the resonance structures associated with its carboxylate and cation moiety were of hyperconjugative n ? r⁄ and r ? r⁄ origin. The n ? p⁄ transitions due to nitro group was responsible for the electronic absorption maximum for gas and water phases of GPNB. The two absorption maxima in crystalline phase were assigned to n ? r⁄ and n ? p⁄ transitions. The electrophilic and nucleophilic centres in GPNB have been identified using MEP. The small HOMO–LUMO gap explains the nature, strength and extend of intramolecular charge transfer interactions which is contributed the NLO activity in GPNB. Also, the high polarizability and global softness values were classify the monomer to soft molecule category. The strong hyperconjugative resonance interactions of r type bonds and large and good charge separation in GPNB are undoubtedly leading to its large dipole moment and low hyperpolarizability with low oscillator strengths. The dipole moment, extended hyperconjugation and strong effectiveness with shorter

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Please cite this article in press as: V. Sasikala et al., Electronic structure, vibrational spectral and intervening orbital interactions studies of NLO material: Guanidinium 4-nitrobenzoate, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/10.1016/j.saa.2014.12.013