Vibrational study of 5-azacytosine and propanedinitrile under high pressure

Vibrational Spectroscopy 78 (2015) 60–65

Contents lists available at ScienceDirect

Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec

Vibrational study of 5-azacytosine and propanedinitrile under high pressure M.A. Puerto a , F. Miotto b , J. Catafesta b , C.A. Perottoni b , N.M. Balzaretti a, * a b

Instituto de Física, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, RS, Brazil Instituto de Materiais Cerâmicos, Universidade de Caxias do Sul, 95765-000 Bom Princípio, RS, Brazil

A R T I C L E I N F O

A B S T R A C T

Article history: Received 2 December 2014 Received in revised form 1 April 2015 Accepted 9 April 2015 Available online 13 April 2015

The effect of high pressure on the vibrational modes of 5-azacytosine (C3H4N4O) and propanedinitrile (CH2(CN)2) was investigated at room temperature using a diamond anvil cell (DAC). In situ infrared and Raman spectroscopy results indicated that the ring structure of 5-azacytosine remained preserved and the band shifts induced by high pressure were fully reversible up to 13 GPa. For the open-chain structure of propanedinitrile, a polymerization reaction was observed by infrared spectroscopy almost instantly at about 15 GPa at room temperature. The same pressure-induced polymerization was observed after a week at 1 GPa, at room temperature. ã 2015 Elsevier B.V. All rights reserved.

Keywords: 5-Azacytosine Propanedinitrile High pressure Vibrational properties

1. Introduction Back in 1989, Liu and Cohen stepped forward the suggestion that a hypothetical b-C3N4 phase could exhibit a bulk modulus of the same order or slightly higher than diamond [1]. However, synthesis of b-C3N4 bulk samples has eluded researchers ever since. In this work, two molecular compounds containing carbon and nitrogen, one with ring structure (5-azacytosine) and the other with open-chain structure (propanedinitrile) were investigated under high pressure looking for new routes for synthesis of covalently bonded, carbonitride high hardness materials. 5-Azacytosine (6-amino-1H-1,3,5-triazin-2-one, C3H4N4O) is a heterocyclic compound containing nitrogen and carbon atoms, similar to cytosine (one of the four DNA’s bases), where the pyrimidine ring is replaced by the 1,3,5-triazin ring (Fig. 1). Podolyan and Rubin [2] performed an ab initio post-Hartree–Fock study of the infrared vibrational spectrum for two of the most stable tautomers of cytosine and 5-azacytosine. The small energy difference between the two tautomers suggests that molecular 5-azacytosine could exhibit keto-enol tautomerism [2]. On the other hand, theoretical and experimental studies using the ultraviolet absorption spectrum of 5-azacytosine indicated that it would be in the enol form in solid state [3]. Experimental data related to the Raman spectrum of 5-azacytosine was not found in the literature.

* Corresponding author. Tel.: +55 51 33086489. E-mail address: [email protected] (N.M. Balzaretti). http://dx.doi.org/10.1016/j.vibspec.2015.04.001 0924-2031/ ã 2015 Elsevier B.V. All rights reserved.

The molecular crystal propanedinitrile (CH2(CN)2) (Fig. 1d) presents four crystalline phases observed from 140 to 295 K, showing unusual re-entrant phase transitions (a sequence of twophase transitions for which the first and third phases have the same symmetry and they are, effectively, identical) [4–10]. Furthermore, propanedinitrile is an important reagent for industrial uses, e.g., in the syntheses of pharmaceutics, herbicides, fungicides, defoliants, dyes and polymers, because it is a weak cyanocarbon acid with an exceptional reactivity [11–12]. Previous studies of propanedinitrile at high pressures indicated polymerization induced by pressure and shear deformation [13–14]. In this work we present a comparative theoretical and experimental study about the ring structure of 5-azacytosine and the open-chain structure of propanedinitrile under high pressure, up to 15 GPa using a diamond anvil cell. A complementary Raman study of 5-azacytosine under pressure was also performed. 2. Materials and methods The samples used in this work were 5-azacytosine (98%, Acros Organics, New Jersey, USA) and propanedinitrile (99%, Sigma– Aldrich). Samples were loaded into a Piermarini–Block diamond anvil cell (DAC) with 400 mm culet diamond anvils (low fluorescence IIa type) and a gasket made of Waspalloy with a 250 mm hole for infrared (IR) and Raman measurements without pressure transmitting medium (except for the IR experiments with 5-azacytosine, in which case the sample was dispersed in KBr). The pressure was measured by the ruby fluorescence method [15].

M.A. Puerto et al. / Vibrational Spectroscopy 78 (2015) 60–65

61

Fig. 1. (a) and (b) chemical structures of the most stable tautomers of 5-azacytosine [2], keto and enol forms, respectively, (c) cytosine, and (d) propanedinitrile. In these structures the color code is: dark gray = carbon; light gray = hydrogen; red = oxygen and blue = nitrogen atoms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Raman spectra at high pressures were obtained using a He–Ne laser with a nominal power of 10 mW using a Horiba Jobin-Yvon iH320 spectrometer with a CCD cooled by liquid nitrogen. Raman spectra at room conditions were obtained with a Horiba LabRAM HR Evolution spectrometer, using a He–Ne laser. Fourier transform infrared (FTIR) spectra (4 cm
1 resolution, 512 scans) were collected in transmittance mode using a Perkin-Elmer Spectrum 400 MIR/FIR and a Bomem MB 100 spectrometers. All measurements were performed in situ at room temperature with the sample inside the DAC for different values of pressure (up to 13 GPa for Raman measurements, and 9 GPa and 15 GPa for IR measurements of 5-azacytosine and propanedinitrile, respectively.).

Fig. 2. X-ray powder diffraction pattern of commercial 5-azacytosine.

The X-ray diffraction pattern (XRD) of 5-azacytosine at ambient conditions was obtained in a Siemens D500 diffractometer, with Cu Ka radiation (l = 1.5404 Å), operating in Bragg–Brentano geometry and equipped with Soller slits, 1 divergence slit and 1 scattering slit in the primary beam, 0.15 mm receiving slit and a graphite monochromator in the secondary beam, with 0.05 per step, from 5 to 40 2u, and an acquisition time of 1 s per step. The band assignments were based on calculations for isolated molecules of 5-azacytosine and propanedinitrile performed at the MP2 level of theory with augmented-cc-pVDZ basis sets, using

Fig. 3. Infrared spectrum for 5-azacytosine: (a) theoretical calculation for enol tautomer (unscaled); (b) theoretical calculation for keto tautomer (unscaled); (c) experimental results. Theoretical results considered a Lorentzian profile with FWHM of 15 cm
1.

62

M.A. Puerto et al. / Vibrational Spectroscopy 78 (2015) 60–65

Gaussian 09 [16], taking into account a previous theoretical study carried out at the MP2/6-31G(d,p) level of theory for 5-azacytosine [2]. Relative Raman intensities were calculated from theoretical Raman activities according to the procedure outlined in [17].

Table 1 Experimental and theoretical IR and Raman vibrational mode frequencies (cm
1) and pressure-induced band shifts for 5-azacytosine (keto tautomer). Figures in parentheses correspond to the uncertainty of the last digit. Experimental

Theoretical

3. Results and discussion

Keto form (this work)

Keto form (Ref. [2])

n(cm
1)

n

IR n Raman n (cm
1) (cm
1)

3.1. 5-azacytosine at ambient conditions Fig. 2 shows the X-ray diffraction pattern of the commercial sample of 5-azacytosine, where the large halos indicate that crystallization was not complete. No previous results about X-ray study of 5-azacytosine were found in the literature nor in X-ray databases. Several attempts were made in order to recrystallize 5azacytosine, using different solvents. However, none of them was successful. A possible explanation for this is the amount of water (2%) present in the commercial sample preventing the complete crystallization of the sample. In fact that, the FTIR spectrum of the pristine sample (Fig. 3c) is identical to the spectrum of the keto tautomer C3H4N4O:1/2H2O (spectral database for organic compounds – CAS 931-86-2). Fig. 3 compares theoretical results for enol (Fig. 3a) and keto (Fig. 3b) tautomers with the experimental results (Fig. 3c) for the infrared spectrum of 5-azacytosine, and Fig. 4 shows the comparison for the Raman spectrum of this sample. The theoretical spectra were calculated using a Lorentzian profile with 15 cm
1 full width at half maximum (FWHM). A possible explanation for the observed mismatch between theoretical and experimental results would be related to the inherent difficulty of comparing theoretical results obtained for isolate molecules under vacuum with experimental results measured for samples in the solid state. It is important to remark that, as far as we know, this is the first time the experimental Raman spectrum of 5-azacytosine is reported. Table 1 shows theoretical normal mode analysis of the MP2/augmented-cc-pVDZ calculations carried out in this work for the keto tautomer. The assignments of the vibrational modes were based on previous theoretical calculations for compounds similar to 5-azacytosine (including cytosine and 6-azacytosine [2,18–20]). The modes with frequencies below 188 cm
1 correspond to lattice modes. The region above 3000 cm
1, related to NH2, OH, NH and CH, contains the most intense modes for the isolated tautomer according to the theoretical calculations [2]. The experimental infrared spectrum of the solid sample (Fig. 3c) shows absorption bands in this region,

Assignment

(cm
1) 3853 3699

nas(NH2) ns(NH2)

3582 3385

3354

3693

n(N1H)

3324

3324

3221

nas(NH2) ns(NH2) n(N1H) n(N1H) ns(NH2) n(C6H)

3260

n(C6H)

3180

1778

n(C2O)

1855

n(C2O)

3178 2633 1711

1691

n(N5C6)

1739

n(N5C6)

1629

3780 3622 3615

1433

n(N3C4) b(NH2) b(scNH2) n(N5C6) n(C4Nam) b(scNH2) n(N3C4) n(C4N5) b(N1H) n(C4Nam) b(scNH2) b(N1H) b(N1H)

1373

b(HNamC4) b(C6H) b(HNamC4) 1411

1636

1563

1484

1276 1172

1082 976 970 937

n(N3C4)

1468

1455

n(C4Nam) b(scNH2) n(C6N1) b(N1H)

1447

1442

0.5(1)

1352

1354/ 1361

1.1(3)

b(C6H)

1276

1277/ 1286

b(C6H) n(C2N3) 1294 b(HNamC4) n(C6N1) 1196 b(N1H) r(NH2) r(NH2) 1110 n(N1C2)

n(N5C6) n(C2N3)

1225

1230

2.8(4)

2.9(3)

g(C6H)

n(N1C2) n(C4N5)

1604

1519

1465

n(C6N1) b(N1H)

1147

b(roNH2)

987

987

985

b(R1)

905

905

952

n(N1C2) n(C4N5)

798

824

n(N1C2) t(R1)

612

615

576

571

761

t(ring)

740

664

g (N1H)

672

586

Twisting (NH2) b(ring) b(ring) b(C4Nam) b(C2O) t(ring) b(C4Nam) b(C2O) g (NH2) t(ring) g (NH2) t(ring) g (NH2)

590

117

1.2(1)

1541

803 762

187 143

13(5) 2.2(2) 1.1(2)

1512

r(NH2) b(ring) t(ring)

415 364

1699/ 1684 1658/ 1646


8.7(4)

b(scNH2) n(N5C6) n(C4Nam) n(N3C4) b(scNH2)

1682

790 780

581 557 525

Fig. 4. Raman spectrum of 5-azacytosine: (a) theoretical calculation for enol tautomer (unscaled); (b) theoretical calculation for keto tautomer (unscaled); (c) experimental results. Simulated Raman bands are represented using Lorentzian profiles with FWHM of 15 cm
1.

dn/dP (cm
1/ GPa)

584 566 530

g(C4Nam) g(C2O) g(C4Nam) g (N1H) g(C4Nam) b(R3) b(R2)

1.5(2) (IR) 3.5(1) (Raman)

543

412 369

408/422

220 174

184/152 136

130

113/96

Notation for in-plane modes: n, stretching; as, asymmetric; s, symmetric; b, bending or ring deformation; r, rocking; notation for out-of-plane modes: g , bending; t , torsion.

M.A. Puerto et al. / Vibrational Spectroscopy 78 (2015) 60–65

63

however at lower wavenumbers, while the experimental Raman spectrum (Fig. 4) presents very weak and broad bands. In fact, infrared spectra provide the best tools to study stretch modes in molecular solids with hydrogen bonded molecules [21–23]. 3.2. Infrared and Raman spectra of 5-azacytosine under high pressure Inside the DAC, only the Raman band at 824 cm
1, related to the ring vibration, was intense enough to enable in situ measurements. This peak shifted to higher wavenumbers under increasing pressure and returned to the original value after decompression, indicating that 5-azacytosine exhibits a reversible behavior along the compression–decompression cycle showing a complete chemical stability of the molecule even under pressures of about 13 GPa. In the case of FTIR, it was possible to measure the spectra of 5-azacytosine inside the DAC for different values of pressure, as shown in Fig. 5a and b. Fig. 6 shows the pressure dependence of some 5-azacytosine IR bands up to 10 GPa. Most of the IR bands shifted to higher wavenumbers, as should be expected due to the reduction of the atomic distances under high pressure. The only exception was the band at about 3385 cm
1 , related to NH2 or N1H stretching modes, for which dn/dP =
8.7

Fig. 6. Effect of pressure on the infrared absorption bands of 5-azacytosine. Closed (open) circles represent data taken upon pressure increase (decrease). The squares represent the effect of pressure on the Raman peak at 824 cm
1.

(4) cm
1/GPa. The shift to lower wavenumbers suggests that these groups were probably forming hydrogen bonds with the CO groups at high pressures. In fact, previous studies have focused on the effect of the formation of hydrogen bonds on the N

H stretching modes, whose wavenumber depends on the N

H  O distance [20]. Accordingly, under the hypothesis that the observed band shift for the NH2 and N1H stretching modes is primarily due to intermolecular hydrogen bonding to oxygen, the compression of 5-azacytosine leads to a roughly estimated reduction of the average intermolecular N

H  O distance of about 0.016 Å/GPa [24]. A similar behavior was also observed for sucrose under high pressure, for which the hydroxyl stretching band shifts according to dn/dP =
4.9 cm
1/GPa [25]. This effect was also reversible under pressure release.

Fig. 5. Infrared absorption spectrum of 5-azacytosine in situ in the DAC. For both panels (a) and (b), corresponding to different spectral ranges, the spectra were taken consecutively from below during the pressure increase up to 9 GPa followed by the pressure decrease down to the complete release. The dotted vertical lines indicate the wavenumbers, at ambient pressure, of the bands for which dn/dP is reported in the Table 1.

Fig. 7. Infrared spectrum of propanedinitrile: (a) theoretical (unscaled) and (b) experimental results. Theoretical results considered a Lorentzian profile with FWHM of 15 cm
1.

64

M.A. Puerto et al. / Vibrational Spectroscopy 78 (2015) 60–65

Fig. 8. Raman spectrum of propanedinitrile: (a) theoretical (unscaled) and (b) experimental results. Simulated Raman bands are represented using Lorentzian profiles with FWHM of 15 cm
1.

3.3. Propanedinitrile – infrared and Raman spectroscopy The sample of propanedinitrile was purified by recrystallization in ethanol until a colorless product was obtained. According to Nakamura et al. [26], the stable phase at room temperature is monoclinic. Figs. 7 and 8 compare theoretical and experimental results for the infrared and Raman spectra of propanedinitrile, respectively. The theoretical spectra were calculated using a Lorentzian profile with FWHM of 15 cm
1. Table 2 presents the proposed assignments for the vibrational modes, in agreement with those reported previously in the literature [9,27,28]. Below 315 cm
1 the modes correspond to lattice vibrations. Fig. 9 shows the infrared absorption spectra measured of propanedinitrile inside the DAC for different values of pressure. According to Hara [14], evidences of polymerization observed at 5 GPa and 300  C in the infrared spectrum were the decrease in intensity of the band at 2270 cm
1, characteristic of the C¼N bond, and the appearance of a C¼N broad band around 1560 cm
1. In the results shown in Fig. 9, the band around 2270 cm
1 is masked by an artifact related to the subtraction of the background due to the absorption of the diamond anvils in this region. On the other hand, the broad band that appeared at 15 GPa, indicated by the arrow in Fig. 9, remained visible after pressure release, near 1560 cm
1. This

Fig. 9. Infrared absorption spectrum of propanedinitrile at the pressures indicated at right. The dotted vertical lines indicate the wavenumbers, at ambient pressure, of the bands for which dn/dP is reported in the Table 2. The arrow points to the

C¼N

band that signals pressure induced polymerization of propanedinitrile. The irregularly shaped feature at around 2350 cm
1 in the spectra taken at 10.5 and 15.0 GPa is an artifact arising from subtraction of the background due to the absorption of the diamond anvils.

band is probably associated with the formation of a C¼N bond due to the polymerization process. The product obtained at high pressure was brown, as can be seen in Fig. 10. In fact, complementary experiments have shown that this reaction starts at even smaller pressures at moderate temperatures (about 2 GPa at 160  C) [14]. According to Zharov, propanedinitrile can react under high pressure (1 GPa) forming C¼N chains at room temperature [13]. The product was brown or black and the shear deformation of the solid sample was a necessary condition for the chemical reaction to occur. In this work, the emergence of the band near 1560 cm
1 indicated the onset of a polymerization reaction at 15 GPa at room temperature evidenced, also, by the observation that the sample darkened inside the DAC. The polymerization reaction usually started at the center of the sample (as can be seen in

Table 2 Band assignment and pressure-induced band shift of propanedinitrile. Figures in parentheses correspond to the estimated uncertainty of the last digit. Theoretical

Experimental IR

Raman

3151 3093 2176 2169 1439 1317 1235 990 929 898

2968 2933 2272

2970 2933 2273/2266

1390 1321 1216 987 930/909 891 710 580

1393 1311 1217 983

566 342 341 315 138

895 581 372/370 365 342 155 86

Assignment

dn/dP (cm
1/GPa)

nas(CH2) ns(CH2) nas(CN) ns(CN) b(CH2)

4.7(5) 3.0(3) 3.8(2)

Wag(CH2) Twist(CH2) n(CCC) r(CH2) n(CCC)


2.0(2) 1.1(2) 0.2(1) 4.6(4) 1.6(2) 2.5(1)

b(CCC) b(CCN) b(CCN) b(CCN) b(CCN)

Notation: n, stretching; as, asymmetric; s, symmetric; b, bending; r, rocking.

Fig. 10. Sample of propanedinitrile in the DAC at 15 GPa. Sample darkening signals the onset of pressure-induced polymerization.

M.A. Puerto et al. / Vibrational Spectroscopy 78 (2015) 60–65

65

propanedinitrile polymerized under pressure (as already pointed out by previous works available in the literature) while the ring structure of 5-azacytosine remained stable. In this context, distinct spatial configurations of compounds containing carbon and nitrogen would lead to different structures under high pressure, as may be expected. Perhaps the polymerization observed for propanedinitrile is a good start for the synthesis of superhard carbonitride structures. Acknowledgments The authors acknowledge financial support from the Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS). Thanks are due also to HORIBA Scientific. The calculations were performed at the Centro Nacional de Supercomputação (CESUP) – Porto Alegre, Brazil. Authors thank the support of Giovani Rech for elaboration of graphics. References

Fig. 11. Pressure dependence of the wavenumber for some selected bands of propanedinitrile. Closed (open) circles represent data taken upon pressure increase (decrease).

Fig. 10). In addition, the C¼N IR remained active, indicating that only part of the sample was actually undertaken pressure-induced polymerization. The sample remained dark after pressure release, a clear indication that an irreversible polymerization had occurred. Fig. 11 shows the pressure dependence of some propanedinitrile IR bands up to 15 GPa. In spite of the onset of an irreversible pressure-induced polymerization, the shift with pressure for the bands associated to chemical groups not directly involved in the polymerization reaction exhibit a completely reversible behavior over the pressure range explored in this work. The negative shift of the propanedinitrile CH2 bending mode could also be caused by pressureinduced intermolecular hydrogen bonding. This, however, should be considered only as a tentative explanation, as the formation of hydrogen bonding usually increases the frequency of bending modes [29]. The effect of hydrogen bonding on bending mode frequencies is not yet very well established as compared, for instance, with its effect on stretching mode frequencies, which always decreases upon hydrogen bond formation, following well defined empirical relationships between frequency shift and donor–acceptor distance [24,30].

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

[17] [18] [19]

4. Conclusion [20]

The evolution of the Raman and infrared spectra of 5-azacytosine as a function of pressure up to 13 GPa showed reversible changes. The stability of this molecule under high pressure possibly results from its ring structure. The vibrational modes shifted to higher energies with increasing pressure, as expected, except for the NH2 and N1H stretching modes, probably due to formation of hydrogen bonds. For propanedinitrile, the appearance of a new absorption band in the infrared spectrum, around 1560 cm
1 and assigned to C¼N bond stretching, revealed the tendency of this molecule to polymerize in an irreversible way under pressure at 15 GPa at room temperature. Evidences of polymerization after 1 week at 1 GPa were also observed (darkening of the sample). In summary, it was observed that

[21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

A.Y. Liu, M.L. Cohen, Science 245 (1989) 841–842. Y. Podolyan, Y.V. Rubin, Int. J. Quant. Chem. 83 (2001) 203–212. K. Raksányi, I. Földváry, J. Fidy, L. Kittler, Biopolymers 17 (1978) 887–896. M.T. Dove, G. Farelly, A.I.M. Rae, L. Wrigths, J. Phys. C: Solid State 16 (1983) L195–L198. T. Wasiutynski, W. Olejarczyk, J. Sciesinski, W. Witko, J. Phys. C: Solid State 20 (1987) L65–L69. M.T. Dove, J. Phys.: Condens. Matter 23 (2011) 1–8. M. Krauzman, R.M. Pick, N. Le Calvé, B. Pasquier, J. Phys. 44 (1983) 849–858. M.T. Dove, A.I.M. Rae, Faraday Discuss. Chem. Soc. 69 (1980) 98–106. R. Savoier, R. Brousseau, C. Nolin, Can. J. Chem. 54 (1976) 3293–3302. A. Le Roy, A. Benkhai, R. Guerin, J. Mol. Struct. 266 (1992) 147–152. A. Fatiadi, J. Synthesis 3 (1978) 165–204. F. Freeman, Chem. Rev. 69 (1969) 591–624. A.A. Zharov, Russ. Chem. Rev. 53 (1984) 236–250. K. Hara, Rev. Phys. Chem. Jpn. 40 (1970) 73–92. G.J. Piermarini, S. Block, J.D. Barnett, R.A. Forman, J. Appl. Phys. 46 (1975) 2774–2780. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S.A.D. Dapprich Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT, 2013. V. Krishnakumar, G. Keresztury, T. Sundius, R. Ramasamy, J. Mol. Struct. 702 (2004) 9–21. S.P. Samijlenko, I.V. Alexeeva, L.H. Palchykivs'ka, I.V. Kondratyuk, A.V. Stepanyugin, A.S. Shalamay, D.M. Hovorun, J. Mol. Struct. 484 (1999) 31–38. M. Rozenberg, G. Shoham, I. Reva, R. Fausto, Spectrochim. Acta A 60 (2004) 463–470. V. Subramanian, K. Chitra, K. Venkatesh, S. Sanker, T. Ramasami, Chem. Phys. Lett. 264 (1997) 92–100. T. Lankau, C.H. Yu, Phys. Chem. Chem. Phys. 13 (2011) 12758–12769. H. Bhatt, C. Murli, N. Garg, M.N. Deo, R. Chitra, R.R. Choudhury, S.M. Sharma, Chem. Phys. Lett. 532 (2012) 57–62. V.V. Struzhkin, A.F. Goncharov, R.J. Hemley, H. Mao, Phys. Rev. Lett. 23 (1997) 4446–4449. G.C. Pimentel, C.H. Sederholm, J. Chem. Phys. 24 (1956) 639–641. G.C. Ribeiro, T.M.H. Costa, A.S. Pereira, L.A. Cassol, C.A. Perottoni, N.M. Balzaretti, Vib. Spectrosc. 57 (2011) 152–156. N. Nakamura, S. Tanisaki, K. Obatake, Phys. Lett. 34A (1971) 372–373. Y.I. Binev, J.A. Tsenov, I.N. Juchnovski, I.G. Binev, J. Molec. Struct. 625 (2003) 207–214. T. Fujiyama, T. Shimanouchi, Spectrochim. Acta 20 (1964) 829–845. S.N. Singh, R.K. Agarwal, M. Katyal, Molecular Structure – A Spectroscopy Approach, Discovery Pub. House, New Delhi, 1990. K. Nakamoto, M. Margoshes, R.E. Rundle, J. Am. Chem. Soc. 77 (1955) 6480.