Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1113–1120

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

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Infrared and Raman spectra, conformational stability and vibrationalassignment of 1-formylpiperazine

1386-1425/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.saa.2013.09.095

⇑ Corresponding author. Tel.: +90 (274) 265 20 31/3116; fax: +90 (274) 265 2014.

E-mail address: cemal.parlak@dpu.edu.tr (C. Parlak).

Gürkan Kes�an a, Cemal Parlak b,⇑a Institute of Physics and Biophysics, Faculty of Science, University of South Bohemia, Branišovská 31, Ceské Budejovice 370 05, Czech Republicb Department of Physics, Dumlupınar University, Kütahya 43100, Turkey

h i g h l i g h t s

� Infrared, Raman and quantumchemical calculations of 1-fp.� Normal chair form with equatorial

substituents is not preferred.� Conformational energy barrier is

independent of the solvent.� Vibrational frequencies and

intensities change when going fromnon-polar to polar solvents.

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

a r t i c l e i n f o

Article history:Received 24 June 2013Received in revised form 10 September2013Accepted 26 September 2013Available online 8 October 2013

Keywords:1-FormylpiperazineVibrational spectraDFTPED

a b s t r a c t

Infrared and Raman spectra of 1-formylpiperazine (1-fp) have been recorded in the region of 4000–50 cm�1. The conformational analysis, optimized geometric parameters, normal mode frequencies andcorresponding vibrational assignments of 1-fp (C5H10N2O) have been examined by means of Becke-3–Lee–Yang–Parr (B3LYP) density functional theory (DFT) method together with 6-31++G(d,p) basis set.Furthermore, reliable conformational investigation and vibrational assignments have been made bythe potential energy surface (PES) and potential energy distribution (PED) analyses, respectively.Calculations are carried out with the possible six conformational isomers of 1-fp, both in gas phaseand in solution using benzene and methanol. The experimental and theoretical results indicate thatB3LYP method provides satisfactory evidence for the prediction vibrational wavenumbers, and thenormal chair conformation with equatorial substituents is not preferred due to the steric interaction.

� 2013 Elsevier B.V. All rights reserved.

Introduction

1-Formylpiperazine, called in the literature by different namessuch as 1-piperazine-carboxaldehyde and 1-piperazinecarbalde-hyde, is a very versatile molecule. It has been the subject of manyscientific studies. For example, 1-fp has been used in synthesizingsome antihypertensive [1] and potential male antifertility agents[2], novel potent neuropeptide YY5 receptor antagonists [3],

compounds for dopamine D4 receptor imaging [4] and 6-nitro-quipazine analogs for serotonin transporter [5,6]. 1-fp has alsobeen employed in the researching a new class of antimalarialagents [7], ruthenium-catalyzed formylation of amines [8], kineticsand mechanisms of the reactions of S-methyl chlorothioformate[9], CO2 capture [10,11].

There are ample examples of 1-fp being used as an intermediateproduct in the literature. However, though 1-fp has wide applica-tions in many areas of science, to the best of our knowledge, thereis no any information present in the literature on vibrational andstructural studies of 1-fp. A detailed, quantum chemical study willaid in making definitive assignments to the fundamental normal

1114 G. Kes�an, C. Parlak / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1113–1120

modes and in clarifying the obtained experimental data of 1-fp.Furthermore, all data presented as theoretically and experimen-tally may be helpful in the context of the further studies. For theabove goals, we have reported vibrational spectra of 1-fp. Thevibrational frequencies with PED values, thermodynamics func-tions, highest occupied and lowest unoccupied molecular orbitals(HOMO and LUMO) and conformational analysis by PES of 1-fpare performed by the B3LYP/6-31++G(d,p) level both in gas phaseand in solution. The results of the theoretical and spectroscopicstudies are here reported.

Fig. 2. Potential energy surface of 1-fp.

Experimental

A commercially available sample of 1-fp in liquid form waspurchased from Aldrich (90%) and used without further purifica-tion. FT-MIR and FT-FIR spectra of 1-fp were recorded in the regionof 4000–400 cm�1 and 400–50 cm�1 with Bruker Optics IFS66v/sFTIR spectrometer at a resolution of 2 cm�1. Raman spectrumwas obtained using a Bruker Senterra Dispersive Ramanmicroscope spectrometer with 532 nm excitation from a 3B diodelaser having 2 cm�1 resolution in the spectral region of 4000–50 cm�1.

Computational details

All the calculations were performed using Gaussian 09.A1 pro-gram package [12] on HP DL380G7 server system and GaussView5.0.8 [13] was used for visualization of the structure and simulatedvibrational spectra. Many possible conformers could be proposedfor 1-fp; however, the discussion was here performed to 6 con-formers of 1-fp, four of them (A: a–e (axial-equatorial), B: a–a, C:e–e, D: e–a, where the former represents NH while the latterstands for formyl group) from the conformations of piperazine[14] and the other two (E and F) from PES analysis (Fig. 1). A–Dforms are considered in axial and equatorial positions accordingto plane formed by C2, C3, C5 and C6 carbon atoms of 1-fp. Forthe PES analysis, it was first investigated the rotation in6CA5CA4NA13H and 5CA6CA1NA7C (Fig. 1) dihedral anglesscanning from 80� to 180�, in 10� increments. Scan process hasconducted for 121 different conformational isomers of 1-fp and ithas been found that 1-fp is the most stable in conformer F (Fig. 2and Table 1). Fig. 2 shows PES graphics for the title molecule which

BA

ED

Fig. 1. Optimized structures o

allowed us to determine its conformational composition with ahigh accuracy.

For these calculations, 6 forms of 1-fp were first optimized byB3LYP level of theory using 6-31++G(d,p) basis set both in thegas phase and in benzene and methanol solvent environments.After the optimization, harmonic vibrational frequencies and cor-responding vibrational intensities for 6 conformers of 1-fp werecalculated by using the same method and basis set and then scaledby 0.955 (above 1800 cm�1) and 0.977 (under 1800 cm�1) [15–17].PED calculations, which show the relative contributions of theredundant internal coordinates to each normal vibrational modeof the molecule and thus make it possible to describe the characterof each mode numerically, were carried out by the VEDA 4 (Vibra-tional Energy Distribution Analysis) [18]. Calculated Raman activi-ties are converted to relative Raman intensities using therelationship derived from the intensity theory of Raman scattering[17,19,20].

Results and discussion

The results of the calculations on the molecular conformationsand geometrical parameters of 1-fp are discussed first. A brief

C

F

f six conformers of 1-fp.

Table 1Conformers and their energies in various medium of 1-fp.

Phase Conformers DE (Hartree) B3LYP/631++G(d,p) Relative energy (kcal/mol) D(C5AC6AN1AC7) D(C6AC5AN4AH13)

Gas A �381.301 5.020 179.9 79.2B �381.302 4.393 79.9 79.2C �381.302 4.393 179.9 179.2D �381.304 3.138 79.9 179.2E �381.307 1.255 129.9 79.2F �381.309 0.000 129.9 179.2

Benzene A �381.306 5.648 179.9 79.2B �381.307 5.020 79.2 79.2C �381.308 4.393 179.9 179.2D �381.309 3.765 79.9 179.2E �381.313 1.255 129.9 79.2F �381.315 0.000 129.9 179.2

Methanol A �381.312 6.275 179.2 79.2B �381.314 5.020 79.9 79.2C �381.314 5.020 179.9 179.2D �381.315 4.393 79.9 179.2E �381.320 1.255 129.9 79.2F �381.322 0.000 129.9 179.2

Table 2Some optimized geometric parameters for 1-fp in various medium.

Parameters Crystal a,b,c,d B3LYP/6-31++G(d.p)

Gas phase Benzene Methanol

A B C D E F E F E F

Bond lenghts (Å)N4AH13 1.015 1.016 1.017 1.016 1.016 1.016 1.016 1.016 1.016 1.016N1AC7 1.359a/1.340b 1.376 1.375 1.374 1.375 1.362 1.362 1.356 1.349 1.356 1.349

1.391e/1.350f

C7@O8 1.231b 1.222 1.223 1.222 1.223 1.225 1.225 1.231 1.238 1.231 1.2381.224e/1.240f

(CAN)pp 1.467c 1.466 1.470 1.471 1.471 1.461 1.464 1.463 1.465 1.466 1.467(CAC)pp 1.540c 1.534 1.541 1.525 1.534 1.539 1.530 1.539 1.538 1.531 1.530(CAH)pp 1.110c 1.097 1.096 1.098 1.097 1.097 1.098 1.095 1.096 1.091 1.097C7AH18 0.980b 1.107 1.107 1.107 1.107 1.106 1.107 1.103 1.103 1.103 1.103

1.090f

Bond angles (�)C2AN1AC7 113.8 116.3 114.0 116.3 121.5 121.5 121.9 122.4 121.9 122.3C6AN1AC7 115.9 118.8 116.1 118.7 122.8 122.8 122.6 122.3 155.5 122.2N1AC7@O8 125.3 125.5 125.2 125.6 125.6 125.6 125.7 125.8 125.7 125.8O8@C7AH18 121.9 122.0 121.9 121.9 122.1 122.1 121.8 121.5 121.8 121.5(CACAN)pp 110.4c 113.4 111.4 111.2 109.3 111.8 109.7 111.1 111.9 109.7 109.8(CANAC)pp 109.0c 114.6 112.5 113.2 111.9 114.5 113.7 114.5 114.3 113.6 113.4(HACAH)pp 109.1c 107.2 107.8 107.8 108.4 107.8 108.4 107.9 108.0 108.4 108.3

Dihedral angles (�)C2AC3AN4AH13 �77.1 �76.8 �177.0 �176.9 �77.1 �177.0 �77.0 �76.8 �177.2 �177.3C6AC5AN4AH13 79.2 79.2 179.2 179.2 79.2 179.2 79.2 79.2 179.2 179.2C3AC2AN1AC7 �177.5 �80.4 �177.6 �80.4 �128.7 �128.7 �128.7 �128.7 �128.8 �128.7C5AC6AN1AC7 142.3d 179.9 79.9 179.9 79.9 129.9 129.9 129.9 129.9 129.9 129.9C2AN1AC7@O8 14.8 �17.8 15.2 �18.4 0.9 0.4 0.9 1.0 0.2 �0.2C6AN1AC7@O8 154.7 �155.1 155.5 �155.1 177.9 178.3 177.8 177.5 178.2 178.2C2AN1AC7AH18 �179.3d �167.2 165.6 �166.9 165.1 �179.4 �179.9 �179.4 �179.2 179.9 179.9C6AN1AC7AH18 �27.3 28.4 �26.7 28.4 �2.4 �2.0 �2.6 177.5 �0.2 �2.0

a 1,4-Bis(chloroacetyl)piperazine [26].b N,N-dimethylformamide (DMF) [27].c Piperazine (pp) [28].d (+)-cis-1-Acetyl-4-(4-{[(2R,4S)-2-(2,4-dichlorophenyl)-2-(1H-imidazol-1-ylmethyl)-1,3-dioxolan-4-yl]methoxy}phenyl)piperazine [(2R,4S)-(+)-ketoconazole] [29].e Gas phase [30].f Liquid phase [31].

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discussion of the experimental and theoretical vibrational frequen-cies and intensities together with the solvent effect is thenpresented.

Geometrical structures

To clarify vibrational frequencies it is essential to examine thegeometry of the compound, as small changes in geometry can

cause substantial changes in frequencies. The conformational andrelative energies of the optimized geometries in gas phase and insolution of six forms of 1-fp with B3LYP/6-31++G(d,p) methodare given in Table 1. Regarding the calculated energies in gas phase,benzene as a non-polar solvent and methanol as a polar solvent,the F conformer is more stable than others. For these six conform-ers, the A–D forms could be neglected for calculation of equilib-rium constant since their energy differences are larger than

Table 3Vibrational frequencies (cm�1) for F and E forms of 1-fp.

Modes Experimental (Liquid) Theoretical/B3LYP/6-31++G(d, p)/gas phase

IR Raman F conformer E conformer

Assignments PEDa (P5) mb IIRc IR

c Assignments PEDa (P5) mb IIRc IR

c

m1 3326 3342 m NH (100 3376 0.74 136.99 m NH (100) 3382 0.61 104.00m2 – – m CH (94) 3001 3.94 47.54 m CH (96) 2990 4.89 58.82m3 – 2966 m CH (91) 2960 22.35 76.60 m CH (99) 2957 35.60 121.31m4 2950 – m CH (96) 2948 35.61 145.18 m CH (97) 2955 36.14 123.01m5 – 2925 m CH (97) 2945 40.06 148.34 m CH (91) 2948 33.88 119.08m6 2911 – m CH (91) 2894 43.33 124.35 m CH (96) 2900 42.54 110.33m7 – 2873 m CH (89) 2882 44.41 110.74 m CH (96) 2889 36.09 80.68m8 2862 – m CH (99) 2833 84.30 105.24 m CH (90) 2864 51.76 220.62m9 2818 – m CH (96) 2819 80.13 163.93 m CH (87) 2852 57.78 128.09m10 – 2839 m CH (96) 2804 69.28 116.39 m CH (97) 2835 75.91 87.28m11 1664 1666 m OC (81) 1714 547.31 18.80 m OC (79) + dHCO (10) 1713 549.01 19.15m12 – – d HCH (88) 1472 1.66 8.28 d HCH (83) 1463 5.21 5.34m13 – – d HCH (77) 1463 0.88 5.62 s HCCN (66) 1460 8.43 6.31m14 – – d HCH (79) 1461 18.46 3.42 d HCH (85) 1451 8.62 2.02m15 – 1453 d HCH (86) 1454 2.82 16.12 d HCH (79) 1447 16.41 7.15m16 – – d HCH (76) 1449 6.55 3.77 d HCH (89) 1445 1.32 17.38m17 1439 1410 d HCN (38) + m CN (23) 1431 98.10 11.33 d HCN (15) + m NC (35) 1427 93.05 15.24m18 1399 – d HCO (54) 1395 25.03 4.63 d HCO (78) 1394 35.55 4.01m19 – 1378 d HCO (76) 1393 8.17 2.10 d HCC (55) 1368 1.94 10.96m20 1362 – d HCC (69) 1364 7.36 1.35 d HCN (67) 1361 6.39 0.63m21 1338 1325 d HCN (64) 1340 14.02 0.89 d HCC (64) 1345 5.59 0.96m22 1318 – d HCN (66) 1318 24.04 1.83 d HCN (68) 1323 0.22 2.72m23 – 1294 d HCN (61) 1280 11.29 12.08 d HCN (77) 1315 2.02 6.94m24 1269 – m NC (34) + s HCCN (14) 1261 41.75 1.00 m NC (26) + HCN (21) 1264 48.40 1.48m25 1216 1223 d HCN (53) + m NC (11) 1205 55.73 13.19 d HCN (45) + m NC (10) 1212 62.19 7.90m26 – 1181 d HCN (76) 1197 10.24 10.87 s HCCN (70) 1188 8.73 7.72m27 1168 – m NC (22) + d HCN (10) 1162 18.85 3.95 m NC (34) + d HCC (13) 1178 39.59 10.91m28 1138 1142 m NC (68) 1131 24.66 1.78 m NC (74) 1128 33.83 0.62m29 1109 1118 d HCC (59) 1099 29.31 2.63 d HCN (51) 1110 21.30 7.37m30 1055 1065 d HCN (66) 1054 0.57 0.09 d HCN (46) 1016 5.00 0.65m31 – 1025 d HCC (38) + m NC (15) 1041 28.18 5.14 d HCN (59) 1005 27.14 1.64m32 1014 – m CC (34) + HCC (11) + d HNC 15) 1003 31.99 3.88 s HCNC (88) 988 2.33 2.68m33 926 – s HCNC (99) 989 0.76 1.95 m CC (74) 982 18.27 8.24m34 – 938 m CC (61) 911 0.17 0.52 m CC (67) 905 0.08 0.68m35 904 908 m NC (68) 872 3.94 5.10 m NC (59) 863 1.37 3.43m36 – – s HCNC (67) 839 0.12 0.68 d HCC (409) 822 0.13 1.28m37 803 – m NC (69) 792 3.02 11.24 m NC (53) 787 4.77 10.80m38 – 819 d HNC (48) + m NC (25) 779 83.48 3.53 d HNC (59) 687 110.47 2.10m39 656 670 d NCN (58) 648 1.71 1.87 s HCCN (10) + d HCN(16) 629 74.10 3.49m40 578 594 d HNC (36) 549 40.31 4.10 s HCNC (39) 569 1.61 1.60m41 476 490 d CCN (67) + s HCNC (13) 464 3.15 1.40 s HCNC (65) 470 1.50 1.76m42 438 451 s HCNC (69) 417 8.76 0.90 s HCNC (42) 410 4.65 2.44m43 410 420 s HCCN (68 388 1.59 1.51 s HCNC (60) 378 8.77 0.70m44 – 349 s CNCO (66) 317 6.90 2.20 s CNCO(62) + d CNC (14) 313 10.08 2.61m45 287 283 s HCCN (60) 292 17.88 1.10 d CNC (51) + s CNCO (13) 290 10.29 1.13m46 – 228 s CNCO (70) 246 0.46 0.85 s HNCO (77) 241 7.26 1.10m47 178 122 s CNCO (27) + d HCH (31) 203 6.39 0.41 s NCCN (72) + d CNC (12) 193 3.02 0.20m48 73 – s HCNC (90) 57 2.04 0.92 s CCNC (56) + s HCNC (35) 59 2.41 0.80

a PED data are taken from VEDA4.b Scaled frequencies with 0.955 above 1800 cm�1, 0.977 under 1800 cm�1.c IIR and IR calculated infrared and Raman intensities.

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2 kcal/mol [15,19,21,22]. Furthermore, the F form is more stablethan E by 1.255 kcal/mol. Consequently, 1-fp in the gas phaseand in solution prefers E and F forms with preference of 11% and89%, respectively.

The crystal structures of piperazine derivatives showed that thesubstituents on a piperazine ring generally occupy in the e–e anda–e position [14]. In the case of E and F forms, the NH group isfound in the axial (79.2�) and equatorial (179.2�) position by6CA5CA4NA13H, respectively. However, neither axial nor equato-rial positions for the formyl group are favored since the dihedralangles of 5CA6CA1NA7C are calculated as 129.9� (Tables 1 and2). This is due to steric effects of substituents on the piperazine.

Some of the optimized geometric parameters (bond lengths,bond and dihedral angles) calculated by B3LYP/6-31++G(d,p) arelisted in Table 2. To the best of our knowledge, the experimentaldata on geometric structure of 1-fp is not available in the literature.

Therefore, the theoretical results have been compared togetherwith the data of related parts of some molecules such as pipera-zine, 1,4-bis(chloroacetyl)piperazine or N,N-dimethylformamide(DMF) [23–28] as given in Table 2. Generally, the calculated bondlengths, bond and dihedral angles for 1-fp are in good agreementwith previously reported experimental data for its fragments[23–28]. The mean absolute deviation or similar investigation ofthese parameters is not performed as there is too few experimentaldata for comparison.

The carbonyl bond length varies upon the physical state of DMFas much 0.01 Å. Further, the bond length of CAN connected next tocarbonyl group decrease upon the states. The bond lengths CAH offormyl group also decrease on the states [24,27,28]. Theoreticallycalculated values for these parameters of 1-fp are consistent withpreviously reported structural data for the related medium(Table 2). The lengthening of C@O, shortening of both CAN and

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CAH bond distances in the condensed phase indicate the strongintermolecular interaction. For the present theoretical data, thesame features are observed but not as much as experimentally re-ported structural. The bond lengths of CAH in crystal are notice-ably shorter than liquid or gas partly because the distance ofhydrogen atom inherently comes shorter through the crystalpacking.

Several thermodynamics parameters (capacity, zero point en-ergy, entropy, etc.) calculated by B3LYP/6-31++G(d,p) for E and Fforms are presented in Table S1. The variation in the zero pointvibrational energy seems to be insignificant. The total energy andchange in total entropy of 1-fp are at room temperature. The dipolemoment is expected to be larger in solution than the correspondingdipole moment in the gas phase. This situation is clearly observedin Table S1. The dipole moment increases gradually from a lower toa higher dielectric and the increases going from gas to non-polar/polar solvents are about 18%/33% (F) and 19%/36% (E).

Vibrational studies of 1-fp

To the best of our knowledge, the vibrational assignments for 1-fp in the middle and far infrared regions of the spectrum have notbeen reported in the literature. All experimental and theoretical

Fig. 3. Experimental (a – liquid phase) and theoretical (b and c: E and F conformer

vibrational frequencies for the E and F conformers of 1-fp, alongwith corresponding vibrational assignments and intensities, are gi-ven in Table 3 and Figs. 3–5 (see also in Tables S2–S3 and Fig. S1).All calculated frequency values presented in this paper are ob-tained within the harmonic approximation. This allows us to de-scribe vibrational motion in terms of independent vibrationalmodes, each of which is governed by a simple one-dimensionalharmonic potential. The 1-fp molecule consists of 18 atoms, having48 normal vibrational modes, and it belongs to the point group C1

with only identity (E) symmetry element or operation. It is verydifficult to determine the vibrational assignments of 1-fp due toits low symmetry. Therefore, the assignments of vibrational modesof the forms investigated for 1-fp have been provided by VEDA 4.According to the present calculations, 5 normal vibrational modesof 1-fp are below 400 cm�1 while 43 modes are between4000 cm�1 and 400 cm�1. The following are some of the importantvibrational motions observed.

NH stretchingNH stretching bands (m1) of 1-fp are attributed to piperazine

group. The free piperazine for the NH equatorial (lone pair axial)conformation has a main NH band at 3351 cm�1 with a shoulderat 3314 cm�1 in the IR spectrum [29]. NH bands of 1-fp are

s – gas phase) IR spectra for 1-fp. (�indicated shoulder band of NH stretching).

Fig. 4. Experimental (a – liquid phase) and theoretical (b and c: E and F conformers – gas phase) Raman spectra for 1-fp. (*indicated shoulder band of NH stretching).

1118 G. Kes�an, C. Parlak / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1113–1120

measured at 3326 cm�1 and 3342 cm�1 with a shoulder at3244 cm�1 and 3313 cm�1 in the IR and Raman spectra, respec-tively (Table 3 and Figs. 3 and 4). The NH stretching fundamentalsare consistent with those previously reported for piperazine. TheNH modes for gas phase have been calculated as 3382 cm�1 (E)and 3376 cm�1 (F).

CH stretchingThe piperazines exhibit intense perturbed CH bands. This is ex-

pected since the substituents are strong electron donors, and allCH2 groups are adjacent to a N atom. The CH absorption of piper-azine is complicated by the existence of conformers with axial orequatorial combinations. The CH2 vibration bands of piperazinewere assigned as follows: asymmetric mode: 2944 cm�1 (normal),2918 cm�1 and 2911 cm�1 (perturbed), symmetric mode:2855 cm�1 (normal), 2825 cm�1 and 2812 cm�1 (perturbed) [29].Similar frequencies are observed for the present molecule. CH2

asymmetric modes (m3–6) of 1-fp are reported at 2966 cm�1 (R),2950 cm�1 (IR), 2925 cm�1 (R), 2911 cm�1 (IR). The theoreticallycalculated values for this mode are between 2960 and2894 cm�1. CH2 symmetric modes (m9–10) of 1-fp are reported at2818 cm�1 (IR) and 2839 cm�1 (R). They are calculated as2804 cm�1 and 2819 cm�1. Furthermore, CH stretching modes(m7–8) due to the formyl group are observed at 2873 cm�1 (R) and2862 cm�1 (IR) while theoretical values are shown at 2882 cm�1

and 2833 cm�1. They are also consistent with those previously re-ported for DMF [30], where these modes were measured at2856 cm�1 (IR) and 2857 cm�1 (R). In the high wavenumber regionof the spectra, the anharmonicity can explain substantialdifferences between the experimental and calculated values.Alternatively, these differences may be due to intermolecularinteractions or to the laser used for Raman.

C@O stretchingThe characteristic feature of an amide species is the amide I

band, which is mainly attributed to C@O stretching and associatedwith CN stretching and weakly with CH in-plane bending. Stretch-ing frequencies (m11) of C@O are observed at 1664 cm�1 in IR and at1666 cm�1 in Raman spectrum. The C@O stretching fundamentalsare consistent with those previously reported for DMF [30], wherethese modes have been observed in the range 1659–1677 cm�1

(IR). For calculations (E and F), they show at 1713 cm�1 and1714 cm�1, respectively.

CN stretchingStretching modes for CN bonds of 1-fp are assigned as follows:

17, 24, 25, 27, 28, 31, 35, 37 and 38. The mode 17 is measured at1439 cm�1 (IR) and 1410 cm�1 (R) while the others are observedin the range 1269–819 cm�1. Similar data (1431 cm�1 and 1261–779 cm�1) have been shown in calculations. The CN stretching

Fig. 5. Theoretical IR (a) and Raman (b) spectra for F form of 1-fp in various medium.

G. Kes�an, C. Parlak / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1113–1120 1119

mode in IR spectra of piperazine were reported in the range 1136–833 cm�1 [31] whereas it was observed at 1388 cm�1 (R) and1386 cm�1 (IR) for the formyl group of DMF [30]. Hence, the CNstretching frequencies observed in this study are also inagreement with the data for similar structures.

Other vibrationsThe HCH, HCN, HCO, HCC bending modes dominate the regions

of 1400–1000 cm�1 while the CC stretching, HCN, HCC or CNCbending and HCNC, CNCO, HNCO, NCCN, HCCN or CCNC torsionmodes are seen in the low frequency region (1000–70 cm�1). Sim-ilar situations have been shown in calculations. Vibrational modesin the low wavenumber region of the spectrum contain contribu-tions of several internal coordinates and their assignment is areduction approximation to one of two of the internal coordinates.

Solvent effect on frequencies and intensitiesAs can be seen from Table 3 and Fig. 5 (see also in Tables S2–S3,

Fig. S1), if the vibrational assignments are investigated one-by-one,the assignments in various medium are generally consistent withone another. As the presence of dielectric medium has a stronginfluence on the vibrational frequencies, there are significant

changes in the presented theoretical vibrational values. Someimportant vibrational motions are here described. The carbonylbond lengths increase on going from the gas phase to the solventphase. Therefore, the C@O stretching frequencies should decrease.It is clearly observed in all tables and figures that these require-ments are substantially fulfilled for 1-fp. These frequencies shiftsare explained in terms of increased positive character on oxygenatom in solvents of high dielectric constant [15]. However, thereis no any changes NH stretching frequencies except for theirintensities.

Regarding the calculated fundamentals, the computed vibra-tional intensities in the gas phase are in reasonable agreementwith the experimental results in both high and low frequency re-gions. It is important to note that calculations have been performedfor a single molecule in the gaseous state, contrary to the experi-mental values recorded in the presence of intermolecular interac-tions. IR intensities are expected to dramatically change whenthe solute is solvated and this is indeed the case in our presentstudy. As can be seen from Table 3, Tables S2–S3, Fig. 5 andFig. S1, the noticeable changes are shown in many modes andthe calculated intensities in solutions are very high when com-pared to those in the gas phase for most cases. Like IR intensities,

1120 G. Kes�an, C. Parlak / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1113–1120

significant changes in Raman intensities are seen when the mole-cule is solvated. For the IR and Raman intensities, the increasesin methanol are generally larger than in benzene. On the otherhand, frontier molecular orbital energy and energy gap of all con-formations in various medium is also given as Supplementarymaterial (Table S4 and Fig. S2).

Conclusion

The experimental and theoretical vibrational investigations of1-fp are successfully performed by using FT-IR, Raman and quan-tum chemical calculations. In conclusion, the following resultscan be summarized:

1. Results of energy calculations for gas phase and solvations showthat the F form is the most stable isomer of 1-fp and the confor-mational energy barrier is independent of the solvent. However,the minimum energies of the structures decrease, or 1-fp tendsto be more stable, as the polarity of the solvents increases.

2. The following (IR/R) mean absolute deviations (MAD) betweenthe experimental and calculated frequencies of F form are foundfor gas phase, benzene and methanol: 15.97/24.52, 14.07/23.59,14.38/23.66 cm�1. Also, the correlation values (IR/R) are foundto be 0.99935/0.99895, 0.99955/0.99908, 0.99949/0.99910,respectively. It can be seen that the B3LYP/6-31++G(d,p) calcu-lation is reliable and makes the understanding of vibrationalspectra of 1-fp easier.

3. Some significant changes are found in the geometric parame-ters when 1-ha in solvated. From lower to higher dielectric,the dipole moment increases. In general, the frequency differ-ences increase when going from non-polar to polar solvents.Also, solvent effects on vibrational intensities are also consider-able and they increase as one goes from lower to higherdielectric constant in the most cases.

4. Any differences observed between the experimental and calcu-lated values may be due to the fact that the calculations havebeen performed for a single molecule in the gas and solvationsstates, whereas the experimental values in the liquid phasehave been recorded in the presence of intermolecularinteractions.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.saa.2013.09.095.

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