A combined experimental and theoretical study on vibrational spectra of 2-acetylpyridine

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 90–95

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A combined experimental and theoretical study on vibrational spectra of 2-acetylpyridine Cuiping Zhai a,b,⇑, Fenghua Cui c, Xuejun Liu a a

Institute of Fine Chemical and Engineering, Henan University, Kaifeng 475004, Henan, China School of Chemistry and Chemical Engineering, Shandong University, Jinan 250010, Shandong, China c Department of Materials Science and Engineering, Xinxiang Vocational and Technical College, Xinxiang 453007, Henan, China b

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

 2-Acetylpyridine is an important food

additive and a flavoring substance.  Vibrational spectra of 2-

acetylpyridine have been determined and assigned for the observed bands.  The cis conformer of 2-acetylpyridine is most stable according to DFT calculations.

a r t i c l e

i n f o

Article history: Received 26 March 2014 Received in revised form 5 May 2014 Accepted 1 June 2014 Available online 24 June 2014 Keywords: 2-Acetylpyridine Vibrational spectra Molecular structure Density functional theory

a b s t r a c t The molecular geometries, FT-IR and Raman spectra of 2-acetylpyridine were studied using Density functional theory (DFT-B3LYP) with the large basis sets. Theoretical calculations indicate the cis conformer of 2-acetylpyridine is most stable though this conformation was seldom found in the crystal structures of coordinated compounds. Based on the stable conformer, comprehensive assignments of the experimental bands were made. The observed and calculated positions are found to be in good agreement with an average deviation of <4 cm1. The assignments provide valuable information for the fingerprint and identification of 2-acetylpyridine. Ó 2014 Elsevier B.V. All rights reserved.

Introduction 2-Acetylpyridine, also named as 1-(2-pyridinyl)ethanone or methyl 2-pyridyl ketone, is a ‘‘popcorn’’-like compound, and often used as the food additive and flavoring substance in tobacco, ice-cream, milk, cooked rice and other food products [1]. Generally, 2-acetylpyridine exists in a colorless to yellowish liquid and can be ⇑ Corresponding author at: Institute of Fine Chemical and Engineering, Henan University, Kaifeng 475004, Henan, China. Tel./fax: +86 0371-23881589. E-mail address: [email protected] (C. Zhai). http://dx.doi.org/10.1016/j.saa.2014.06.086 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

naturally found in rice, hazelnuts, Yahonkaoluo leaves and cocoa [1–3]. Studies have also suggested that it can be used as an intermediate to synthesize a series of chemical compounds with antiviral activity [4–6]. For example, Abid et al. [4] prepared the various oxime ether derivatives of 2-acetylpyridine, and evaluated theirs antiamoebic activities against HM1:IMSS strain of E. histolytica. Although its industrial and medical importance, the structure and vibrational spectroscopic analysis for 2-acetylpyridine are limitedly reported, and the crystal structure of pure 2-acetylpyridine molecule is still unavailable. As it bounds to the metal ion and proton, the crystal structure of 2-acetylpyridine [7–11] often

C. Zhai et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 90–95

shows a trans conformation as given in Fig. 1, and both the carbonyl O atom and ring N atom simultaneously coordinate with the metal ion. Its cis conformation, obtained by 180° rotation around the C7AC2 bond, was found in the only one entry of bis(2-acetylpyridine)-tetraphenylporphyrin-zinc(ii) [12], in which the carbonyl O atom individually interacts with the center zinc ion. Therefore, the structure of isolated 2-acetylpyridine is still unclear and deserved paying more attention. On the other hand, Medhi [13] determined and empirically assigned the IR and Raman bands of 2-acetylpyridine in 1977. Recently, Sett and coworkers [14] have assigned again these bands using empirical force fields based on pyridine ring and acetyl group approximation. However, this approximation often leads to the questionable agreement between experiment and theory in view of the different molecular conformation and the effect of substituent. For example, the substitution at the 2 position of pyridine ring strengthens and shortens the NAC(2) bond, but where there is little effect upon substitution at the 3 position [15]. More recently, Density functional theory (DFT) calculation has been proved to be a powerful tool in prediction of molecular structure and vibrational spectra due to its high computational accuracy and low time cost [16–21]. In the present work, the geometric structures of 2-acetylpyridine were optimized using Becke3–Lee–Yang–Parr (B3LYP) functional with aug-cc-pvtz basis set. The relative stability of the cis and trans conformations was examined, and compared with the reported structures determined by single-crystal X-ray diffraction. The infrared and FT-Raman spectra were experimentally recorded, and the observed bands were analyzed and assigned based on the lowest energy conformer. Also, the various thermodynamic functions such as entropy, heat capacity and energy, molecular electrostatic potential, and HOMO–LUMO analysis were provided. Experimental details Liquid 2-acetylpyridine (>97% purity) was purchased from Sigma, and used as such without further purification. FT-IR spectrum of the sample was collected in the region from 400 to 4000 cm1 at room temperature on a Bruker IFS 66 V FTIR spectrometer using liquid film technique with two KBr windows. The FT-Raman spectrum of 2-acetylpyridine was recorded on a FRA106 FT-Raman accessories using Nd:YAG laser source and with a liquid nitrogen-cooled Ge-diode detector. The sample was sealed in an NMR tube, and the number of scans is 300. Spectral resolutions of both IR and Raman spectra are 2 cm1. Computational details Density functional theory (DFT) computations for the geometric optimization and frequency calculation were performed using Gaussian 03 program [22]. The initial geometries of 2-acetylpyri-

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dine were obtained using potential energies scan (PES) at B3LYP/ 6-311++G(d,p) level. The structures and vibrational characteristics reported in this paper were further calculated at B3LYP/augcc-pvtz level recommended by Zvereva and co-workers [23]. This calculated level is the most cost-effective choice for the prediction of vibrational frequencies, relative IR intensities and Raman activities for medium-size isolated molecules [23,24]. Assignments of vibrational modes studied in this work were performed on the basis of potential energy distribution (PED) analysis obtained from Molvib program (version V7.0-G77) [25,26]. Results and discussion Conformational analysis As mentioned above, two possible conformers of 2-acetylpyridine were found in the determination of crystal structures [7– 12]. A rigid PES scan through the dihedral angles around the C2AC7 bond was firstly carried out at B3LYP/6-311++G(d,p) level, and the results give two conformers as shown in Fig. 1, corresponding to the cis and trans conformations. Further optimization without any symmetry constraint at B3LYP/aug-cc-pvtz level indicates that the energy of trans conformer is higher 25.1 kJ/mol than that of cis conformer. This result is different to the crystal studies, in which the trans conformation is the most common and often behaves as a bidentate chelating ligand to stabilize the coordination compounds. The conformer stability of pure 2-acetylpyridine can be explained by the NBO analysis. The NBO charges on N and O atoms are 0.41714 and 0.53966, 0.37787 and 0.50141 for cis and trans conformers, respectively. Considering the short distance between N and O atom in trans conformer, the more positive charge indicates a strong repulsion as both N and O atoms are electronegative. The stability of the cis conformer in relation to the trans was further verified by molecular electrostatic potential map as provided in the supporting information. The charges on N1 and O8 atoms are clearly responsible for the lower stability of the trans conformer. Geometrical parameters Due to the larger difference in energy between the cis and trans conformers, Boltzman population ratio of the trans conformer is thus very low, and it can be neglected in the liquid phase. This is greatly different to the studies of crystal structure. To the best of our knowledge, Byrn et al. [12] reported the only one case of the cis structure of 2-acetylpyridine, then its structural parameters was selected to assess the calculated result. In order to determine the accurate geometry of the cis conformer, the further calculation was carried out. We found that a planar geometry (Cs point group) for cis conformer has the lowest energy. The other geometries such as the structure with C1 point group are not preferred in energy

Fig. 1. Molecular model of 2-acetylpyridine along with numbering of atoms.

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(see the supporting information). Table 1 summarized the selected bond lengths, bond angles and dihedral angles of 2-acetylpyridine derived from X-ray diffraction and DFT calculation. In general, it is expected that the bond lengths obtained by DFT method are longer than those in the crystal structure [15,27]. More importantly, the O atom of the coordinated 2-acetylpyridine molecule in the crystal of bis(2-Acetylpyridine)-tetraphenylporphyrin-zinc(ii) interacts with zinc ion, and the whole 2-acetylpyridine molecule is closely surrounded by tetraphenylporphyrin molecules. Thus, the deviation, indicated in Table 1, in bond lengths between isolated gaseous 2-acetylpyridine molecules and that in the crystal solid is expected. For example, the biggest difference of 0.035 Å for the ring C5AC6 bond is acceptable. In addition, all of the bond angles and dihedral angles are reasonably close to the experimental values of the substituted pyridine [15,28,29]. The calculated dihedral angles indicate that the atoms on the pyridine ring stay in the same one plane, which is in agreement with the experimental result. Due to the free rotation of C2AC7 bond, value of dihedral angle N1C2C7C9 is about 8.6°, slightly larger than the calculated. In brief, the calculated geometric parameters of 2-acetylpyridine are consistence with the experimental data, and the subsequent calculation and discussion are based on these data. Analysis of vibrational spectra Although Sett and co-workers [14] have reported and assigned the experimental bands for 2-acetylpyridine, their assignment may remain some uncertainties since the substitution at the 2 position strongly effects on the ring bond parameters [15,28]. Also, Simplify of methyl group [14] may lead to the assignment uncertainty. This can be improved by normal coordinate analysis based on a high accurate DFT calculation. For this purpose, a detailed description of vibrational modes is firstly necessary. Therefore, the full set of 55 internal coordinates given in Table 2 was used to construct the local symmetry coordinates for the normal modes of 2-acetylpyridine molecule. The local symmetry coordinates, recommended by Pulay et al. [30,31], were summarized in Table 3, and used to represent the potential energy distribution (PED) to assign the observed bands. The force constant matrix obtained from the Gaussian 03 [22] output file is transformed to the force field, and then refined by means of Molvib program written by Sundius [25,26]. The 2-acetylpyridine molecule consists of 16 atoms, thus it has 42 normal vibrational modes. Since it belongs to the Cs point group, these normal modes are distributed with 28 in-plane and 14 out-of-plane vibrations. The recorded FT-infrared and Raman spectra of 2-acetylpyridine in liquid are displayed in Figs. 2 and 3, respectively. The corresponding experimental data are collected

in Table 4. The theoretical IR intensities, Raman scattering activities, unscaled and scaled vibrational wavenumbers are also presented in Table 4. The simulated IR and Raman spectra obtained from the scaled wavenumbers using a Lorentzian function with half-width at half-maximum of 10 cm1 are provided in the supporting information. Since the ab initio calculation often predicts the higher vibrational frequencies, the scaled vibrational frequencies changes with the different scaled methods [17,32]. In order to evaluate the scaled frequencies, the root mean square (RMS) value, defined by qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn cal exp 2 1 the RMS ¼ ðn1Þ Þ equation, was calculated. The i ðv i  v i RMS value of this work is about 3.3 cm1, indicating the scaled vibrational frequencies match well with the experimental data. In addition, it can also be seen in Table 4 that the assignments of most of the bands are straight, thus our attentions were paid on some important modes.

CAH group modes 2-Substituted pyridine has aryl-type mCAH stretching bands in the 3000–3100 cm1 region, and all of them are asymmetric [33]. One weak band at 3056 cm1 and a stronger Raman band at 3062 cm1 are assigned to the ring antisymmetric CAH stretching modes of 2-acetylpyridine. The weak infrared band was not previously reported by other authors [13,14]. Two aryl CAH in-plane deformation modes are observed, and their positions clearly are 1436 (IR) and 1438 (Raman) cm1, and 1148 (IR) and 1150 (Raman) cm1, respectively. The former has been ascribed to ring CC/CN stretching mode [14]. Another two bands, 1465 cm1 (IR), 1296 cm1 (IR) and 1297 cm1 (Raman) are attributed to the mixture of aryl CAH in-plane deformation and ring CC/CN stretching mode. The later was previously assigned to the mCringAC string mode [14]. Similar to the result of Sett et al. [14], the features located at 954 and 907 cm1 in the IR spectrum, 955 cm1 in the Raman spectrum are assigned to the ring CAH wagging modes, respectively. The CAH vibrational mode of the methyl group is not assigned in the Sett and co-workers’ work due to the simplicity of calculated model [14]. Two pairs of IR and Raman bands (see Table 4) in the region from 2900 to 3020 cm1 are directly assigned to the methyl group mCAH stretching mode, and this is supported by Medhi [13]. The infrared bands at 1419, 1357, and 1020 cm1 are respectively attributed to the in-plane-bending, and symmetric bending, and rocking modes. Medhi [13] has assigned the former to CAC stretching mode, but our result is more reasonable since this type band often locates within 1440–1410 cm1 region with a weak to medium intensity [33].

Table 1 The selected experimental and calculated geometric parameters for 2-acetylpyridinea.

Bond lengths (Å) R(N1C2) R(C2C3) R(C3C4) R(C4C5) R(C5C6) R(C6N1) R(C2C7) R(C7O8) R(C7C9) a b c

X-rayb

DFTc

Parameters

X-rayb

DFTc

1.353 1.356 1.408 1.380 1.356 1.309 1.501 1.207 1.512

1.337 1.395 1.386 1.389 1.391 1.331 1.509 1.214 1.506

Bond angles (°) \(N1C2C3) \(C2C3C4) \(C3C4C5) \(C4C5C6) \(C5C6N1) \(C6N2C2) \(N1C2C7) \(C2C7O8) \(C2C7C9) \(O8C7C9)

123.6 118.0 117.8 119.4 124.1 117.1 115.4 118.9 119.2 121.9

122.9 118.7 118.6 118.6 123.2 118.0 117.5 119.9 117.8 122.3

For numbering of atom see Fig. 1. Ref. [12]. B3LYP/aug-cc-pvtz level.

Dihedral angles (°) D(N1C2C3C4) D(C2C3C4C5) D(C3C4C5C6) D(C4C5C6N1) D(C5C6N1C2) D(C6N1C2C7) D(N1C2C7C3) D(N1C2C7C9) D(C2C7O8C9) D(N1C2C7O8)

X-rayb

DFTc

0.0 1.1 0.8 0.6 1.7 179.8 178.9 8.6 178.7 172.6

0.0 0.0 0.0 0.0 0.0 180.0 180.0 0.0 180.0 180.0

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Table 2 Definition of internal coordinates of 2-acetylpyridine.

Stretching 1–2 3–6 7 8 9 10–13 14–16 In-plane 17–22 23–25 26–27 28–35 36–38 39–41 Out-of-plane 42–47 48 49 50–53 54–55 a

Symbol

Type

Definitiona

Ri Ri Ri Ri Ri Qi Qi

CAN (ring) CAC (ring) CAC (aro) CAC (methyl) CAO CAH(aro) CAH (methyl)

C2AN1, N1AC6 C2AC3, C3AC4, C4AC5, C5AC6 C2AC7 C7AC9 C7AO8 C4AH11, C3AH10, C6AH13, C5AH12 C9AH14, C9AH15, C9AH16

ai bi bi bi xi xi

Ring CCC CCO CAH (aro) CAH (methyl) HCH

C6AN1AC2, N1AC2AC3, C2AC3AC4, C3AC4AC5, C4AC5AC6, C5AC6AN1 C7AC2AC3, C7AC2AN1,C2AC7AC9 C2AC7AO8, C9AC7AO8 H11AC4AC3, H11AC4AC5, H10AC3AC2, H10AC3AC4, H13AC6AN1, H13AC6AC5, H12AC5AC4, H12AC5AC6 C7AC9AH14, C7AC9AH15, C7AC9AH16 H14AC9AH15, H14AC9AH16, H15AC9AH16

si xi xi xi si

s Ring OAC CAC HAC CAC

C6AN1AC2AC3,N1AC2AC3AC4,C2AC3AC4AC5,C3AC4AC5AC6, C4AC5AC6AN1, C5AC6AN1AC2 O8AC7AC9AC2 C7AC2AC3AN1 H11AC4AC3AC5, H10AC3AC2AC4,H13AC6AC5AN1, H12AC5AC4AC6 C2(O8)AC7AC9AH14(H15,H16), C9(O8)AC7AC2AC3(N1)

Atom numberings are given in Fig. 1.

Table 3 Definition of local symmetry coordinates of 2-acetylpyridinea. No(i)

Type

Definition

1–2 3–6 7 8 9 10–13 14

CAN (ring) CAC (ring) CAC (aro) CAC (methyl) CAO CAH (aro) CH3 sym

15

CH3 asym

16

CH3 sym

R1,R2 R3, R4, R5, R6 R7 R8 R9 R10, R11, R12, R13 pffiffiffi (R14 + R15 + R16)/ 3 pffiffiffi (2R14  R15  R16)/ 6 pffiffiffi (R15  R16)/ 2

In-plane 17

bRing1

pffiffiffi (a17  a18 + a19  a20 + a21  a22)/ 6

18

bRing2

19 20–21

bRing3 bCCC

22

bCO

23

bCH (aro)

24

bCH (aro)

25

bCH (aro)

26

bCH (aro)

27

CH3 sb

28

CH3 ipb

29

CH3 opb

30

CH3 ipr

31

CH3 opr

Out-of-plane 32 sRing1 33 34 35 36 37–40 41–42 a

sRing2 sRing3 xCO xCC xCH sCC

pffiffiffiffiffiffi (a17  a18 + 2a19  a20  a21 + 2a22)/ 12 (a17  a18 + a20  a21)/2 pffiffiffi (b23  b24)/ 2, b25 pffiffiffi (b26  b27)/ 2 pffiffiffi (b28  b29)/ 2 pffiffiffi (b30  b31)/ 2 pffiffiffi (b32  b33)/ 2 pffiffiffi (b34  b35)/ 2 pffiffiffi (x36  x37  x38 + x39 + x40 + x41)/ 6 pffiffiffi (  x39  x40 + 2x41)/ 6 pffiffiffi (x39  x40)/ 2 pffiffiffi (2x36  x37  x38)/ 6 pffiffiffi (x37  x38)/ 2

Fig. 2. FT-infrared spectrum of 2-acetylpyridine.

pffiffiffi (D42  D43 + D44  D45 + D46  D47)/ 6 (D42  D44 + D45  D47)/2

pffiffiffiffiffiffi (D42 + 2D43  D44  D45 + 2D46  D47)/ 12 x48 x49 x50,x51,x52,x53 s54,s55

The internal coordinates used here are defined in Table 2.

C@O group modes Very strong bands at 1699 cm1 in both the IR and Raman spectra of 2-acetylpyridine are attributed to the mC@O stretching mode. This position is very similar to the calculated value (1703 cm1).

Fig. 3. FT-Raman spectrum of 2-acetylpyridine.

Carbonyl stretching band was observed at 1696–1670 cm1 for 2,6-diacetylpyridine [34], 1686–1696 cm1 for 2-acetylpyridine [13], 1694–1696 cm1 for 4-acetylpyridine [13,35], respectively. According to the PED analysis, the strong IR band at 592 cm1 is

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Table 4 Experimental and computed vibrational spectral parameters and PED analysis for 2-acetylpyridinea,b. No.

Observeda IR

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Raman 3062s

3056w 3007w

3011m

2921w 1699s 1584m 1568m 1465w 1436m 1419w

2925s 1699s 1585s 1570m

1357s 1296m 1282s 1238s 1148w 1101m 1043m 1020w 995w 954w

1438m

1297m 1284m 1239m 1150w 1102w 1087w 1044s

996s 955w

907w 780s 750s 741w 622w 592s

PEDd

Computed

623m 592w 493w 416w 366m 165s

DFT

c

3210 3193 3176 3152 3146 3100 3046 1755 1622 1611 1500 1468 1463 1457 1391 1322 1311 1276 1171 1124 1108 1064 1047 1036 1015 1000 959 936 803 765 762 636 605 600 484 433 409 362 209 153 114 57

IR

Raman

I

S

Scaled

2.866 11.704 7.394 14.266 14.384 3.819 0.534 178.013 10.664 16.195 4.184 10.020 9.733 13.871 49.300 11.644 30.307 89.406 0.912 20.375 1.198 7.730 0.681 0.152 6.576 0.852 35.603 0.556 36.574 12.230 0.270 2.051 9.929 34.379 0.197 1.752 1.669 0.439 9.879 0.268 0.004 6.932

105.616 185.588 83.865 101.305 78.236 43.760 177.059 49.117 83.311 7.837 1.632 10.000 4.511 5.084 1.608 5.395 7.800 29.235 4.887 6.964 7.338 24.420 0.050 0.070 28.954 0.168 5.238 0.225 0.732 0.047 10.961 5.410 0.427 1.628 1.711 0.169 0.004 3.190 0.219 1.788 0.218 0.283

3089 3073 3056 3033 3011 2968 2916 1703 1580 1569 1469 1437 1423 1417 1353 1294 1279 1243 1148 1096 1084 1037 1021 993 986 954 932 894 779 745 742 626 593 589 475 424 402 354 206 150 114 57

mCAH(93) mCAH(93) mCAH(93) mCAH(92) masCAH(98) masCAH(100) msymCAH(98) mC@O(80) mCC(51) + mCN(13) + bRing3(10) mCC(31) + mCN(14) + bRing2(10) bCH(54) + mCN(18) bCH(68) CH3 ipb (70) + CH3 opb (23) CH3 opb (66) + CH3 ipb (22) CH3 sb (83) + mCAC(11) bCH(38) + mCC(16) + mCN(14) mCN(35) + mCC(aro)(13) mCN(30) + mCC(aro)(16) + mCC(13) bCH(68) + mCC(13) mCC(29) + bCH(15) + CH3 opr (12) bRing1 (22) + bCH(19) mCC(50) + bRing1 (17) + bCH(10) CH3 ipr (44) + xCO(20) + CH3 opr (16) xCH (79) + sRing1 (12) bRing1 (46) + mCN(24) xCH (85) mCAC (35) + CH3 opr (28) xCH (85) sRing1(47) + xCC(18) + xCH(18) sRing1 (43) + xCH (39) bRing2(23) + bRing3(23) + mCAC(aro)(19) + mCAC(12) bRing2 (56) + bRing3 (28) xCO (34) + CH3 ipr (16) + sRing1 (14)+xCC (10) bCO (49) + bRing3 (20) + mCAC(14) bCCC(69) sRing2(40) + sRing3(22) + xCC(16) sRing3(60) + sRing2(15) bCCC(22) + mCAC (aro)(21) + bCO (19) + bRing3(14) bCCC(90) sRing2(36) + xCC (31) + xCO(14) sCC(70) + CH3 ipr (11) sCC(88)

a

Frequency (cm1), IR intensities, IIR (km mol1); Raman scattering activities, SRaman (A amu1). Abbreviations: s, strong; m, medium; w, weak; v, stretching; b, bending; x, wagging; s, torsion; as, asymmetric; opr, out-of-plane-rotation; ipr, in-plane-rotation; opb, out-of-plane-bending; ipb, in-palne-bending; sym, symmetric. c B3LYP/aug-cc-pvdz level. d Contributions larger than 10% are given. b

assigned to the deformation mode of carbonyl group, which is in the same position with the weak intensity in Raman spectrum and strongly disturbed by the other vibrational mode. This assignment is in agreement with the result of Sett et al. [14] and Medhi [13]. Ring modes The ring characteristic vibrational modes of mono-substituted pyridine have been summarized in literature [36]. In our work, the medium intensity IR bands at 1584 and 1568 cm1 are assigned to the ring CC stretching modes. These positions are within their normal range, and consistent with the results reported by Sett et al. [14]. In Raman spectrum, these two bands with high intensities locate the similar position. Two stronger IR 1282 and 1238 cm1 bands are mainly due to the ring CN stretching, and both of them are the typical mixed vibrational modes based on PED analysis. Their Raman bands slightly shift to higher wavenumber by 1–2 cm1. The band at 1043 cm1 in IR spectrum and 1044 cm1 in Raman spectrum are ring in-plane deformation according to the PED result, previously assigned to ring CAH

in-of-plane deformation by Sett et al. [14]. The very strong band at 985–1000 cm1 is the distinguishing band for 2- and 4-monosubtituted pyridine [33,36]. In our work, the ring breathing mode at 995 cm1 in IR spectrum and 996 cm1 in Raman spectrum were observed, and very close to the corresponding values in 4-acetylpyridine (993 cm1) [13]. In the region below 1000 cm1, one strong IR band at 780 cm1 is assigned to the ring adjacent hydrogen wag mode. This value agrees well with the calculated position (779 cm1). Another pyridine ring twisted mode appears at 416 cm1 with weak intensity in Raman spectrum. This assignment is different to the previously result of C@O in-plane deformations [13]. A weak infrared band at 622 cm1 and medium Raman band at 623 cm1 originate the mixture of two different ring deformations as shown in Table 4. However, the assignment of the weak IR band at 741 cm1 and strong Raman band at 750 cm1 is very difficult since the small observed separation. According to the calculated intensity, a reasonable result is that the band at 750 cm1 is assigned to the ring torsion mode, and the band at 741 cm1 to ring bending mode. This result is greatly different to those reported by Sett et al. [14] and Medhi [13].

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The other modes It can be seen that some observed bands list in Table 4 are contributed to the mixture of internal vibrations. For simplicity, these assignments are described approximately by the major internal vibrations. A very typical band located at 165 cm1 with the high intensity in the Raman spectrum is assigned to the ring torsional mode. Strictly speaking, this band should be associated with the ring and C@O skeletal modes for a reason of a comparable contribution of CAC and C@O torsions. Other molecular properties In the study of chemical reaction and chemical equilibrium, thermodynamic functions, electronic frontier molecular orbitals, and their energy gap are of great importance. The knowledge and understanding of these properties are very valuable for the chemist, but beyond the scope of the molecular spectroscopists. Therefore, theoretical results on these properties are provided in the supporting information. Conclusion In this work, we investigated the molecular structure and vibrational spectra of 2-acetylpyridine using B3LYP method with aug-cc-pvtz level. The result indicates that cis conformer of 2acetylpyridine is the most stable structure, and the trans conformer is less stable due to the repulsion between the oxygen atom of the carbonyl group and nitrogen atom on pyridine ring. The predicted geometric parameters are in agreement with the crystal structural data. The FT-IR and Raman bands of 2-acetylpyridine were also recorded and compared with the theoretical data. Detailed assignments were made based on PED analysis. The RMS between experimental and scaled frequencies is only 3.3 cm1, indicating the excellent assignment. Acknowledgement Financial support from the Science and Technology Development Program of Henan (144300510034, 142300410125) is gratefully acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.06.086. Reference [1] G. Jianming, Food Chem. 78 (2002) 163–166. [2] T.E. Kinlin, R. Muralidhara, A.O. Pittet, A. Sanderson, J.P. Walradt, J. Agric. Food Chem. 20 (1972) 1021–1028. [3] J.A. Maga, J. Agric. Food Chem. 29 (1981) 895–898. [4] M. Abid, K. Husain, A. Azam, Bioorg. Med. Chem. Lett. 15 (2005) 4375–4379.

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