Theoretical and experimental vibrational spectroscopic study of 3-piperidino-propylamine

Journal of Molecular Structure 923 (2009) 120–126

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Theoretical and experimental vibrational spectroscopic study of 3-piperidino-propylamine Özgür Alver a,b,*, Cemal Parlak a,b, Mustafa Sßenyel a a b

Department of Physics, Science Faculty, Anadolu University, 26470 Eskisßehir, Turkey Plant, Drug and Scientific Research Centre, Anadolu University, 26470 Eskisßehir, Turkey

a r t i c l e

i n f o

Article history: Received 15 May 2008 Received in revised form 11 February 2009 Accepted 13 February 2009 Available online 24 February 2009 Keywords: 3-Piperidino-propylamine IR and Raman spectra HF DFT

a b s t r a c t FT-IR and Raman spectra and the vibrational spectral assignments of 3-piperidino-propylamine (3-pipa) have been reported in the region of 4000–400 cm1 for the first time. The molecular geometry (bond lengths, bond angles and dihedral angles) and vibrational frequencies of 3-pipa have been calculated in the ground state by using the Hartree–Fock and density functional methods (BLYP and B3LYP) with the 6-31G(d) basis set. Comparison of the observed and the calculated wavenumbers of 3-pipa indicates that B3LYP is superior to the scaled BLYP and Hartree–Fock approach for predicting vibrational wavenumbers and the Hartree–Fock method seems very good at explaining NH2 and CH2 antisymmetric and symmetric vibrations in the region 3400–2800 cm1. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction 3-Piperidino-propylamine molecule is an organic compound with the molecular formula C8H18N2. Piperidine is a structural element of many pharmaceutical drugs such as raloxifene and minoxidil [1]. The vibrational modes of piperidine and its derivatives and propylamine have been studied by several workers [1–4]. However, no vibrational assignment has been performed for 3-pipa molecule up till now. Density functional theory (DFT) calculations have provided excellent agreement with experimental vibrational frequencies of organic compounds, if the calculated frequencies are scaled to compensate for the approximate treatment of electron correlation, basis set deficiencies and anharmonicity [5–14]. It is known that piperidine molecule exists in two possible chair conformations, which differ in the axial or equatorial positions of the N–H group. The gas phase electron diffraction experiments indicate that the equatorial form of piperidine is the most stable one [15,16]. Propylamine has five possible conformers. These are TT, TG, GT, GG1 and GG2 [2]. T and G letters refer to trans and gauche, respectively. At ambient temperature, the amount of TT and TG conformers are much more than the other three conformers [2].

* Corresponding author. Address: Department of Physics, Science Faculty, Anadolu University, 26470 Eskisßehir, Turkey. Tel.: +90 222 335 2952; fax: +90 222 335 0127. E-mail address: [email protected] (Ö. Alver). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.02.012

In this work, we have reported infrared and Raman spectra of 3-pipa molecule. We have also calculated the vibrational frequencies, bond lengths, bond angles and dihedral angles of the title molecule by using HF and DFT (B3LYP, BLYP) methods in the ground state. 2. Experimental details A commercial sample of 3-pipa was purchased and used without further purification. IR and Raman spectra of 3-pipa have been recorded in the region of 4000–400 cm1 via Perkin–Elmer FT-IR 2000 spectrometer at a resolution of 4 cm1 and Bruker Senterra Dispersive Raman Microscope using the 532 nm line of a 3B diode laser, respectively. 3. Computational details For the vibrational calculations, molecular structure of 3-pipa was optimized by HF, BLYP and B3LYP with the 6-31G(d) basis set while N(5)–C(7) in equatorial position and propylamine in TG form (Fig. 1). By using the same methods and the basis set the vibrational frequencies of 3-pipa were calculated, and then scaled by 0.8929 [9–14], 0.9940 [11–14] and 0.9613 [11–14] (HF, BLYP and B3LYP, respectively). Optimized geometry has been used to calculate all parameters of 3-pipa. All the calculations were performed by using Gaussian 03 program [17] on a personal computer and GaussView program [18] was used for visualization of the structure.

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4. Geometrical structures The optimized geometric parameters calculated by HF, BLYP and B3LYP with the 6-31G(d) as the basis set are listed in Table 1. To the best of our knowledge, experimental data on the geometric structure of 3-pipa is not available in the literature. However, Vayner and Ball [19] presented some bond distances, bond angles and dihedral angles for piperidine molecule. Therefore, we could only compare the calculation results with piperidine molecule as given in Table 1. The calculated structural parameters for piperidine in 3-pipa do not differ much from Fig. 1. The optimized molecular structure of 3-pipa at used methods. Table 1 The optimized geometric parameters of 3-pipa calculated by HF, BLYP and B3LYP with 6-31G(d) basis set. a

Parameters

Experimental piperidine

b

Theoretical piperidine

c

Theoretical propylamine

HF 6-31G(d)

BLYP 6-31G(d)

B3LYP 6-31G(d)

1.457 1.457 1.526 1.527 1.528 1.455 1.528 1.526 1.458 1.095 1.086 1.088 1.084 1.086 1.092 1.005 1.005 1.089d 1.085 1.097 1.087 1.087 1.087 1.089 1.087 1.087 1.082 1.097

1.483 1.483 1.544 1.544 1.545 1.481 1.545 1.542 1.486 1.118 1.106 1.107 1.104 1.106 1.114 1.032 1.033 1.108d 1.105 1.119 1.105 1.106 1.105 1.108 1.105 1.106 1.108 1.120

1.468 1.468 1.532 1.532 1.533 1.466 1.533 1.531 1.471 1.110 1.098 1.099 1.096 1.099 1.106 1.022 1.023 1.100d 1.097 1.111 1.098 1.098 1.097 1.100 1.098 1.098 1.094 1.111

112.9 110.7

106.8e 106.9 107.3 106.9 107.3 106.9 106.4 107.0 106.7 106.1 110.9 111.0 111.1 109.8 111.4 111.4 112.1 110.6

106.6e 106.8 107.3 106.8 107.3 106.9 106.3 106.9 106.4 104.5 110.4 110.9 111.0 110.0 111.4 111.4 112.4 110.6

106.7e 106.8 107.3 106.8 107.3 106.9 106.3 106.9 106.4 105.2 110.6 110.9 111.0 109.9 111.5 111.4 112.3 110.7

181.5 66.0 175.9

177.7 65.8 177.7

176.4 67.3 180.0

176.5 66.4 179.3

422.061965 1.5861

424.709142 1.4906

424.957396 1.4957

0

Bond lengths (Å A) N5–C4 N5–C6 C4–C3 C6–C1 C1–C2 N5–C7 C7–C8 C8–C9 C9–N10 C7–H21 C7–H22 C8–H23 C8–H24 C9–H25 C9–H26 N10–H27 N10–H28 \C—H ðringÞ C4–H17 C4–H18 C3–H15 C3–H16 C2–H13 C2–H14 C1–H11 C1–H12 C6–H19 C6–H20 Bond angles (°) \H—C—H ðringÞ H17–C4–H18 H15–C3–H16 H13–C2–H14 H11–C1–H12 H19–C6–H20 H21–C7–H22 H23–C8–H24 H25–C9–H26 H27–N10–H28 C4–N5–C6 C4–C3–C2 C6–C1–C2 C1–C2–C3 N5–C4–C3 N5–C6–C1 C7–C8–C9 C8–C9–N10

1.469

1.464

1.530

1.532

1.530

1.536 1.528 1.525 1.456 1.085 1.089 1.086 1.093 1.002 1.002

1.098

1.100

110.0

107.1

110.7 109.6

112.0 110.6

111.1 110.5

110.7 109.5

Dihedral angles (°) C7–C8–C9–N10 C8–C9–N10–H28 C8–C9–N10–H27 Optimized energy (au) Dipole moment (D) a b c d e

Taken from Ref. [3]. Taken from Ref. [1]. Taken from Ref. [2]. Average value of C–H bond lengths for piperidine. Average value of H–C–H bond angles for piperidine.

251.9043717 0.9071

172.323242994

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Ö. Alver et al. / Journal of Molecular Structure 923 (2009) 120–126

the previously reported data (Table 1). Many possible conformers could be suggested for 3-pipa molecule such as Eq-TT, Ax-TT, Eq-TG, Ax-TG, Eq-GT, Ax-GT, Eq-GG1, Ax-GG1, Eq-GG2, Ax-GG2. Eq and Ax letters refer to equatorial and axial, respectively. If compared to its equatorial forms, relative stabilities of the axial conformers of 3-pipa are bigger than 2 kcal/mol. Therefore, they can be ignored. All the equatorial forms are comparable regarding their sum of electronic and thermal free energies. Here, we confined our discussion for the equatorial piperidine conformer and TG propylamine conformer. Dihedral angle D(1;6;5;7) or D(3;4;5;7) for the optimized 3-pipa structure is 176.3° which indicates that N(5)–C(7) bond almost oriented in equatorial position which is the lowest energy configuration for piperidine. The optimized structural parameters for five possible conformations of n-propylamine were calculated by Durig et al. [2]. TT and TG conformers for n-propylamine are the most favorable ones. The optimized structural parameters in present study for n-propylamine in 3-pipa are chosen similar to the TG conformer of propylamine. By choosing

the minimal energy cases for both n-propylamine and piperidine fragments of 3-pipa, we tried to optimize the title molecule. Theoretically calculated structural data for 3-pipa are consistent with its fragments’ (piperidine and propylamine) previously reported structural data (Table 1) [1–2]. Furthermore, experimentally obtained vibrational frequencies are in good agreement with the theoretical results (Table 2). Henceforth, selected equatorial and TG conformations for piperidine and propylamine seem reasonable. 5. Results and discussion 3-Pipa molecule belongs to the Cs point group. It has 78 vibrational modes distributed as 53A0 + 25A00 . It has also 2A0 + A00 translational and A0 + 2A0 0 rotational motion distribution. The infrared and Raman spectra of the liquid 3-pipa molecule are given in Figs. 2 and 3. According to the calculations, 10 normal vibrational modes of the title molecule are below 400 cm1. All the experimental and theoretical vibrational

Table 2 The measured IR and Raman wavenumbers (cm1) for 3-pipa molecule together with the data for piperidine and propylamine molecules. Assignmenta

Assignmentc

Piperidine IR

a

R

b

Propylaminec IR

3-Pipa R

IR

R

3364 s, br

3363 w

3287 s, br 2933 vs

3308 m 2922 vs

vs s s vw s, br

2846 2792 2757 2729 –

A0

3341 s, br

3317 w

NH2 a-str

A0 A0

3309 s, br 2931 s, br

NH2 s-str CH2 a-str

A0 + A0 0 A0 0 A0

2850 vs 2803 vs 2730 s

3284 2962 2946 2915 2804 2790

3316 3255 3192 2929

CH2 a-str CH2 s-str CH2 s-str

2897 s, sh 2854 s, sh 2832 m, sh

2899 vs – –

–

–

–

NH2 def

–

A0 0 A0 A0 A0

1468 1444 1386 1365

s s, sh m m

1461 vw – – –

CH2 CH2 CH2 CH2

1470 s 1421 s 1368 w, br –

1468 1442 1376 1350

s vs vs vs

– 1438 s 1385 w –

A0 A0 0

m, sh s s

– –

1325 w 1302 vs

1339 w –

CH2 twist – – – –

1294 vs – – – –

1293 m – – – –

1270 1258 1190 1154 1124

1281 1260 – 1144 1112

– A0 A0 A0 0 – – A0

– 1035 s 1006 m 964 m 917 w, br – 859 s

1345 vw 1312 vw 1298 vw – – 1178 vw – 1113 vw 1103 vw – – – – – – 865 w

– –

– A0 0 A0 0 A0 /A0 0 A0 0

1346 1332 1318 – 1258 1191 1164 1115

1616 1605 1473 1421 1366 1357 1349 – –

2853 2801 2762 2736 1600

CN str CCC a–str – – – CCC s–str CH2 rock

1056 vw 1030 m – 961 vw – – 848 m

820 vw

–

815 m

–

– A0 A0 –

822 810 – 743 546 –

1075 w 1026 m – – – – 860 m 856 m –

1070 m 1040 vs 995 m 963 m 909 w 884 s, sh 861 s

A0 /A0 0

1075 m, sh 1022 vs – – – 877 s 858 m 854 m –

– – – –

NH2 wag CH2 rock – CCN bend

– – – 476 m 451 w

782 753 588 497 461

773 739 572 484 449

A0

432 m

423 vw

–

765 747 – 473 455 447 –

–

A0

404 w

–

–

–

424 m 415 vw 403 w

m m m/1146 s vs

m/ m,sh vs s

w sh m vw w vw

def def wag twist + CH2 wag

vs: very strong, s: strong, m: medium, w: weak, vw: very weak, br: broad, sh: shoulder. a Taken from Ref. [3]. b Taken from Ref. [4]. c Taken from Ref. [2].

vs s s vs

vs vs vs w s m w

vs vs vs s, sh vs

3308 3248 3186 2930

–

vs m, br m s, sh

s s vw vs vs

m m w m w

s w m vw

s w w vw

m w w vw w

410 m –

Ö. Alver et al. / Journal of Molecular Structure 923 (2009) 120–126

123

Fig. 2. FT-IR spectrum of 3-pipa in the region 4000–400 cm1.

Fig. 3. Raman spectrum of 3-pipa in the region 4000–400 cm1.

modes obtained in this study have been given in Tables 1 and 3. The assignments of the vibrational modes of the title molecule are provided by animation option of the GaussView package program for the B3LYP/6-31G(d) level of calculation. Using the animation we have identified the vibrational motions of the studied molecule. Regarding our calculations and previously reported infrared and Raman spectra [1–4], we provided a reliable one-to-one correspondence between our fundamentals and calculated frequencies with HF, BLYP and B3LYP.

NH2 antisymmetric and symmetric stretching fundamentals of 3-pipa have been observed at 3364 cm1 (IR), 3363 cm1 (R) and 3287 cm1 (IR), 3308 cm1 (R), respectively (Table 3). NH2 antisymmetric and symmetric modes have been calculated at 3342 cm1, 3324 cm1, 3341 cm1 and 3274 cm1, 3247 cm1, 3268 cm1 for HF, BLYP and B3LYP, respectively. These are usual range of appearance for NH2 stretching vibrations. As in the case reported by Kurt et al. [20], in the high frequency region, calculated HF values are more reliable than B3LYP and BLYP values for the present study.

Mode

m49 m48 m47 m46 m45 m44 m43 m42 m41 m40 m39 m38 m37 m36 m35 m34 m33 m32 m31 m30 m29 m28 m27

NH2 a-str NH2 s-str C(6;8)–H2 a-str C(6;7;8)–H2 a-str C(1;2;3)–H2 a-str C(1;3)–H2 a-str C(1;2;3)–H2 a-str C(2;4;7)–H2 a-str C(7;8;9)–H2 a-str C(4;7;8)–H2 a-str C(2)–H2-a-str + C(1;3)–H2 s-str C(1;3;4)–H2-s-str C(8;9)–H2-s-str C(2)–H2-s-str + C(1;3)–H2-a-str C(9)–H2-s-str C(4;6;7)–H2-s-str C(4;6;7)–H2-s-str C(4;6)–H2-s-str NH2-sciss C(7;8;9)–H2-def C(4;6;7;9)–H2-def C(2;6;7;8;9)–H2-def C(4;6;8)–H2-def C(1;2;3;6;8)–H2-def C(4;7;8)–H2-def C(1;2;3)–H2-def C(1;3)–H2-def C(4;6;7;8;9)–H2-wag C(8)C(9)N(10) a-str + NH2-twist + C(4;6;7;8;9)– H2-wag C(6;7;8;9)–H2-wag + C(8)C(7)N(5)-a-str C(4;6)–H2-twist + C(1;3)–H2-wag + C(1)C(2)C(3)s-str C(2;4;6;7)–H2-wag + ring str CCH-bend (ring) + ring str CH2-wag (ring) C(7;8;9)–H2-twist + NH2-twist C(7;8;9)–H2-twist + C(4)–H2-twist + NH2-twist CH2-twist (ring + chain) C(7;8;9)–H2-twist + NH2-twist C(2;4;6)–H2-twist C(1;3;8)–H2-twist + NH2-twist C(1;3;4;8)–H2-twist + NH2-twist C(1;2;3)–H2-twist + ring str + C(7;8)–H2-wag C(7;8;9)–H2-twist + NH2-twist + opp ring bend Opp ring bend + CH2 twist (ring) C(5)N(7)-str + ring bend + NH2-twist Ring-str + C(7)C(8)C(9)-s-str + CH2-twist (ring) N(5)C(7)-str + ring def N(5)C(7)-str + ring-str+C(7)-H2-twist C(8)C(9)N(10)-a-str + C(7)C(8)C(9)-a-str + ring str C(9)N(10)-str + ring str Ring-str C(7)C(8)-str + ring torsion

Exp. IRb

Exp. Ramanb

HF/6-31G(d)

BLYP/6-31G(d)

B3LYP/6-31G(d)

Scaled Freq.

Calc. IIR

Calc. IR

Scaled Freq.

Calc. IIR

Calc. IR

Scaled Freq.

Calc. IIR

Calc. IR

3344 3274 2930 2907 2895 2892 2890 2886 2884 2874 2856 2854 2853 2840 2800 2775 2763 2757 1620 1490 1486 1478 1471 1465 1460 1457 1448 1424 1411

6.3 3.9 77.7 123.2 82.8 108.3 62.5 105.5 3.6 70.8 22.1 24.8 50.1 42.0 108.1 207.0 56.5 22.8 48.0 1.8 1.9 1.0 1.5 5.0 1.1 4.3 0.7 23.4 35.4

131.7 188.8 187.0 23.3 205.8 276.3 110.3 241.0 61.3 122.1 193.6 290.8 80.4 156.8 151.7 296.7 81.4 17.2 11.7 17.8 4.1 7.0 19.6 9.3 33.0 23.7 35.3 4.6 7.2

3348 3263 2996 2983 2977 2972 2951 2948 2940 2936 2933 2932 2930 2918 2806 2799 2785 2779 1614 1498 1489 1478 1472 1470 1462 1451 1454 1397 1393

0.1 0.6 68.7 72.8 99.7 36.8 65.9 90.5 65.8 13.8 31.4 31.0 15.0 35.3 113.3 240.7 67.3 39.1 28.1 0.8 2.9 1.1 1.1 3.9 3.1 5.3 0.8 4.8 31.0

178.2 275.0 204.0 44.3 159.1 156.7 405.8 212.8 54.6 127.5 217.6 113.9 213.8 223.5 197.8 354.6 93.6 45.7 21.0 13.4 5.4 6.1 67.3 14.5 24.6 23.2 31.6 7.7 9.2

3342 3268 2981 2968 2962 2957 2950 2945 2930 2918 2917 2915 2912 2903 2832 2796 2784 2779 1611 1486 1478 1467 1461 1459 1453 1444 1443 1396 1388

1.3 0.4 62.5 70.9 89.4 34.9 60.6 84.0 60.0 13.8 29.2 29.4 17.3 34.4 104.6 332.2 60.4 35.1 35.7 1.4 3.4 1.3 0.5 5.4 3.2 6.3 1.3 13.7 26.4

158.6 238.9 186.4 43.7 150 145.4 361.6 206.3 66.2 119.4 216.2 107.3 197 210.3 183.8 0.2 87.3 37.1 17.0 15.4 3.8 3.3 51.2 17.3 24.0 22.6 32.0 7.4 8.2

3364 s,br 3287 s,br

3363 w 3308 m

2933 vs

2922 vs

2853 vs

2846 s

2801 s 2762 s

2792 w 2757 m

1600 s,br

–

1468 s

–

1442 vs

1438 s

1376 vs

1385 w

1400 1369

16.8 14.0

1.4 3.9

1374 1353

19.3 6.8

6.5 10.8

1373 1351

17.7 3.2

4.1 7.7

1350 vs

–

1325 w 1302 vs

1339 w –

1270 s 1258 s

1281 s 1260 w

1190 w

–

1154 vs

1144 w

1124 vs

1112 vw

1070 m 1040 vs

1056 vw 1030 m

995 m

–

1367 1363 1332 1325 1306 1292 1280 1267 1262 1249 1209 1165 1151 1144 1128 1110 1082 1046 1031 1014 1006

4.9 1.7 1.4 18.2 8.2 5.2 8.5 13.2 2.4 12.5 30.8 13.3 27.4 5.8 36.0 36.0 13.2 19.1 1.1 12.5 3.9

1.0 2.7 0.7 0.7 17.0 68.9 4.5 6.7 29.0 10.1 5.7 10.0 11.0 5.2 7.4 9.0 7.5 24.7 8.0 17.8 5.3

1348 1346 1324 1315 1296 1286 1274 1263 1256 1250 1199 1161 1146 1138 1117 1087 1059 1046 1028 1012 1007

21.3 7.1 3.8 4.0 11.1 3.8 7.1 12.8 9.6 7.9 20.8 7.3 4.6 15.6 18.0 42.3 23.2 9.1 3.4 12.8 1.2

14.8 8.0 2.5 6.4 32.0 63.0 4.2 32.5 9.1 7.5 3.4 9.1 9.5 5.4 2.9 12.8 3.1 24.5 4.9 13.2 4.1

1348 1345 1319 1310 1293 1282 1270 1258 1253 1245 1199 1158 1141 1139 1116 1100 1074 1047 1034 1013 1012

20.1 4.6 2.5 8.9 11.0 3.8 6.0 13.1 5.5 11.6 23.1 8.7 4.4 26.1 22.9 31.2 23.2 9.7 0.4 11.3 3.2

8.4 5.6 1.5 3.7 19.7 72.0 3.0 30.7 7.1 8.3 3.2 8.3 7.4 8.6 3.1 9.5 5.2 22.5 4.8 12.3 5.2

Ö. Alver et al. / Journal of Molecular Structure 923 (2009) 120–126

m78 m77 m76 m75 m74 m73 m72 m71 m70 m69 m68 m67 m66 m65 m64 m63 m62 m61 m60 m59 m58 m57 m56 m55 m54 m53 m52 m51 m50

Approximate mode descriptionsa

124

Table 3 Comparison of the experimental and the calculated vibrational wavenumbers (cm1) of 3-pipa.

410 m – 424 m/415 vw 403 w

m w w vw/449 w 773 739 572 484 782 753 588 497

m m w m/461 w

– 815 m

IIR and IR: Infrared intensities (km/mol), Raman scattering activities (Å4 amu1)]. Exp.: experimental, Freq.: frequency; Calc.: calculated, v: very, s: strong, m: medium, w: weak, vw: very weak, sh: shoulder, br: broad. Str: stretching, bend: bending, def: deformation; sciss: scissoring, twist: twisting, wag: wagging, s: symmetric, a: antisymmetric; opp: out of plane. Scaling factor = 0.8929 [9–14], 0.9940 [11,14] and 0.9613 [11,14] for HF, BLYP and B3LYP with 6-31G(d) basis set, respectively. a Our vibrational frequency assignment on the basis of DFT calculations. b Our experimental IR and Raman frequencies.

4.0 2.0 4.5 1.0 7.4 4.1 1.3 17.7 0.6 0.9 2.2 1.1 2.1 1.7 1.9 117.5 29.1 27.7 9.8 31.7 0.4 15.2 4.9 0.5 11.1 0.6 2.6 8.6 949 906 879 851 837 824 793 759 743 562 476 452 436 398 18.46 11.7 2.4 4.6 0.9 4.9 7.6 1.2 4.7 12.5 1.2 2.8 1.4 2.4 1.9 7.3 121.9 28.2 19.4 28.0 7.0 0.6 3.4 18.7 0.7 11.2 0.6 2.7 8.4 946 906 877 852 838 825 796 773 747 565 476 454 428 402 19.08 14.0 1.1 3.4 0.6 7.0 5.9 2.2 30.2 0.6 1.2 1.5 0.8 2.5 1.2 16.7 136.8 18.4 23.4 18.1 26.9 0.4 13.0 3.1 0.5 11.2 0.7 2.4 9.5 944 906 871 850 832 822 795 758 729 552 474 448 446 391 19.12 – – 848 m 909 w 884 s,sh 861 s

m24 m23 m22 m21 m20 m19 m18 m17 m16 m15 m14 m13 m12 m11 r

m26 m25

C(5)N(7)-str + ring str + NH2 twist N(10)–H2-twist + C(8)C(9)N(10)-s-str + opp ring bend Opp ring bend + C(8)C(9)-str + NH2 twist NH2-wag + C(8;9)-twist + ring str NH2-wag + ring str + C(7)C(8)C(9)-s-str C(2;4;9)-H2-rock + H2-wag + ring bend Ring-str + C(2)–H2-rock CH2-rock (ring) + C(8) H2-twist + NH2-wag Opp ring bend + CH2-rock (ring) Ring breath. + C(7)N(5)-str C(7;8;9)-rock Ring torsion C(8)C(7)N(5)-bend. + C(8)C(9)N(10)-bend Ring bend + C(2)H2-rock Ring torsion Ring bend

963 m

961 vw

966 948

7.4 1.3

1.7 1.7

967 955

11.0 1.2

0.9 1.7

966 952

9.4 6.7

1.0 9.5

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CH2 antisymmetric and symmetric vibrations of 3-pipa have been observed at 2933 cm1 (IR), 2922 cm1 (R) and 2853 cm1 (IR), 2846 cm1 (R), respectively. The theoretically calculated values for these antisymmetric and symmetric CH2 vibrations are 2895 cm1, 2977 cm1, 2962 cm1 and 2881 cm1, 2959 cm1, 2945 cm1 for HF, BLYP and B3LYP, respectively. The fundamental CH2 vibrations which are scissoring, wagging and rocking appear in the expected wavenumber region 1600–800 cm1[3]. We have made measurements in the wavenumber region 1600–400 cm1. These vibrations have revealed to be mixed CH2 wagging, CH2 twisting, CH2 rocking, NH2 wagging, C–C–C stretching and C–C–N bending. Experimentally determined NH2 deformation vibration for infrared measurement is 1600 cm1 and this vibration has been found as Raman inactive. We have calculated NH2 deformation at 1620 cm1, 1621 cm1 and 1611 cm1 (HF, BLYP and B3LYP, respectively). In order to make a comparison with experimental frequencies, we have also calculated root mean square deviation (RMSD) based on the calculation at the bottom of Table 3. Within the obtained results, the best reliable RMS value is 18.46. Furthermore, correlations of computed frequencies from the experiment are found to be 0.99950, 0.99951 and 0.99955 for HF, BLYP and B3LYP, respectively (Fig. 4). The results of B3LYP, BLYP and HF methods for 3-pipa are comparable and B3LYP is slightly better than HF and BLYP methods. However, HF method gives more accurate results in the high frequency region when compared to B3LYP and BLYP. 6. Conclusion We have performed the experimental and theoretical vibrational analysis of 3-pipa for the first time. The structural parameters, IR–Raman wavenumbers, IR intensities and Raman scattering activities of vibrational bands of 3-pipa have been calculated with HF, BLYP and B3LYP methods with the 6-31G(d) basis set. Comparison of the experimentally determined fundamental vibrational frequencies of 3-pipa and the results calculated by HF and density functional B3LYP and BLYP methods show that each of three methods gives reasonable results and B3LYP is slightly superior to the scaled HF and BLYP approach. As for the high frequency region HF method seems more accurate. The results obtained in this study also indicate that B3LYP/6-31G(d) method is reliable and it makes easier the understanding of vibrational spectrum and structural parameters of 3-pipa.

Fig. 4. Plot of calculated vs. experimental wavenumbers of 3-pipa.

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