Vibrational spectroscopy (FTIR and FTRaman) investigation using ab initio (HF) and DFT (B3LYP and B3PW91) analysis on the structure of 2-amino pyridine

Spectrochimica Acta Part A 77 (2010) 73–81

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Vibrational spectroscopy (FTIR and FTRaman) investigation using ab initio (HF) and DFT (B3LYP and B3PW91) analysis on the structure of 2-amino pyridine S. Ramalingam a,∗ , S. Periandy b , S. Mohan c a b c

Department of Physics, A.V.C. College (Autonomous), 14, Sivasakthi Nagar, Mappadugai, Mayiladuthurai, Nagai District, India Department of Physics, Tagore Arts College, Puducherry, India Department of Mathematical and Physical Sciences, Hawasa University, Ethiopia

a r t i c l e

i n f o

Article history: Received 19 March 2010 Received in revised form 21 April 2010 Accepted 27 April 2010 Keywords: 2-Amino pyridine Bruker IFS Ab initio Hartree–Fock B3LYP/B3PW91 FTIR FTRaman

a b s t r a c t The FTIR and FTRaman spectra of 2-amino pyridine (2-AP) molecule have been recorded using Bruker IFS 66 V spectrometer in the range of 4000–100 cm−1 . The molecular geometry and vibrational frequencies in the ground state are calculated by using the ab initio Hartree–Fock (HF) and DFT (B3LYP and B3PW91) methods with 6-31++G (d, p) and 6-311++G (d, p) basis sets. The computed values of frequencies are scaled using a suitable scale factor to yield good coherence with the observed values. Making use of the recorded data, the complete vibrational assignments are made and analysis of the observed fundamental bands of molecule is carried out. The geometries and normal modes of vibrations obtained from ab initio HF and B3LYP/B3PW91 calculations are in good agreement with the experimentally observed data. The differences between the observed and scaled wave number values of most of the fundamentals are very small in B3LYP than HF. The influence of N atom and amine group in the skeletal ring vibrations of the title molecule has also been discussed. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction Heterocyclic nitrogen containing compounds, such as pyridine and its derivatives are commonly present in synthetic and natural products [1,2]. The study of the vibrational spectra of substituted pyridine mainly amino pyridine attracts the attention of many spectroscopists due to their wide application in pharmacology and agro-chemistry. Pyridine heterocycles are a repeated moiety in many large molecules with interesting photo physical, electrochemical and catalytic applications [3–10]. They serve as good anesthetic agent and hence are used in the preparation of drugs for certain brain disease. These pharmaceutically acceptable sults and the pre-drugs are used for the treatment (or) prevention of diabetic neuropathy [11,12]. The pharmaceutical development of amino pyridine derivatives has received considerable attention, since they have been fully employed as chiral nucleophilic catalysts in a wide range of asymmetric synthetic process [13]. The vibrational spectra of substituted pyridine have been the subject of several investigations [14–16]. More recently [17,18], FTIR and FTRaman spectra of amino pyridine and amino picoline have been reported together with the vibrational assignments of the vibrational modes. However, the detailed

∗ Corresponding author. Tel.: +91 4364 222264; fax: +91 4364 222264. E-mail address: [email protected] (S. Ramalingam).

HF/B3LYP/B3PW91 at 6-31++G (d, p) and 6-311++G (d, p) comparative studies on the complete FTIR and FTRaman spectra of 2-amino pyridine have not been reported so far. In this study, molecular geometry, optimized parameters and vibrational frequencies are computed and the performance of the computational methods for ab initio (HF), hybrid density functional methods (B3LYP and B3PW91) at 6-31++G (d, p) and 6-311++G (d, p) basis sets are compared. These methods predict relatively accurate molecular structure and vibrational spectra with moderate computational effort. In particular, for polyatomic molecules the DFT methods lead to the prediction of more accurate molecular structure and vibrational frequencies than the conventional ab initio Hartree–Fock calculations. In DFT methods, Becke’s three parameter exact exchange-functional (B3) [19] combined with gradient-corrected correlational functional of Lee, Yang and Parr (LYP) [20,21] and Perdew and Wang (PW91) [22,23] are the best predicting results for molecular geometry and vibrational wave numbers for moderately larger molecule [24–26].

2. Experimental details The compound under investigation namely 2-amino pyridine is purchased from Sigma–Aldrich chemicals, U.S.A. which is of spectroscopic grade and hence used for recording the spectra as such without any further purification. The FTIR spectra of the compounds are recorded in Bruker IFS 66 V spectrometer in the range of

1386-1425/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.04.027

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S. Ramalingam et al. / Spectrochimica Acta Part A 77 (2010) 73–81

4000–100 cm−1 . The spectral resolution is ±2 cm−1 . The FTRaman spectra of these compounds are also recorded in the same instrument with FRA 106 Raman module equipped with Nd:YAG laser source operating at 1.064 ␮m line widths with 200 mW power. The spectra are recorded in the range of 4000–100 cm−1 with scanning speed of 30 cm−1 min−1 of spectral width 2 cm−1 . The frequencies of all sharp bands are accurate to ±1 cm−1 . 3. Computational methods The molecular structure of the title compound in the ground state is computed by performing both ab initio-HF and DFT (B3LYP/B3PW91) with 6-31++G (d, p) and 6-311++G (d, p) basis sets. The optimized structural parameters are used in the vibrational frequency calculations at HF and DFT levels. The minimum energy of geometrical structure is obtained by using level 6-31++G (d, p) and 6-311++G (d, p) basis sets. All the computations have been done by adding polarization function d and diffuse function on heavy atoms [27] and polarization function p and diffuse function on hydrogen atoms [28], in addition to triple split valence basis set (6-311++G (d, p)), for better treatment of polar bonds of amino group. Therefore, we had a discussion on calculated values using these sets. The calculated frequencies are scaled by 0.873 and 0.890 for HF/6-31++G (d, p) and 0.882 and 0.896 for HF/6311++G (d, p) [29]. For B3LYP with 6-31++G (d, p) set is scaled with 0.932, 0.976, 0.948, 0.872 and 0.963, B3LYP/6-311++G (d, p) basis set is scaled with 0.934, 0.940, 0.982, 0.952, 0.899 and 0.969 and B3PW91/6-311++G (d, p) is scaled by 0.927, 0.935, 0.980, 0.956, 0.866 and 0.967 [30]. The theoretical results have enabled us to make the detailed assignments of the experimental IR and Raman spectra of the title molecule [31]. HF/DFT calculations for 2-amino pyridine are performed using Gaussian 03 W program package on Pentium IV processor personal computer without any constraint on the geometry [32–35]. 4. Results and discussion 4.1. Molecular geometry The molecular structure of the 2-amino pyridine belongs to C1 point group symmetry. The optimized molecular structure of title molecule is obtained from GAUSSAN 98W and GAUSSVIEW programs are shown in the Fig. 1. The molecule contains amino group connected with pyridine ring. The structure optimization zero point vibrational energy of the title compound in HF/6-31++G (d, p), HF/6-311++G (d, p), B3LYP/6-31++G (d, p), B3LYP/6-311++G (d, p) and B3PW91/6-311++G (d, p) are −296677.4, −295154.7, 276903.0, 275915.7 and −276866.4 J/mol and 70.90, 70.54, 66.18, 65.94 and 66.17 Kcal/mol respectively. The comparative optimized structural parameters such as bond lengths, bond angles and dihedral angles are presented in Table 1. From the theoretical values, it is found that most of the optimized bond lengths are slightly larger than the experimental values, due to that the theoretical calculations belong to isolated molecules in gaseous phase and the experimental results belong to molecules in solid state [36]. Comparing bond angles and lengths of B3LYP/B3PW91 with those of HF, as a whole the formers are bigger than later and the B3LYP/B3PW91 calculated values correlates well compared with the experimental data. Although the differences, calculated geometrical parameters represent a good approximation and they are the bases for the calculating other parameters, such as vibrational frequencies and thermodynamics properties. Optimized geometrical parameters, namely, bond lengths and bond angles at HF/6-31++G (d, p), HF/6311++G (d, p), B3LYP/6-31++G (d, p), B3LYP/6-311++G (d, p) and B3PW91/6-311++G (d, p) levels are given in Table 1.

Fig. 1. Molecular structure of 2-amino pyridine.

Some experimental results of pyridine and its other derivatives are also included in the Table 1 for comparison. The comparative graphs of bond lengths, bond angles and dihedral angles of title molecule for five sets are presented in Figs. 6–8 respectively. Optimized structure yields fairly accurate bond length pairs for the bonds N1–C2 and N1–C6, N11–H12 and N11–H12, C3–H8 and C4–H7 at all five levels of calculations. Bond lengths of all pairs decrease in going from HF/6-311++G (d, p) to HF/6-31++G (d, p) to B3PW91/6-311++G (d, p) to B3LYP/6-311++G (d, p) to B3LYP/631++G (d, p). According to the experimental values [37], the bond lengths N1–C2 and N1–C6 are equal (1.340 Å) whereas in the case of DFT calculation, the value of bond length N1–C2 is 0.001 Å at B3LYP/6-31++G (d, p) level and 0.002 Å at B3PW91/6-311++G (d, p) greater than bond length N1–C6. In HF calculations, this effect is reversed. This increase of bond length is exactly at the substitution place and also may be due to the single (C–N) and double (C N) bond lengths in the ring. The bond lengths of C2–C3, C5–C6 are equal (expt. 1.395 Å). The calculated bond length of C2–C3 (1.412 Å) is slightly differed from bond C5–C6 (1.393 Å) at B3LYP/6-31++G (d, p) level. From the observation, it is clear that the bond length C2–C3 is 0.017 Å greater and C5–C6 is 0.002 Å lesser than the experimental values [38–40]. This may be due to the substitution of NH2 instead of H. The bond length N1–C2 is 0.058 Å (expt.) and 0.043 Å (cal.) less than the bond length C2–N11 since N1–C2 bond in and C2–N11 out of the ring. The ring angle C2–N–C6 is 117.30 Å (expt.) approximately coincide with the calculated values 117.95 Å (B3LYP/6-31++G (d, p)) and B3PW91/6-311++G (d, p) than other levels. The bottom ring angle C2–N1–C6 is 117.30 Å (expt.), 2.10 Å less than the top ring angle C3–C4–C5 (119.40 Å) since the replacement of C by N in the ring. And also the ring carbon atom exerts a large attraction on valence electron cloud of nitrogen resulting in an increase in C–N force constant and decrease in the corresponding bond length [41]. The hetero aromatic ring appears little distorted and angles slightly out of perfect hexagonal structure. It is due to the substitution of the amino groups in the place of hydrogen. The breakdown of hexagonal structure of the hetero aromatic ring is obvious from the dihedral angle C4–C5–C6–N1 is 0.200 Å (cal. B3LYP/6-311++G (d, p)), 0.110 Å squeezed than the angle C6–N1–C2–C3 (cal. 0.330 Å at B3LYP/6-311++G (d, p)). From the above observation, it is clear that DFT (B3LYP/B3PW91) estima-

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Table 1 Optimized geometrical parameters for 2-amino pyridine computed at HF/6-31++G (d, p), HF/6-311++G (d, p), B3LYP/6-31++G (d, p) and 6-311++G (d, p) and B3PW91/6-311++G (d, p) basis sets. Geometrical parameters

Methods HF/6-31++G (d, p)

Bond length (Å) N1–C2 N1–C6 C2–C3 C2–N11 C3–C4 C3–H8 C4–C5 C4–H7 C5–C6 C5–H9 C6–H10 N11–H12 N11–H13 Bond angle (◦ ) C2–N1–C6 N1–C2–C3 N1–C2–H11 C3–C2–H11 C2–C3–C4 C2–C3–H8 C4–C3–H8 C3–C4–C5 C3–C4–H7 C5–C4–H7 C4–C5–C6 C4–C5–H9 C6–C5–H9 N1–C6–C5 N1–C6–H10 C5–C6–H10 C2–N11–H12 C2–N11–H13 H12–N11–H13 Dihedral angles (◦ ) C6–N1–C2–C3 C6–N1–C2–N11 C2–N1–C6–C5 C2–N1–C6–H10 N1–C2–C3–C4 N1–C2–C3–H8 N11–C2–C3–C4 N11–C2–C3–H8 N1–C2–N11–H12 N1–C2–N11–H13 C3–C2–N11–H12 C3–C2–N11–H13 C2–C3–C4–C5 C2–C3–C4–H7 H8–C3–C4–C5 H8–C3–C4–H7 C3–C4–C5–C6 C3–C4–C5–H9 H7–C4–C5–C6 C7–C4–C5–H9 C4–C5–C6–N1 C4–C5–C6–H10 H9–C5–C6–N1 H9–C5–C6–H10

1.319 1.326 1.403 1.376 1.374 1.074 1.393 1.076 1.378 1.073 1.076 0.994 0.995

Experimental value HF/6-311++G (d, p) 1.316 1.324 1.402 1.377 1.372 1.074 1.392 1.076 1.377 1.073 1.076 0.994 0.995

B3LYP/6-31++G (d, p) 1.341 1.340 1.412 1.384 1.387 1.086 1.401 1.086 1.393 1.084 1.088 1.009 1.010

118.39 122.49 116.68 120.79 118.15 120.41 121.42 119.68 119.89 120.42 117.16 121.84 120.98 124.11 115.51 120.36 117.22 114.45 114.64

118.36 122.52 116.70 120.73 118.14 120.40 121.45 119.67 119.89 120.43 117.14 121.88 120.96 124.14 115.56 120.28 116.82 114.16 114.33

117.95 122.52 116.19 121.23 118.37 120.42 121.19 119.54 119.89 120.55 117.53 121.70 120.76 124.06 115.47 120.46 117.89 114.66 115.24

0.319 −177.73 −0.44 179.83 0.008 −179.37 177.98 −1.39 −154.91 −16.40 27.0 165.50 −0.22 179.97 179.14 −0.65 0.11 −179.89 179.91 −0.09 0.23 179.93 −179.75 −0.05

0.308 −177.67 −0.404 179.81 0.015 −179.38 177.92 −1.47 −154.16 −17.02 27.80 164.94 −0.25 179.98 179.13 −0.62 0.17 −179.85 179.93 −0.09 0.16 179.93 −179.81 −0.03

0.235 −177.41 −0.458 179.82 0.031 −179.48 177.66 −1.84 −156.69 −15.96 25.53 166.25 −0.27 179.89 179.23 −0.59 0.15 −179.90 179.98 −0.06 0.220 179.91 −179.72 −0.02

tion is better than HF, which under estimate the bond length and bond angle than experimental values. 4.2. Vibrational assignments The title molecule consists of 13 atoms, which undergoes 33 normal modes of vibrations. Of the 33 normal modes of vibrations, 22 modes of vibrations are in plane and remaining 11 are out of plane. The bands that are in the plane of the molecule is represented

B3LYP/6-311++G (d, p) 1.338 1.337 1.409 1.382 1.384 1.084 1.397 1.084 1.389 1.082 1.086 1.007 1.009

B3PW91/6-311++G (d, p) 1.335 1.333 1.407 1.377 1.381 1.085 1.395 1.085 1.387 1.083 1.087 1.006 1.008

118.02 122.45 116.28 121.22 118.40 120.42 121.16 119.53 119.92 120.54 117.55 121.68 120.76 124.01 115.54 120.43 117.83 114.60 115.21

117.95 122.50 116.19 121.25 118.36 120.47 121.15 119.55 119.89 120.55 117.47 121.71 120.80 124.14 115.48 120.37 117.88 114.54 115.35

0.33 −177.46 −0.46 179.82 0.02 −179.47 177.72 −1.77 −156.52 −16.03 25.64 166.13 −0.28 179.89 179.21 −0.06 0.17 −179.89 179.99 −0.07 0.20 179.91 −179.72 −0.02

0.33 −177.50 −0.46 179.83 0.03 −179.46 177.76 −1.73 −156.69 −15.95 25.43 166.17 −0.28 179.89 179.21 −0.61 0.16 −179.90 180.00 −0.08 0.21 179.90 −179.70 −0.01

1.340 1.340 1.395 1.398 1.374 1.081 1.394 1.081 1.395 1.081 1.081 1.071 1.071 117.30 123.60 – – 118.50 120.20 – 119.40 – – 118.10 121.0 – 123.30 – – – – – – – – – – – – – – – – – – – – – – – – – – – – –

as A and out-of-plane as A . Thus the 45 normal modes of vibrations title molecule are distributed as  Vib = 22 A + 11 A . In agreement with C1 symmetry all the 33 fundamental vibrations are active in both Raman scattering and IR absorption. The harmonic-vibrational frequencies calculated for 2-amino pyridine at HF and DFT (B3LYP and B3PW91) levels using the triple split valence basis set along with the diffuse and polarization functions, 6-31++G (d, p) and 6311++G (d, p) observed FTIR and FTRaman frequencies for various modes of vibrations have been presented in the Table 2.

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Table 2 Observed and HF/6-31++G (d, p), HF/6-311++G (d, p), B3LYP/6-31++G (d, p) and B3LYP/6-311++G (d, p) and B3PW91/6-311++G (d, p) level calculated vibrational frequencies of 2-amino pyridine. Symmetry species C1

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

A A A A A A A A A A A A A A A A A A  A  A  A  A  A  A A A A A A  A  A  A  A 

Observed frequency

Calculated frequency (cm−1 ) with HF/6-31++G (d, p)/HF/6-311++G (d, p)

Calculated frequency (cm−1 ) with B3LYP/6-31++G (d, p)/B3LYP/6-311++G (d, p)

FTIR

FTRaman

Unscaled value

Scaled value

Unscaled value

Scaled value

3450vs – 3020s – 2960s 2950s 1610vs 1600vs 1590vs – 1430vs – 1270vs 1160vs – 1070w 1030m 1020vs 980s – 960m 840m – 760vs – 610m – 520vs – 400vs 360vs 330vs –

– 3330w – 2980w – 2950s – 1600vs 1590vs 1470vs – 1310vs 1270vs – 1110vs – – – 980s 970vs – 840vs 810m – 730w – 580w 520w 470w – – – 200m

3912/3915 3819/3802 3385/3362 3367/3344 3350/3324 3344/3318 1806/1798 1786/1780 1771/1762 1650/1641 1599/1590 1460/1455 1436/1427 1332/1318 1242/1238 1217/1206 1141/1135 1136/1128 1122/1114 1106/1103 1079/1076 953/949 919/915 866/865 824/822 686/685 622/628 599/600 537/543 458/457 432/430 378/376 228/227

3441/3437 3334/3338 3005/2999 2989/2982 2975/2965 2969/2959 1603/1611 1586/1594 1572/1578 1465/1470 1420/1424 1296/1303 1275/1278 1182/1181 1102/1109 1080/1080 1013/1017 1008/1010 996/998 982/988 958/964 846/850 816/820 769/775 731/736 609/614 552/562 531/537 476/486 406/409 383/385 335/337 202/203

3710/3693 3586/3579 3216/3199 3197/3180 3181/3164 3167/3147 1657/1649 1633/1629 1619/1610 1518/1511 1479/1474 1352/1349 1340/1335 1329/1311 1175/1173 1144/1143 1068/1066 1055/1054 1000/1001 995/990 976/974 865/864 860/858 785/781 747/746 639/642 571/573 546/547 484/485 418/418 404/404 366/367 201/200

3446/3450 3331/3342 2997/3007 2979/2989 2964/2974 2951/2958 1617/1616 1597/1596 1580/1577 1481/1453 1443/1444 1319/1322 1270/1270 1158/1162 1131/1097 1101/1069 1028/1032 1015/1021 963/970 958/959 939/943 832/837 828/831 722/756 719/722 615/591 571/555 526/530 484/470 418/418 364/363 329/329 201/200

VS, very strong; S, strong; m, medium; w, weak; as, asymmetric; s, symmetric; , stretching; ␦, in plane bending; ␥, out plane bending; ␶, twisting.

Calculated frequency (cm−1 ) with B3PW91/6-311++G (d, p)

Unscaled value 3719 3600 3210 3191 3175 3154 1662 1631 1625 1517 1478 1351 1342 1335 1170 1143 1071 1055 1002 990 975 866 860 789 747 638 570 546 480 415 403 368 199

Vibrational assignments

Scaled value 3447 3337 3007 2989 2974 2955 1628 1598 1592 1486 1448 1323 1269 1156 1013 1068 1035 1020 968 957 942 837 831 763 722 590 557 527 464 401 389 330 199

(N–H) as  (N–H) s  (C–H)  (C–H)  (C–H)  (C–H)  (N–H) ␦ (N–H) ␦ (C C)  (C N)  (C C)  (C–C)  (C–C)  (C–N)  (C–H) ␦ (C–H) ␦ (C–H) ␦ (C–H) ␦ (N–H) ␥ (N–H) ␥ (C–H) ␥ (C–H) ␥ (C–H) ␥ (C–H) ␥ (CCC) ␦ (CCC) ␦ (C–N) ␦ (C-NH2 ) ␦ (C–N–C) ␦ (CCC) ␥ (CCC) ␥ (C–N–C) ␥ (C-NH2 ) ␥ (NH2 ) ␶

S. Ramalingam et al. / Spectrochimica Acta Part A 77 (2010) 73–81

S. no.

S. Ramalingam et al. / Spectrochimica Acta Part A 77 (2010) 73–81

Fig. 2. Experimental [A], calculated [B], [C], [D] and [E] FTIR spectra of 2-amino pyridine.

Comparison of frequencies calculated at HF, B3LYP and B3PW91 with the experimental values reveals the over estimation of the calculated vibrational modes due to the neglect of anharmonicity in real system. Inclusion of electron correlation in the Density functional theory (B3LYP/B3PW91) to certain extends makes the frequency values bigger in the comparison with the HF frequency data. Reduction in the computed harmonic vibrations, although basis set sensitive are only marginal as observed in the DFT (B3LYP) values using 6-311++G (d, p). Any way not withstanding the level of calculations, it is customary to scale down the calculated harmonic frequencies in order to develop the agreement with the experiment. The scaled calculated frequencies minimize the root-mean square difference between calculated and experimental frequencies for bands with definite identifications. The descriptions concerning the assignment have also been indicated in Table 2. The comparative IR and Raman spectra of experimental and calculated (HF/B3LYP/B3PW91) are given in Figs. 2 and 3. 4.2.1. Computed vibrational frequency analysis The standard deviation (SD) calculation made between experimental and computed (HF/DFT) for the title molecule is presented in the Table 4. According to the SD, the computed frequency deviation decrease in going from HF/6-31++G (d, p) to HF/6-311++G (d, p) to B3PW91/6-311++G (d, p) to B3LYP/6-31++G (d, p) to B3LYP/6311++G (d, p). The deviation ratio of HF and B3LYP is 1.030 at 6-311++G (d, p) level and B3PW91 and B3LYP is 1.003 at 6-311++G (d, p). The comparative graph of calculated vibrational frequencies by HF and DFT methods at HF/6-31++G (d, p)/6-311++G (d, p), B3LYP/631++G (d, p)/6-311++G (d, p) and B3PW91/6-311++G (d, p) basis sets with frequencies for the title molecule are given in Fig. 9. From the figure, it is found that the calculated (unscaled) frequencies by B3LYP with 6-311++G (d, p) basis set are closer to the experimental frequencies than HF method with 6-311++G basis set. 4.2.2. Computed IR intensity and Raman activity analysis Computed vibrational spectral IR intensities and Raman activities of the title molecule for corresponding wave numbers by HF and DFT (B3LYP and B3PW91) methods at 6-31++G (d, p) and 6311++G (d, p) basis sets have been collected in the Table 3. The

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Fig. 3. Experimental [A], calculated [B], [C], [D] and [E] FTRaman spectra of 2-amino pyridine.

title molecule is a non-polar and belongs to C1 point group. Comparison of IR intensities and Raman activities calculated by HF and DFT (B3LYP and B3PW91) at 6-31++G (d, p) and 6-311++G (d, p) levels with experimental values exposes the variation of IR intensities and Raman activities. In the case of IR intensities, the values of HF are found to be higher than B3PW91 and B3LYP at 6-311++G (d, p) levels whereas in the case of Raman activities the effect is reversed. The comparative graphs of IR intensities and Raman activities for three sets are presented in Figs. 4 and 5 (Figs. 6–9 and Table 4). 4.2.3. C–H vibrations The hetero aromatic organic compounds commonly exhibit multiple week bands in the region 3100–3000 cm−1 due to C–H stretching vibrations [42–44]. In the title molecule, the bands have been assigned at 3020, 2980, 2960, and 2950 cm−1 to C–H ring stretching vibrations. The last three bands are deviated trivially from the expected range. This is clearly due to the influence of N–H stretching vibrations which is found to be very dominating in this molecule. The scaled values computed by B3LYP/6-31++ G (d, p) and HF/6-311++G (d, p) levels coincide well with the experimental observation. The C–H in-plane ring bending vibrations normally occurred as a number of strong to weak intensity sharp bands in the region 1300–1000 cm−1 [45,46]. The bands for C–H in-plane bending vibrations of the title compound identified at 1270, 1160, 1110, 1070 and 1020 cm−1 . The theoretically computed frequencies for C–H in-plane bending vibrations by B3LYP/6-311++G (d, p) and B3PW91/6-311++G (d, p) methods shows excellent agreement with recorded spectrum as well as literature data. The C–H out-of-plane bending vibrations are strongly coupled vibrations and normally observed in the region 950–809 cm−1 [47–52]. In the present case, the bands are identified at 980, 970, 960, 840 and 810 cm−1 for C–H out-of-plane bending. Except first two bands, the assigned frequencies are found to be well within their characteristic regions. In the case of C–H vibrations of present molecule, the stretching vibrations are slightly pushed down and out-of-plane bending pulled up fairly. This view of push and pull of vibrations is purely by the heavy mass of amine group vibration.

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Table 3 Comparative values of IR intensities and Raman activities between HF/6-31++G (d, p), HF/6-311++G (d, p), B3LYP/6-31++G (d, p) and B3LYP/6-311++ G (d, p) and B3PW91/6-311++ G (d, p) of 2-amino pyridine. Sl. no.

Calculated with B3LYP/6-31++G (d, p)

Calculated with B3LYP/6-311++G (d, p)

Calculated with B3PW91/6-311++G (d, p)

IR intensity (Ai)

Raman activity (I)

IR intensity (Ai)

Raman activity (I)

IR intensity (Ai)

Raman activity (I)

IR intensity (Ai)

Raman activity (I)

IR intensity (Ai)

Raman activity (I)

39.79 48.49 12.05 20.94 14.17 14.14 396.60 32.06 56.30 69.44 116.67 20.11 50.54 12.97 9.38 29.02 2.32 10.76 0.51 0.28 9.55 7.20 5.09 91.60 17.23 1.37 74.01 31.97 234.67 3.10 2.07 49.56 9.86

45.13 134.94 175.09 72.55 111.43 50.65 39.91 3.86 12.91 1.35 2.72 1.10 14.37 8.25 9.73 2.44 16.69 1.19 0.48 0.64 24.32 0.98 19.17 2.08 0.07 4.03 6.35 1.04 1.42 0.24 0.59 0.49 0.74

36.68 46.09 11.15 19.07 12.13 17.15 374.49 36.18 60.19 71.11 119.30 18.47 51.88 12.79 9.78 27.96 1.92 12.52 0.34 0.32 10.24 7.78 5.00 84.40 18.81 1.27 112.35 26.19 193.51 3.16 1.51 43.49 9.26

42.82 133.14 171.42 71.94 94.45 61.48 36.76 4.26 13.71 1.23 2.60 0.92 13.20 7.80 9.44 3.01 17.05 1.15 0.41 0.51 23.81 1.02 18.90 1.95 0.09 4.00 6.10 1.65 1.20 0.24 0.53 0.44 0.73

30.94 37.81 11.86 20.38 5.94 22.93 331.96 9.12 34.86 69.43 85.13 1.45 49.93 15.23 11.22 1.46 6.68 2.16 6.18 0.01 0.35 2.87 3.28 57.01 14.44 1.09 19.01 39.79 229.44 2.75 12.97 58.83 7.67

55.89 196.65 203.39 99.96 97.37 100.64 34.18 2.84 8.47 3.70 2.25 1.47 10.01 5.06 2.74 7.34 20.07 1.49 19.19 0.38 0.26 6.74 6.99 1.15 0.15 3.76 7.34 0.28 0.98 0.18 1.32 0.19 0.60

30.62 38.85 10.27 17.93 5.51 22.01 322.65 7.99 35.21 69.85 89.59 5.40 47.25 16.05 11.45 1.79 7.01 2.42 6.54 0.06 0.41 3.01 3.35 52.33 19.39 1.12 19.71 41.56 220.51 2.89 12.61 55.17 7.38

53.22 191.68 199.01 97.76 88.58 101.31 31.66 3.99 8.64 3.28 2.08 1.01 8.98 6.19 2.70 6.97 18.64 1.75 19.06 0.33 0.11 7.35 16.00 0.95 0.19 3.73 7.41 0.30 0.99 0.16 1.27 0.18 0.58

32.97 41.36 8.73 16.02 5.45 21.88 321.92 43.90 28.24 73.79 79.09 3.31 29.26 22.15 10.59 1.87 7.39 2.33 6.02 0.10 0.42 3.84 2.42 50.18 22.10 1.07 16.05 35.08 218.53 2.79 19.49 59.24 7.54

51.52 186.53 192.49 98.80 83.91 102.89 33.84 0.97 6.76 3.31 2.09 5.07 3.45 6.35 2.72 7.05 18.80 1.95 18.78 0.29 0.09 15.74 6.18 0.78 0.18 3.75 7.07 0.29 0.94 0.17 1.31 0.16 0.56

S. Ramalingam et al. / Spectrochimica Acta Part A 77 (2010) 73–81

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

Calculated with HF/6-311++G (d, p)

Calculated with HF/6-31++G (d, p)

S. Ramalingam et al. / Spectrochimica Acta Part A 77 (2010) 73–81

79

Fig. 4. Comparative graph of IR intensities by HF and DFT (B3LYP/B3PW91). Fig. 6. Bond length differences between theoretical [HF and DFT] approaches.

Fig. 5. Comparative graph of Raman activities by HF and DFT (B3LYP/B3PW91).

4.2.4. C C vibrations The ring stretching vibrations (C C) are very much prominent in the spectrum of pyridine and its derivatives and are highly characteristic region of the hetero aromatic ring itself [53,17,54]. The bands between 1590 and 1650 cm−1 in benzene derivatives usually assigned to C C stretching modes [55]. Since the present molecule is hetero aromatic, two C C and two C–C stretching vibrations are possible. The C C stretching vibrations have been found at 1590 and 1470 cm−1 in this molecule. The first assigned value is in expected range whereas the second is shifted away from the range. The shift of frequency with the mass of substitution has been reverse in these modes. This shows that, there is a gain in energy instead of a loss as it is in the previous cases. In the present case, ring C C stretching vibrations suppressed due to the bonding of N in the ring.

Fig. 7. Bond angle differences between theoretical [HF and DFT] approaches.

4.2.5. C–C vibrations Ring C–C stretching vibrations normally occur in the region 1590–1430 cm−1 [56]. In the present case, the C–C stretching vibrations have been assigned at 1430 and 1310 cm−1 . When compared to the literature range cited above, there is a considerable decrease in frequencies which is also worsening with the increase of mass of substitutions. In the present work, two strong bands present at 760 and 730 cm−1 assigned to CCC in-plane bending and two supplementary bands assigned at 470 and 400 cm−1 to CCC out-of-plane bending. These assignments are in line with the assignments proposed by the literature [57]. A Raman band observed at 520 cm−1 for C–N–C trigonal bending and a band with medium intensity assigned at 360 cm−1 for C–N–C ring breathing mode. These assign-

Table 4 Standard deviation of frequencies by HF/DFT (B3LYP/B3PW91) at 6-31++G (d, p) and 6-311++G (d, p) basis sets. S. no.

Basic set levels

Total values

Average

Standard deviation

1 2 3 4 5

Experimental HF/6-31++(d, p) HF/6-311++(d, p) B3LYP/6-31++(d, p) B3LYP/6-311++(d, p) B3PW91/6-311++(d, p)

43880 49562 49333 46283 46116 46227

1329.70 1501.88 1494.94 1402.51 1397.45 1402.33

– 996.90 993.48 967.42 964.43 967.52

Deviation ratio

1.030 1.003

80

S. Ramalingam et al. / Spectrochimica Acta Part A 77 (2010) 73–81

4.2.7. C–N and C-NH2 vibrations The pyridine absorbs strongly in the region 1600–1500 cm−1 due to the C N ring stretching vibrations [66]. Accordingly, a strong band is observed at 1590 cm−1 in the title molecule and is coupled with the C C vibration. The C–N stretching vibration is always mixed with other bands and is usually assigned in the region 1266–1382 cm−1 [67,68]. The C–N stretching is observed strongly in at 1270 cm−1 and is mixed with C–H in-plane bending vibration. This frequency is also at the lower end of the expected range which may be also due to the interaction of C–C vibration, whose frequency extends up to this value. This view is supported by the literature [69]. The C-NH2 in-plane bending and C-NH2 out-of-plane bending observed at 580 and 330 cm−1 respectively. These assignments are found to be satisfactory [70,71]. The C–N vibration in this work is observed strongly and coupled with C–C vibration. The substitution of amine group in the ring does not have any impact of C N vibrations. Fig. 8. Dihedral angle differences between theoretical [HF and DFT] approaches.

ments are in line with the literature [58,59]. The theoretically computed values by B3LYP/6-311++G (d, p) method for CCC out-ofplane bending (with out scaling) are approximately coincide with FTIR experimental values.

4.2.6. Amino group vibrations In primary amines, usually the N–H stretching vibrations occur in the region 3500–3300 cm−1 [60,61,49]. The NH2 group has two vibrations; one is being asymmetric and other symmetric. The frequency of asymmetric vibration is higher than that of symmetric one. In the present study, the asymmetric and symmetric vibrations of N–H stretching are assigned to the bands at 3450 and 3330 cm−1 respectively. These assignments are in line with the literature. The N–H in-plane bending vibrations (scissoring) are usually observed in the region 1610–1630 cm−1 and the out-of-plane bending vibrations are normally identified in the region 1150–900 cm−1 [62–65]. In the title molecule, the N–H in-plane bending vibrations are assigned at 1610 and 1600 cm−1 and the out-of-plane bending vibrations assigned at 1020 and 980 cm−1 . According to the literature, these assignments are agreed well. The N–H vibrations in this present molecule affect the vibrations of other substitutions. But the N–H vibrations did not affect by vibrations of other substitutions.

Fig. 9. Comparative graph of computed frequencies [HF and DFT] with experimental values.

5. Conclusion Attempts have been made in the present work for the molecular parameters and frequency assignments for the compound 2-amino pyridine from the FTIR (solid and gas phase) and FTRaman spectra. Vibrational frequencies, infrared intensities and Raman activities are calculated and analyzed by HF and DFT (B3LYP and B3PW91) levels of theory utilizing 6-31++G (d, p) and 6-311++G (d, p) higher basis sets. The difference between the observed and scaled wavenumber values of most of the fundamentals is very small. Comparison between the calculated and experimental structural parameters indicates that B3LYP results are in good agreement with experimental values. On the basis of agreement between the calculated and observed results, assignments of fundamental vibrational modes of 2-amino pyridine are examined. Therefore, the assignments made at higher level of theory with higher basis set with only reasonable deviations from the experimental values seem to be correct. From the vibrational discussion, it was concluded that the Substitution of H atom by the NH2 group distort the ring geometries to small extent and the planarity of the molecule. References [1] T.L. Gilchrist, Heterocylic Chemistry, John Wiley & Sons, New York, 1988. [2] J.A. Fallas, L. Gonzalez, I. Corral, Journal of Tetrahedron Letters 65 (2009) 232–236. [3] F. Zucchi, G. Trabanelli, N.A. Gonzalez, Journal of Archaeological Modern Chemistry 132 (1995) 4579. [4] B.T. Khan, S.R.A. Khan, K. Annapoorna, Indian Journal of Chemical Society 34 (1995) 11878. [5] M.E. Lizarraga, R. Navarro, E.P. Urriolabeitia, Journal of Organo Metallic Chemistry 542 (1997) 51–55. [6] A.S. Georgopoulou, S. Ulvenlund, D.M.P. Mingos, I. Baxter, D.J. Williams, Journal of Chemical Society 4 (1999) 547–551. [7] W. Liaw, N. Lee, C. Chen, C. Lee, G. Lee, S. Peng, Journal of American Chemical Society 122 (2000) 488–492. [8] P.J. Trotter, P.A. White, Journal of Applied Spectroscopy 32 (1978) 323–327. [9] K.Y. Rajpure, C.H. Bhosale, Journal of Materials Chemistry and Physics 64 (2000) 70–76. [10] S. Licht, Journal of Solar Energy Materials and Solar Cells 38 (1995) 305–310. [11] J.M. Altenburger, G.Y. Lassalle, M. Matrougui, D. Galtier, J.C. Jetha, Z. Bocskei, C.N. Berry, C. Lunven, J. Lorrain, J.P. Herault, P. Schaeffer, S.E. O’Connor, J.M. Herbert, Bioorganic and Medicinal Chemistry Letters 12 (2004) 1713. [12] H. Camp, J. Perk, Hand Book of American Chemical Society (2000) 31–32. [13] R. Murugan, E.F.V. Scrivan, Aldrichim Acta 36 (2003) 21–24. [14] A. Topacli, S. Bayari, Spectrochimica Acta Part A 57 (2001) 1385–1389. [15] R.N. Medhi, R. Barman, K.C. Medhi, S.S. Jois, Spectrochimica Acta Part A 56 (2000) 1523–1528. [16] I. Lopez Tocon, M.S. Wooley, J.C. Otero, J.I. Marcos, Journal of Molecular Structure 470 (1998) 241. [17] Sujin P. Jose, S. Mohan, Spectrochimica Acta Part A 64 (2006) 240–245. [18] Adel Mostafa, Hussan S. Bazzi, Spectrochimica Acta Part A 74 (2009) 180–187. [19] A.D. Becke, Physics Review A 38 (1988) 3098. [20] C. Lee, W. Yang, R.G. Parr, Physics Review B. 37 (1988) 785.

S. Ramalingam et al. / Spectrochimica Acta Part A 77 (2010) 73–81 [21] A.D. Becke, Journal of Chemical Physics 98 (1993) 5648. [22] J.P. Perdew, K. Burke, Y. Wang, Physics Review B 54 (1996) 16533. [23] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Physics Review B 48 (1993) 4979(E). [24] Z. Zhengyu, Du. Dongmei, Journal of Molecular structure (Theochem.) 505 (2000) 247–249. [25] Yannick Carissan, Wim Klopper, Journal of Molecular Structure (Theochem.) 940 (2010) 115–118. [26] M.H. JamroÂz, J.Cz. Dobrowolski, Journal of Molecular Structure 565–566 (2001) 475–480. [27] T. Clark, J. Chandrasekhar, G.W. Spitznagel, P.V.R. Schleyer, Journal of Computational Chemistry (1983) 294. [28] M.J. Frisch, J.A. Pople, J.S. Binkley, Journal of Chemical Physics 80 (1984) 3265. [29] D.C. Young, Computational Chemistry: A Practical Guide for Applying Techniques to Real World Problems (Electronic), John Wiley and Sons Inc., New York, 2001. [30] N. Sundaraganesan, S. IIlakiamani, H. Saleem, P.M. Wojiciechowski, D. Michalska, Spectrochimica Acta A 61 (2005) 2995. [31] Mehmet Karabacak, Dilek Karagoz, Mustafa Kurt, Spectrochimica Acta A 61 (2009). [32] Gaussian 03 Program, Gaussian Inc., Wallingford, CT, 2000. [33] M.J. Frisch, A.B. Nielsen, A.J. Holder, Gauss View Users Manual, Gaussian Inc., Pittsburgh, PA, 2000. [34] M.J. Frisch, A.B. Nielsen, A.J. Holder, Gauss View Users Manual, Gaussian Inc., Pittsburgh, PA, 2004. [35] Gaussian 03 Program, Gaussian Inc., Wallingford, CT, 2004. [36] N. Sundaraganesan, S. Kalaichelvan, C. Meganathan, B. Dominic Joshua, J. Cornard, Spectrochimica Acta Part A 71 (2008) 898–906. [37] Ming Chao, Ellory Schempp, Acta Crystallography B33 (1977) 1557. [38] L.M. Sverdlav, M.A. Kovner, E.P. krainov, Vibrational Spectra of Polyatomic Molecules, Nauka, Moscow, 1970. [39] S.D. Sharma, S. Doraiswamy, Chemical Physics Letters 41 (1976) 192. [40] P.R. Wei, T.C.W. Mak, Journal of Chemical Crystallography 26 (1996) 133–135. [41] N. Sundaraganesan, J. Karpagam, S. Sabastian, J. Cornard, Spectrochimica Acta Part A 71 (2009) 11–19. [42] V. Krishnakumar, V. Balachandran, T. Chithambarathann, Spectrochimica Acta Part A 62 (2005) 918–925. [43] W.O. George, P.S. Mcintyre, Infrared Spectroscopy, John Wiley & Sons, London, 1987. [44] J. Coates, R.A. Meyers, Interpretation of Infrared Spectra: A Practical Approach, John Wiley and Sons Ltd., Chichester, 2000. [45] Socrates George, Infrared and Raman Characteristics Group Frequencies, 3rd ed., Wiley, New York, 2001. [46] N. Sundaraganesan, H. Saleem, S. Mohan, M. Ramalingam, V. Sethuraman, Spectrochimica Acta Part A 62 (2005) 740–751.

81

[47] V. Krishna kumar, R. John Xavier, Indian Journal of Pure and Applied Physics 41 (2003) 597–602. [48] V. Krishna kumar, N. Prabavathi, Spectrochimica Acta Part A 71 (2008) 449– 457. [49] A. Altun, K. Golcuk, M. Kumru, Journal of Molecular Structure (Theochem.) 637 (2003) 155. [50] S.J. Singh, S.M. Pandey, Indian Journal of Pure and Applied Physics 12 (1974) 300–304. [51] Yu-Xi Sun, Qing-Li Hao, Zong-Xue Yu, Wen-Jun Jiang, Lu-De Lu, Xin Wang, Spectrochimica Acta Part A 73 (2009) 892–901. [52] N. Sundaraganesan, B. Dominic Joshua, T. Rajakumar, Indian Journal of Pure and Applied Physics 47 (2009) 248–258. [53] G. Varsanyi, Vibrational Spectra of Benzene Derivatives, Akademiai Kiado, Budapest, 1969. [54] N.P. Singh, R.A. Yadav, Indian Journal of Physics B75 (4) (2001) 347–352. [55] D.N. Sathyanarayana, Vibrational Spectroscopy Theory and Application, New Age International Publishers, New Delhi, 2004. [56] S. Periandy, S. Mohan, Proceedings National Academic Science India (1998), 68 (A), III. [57] Abdel-shaty Hussein, K. Howard, Journal of Molecular Structure (GB) 42 (37) (1977). [58] A. Fu, D. Du, Z. Zhou, Spectrochimica Acta A 59 (2003) 245. [59] N. Sundaraganesan, K. Sathesh Kumar, C. Meganathan, B. Dominic Joshua, Spectrochimica Acta Part A 65 (2006) 1186–1196. [60] Y. Wang, S. Saebo, C.V. Pittman, Journal of Molecular Structure (Theochem.) 281 (1993) 91–96. [61] N. Puviarasan, V. Arjunan, S. Mohan, Turkey Journal of Chemistry 26 (2002) 323. [62] H.F. Hameka, J.O. Jensen, Journal of Molecular Structure (Theochem.) 362 (1996) 325. [63] J.R. During, M.M. Bergana, H.V. Phan, Journal of Raman Spectroscopy 22 (1991) 141. [64] G. Varsanyi, Vibrational Spectra of Benzene Derivatives, Academic Press, New York, 1969. [65] Z. Niu, K.M. Dunn, J.E. Boggs, Journal of Molecular Physics 55 (1985) 421. [66] G. Socrates, Infrared and Raman Characteristics Group Frequencies Tables and Charts, 3rd ed., Wiley, Chichoster, 2001. [67] N. Sundaraganesan, C. Meganathan, Mustafa Kurt, Journal of Molecular Structure 891 (2008) 284–291. [68] M. Silverstein, G. Clayton Basseler, C. Moril, Spectro Metric Identification of Organic Compounds, Wiley, New York, 1981. [69] R. Shanmugam, D. Sathayanarayana, Spectrochimica Acta A 40 (1984). [70] V. Arjunan, S. Mohan, Spectrochimica Acta Part A 72 (2009) 436–444. [71] J. Swaminathan, M. Ramalingam, N. Sundaraganesan, Spectrochimica Acta Part A 71 (2009) 1776–1782.