Experimental and theoretical investigations of spectroscopic properties of N-acetyl-5-methoxytryptamine

Tags: Wavenumber Rel, R. Arun Balaji, J. Mol, deformation, melatonin, Cristina Giordano, J.J. Dannenberg, Angela Ruggirello, R. M. Uppu, Figen Erkoc, V. Krishnakumar, Spectrochim, M.J. Frisch, Vincenzo Turco Liveri, G. Socrates, chemical shifts, CH2, structural unit, vibration, rocking, J.C. Burant, W. A. Pryor, K. Toyota, Leopoldo Ceraulo, M. T. Lamy- Freund, R. Van Leeuwen, S. Srinivasan, A. M. L. Castrucci, K. J. F. Hilton, R. J. Reiter, spectroscopic properties, theoretical investigations, Canadian Journal of Analytical Sciences, S. Gunasekaran, S. Kumaresan, C. S. Shida, M. Challacombe, B. Lakshmaiah, R. Fukuda, Sakir Erkoc, G. Clayton Bassler, Felice Filizzola, B. Vander Veken, David Bongiorno, J.A. Montgomery, Jr., K. Wolinski, Terence C. Morill
Content: Experimental and theoretical investigations of spectroscopic properties of N-acetyl-5-methoxytryptamine S. Gunasekarana, R. Arun Balajia, S. Kumaresanb, G. Anandb, S. Srinivasanc aSpectrophysic Research Laboratory, Pachaiyappa's College, Chennai ­ 600 030, India. bArulmigu Meenakshi Amman College of Engineering, Vadamavandal ­ 604 410, India. cL.N. Government Arts and Science College, Ponneri, Tamil Nadu, India.
Received: February 8, 2008
Accepted (in revised form): May 10, 2008
Abstract Ab initio and density functional computations of the vibrational (IR) spectrum, molecular geometry, the atomic charges and molecular polarizabilities were carried out on melatonin. The FTIR spectrum of melatonin is recorded in solid phase. Assignments were made in accordance with the calculated and experimental spectra. The UV spectrum was measured in methanol. In order to gain some insight into the recorded spectrum, the quantum mechanical calculations were performed for melatonin using both ZINDO/CIS and TD DFT with B3LYP/6-31G* basis set. In addition, isotropic 1H- and 13C-nuclear magnetic shielding constants of this compound were calculated by employing the direct implementation of the gauge including-atomic-orbital (GIAO) method at the B3LYP density functional theory using 6-311G basis set. Theoretical values are compared to the experimental data. Keywords: FTIR, DFT, Melatonin, HOMO, LUMO, UV Rйsumй Nous avons menй des simulations de calcul ab initio et de densitй fonctionnelle du spectre de vibration (IR), de la gйomйtrie molйculaire, des charges atomiques et des polarisabilitйs molйculaires de la mйlatonine. Le spectre FTIR de la mйlatonine a йtй obtenu en phase *Author to whom correspondence should be addressed: e-mail: [email protected] (S. Gunasekaran) Phone: 919843092789; Fax: 04182247516
solide. L'attribution des pics a йtй effectuйe en accord avec les spectres calculйs et expйrimentaux. Le spectre UV a йtй mesurй dans le mйthanol. Afin d'obtenir plus d'information sur le spectre obtenu, les calculs de mйcanique quantique ont йtй faits pour la mйlatonine en se servant а la fois de ZINDO/CIS et TD DFT avec base de donnйes B3LYP/6-31G*. De plus, nous avons calculй les constantes isotropes de blindage magnйtique nuclйaire 1H et 13C de ce composй, en utilisant l'implйmentation directe de la mйthode qui incorpore l'orbitale atomique (GIAO) а la densitй de thйorie fonctionnelle B3LYP et la base de donnйes 6-311G. Nous avons comparй les valeurs thйoriques aux donnйes expйrimentales. Introduction Melatonin (MLT), N-acetyl-5-methxytryptamine, is a hormone having a indolic structure [1] which is principally secreted by the pineal gland in the brain that helps to regulate other hormones and maintains the body's circadian rhythm. It mediates various neuroendocrine and physiologic cellular processes, controls some neural circadian effectors regulating sleep [2] and seasonal reproduction cycles [3] as well as other Endocrine glands, and has anti-tumoral and anti-degradation properties. Moreover, MLT is the unique hormone which possesses strong anti-oxidant properties. In particular, it acts as a powerful inhibitor of free radicals and of oxygenated active species resulting from the exposure of living organisms to external agents such as ultraviolet radiations, ozone, tobacco, alcohol, asbestos and pesticides. Oxygenated active species are also generated from the breathed oxygen. Up to 5% of the oxygen absorbed from
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mitochondrions comes out as oxygenated free radicals. At muscular level they damage and destroy membranes, disarm enzymes, and alter the genetic map; all these effects occur in a very short time (about 1ns). In the absence of anti-oxidant agents, their action is devastating and relentless [4] determining irreversible chemical changes and could lead to a series of diseases such as cancer [5, 6], AIDS [7], cataract [8] cardiac [9], Parkinson's and Alzheimer's [10, 11] diseases. MLT is five times more effective than glutathione in capturing hydroxyl radicals and five hundreds times more effective than dimethyl sulfoxide in safeguarding chromosomes from radiation induced damages [12]. Besides, it inactivates lipoperoxyl radicals [13] and nitrogen monoxide [14, 15]. Since MLT is a small size amphiphilic substance, it is soluble both in fats [16] and water [17], and it is preferentially located at hydrophilic/hydrophobic interfaces and can easily cross all the anatomic barriers including hematoencephalic and placental ones. These properties make melatonin able to protect all the cellular structures from oxidant agents. The structural and electronic properties of melatonin and its six hydroxyl isomers have been previously investigated by Sakir Erkocet et al [18]. Vasilescu and Broch had reported the four minimal energy conformations of melatonin [19]. David Bongiorno et al [20] had recorded FTIR, X-ray and 1H-NMR spectra of melatonin confined in membrane models. In this study, an attempt has been made to interpret the vibrational spectra of melatonin by applying ab initio and density functional theory calculations based on Hartree-Fock and Becke3- Lee-Yang-Parr (B3LYP) level using 6-31G(d,p) basis set. Further, the calculation of electronic excitations, particularly for valence-like transitions and oscillator strength of melatonin, were calculated employing the all valence electron ZINDO and TDDFT methods. In addition to these 1Hand 13C- chemical shifts were calculated with GIAO method [21,22] using corresponding TMS shielding calculated at the B3LYP/6-311G level. Experimentally observed spectral data of the title compound is found to be well comparable with the data obtained by quantum mechanical methods. Experimental The fine samples of melatonin (Aldrich, 99.5%) was obtained and used as such for the spectral measurements. The FT-IR spectrum of this compound was recorded at room temperature in the region 400 ­ 4000 cm-1 on Brucker Model IFS 66V spectrophotometer using KBr pellet technique with a spectral resolution of 4.0 cm-1. The
electronic absorption spectrum of melatonin was measured in methanol as a solvent using the ELICO SL159 UV/VIS spectrophotometer. The observed experimental FT-IR and Electronic absorption spectra were shown in Figs. 2 and 5. Method of Calculation The entire calculations were performed using the G03 [23] package of programs. Initial geometry generated from standard geometrical parameters [24] and full optimization was carried out at Hartree-Fock level of theory employing 6-31G(d,p) basis set. All the geometries were then reoptimized using 6-31G(d,p) basis set using density functional theory (DFT) [25] employing the Becke's three ­ parameter hybrid functional [26] combined with Lee-Yang-Parr correlation [27] functional (B3LYP) method. The optimized structural parameters were used in the vibrational frequency calculations at the HF and DFT levels to characterize all stationary points as minima. Vibrationally averaged positions of melatonin were used for harmonic frequency calculations resulting in IR frequencies together with intensities. By combining the results of the Chemcraft program [28] with symmetry considerations, vibrational frequency assignments were made. Vertical excitation energies and oscillator strengths were calculated using ZINDO/S and TDDFT [29, 30] methods. Solvent effects were included using B3LYP/631G* basis set. The analysis of the character of the transitions in terms of occupied and virtual orbitals was performed using the transition vectors as the solutions of the finite dimensional time-dependent kohn-Sham eigen value problem [31-33]. 1H- and 13C- chemical shifts were calculated with GIAO method using corresponding TMS shielding calculated at the B3LYP/6-311G level. Results and Discussion Molecular geometry The optimized structure parameters of melatonin calculated by ab initio and DFT-B3LYP levels are listed in the Table 1 in accordance with the atom numbering scheme given in Fig 1. Table 1 compares the calculated bond lengths and angles for melatonin with those experimentally available data [24]. From the theoretical values, we can find that most of the optimized bond angles are slightly larger than the experimental values due to the theoretical calculations belonging to isolated molecules in gaseous phase and the Experimental results
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Experimental and theoretical investigations of spectroscopic properties of N-acetyl-5-methoxytryptamine 151 Fig. 1 Geometry of the melatonin optimized at the B3LYP/6-31G(d,p).
Fig. 2 FT-IR spectrum of melatonin.
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Table 1. Optimized and experimental geometries of the title compound in the ground state.
Parameter Exp. HF DFT-B3LYP
6-31G(d,p ) 6-31G* 6-31G(d,p)
Bond distance (Е)
R(1-2)
1.377 1.36 1.377 1.377
R(1-5)
1.465 1.461 1.469 1.469
R(1-18) 1.014 1
1.018 1.018
R(2-3)
1.225 1.219 1.241 1.241
R(2-4)
1.523 1.517 1.529 1.529
R(4-19) 1.096 1.084 1.096 1.096
R(4-20) 1.096 1.083 1.095 1.095
R(4-21) 1.091 1.079 1.09 1.09
R(5-6)
1.547 1.547 1.557 1.557
R(5-22) 1.093 1.079 1.092 1.092
R(5-23) 1.097 1.082 1.095 1.095
R(6-7)
1.501 1.501 1.502 1.502
R(6-24) 1.098 1.085 1.097 1.097
R(6-25) 1.099 1.085 1.097 1.097
R(7-8)
1.375 1.353 1.377 1.377
R(7-17) 1.443 1.448 1.448 1.448
R(8-9)
1.379 1.383 1.395 1.395
R(8-26) 1.082 1.067 1.078 1.078
R(9-10) 1.382 1.379 1.388 1.388
R(9-27) 1.008 0.994 1.011 1.011
R(10-11) 1.4 1.392 1.4 1.4
R(10-17) 1.418 1.397 1.423 1.423
R(11-12) 1.385 1.368 1.384 1.384
R(11-28) 1.087 1.072 1.084 1.084
R(12-13) 1.416 1.405 1.416 1.416
R(12-29) 1.085 1.069 1.082 1.082
R(13-14) 1.372 1.377 1.392 1.392
R(13-16) 1.391 1.37 1.389 1.389
R(14-15) 1.415 1.433 1.456 1.456
R(15-30) 1.092 1.078 1.09 1.09
R(15-31) 1.099 1.084 1.098 1.098
R(15-32) 1.099 1.084 1.098 1.098
R(16-17) 1.411 1.399 1.407 1.407
R(16-33) 1.084 1.069 1.082 1.082
Parameter Exp. HF DFT-B3LYP 6-31G(d,p ) 6-31G* 6-31G(d,p) Bond angle (°) A(2-1-5) 127.7 128 127.4 127.4 A(2-1-18) 112.6 114.3 114.3 114.3 A(1-2-3) 121.8 121.6 121.8 121.8 A(1-2-4) 115.1 116.6 115.6 115.6 A(5-1-18) 117.6 117.6 117.9 117.9 A(1-5-6) 113.0 112.6 113 113 A(1-5-22) 109 109.9 109.3 109.3 A(1-5-23) 108.5 108.5 108.9 108.9 A(3-2-4) 121.9 121.8 122.6 122.6 A(2-4-19) 111.5 110.7 110.9 110.9
Parameter Exp. HF DFT-B3LYP 6-31G(d,p ) 6-31G* 6-31G(d,p) A(2-4-20) 111.7 111.3 111.5 111.5 A(2-4-21) 107.7 107.5 107.2 107.2 A(19-4-20) 107.8 108.5 108.3 108.3 A(19-4-21) 109.1 109.3 109.3 109.3 A(20-4-21) 109 109.5 109.5 109.5 A(6-5-22) 109.9 109.8 109.4 109.4 A(6-5-23) 109 108.7 108.8 108.8 A(5-6-7) 112.8 111.1 111.1 111.1 A(5-6-24) 108.6 108.6 108.7 108.7 A(5-6-25) 108.6 109.1 108.5 108.5 A(22-5-23) 106.2 107.2 107.3 107.3 A(7-6-24) 110 110.4 110.6 110.6 A(7-6-25) 110.5 110.4 110.4 110.4 A(6-7-8) 126.7 127.3 127.3 127.3 A(6-7-17) 127 126.3 126 126 A(24-6-25) 106.1 107.1 107.3 107.3 A(8-7-17) 106.3 106.4 106.7 106.7 A(7-8-9) 110.1 110.2 109.7 109.7 A(7-8-26) 129.4 129 129.2 129.2 A(7-17-10) 107.3 107.2 107.3 107.3 A(7-17-16) 133.3 132.9 132.9 132.9 A(9-8-26) 120.5 120.8 121 121 A(8-9-10) 109 108.6 109 109 A(8-9-27) 125.4 125.5 125.3 125.3 A(10-9-27) 125.5 125.9 125.6 125.6 A(9-10-11) 130.9 131.3 131.6 131.6 A(9-10-17) 107.3 107.7 107.3 107.3 A(11-10-17) 121.8 121 121.1 121.1 A(10-11-12) 117.9 118.3 118.3 118.3 A(10-11-28) 121.5 121.2 121.4 121.4 A(10-17-16) 119.5 119.9 119.8 119.8 A(12-11-28) 120.6 120.5 120.4 120.4 A(11-12-13) 121.2 121.3 121.5 121.5 A(11-12-29) 121 121.3 121.3 121.3 A(13-12-29) 117.8 117.4 117.3 117.3 A(12-13-14) 114.4 114.8 114.6 114.6 A(12-13-16) 121 120.6 120.5 120.5 A(14-13-16) 124.6 124.6 124.9 124.9 A(13-14-15) 118 120.5 117.6 117.6 A(13-16-17) 118.6 118.8 118.9 118.9 A(13-16-33) 121.3 121.2 121.2 121.2 A(14-15-30) 106.1 105.7 105.1 105.1 A(14-15-31) 111.7 111.4 111.7 111.7 A(14-15-32) 111.7 111.4 111.8 111.8 A(30-15-31) 109.2 109.6 109.6 109.6 A(30-15-32) 109.2 109.5 109.6 109.6 A(31-15-32) 108.9 109.2 109 109 A(17-16-33) 120.1 119.9 119.9 119.9
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Experimental and theoretical investigations of spectroscopic properties of N-acetyl-5-methoxytryptamine 153
Fig. 3 Computed IR spectrum of melatonin at B3LYP and HF level using 6-31G(d,p) basis set.
1100 cm-1 for anisole and its derivatives [45]. In this work, the O-CH3 stretching mode is assigned to medium weak IR band at 1025 cm-1. The theoretically computed value at 1027 cm-1 exactly coincides with the experimental results. The C-O-CH angle bending mode is 3 assigned near 300 cm-1 for anisole and at 421 cm-1 for p-methoxybenzaldehyde by Campagnaro and Wood [46]. Owen and Hester [47], Sundaraganesan et al. [43] and Ramana Rao et al. [48-50] have proposed assignment for C-O-CH angle bending mode in the region 300-670 cm-1 3 for anisole and its derivatives. As this mode lies in the region of the ring planar C-C-C angle bending modes, a strong mixing among these two modes and other planar modes is expected. Accordingly, we have assigned the theoretically calculated value by B3LYP/6-31G(d,p) at 517 cm-1 to C-O-CH angle bending mode. The theoreti- 3 cal computations at the ab initio and density functional levels of the vibrational modes of molecule studied are given in Table 2. For the better comparison between experimental and theoretical IR wavenumbers the plots are drawn, pure Lorentzian band shapes were used with a band width of 10 cm-1, and shown in Fig 3. All the bands are well predicted with experimental values. Charge density The gross atomic charges of melatonin with chemical shifts are given in Tables 3. This data would be used to
explain the preferred position of nucleophilic attack of this molecule. In the present study, we employed HartreeFock and DFT levels to calculate the atomic charges. The gross atomic charges at the carbon C2 attached to nitrogen and oxygen atoms are electron deficient compared to other carbon atoms. In general, electron deficient atoms have the higher value than an electron rich atom. The carbon attached to N and O has lesser electron densities than the other carbons and hence these are more unshielded as shown in Table 3. So the C2 position is the preferred nucleophilic center. The net electron densities on all hydrogen atoms attached to the carbon are electropositive. In general, electron deficient atoms are more unshielded and hence they absorb at downfield. The chemical shift at 170 was assigned to the carbonyl carbon (C-2) and the presence of a carbonyl group was confirmed through the IR absorption at 1681 cm-1. 1H NMR absorption in this molecule can be explained with the atomic charges on the hydrogen atom. The hydrogen atom attached to the nitrogen has higher electron densities than that of carbon. The hydrogen atom attached to the nitrogen (N1, N9) possesses low values of chemical shift compared to hydrogen atoms attached to carbon atoms. Hence, with increase in the electron density of the hydrogen atom the shielding increases and hence values for such proton decrease as show in Table 3. We attempted to correlate the total charge density Canadian Journal of Analytical Sciences and Spectroscopy
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Table 2. Observed and calculated IR wavenumbers (relative intensities) for melatonin using B3LYP/6-31G(d,p), B3LYP/6-31G* and HF/6-31G(d,p) methods.
S. Gunasekaran, R. Arun Balaji, S. Kumaresan, G. Anand, S. Srinivasan
Species W1(A) W2(A) W3(A) W4(A) W5(A) W6(A) W7(A) W8(A) W9(A) W10(A) W11(A) W12(A) W13(A) W14(A) W15(A) W16(A) W17(A) W18(A) W19(A) W20(A) W21(A) W22(A) W23(A) W24(A) W25(A) W26(A) W27(A) W28(A) W29(A) W30(A) W31(A) W32(A) W33(A) W34(A) W35(A)
Exp. B3LYP 6-31G(d,p) Wavenumber Rel. Unscal. Scaled Inten. 26 25 0 32 31 0 52 50 1 76 73 0 82 79 1 106 102 2 158 152 0 173 166 1 187 180 0 206 198 1 246 236 0 263 253 1 303 291 0 336 323 1 353 339 0 401 385 1 431 414 3 460 442 1 475 456 2 498 478 2 501(w) 530 509 1 513(w) 539 518 40 526(s) 550 528 10 549(vw) 567 545 6 580(m) 612 588 0 601(vw) 634 609 0 652(w) 676 650 3 667(w) 704 676 38 703(w) 724 696 4 739(vw) 761 731 0 770(vw) 790 759 1 793(w) 804 772 1 796(w) 809 777 3 806(w) 830 797 7 823(vw) 846 813 2
6-31G* Wavenumber Rel. Unscal. Scaled Inten. 27 26 0 67 64 0 92 89 0 103 99 3 111 107 1 121 116 1 131 126 0 146 140 2 182 175 2 212 204 0 265 255 2 272 262 2 278 267 0 306 294 27 315 303 2 342 329 6 392 377 1 415 399 13 421 405 9 427 411 1 449 432 4 486 468 1 517 497 1 568 546 6 598 575 1 616 593 11 626 602 10 662 637 5 722 695 3 732 704 1 778 748 0 790 760 5 803 772 1 829 797 3 835 803 9
HF 6-31G(d,p) Wavenumber Rel. Unscal. Scaled Inten. 29 26 0 34 31 0 56 51 1 75 68 1 85 77 0 114 103 1 170 153 0 184 166 1 199 180 0 223 201 1 263 237 0 279 252 1 321 290 0 354 320 0 385 348 1 427 385 1 478 431 4 508 459 1 514 464 1 533 481 2 567 512 1 598 540 3 608 549 30 609 550 10 663 598 1 697 629 1 748 675 1 777 701 38 782 706 2 815 736 1 850 767 1 862 778 2 901 813 3 912 823 0 980 885 18
Assignments
CH torsion 3
CH torsion 3
CH2/CNH/CCN
CH3 rocking
CH2-CH2 seesaw
NH/ CH2/CNH
CH /structure deformation 2
CH -CH butterfly/CH rocking
2
2
3
NH/CCH/CNH
CH2/CH3 rocking CH2 rocking CH2 rocking/CNH NH
CH /CH wagging/NH
2
2
NH
CH3 umbrella/CH2 rocking CH3 rocking/NH NH/CH3 rocking/CH2 wagging CCH
CH rocking/CH rocking/CNC
3
2
CH2 deformation
CH2/CH3 deformation
C-O-CH3/CCC
CH
NH/CH rocking 3 CCH/CH rocking/CNC 3 CH
CH2 CH2-CH2 rocking CH
CH/ring deformation
C-CH /ring deformation 3 C-CH3 / CH2-CH2 CH
C-CH2
Experimental and theoretical investigations of spectroscopic properties of N-acetyl-5-methoxytryptamine 155 Canadian Journal of Analytical Sciences and Spectroscopy
Table 2 continued. Observed and calculated IR wavenumbers (relative intensities) for melatonin using B3LYP/6-31G(d,p), B3LYP/6-31G* and HF/6-31G(d,p) methods.
Species W36(A) W37(A) W38(A) W39(A) W40(A) W41(A) W42(A) W43(A) W44(A) W45(A) W46(A) W47(A) W48(A) W49(A) W50(A) W51(A) W52(A) W53(A) W54(A) W55(A) W56(A) W57(A) W58(A) W59(A) W60(A) W61(A) W62(A) W63(A) W64(A) W65(A) W66(A) W67(A) W68(A) W69(A) W70(A)
Exp. B3LYP 6-31G(d,p) Wavenumber Rel. Unscal. Scaled Inten. 837(w) 862 828 11 861(w) 928 892 11 897(w) 941 904 4 925(w) 996 957 5 988(w) 1002 963 0 993(w) 1006 967 4 1009(w) 1027 987 14 1042(w) 1056 1015 3 1056(m) 1087 1044 7 1091 1048 7 1109 1066 5 1117 1073 9 1137(m) 1162 1116 0 1146(w) 1177 1131 0 1159(w) 1195 1148 1 1218 1170 41 1187(m) 1245 1196 14 1256(m) 1250 1201 29 1278(vw)1302 1251 8 1294(vw)1324 1272 7 1317(w) 1338 1286 8 1334(vw)1342 1289 17 1359 1306 16 1369(w) 1374 1320 7 1388 1334 5 1387(w) 1411 1356 4 1403(w) 1431 1375 7 1429(m) 1446 1389 3 1437(w) 1485 1427 3 1457(w) 1491 1433 20 1466(w) 1500 1441 7 1478(w) 1521 1461 12 1499(w) 1522 1462 11 1502(vw)1543 1483 1 1517(vw)1554 1493 0
6-31G* Wavenumber Rel. Unscal. Scaled Inten. 863 830 2 900 866 9 956 920 3 976 939 0 987 949 6 1023 984 2 1052 1012 3 1069 1028 3 1072 1031 4 1097 1055 10 1117 1075 1 1124 1081 10 1147 1103 0 1180 1135 1 1183 1138 1 1222 1176 21 1260 1212 8 1275 1227 40 1304 1254 3 1320 1270 0 1335 1284 12 1357 1305 16 1372 1320 15 1382 1329 1 1410 1356 6 1418 1364 28 1428 1374 6 1460 1405 1 1482 1426 14 1495 1438 7 1512 1455 5 1518 1460 4 1533 1475 2 1545 1486 1 1547 1488 3
HF 6-31G(d,p) Wavenumber Rel. Unscal. Scaled Inten. 1005 907 1 1010 912 3 1065 961 11 1077 972 4 1095 988 3 1124 1015 7 1143 1032 4 1155 1043 0 1178 1063 18 1183 1068 2 1191 1075 5 1200 1083 6 1263 1140 0 1273 1149 0 1292 1166 8 1320 1191 14 1328 1199 13 1349 1218 34 1368 1235 1 1404 1267 3 1426 1287 8 1450 1309 30 1453 1311 5 1480 1336 11 1495 1349 11 1532 1383 3 1555 1404 3 1558 1406 7 1612 1455 9 1621 1463 0 1623 1465 18 1633 1474 6 1648 1487 11 1654 1493 4 1663 1501 3
Assignments
CH
ring deformation
CCH
CH rocking/CH rocking
3
2
CH -CH deformation
2
2
CH3/CH2 rocking
CH3 rocking/ O-CH3
CH3 rocking
NH/ CNH
CNH/ NCH
CCN/ CNC/ HCH
CH2 rocking in CH3 CH3 CCH
CH2-CH2 Indole deformation
C-O / COC
C-N
C-N /C-CH2 stretching C-CH2 CCH/ HCN
CH wagging 2 NCH
CH3/ CH2 CH3 CNH
CCH/ CNH
CNH
CC+ CH 3 CH3 CCH/ CNH
CCC / CCH
CH2 scissoring CH deform 3 CH scissoring 2
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Table 2 continued. Observed and calculated IR wavenumbers (relative intensities) for melatonin using B3LYP/6-31G(d,p), B3LYP/6-31G* and HF/6-31G(d,p) methods.
S. Gunasekaran, R. Arun Balaji, S. Kumaresan, G. Anand, S. Srinivasan
Species W71(A) W72(A) W73(A) W74(A) W75(A) W76(A) W77(A) W78(A) W79(A) W80(A) W81(A) W82(A) W83(A) W84(A) W85(A) W86(A) W87(A) W88(A) W89(A) W90(A) W91(A) W92(A) W93(A)
Exp. B3LYP 6-31G(d,p) Wavenumber Rel. Unscal. Scaled Inten. 1552(w) 1556 1495 6 1558(w) 1561 1500 3 1569(w) 1571 1509 11 1586(w) 1580 1518 1 1601(w) 1614 1551 9 1647(w) 1655 1590 10 1681(s) 1741 1673 100 2923(w) 3019 2901 11 2955(w) 3052 2932 7 2970(w) 3062 2942 2 3069 2949 14 3001(w) 3077 2956 9 3033 (w) 3092 2971 5 3048(w) 3118 2996 4 3052(w) 3143 3020 5 3152 3028 7 3091(w) 3174 3050 3 3203 3077 3 3233 3106 3 3236 3109 1 3278 3150 0 3373(w) 3525 3387 4 3609 3468 26
6-31G* Wavenumber Rel. Unscal. Scaled Inten. 1549 1490 5 1559 1500 41 1565 1506 5 1638 1576 1 1675 1611 6 1703 1638 12 1858 1787 100 3143 3024 12 3164 3044 7 3170 3050 2 3195 3074 6 3201 3079 13 3207 3085 5 3235 3112 4 3262 3138 4 3272 3148 6 3285 3160 3 3318 3192 3 3349 3222 3 3354 3227 1 3386 3257 0 3531 3397 4 3596 3459 25
HF 6-31G(d,p) Wavenumber Rel. Unscal. Scaled Inten. 1671 1508 1 1678 1515 2 1692 1527 3 1712 1545 1 1757 1586 7 1798 1623 8 1897 1712 100 3193 2882 6 3209 2896 5 3219 2905 1 3238 2923 3 3245 2929 10 3250 2933 6 3276 2957 3 3304 2982 5 3311 2989 7 3325 3001 2 3368 3040 2 3399 3068 2 3405 3073 1 3437 3102 0 3785 3416 7 3859 3483 26
Assignments
CC+ NH-CO
CC+ CCC
CCC
CC
CC ring
CC ring
C=O
CH3 sym, II CH3
CH sym, II CH
2
2
CH sym, I CH
3
3
CH2 sym, II CH2/ CH2 asym, I CH2
CH3 asym, II CH3
CH2 sym, I CH2 / CH2 asym, II CH2
VCH3 asym, I CH3
CH asym, II CH
2
2
CH asym, II CH
3
3
CH3 asym, I CH3
C-H, indole
C-H, indole
C-H, indole
C-H, indole
N-H,
N-H, indole.
Experimental and theoretical investigations of spectroscopic properties of N-acetyl-5-methoxytryptamine 157
Table 3. Gross atomic charge density of melatonin.
Atom 1N 2C 3O 4C 5C 6C 7C 8C 9N 10 C 11 C 12 C 13 C 14 O 15 C 16 C 17 C 18 H 19 H 20 H 21 H 22 H 23 H 24 H 25 H 26 H 27 H 28 H 29 H 30 H 31 H 32 H 33 H
B3LYP/6-31G(d,p) -0.9139 0.8483 -0.6304 -0.7085 -0.1365 -0.4605 -0.0919 0.1417 -0.9972 0.4136 -0.2064 -0.2497 0.4005 -0.7372 -0.2666 -0.2801 -0.1135 0.3581 0.2345 0.2261 0.2586 0.2372 0.2348 0.2224 0.2270 0.2604 0.3639 0.2423 0.2623 0.2316 0.1956 0.1916 0.2418
Atomic Charge density
B3LYP/6-31G* HF/6-31G(d,p)
-0.6886
-0.7014
0.6281
0.6420
-0.4991
-0.5003
-0.6610
-0.6641
-0.1839
-0.1868
-0.4688
-0.4719
-0.0068
-0.0053
0.0759
0.0851
-0.8132
-0.8303
0.3680
0.3774
-0.1891
-0.1813
-0.1920
-0.1913
0.3096
0.3203
-0.5551
-0.5761
-0.3289
-0.3217
-0.2766
-0.2713
-0.0144
-0.0216
0.3110
0.3119
0.2151
0.2154
0.2125
0.2110
0.2284
0.2305
0.2249
0.2254
0.2122
0.2150
0.1960
0.1983
0.2074
0.2096
0.2035
0.2024
0.3187
0.3162
0.1790
0.1780
0.1987
0.1978
0.2184
0.2201
0.1940
0.1942
0.1910
0.1915
0.1852
0.1812
1H & 13C NMR Chemical ()shift/ppm 170 23 40 28 110 124 128 112 109 155 55 102 128 8.0 1.8 1.8 1.8 3.4 3.4 2.8 2.8 6.8 10.5 7.0 6.8 3.7 3.7 3.7 6.8
Table 4. The UV-Vis excitation energies (E) and oscilaator strengths (f) for melatonin calculated at the TDDFT and ZINDO methods.
States Observed
obs (nm)
S1
298
S2
277
S
225
3
E (nm) 334(- *) 306( - *) 264( - *)
Calculated
ZINDO/S
f
E (nm)
0.2989 272( - *)
0.0112 247( - *)
0.0160 229 (- *)
DFT f 0.0116 0.0930 0.0001
Table 5. Calculated polarizabilities for melatonin
Basis set B3LYP/6-31G(d,p) B3LYP/6-31G* HF/6-31G(d,p)
Dipole moment XX
4.2837
173.572
4.4398
178.486
4.5677
161.765
XY -0.353 0.378 1.082
YY
XZ
178.625 16.36
184.433 17.6
165.931 15.829
YZ 1.757 1.592 1.732
ZZ 66.281 67.357 64.465
() 139.492 143.425 130.721
Canadian Journal of Analytical Sciences and Spectroscopy
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S. Gunasekaran, R. Arun Balaji, S. Kumaresan, G. Anand, S. Srinivasan
belonging to the molecule in the solid state. Comparing bond angles and lengths of B3LYP with those of HF, as a whole the former are on higher side than the latter and the HF calculated values correlates well compared to those with the experimental results. In spite of the differences, calculated geometric parameters represent a good approximation and they are the basis for calculating other parameters, such as vibrational frequencies and thermodynamic properties. Vibrational assignments According to the theoretical calculations, melato- nin has a structure of C1 point group symmetry. The molecule has 33 atoms and 93 modes of fundamental vibrations. The calculated fundamental vibrational modes together with intensities were calculated on the basis of HF and DFT methods are collected in Table 2. Table 2 also presents observed FTIR and scaled wave numbers with relative intensities. The relative intensities were obtained by dividing the computed value by the intensity of the strongest C=O stretch. The computed intensities show marked deviations from the observed values. One may note that the computed wavenumbers correspond to the isolated molecular state whereas the observed wave numbers correspond to the solid state spectra. Chemcraft [28], a graphical interface, was used to assign the calculated harmonic wavenumbers using scaled displacement vectors to identify the motion of modes. The calculated vibrational wave numbers using different basis sets were compared with experimentally observed values. Some bands found in the predicted IR spectra were not observed in the experimental spectrum of melatonin. The correlation graphs between the unscaled calculated and observed results for the assigned fundamentals in the fingerprint region (500-1700 cm-1) are shown in Fig 4. NH and C-H stretching frequencies were not used as input data to the corelasyon procedure. These neglected fundamentals often show significant uncertainties. Theoretical harmonic frequencies typically overestimate observed fundamentals due to the neglect of mechanical anharmonicity, electron correlation and basis set effects. Therefore, we have used the scaling factor values of 0.9062 and 0.9679 for HF and B3LYP, respectively [34, 35]. If a CH3 group is present in a compound it provokes to exist two asymmetric and one symmetric stretching vibration. In the present study, two methyl groups which are in conjugation with the adjacent carbonyl group may alter the electronic structure of the compound and influence the vibrational frequencies and intensities
[36, 37]. The IR bands observed at 3091 and 3001 cm-1 are assigned to asymmetric stretching and the bands observed at 2970 and 2923 cm-1 are assigned to symmetric stretching. The bands corresponding to torsion, twisting, wagging and bending vibrations of CH3 groups are summarized in Table 2. For the assignments of CH group frequencies, 2 basically six fundamentals can be associated to each CH2 group namely CH2 symmetric stretch; CH2 asymmetric stretch; CH2 scissoring and CH2 rocking which belongs to in-plane vibrations and two out-of-plane vibrations viz., CH2 wagging and CH2 twisting modes, which are expected to be depolarized. The asymmetric CH stretching vibrations are generally observed in the 2 region 3100-3000 cm-1, while the symmetric stretch will appear between 3000 and 2900 cm-1 [38-40]. The CH2 asymmetric vibrations were observed in FTIR at 3052 and 3033 cm-1 and symmetric stretch appeared at 2955 cm-1, respectively. The bands corresponding to scissoring and bending vibrations of CH group are presented 2 in Table 2. The structural unit C=O has an excellent group frequency, which is described as a stretching vibration. Since the C=O group is a terminal group, only the carbon is involved in a second chemical bond. This reduces the number of force constants determining the spectral position of the vibration. The C=O stretching vibration usually appears in a Frequency range that is relatively free of other vibrations. For, example, in many carbonyl compounds the double bond of the C=O has a force constant different from those of such structural units such as C=C, C-C, C-H, etc.; only structural units such as C=C have force constants of magnitudes similar to that of the C=O group. The C=C vibration could interact with the C=O if it was of the same species, but generally it is not. Almost all carbonyl compounds have a very intense and narrow peak in the range of 1800-1600 cm-1. This is why this region is considered as a very important region by organic chemists. In this present study the C=O stretching vibration observed at 1681 cm-1 and the C-O stretch observed at 1187 cm-1 are in excellent agreement with theoretically predicted frequency obtained in B3LYP/631G(d,p) values [41, 42]. The identification of C-N vibrations are a difficult task since the mixing of vibrations possible in this region. The IR bands appearing at 1256 and 1278 cm-1 are assigned to C-N vibrations. All these results agree with Sundaraganesan et al. [43] and Krishnakumar et al. [44]. The O-CH mode is assigned in the region 10003
Volume 53, No. 4, 2008
Experimental and theoretical investigations of spectroscopic properties of N-acetyl-5-methoxytryptamine 159 Fig. 4 Scaled calculated vibrational frequencies in comparison to the experimentally obtained data. All units in (cm-1). Canadian Journal of Analytical Sciences and Spectroscopy
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S. Gunasekaran, R. Arun Balaji, S. Kumaresan, G. Anand, S. Srinivasan
Fig. 5 UV spectrum of melatonin dissolved in methanol. on the hydrogen and carbon atoms with the chemical shifts obtained in NMR spectrum [51, 52]. The 1H- and 13C- chemical shifts were calculated with the B3LYP/6311G optimized geometries by GIAO method [21]. In order to obtain the calculated results comparable with the experimental data, we have transformed the absolute shieldings returned by the program in chemical shifts subtracting to the absolute shielding of TMS and the absolute shieldings of the molecule under examination: rel = rel - abs. In particular, each value of the absolute shieldings of the TMS was obtained with the same level of the absolute shieldings of the TMS, which was found with the same level of theory used in the determination of absolute shielding of the compound. There is satisfactory correlation between the total charge density and chemical shifts (Experimental & Theoretical). Electronic spectrum of melatonin In Fig 5, we present the recorded spectrum of melato- nin, dissolved in methanol. In Table 4, we compare excitation energies of - * transition with the experimental values and present the results obtained using TDDFT method with B3LYP functional as well as ZINDO/S method. As expected, the ZINDO method overestimates excitation energy of the - * transition for all studied indole derivatives. The difference between the experimental band maxima and ZINDO values deviates from 36 nm. Since the ZINDO results are of a qualitative character, we compare the results for the second singlet state S2 (- *) only. Recent papers of Robert J. Cave et al. [53] , C. Jamorski et al. [54] and R. Auernschmitt et al. [55] have shown that the Becke's three-parameter Volume 53, No. 4, 2008
LYP functional combined with appropriate basis set can provide more accurate excitation energies. This observation is consistent with the results obtained in the present study. The differences between the experimental band maxima for the S2(- *) state and the B3LYP excitation energies for molecule was 25 nm. For comparison, the respective difference for the ZINDO method is -39 nm. It is evident that the B3LYP excitation energies are more accurate than ZINDO values. For this reason, the frontier orbital analysis of the nature of excitations is performed using the B3LYP method. In order to gain some insight into the nature of the S (- *) excited states 2 for investigated molecules, we performed the analysis in the spirit of the single excited configurations. In the case of this molecule, we found that the -* transition is dominated by the HOMO-LUMO transition. The plot of the HOMO and LUMO contour surfaces for melatonin is presented in Fig 6.
Molecular polarizability
One of the objectives of this investigation is to study
the effect of the basis set on molecular polarizability of
melatonin using Gaussian 03W. In this study the com-
putation of molecular polarizability of melatonin with
different levels are reported. Here, is a second rank
tensor property called the dipole polarizability and mean
polarizability () are evaluated using Eq. (1) [56].
()=1/3(xx+ YY + ZZ )
(1)
The calculated polarizabilities using different basis
sets for melatonin molecule are summarized in Table
5.
Experimental and theoretical investigations of spectroscopic properties of N-acetyl-5-methoxytryptamine 161
Conclusion The above-discussed results of the study lead to the following conclusions: 1. The frequency assignments performed for the first time from FTIR spectrum recorded for Melatonin. Theoretical DFT and ab initio calculations of the vibrational spectra of the molecule presented in this paper compared with the FTIR spectrum. 2. Geometries reported within the limits of accuracy of available experimental data. The molecular geometry of Melatonin is best at the B3LYP level of DFT. 3. Mulliken charges of Melatonin at different levels were calculated and results discussed. 4. Molecular polarizability of Melatonin discussed and reported. References 1. A. B. Lerner, J. D.Case, Y.Takahashi, T. H. Lee, W. Mori, J. Am. Chem. Soc., 80, 2587 (1958). 2. A. B. Dollins, I. V. Zhdanova, R. J. Wurtman, H. J. Lynch, M. H. Deng, Proc. Natl. Acad. Sci. U.S.A., 91, 1824 (1994). 3. R. J. Reiter, News Physiol. Sci., 6, 223 (1991). 4. R. J. Reiter, J. FASEB., 9, 526 (1995). 5. P. Lissoni, S. Meregalli, L. Nosetto, S. Barni, G. Tancini, V. Fossati, G. Maestroni, Oncology, 53, 43 (1996). 6. P. Lissoni, S. Barni, F. Brivio, F. Rossini, L. Fumagalli, A.Ardizzoia, G.Tancini, Oncology, 52, 360 (1995). 7. G. J. Maestroni, V.Covacci, A.Conti, Cancer Res., 54, 2429 (1994). 8. M. Abe, R. J. Reiter, P. B. Orhii, M. Hara, B. Poeggeler, J. Pineal Res., 17, 94 (1994). 9. W. D. Flitter, Brit. Med. Bull., 49, 545 (1993). 10. M. A. Pappolla, R. J. Reiter, T. K. Bryant-Thomas, B. Poeggeler, Curr. Med. Chem., 3, 33 (2003). 11. K. Mishima, M. Okawa, Y. Hishikawa, S. Hozumi, H. Hori, K. Takahashi, Acta Psychiat. Scand., 89, 1 (1994). 12. Vijayalaxmi, R. J. Reiter, M. L. Meltz, Mutat. Res., 346, 23 (1995). 13. M. A. Livrea, L. Tesoriere, D. D'Arpa, M. Morreale, Free Radical Biol. Med., 23, 706 (1997). 14. A. G. Turjanski, F. Leonik, D. A Estrin, R. E. Rosenstein, F. Doctorovich, J. Am. Chem. Soc., 122, 10468 (2000).
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