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Molecular Properties of the Valence Isomers of Diazines: Density Functional Theory (DFT) and Møller Plesset (Mp2) Methods

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Molecular Properties of the Valence Isomers of Diazines: Density Functional Theory (DFT) and Møller Plesset (Mp2) Methods International Journal of Modern Chemistry, 2018, 10(1): 117-137 International Journal of Modern Chemistry ISSN: 2165-0128 Journal homepage: www.ModernScientificPress.com/Journals/IJMChem.aspx Florida, USA Article Molecular Properties of the Valence Isomers of Diazines: Density Functional Theory (DFT) and Møller Plesset (mp2) Methods IBRAHIM, Asiata Omotayo, OYEBAMIJI Abel Kolawole and SEMIRE Banjo* Department of Pure and Applied Chemistry, Faculty of Pure and Applied Sciences, Ladoke Akintola University of Technology, PMB 4000, Ogbomoso, Oyo State, Nigeria * Author to whom correspondence should be addressed; E-Mail: [email protected] Article history: Received 20 February 2018, Revised 25 April 2018, Accepted 30 April 2018, Published 21 May 2018. Abstract: Density functional theory (B3LYP) and Moller-Plesset perturbation theory to the second order (MP2) methods with 6-31G(d),6-31+G(d,p), 6-311G (d,p), 6-311+G(d,p) and 6-311++G(d,p) basis sets have been used to investigate some diazine isomers. The equilibrium geometries, relative stabilities, strain energies, vibrational frequencies and global nucleophilicity indices for fifteen isomers are reported. The relative energies show that Dewar isomers (except 1,4-DBHD) are more stable compared to the Diazyl-isomers. The isomers with N-N and/or N-C bonds in the center are less stable, this is observed for both Dewar and Diazyl-isomers; therefore nitrogen atom(s) in the center of the isomers decreases the stability. Keywords: Diazines, relative stabilities, nucleophilicity index, quantum mechanical methods 1. Introduction Pyridazines are diazines group of compounds that are formally derived from benzene by the Copyright © 2018 by Modern Scientific Press Company, Florida, USA Int. J. Modern Chem. 2018, 10(1): 117-137 118 replacement of two of the ring carbon atom by nitrogen. Three isomeric diazines are possible with the nitrogen atoms in 1-2, 1-3, or 1-4 relationship, giving rise to the pyridazines, pyrimidines and pyrazines respectively. Pyridazine (1,2-diazine) and its benzo analogues like cinnoline (1,2-diazanaphthalene), phthalazine and benzo(d)pyridazine have been known since the nineteenth century, although the basic synthetic principles and reactivity were investigated in the early years, interest in these compounds revived only during the past 35 years because of their biological activities [1,2] On the other hand, the classes of pyrimidines possess a broad spectrum of biological effectiveness such as anti-tubercular [3], calcium channel blockers [4-6], antitumor [7], antimicrobial [8] and cardiovascular agent [9] and many classes of chemotherapeutic agents containing pyrimidine nucleus are in clinical use. More recently, a new series of pyrimidine derivatives were synthesized and screened for biological activities [10]. Gas-phase electron diffraction and some other methods that had been used to carry out analysis on the molecular structures of pyrazine, pyridazine and pyrimidine and the geometrical structures were effectively compared [11, 12]. The infrared (IR) spectra collected, in the 2000–750 cm-1 range, for the compounds at room temperature, were first reported in the book by Barnes et al., in 1944 [13]. However, the IR spectrum of pyrimidine was discussed briefly by Brownlie [14] and Short and Thompson [15]. The first systematic comparative studies on the vibrational spectroscopy of these diazines namely pyrazine, pyridazine and pyrimidine have been reported [16,17]. In 1957 the complete infrared and Raman assignment of pyridazine was reported [18-21]. Several other researchers have carried out works on these three diazines by using various experimental and theoretical methods for the vibrational assignment of the compounds [22-30]. In the recent time, the role diazine derivatives plays in organic synthesis of diverse biological compounds of pharmaceutical importance cannot be over emphasized. Therefore, it would be of interest to study some other valence isomers of pyridazine, pyrimidine and pyrazine to shed light on the nucleophilicity and thermodynamic stability of these isomers. On this purpose, quantum mechanical methods were employed to study the geometries and electronic properties of fourteen diazine isomers of which three were well known. From the theoretical point of view, especially the Dewar and Diazyl- isomers would be compared to the known diazines, the role diazine derivatives plays in organic synthesis of diverse biological compounds of pharmaceutical importance cannot be over emphasized as a contribution to the understanding of diazine isomers other than pyrazine, pyridazine and pyrimidine as shown in Figure 1. To the best of our knowledge, experimental and theoretical calculations of these isomers have not been reported. However, Dewar and Aza-isomers of pyridine have been studied both at experimental and theoretical levels [31-33]. Copyright © 2018 by Modern Scientific Press Company, Florida, USA Int. J. Modern Chem. 2018, 10(1): 117-137 119 1 1 1 1 4 1 4 1 2 2 3 2 4 2 3 2 2 3 2,5-Diaza-bicyclo[2.2.0]hexa-2,5-di pyrazine (PYZ), D2h pyridazine (PYD), C2V ene (2,5-DBHD), C2 1 4 1 1 2 1 1 2 2 3 3 4 2 3 4 1 2 2 1-(azirin-1-yl)azirine 2-(2H-azirin-2-yl)-2H-azirine 3-Cycloprop-2-enyl-3H-diazirine trans-BZY, C2h (cis-2,2-BZY), C2 (i) trans-CEZ, Cs (ii) cis-CEZ, C1 1 1 2 1 1 4 1 4 2 3 3 4 1 2 2 2 2 2,3-Diaza-bicyclo[2.2.0]hexa-2,5-di 3 2-(2H-azirin-2-yl)-2H-azirine ene (trans-2,2-BZY), C1 pyrimidine (PYM), C2V (2,3-DBHD, Cs) 1 1 2 2 4 1 1 2 1 3 3 2 3 4 2 2 4 1-(2H-azirin-2-yl)azirine 1,4-Diaza-bicyclo[2.2.0]hexa-2,5-di 3 (i) trans-1,2-BZY), C1 ene (1,4-DBHD),D2h (ii) cis-1,2-BZY, C1 2,6-Diaza-bicyclo[2.2.0]hexa-2,5-di ene (2,6-DBHD), Cs Figure 1: Schematic structures of isomeric Diazines 2. Computational Details The conformation equilibrium searching that replaces global equilibrium geometry with the lowest energy equilibrium conformation was performed on each isomer with Monte Carlo methods using MMFF force field [34].The lowest equilibrium conformation of each isomer was further optimized at the DFT level of theory with the standard 6-31G(d), 6-31+G(d,p), 6-311G (d,p), 6-311+G(d,p) and 6- 311++G(d,p) basis sets. The DFT calculations were carried out with the three-parameter B3LYP density functional, which includes Becke’s gradient exchange correction [35] and the Lee, Yang, Parr correlation functional [36]. Moller-Plesset perturbation theory (MP2) calculations were performed on DFT optimized geometries at each basis set [37]. Single point energy calculation was carried out on each isomer at the same level of theory [38, 39]. The absorption transitions were calculated from the optimized geometry in the ground S0 state at TD-DFT/6-311++G (d,p) theory. We also examined HOMO and LUMO levels; the energy gap was evaluated as the difference between the HOMO and LUMO energies. Copyright © 2018 by Modern Scientific Press Company, Florida, USA Int. J. Modern Chem. 2018, 10(1): 117-137 120 All calculations were performed by using Spartan 14 program [40]. 3. Result and Discussion 3.1. Geometries and Stability The studied diazine valance isomers were optimized geometries at the B3LYP(DFT) and MP2 with 6-31G(d), 6-31+G(d,p), 6-311G (d,p), 6-311+G(d,p) and 6-311++G(d,p) basis sets, although only skeletal geometrical parameters calculated at B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) are listed in Tables 1 and 2. The calculated bond lengths and bond angles are in good agreement with those reported in the literatures for PYM, PYD and PYZ. For instance, the bond lengths calculated at B3LYP/6- 311++G** (MP2/6-311++G**) level for N1-C1 and C1-C2 are 1.335 (1.343) and 1.395 Å (1.399 Å) respectively in PYZ. These bond lengths are experimentally observed at 1.335 and 1.398 Å [41]. For PYD, the N1-N2, N1-C1, C1-C2 and C3-C4 bond lengths are calculated to be 1.381, 1.334, 1.386 and 1.381 Å at B3LYP/6-311++G**; 1.341, 1.344, 1.400 and 1.389 Å at MP2/6-311++G**; 1.337, 1.338, 1.400 and .1.385 Å respectively as observed experimentally [42]. For PYM, the N1-C1, N1-C4 and C1- C2 bond lengths are calculated to be 1.335, 1.336 and 1.391Å at B3LYP/6-311++G**; 1.343, 1.342 and 1.395 at MP2/6-311++G** and these are reported as 1.340, 1.393 and 1.393 Å respectively [43,44]. However, Almenningen et al., 1977 reported the experimental values for C-C and N-C bonds as 1.393Å and 1.341Å for PYM [45]. The bond distances calculated at MP2/6-311++G** level of theory are more close to the experimental values than that of B3LYP/6-311++G** results but both methods reproduced the experimental values in terms of bond angles. For instance, N1-C1-C2 (N1-C4-N2) bond lengths obtained at B3LYP/6-311++G**, MP2/6-311++G** are 122.22 (127.00) and 122.24 (127.45) respectively compared to experimental values of 122.30 (127.02) for PYM [43]; N1-C1-C2 (N1-N2-C4) are 123.63 (119.49), 124.07 (119.49), (123.62 (119.50) for B3LYP/6-311++G**, MP2/6-311++G** and experimental respectively for PZD [42]; N1-C1-C2 (C2-N2-C3) are 121.97 (116.07) and 122.42 (115.17) for B3LYP/6-311++G** and MP2/6-311++G** respectively compared to 121.98 (115.40) obtained experimentally for PYZ [41].
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