Quantum mechanical and spectroscopic (FT-IR, FT-Raman,1H,13C NMR, UV-Vis) studies, NBO, NLO, HOMO, LUMO and Fukui function analysis of 5-Methoxy-1H-benzo[d]imidazole-2(3H)-thione by DFT studies
Graphical abstract
The harmonic vibrational frequencies, infrared intensities, Raman activities, bond lengths, bond angles were calculated for 5-Methoxy-1H-benzo[d]imidazole-2(3H)-thione molecule by DFT method using 6-311++ G(d, p) basis set. The calculated geometrical parameters and frequencies were in good agreement with experimental values. 1H, 13C NMR, UV-Vis spectral analyses of the title molecule were conducted by experimental and Density Functional Theory (DFT). The HOMO, LUMO energies, Fukui function and NBO analysis were also worked out.
Introduction
Benzimidazole is an important bioactive compound owing to its presence in a wide range of therapeutic agents like antihypertensives, antihistaminics, proton pump inhibitors, antiparasitics, antiviral, antidiabetic, anticancers, antifungals, anti-inflammatory agents, anticonvulsants, analgesics and anticoagulants [1], [2], [3], [4], [5], [6], [7]. It is also a part of several drug molecules like albendazole, mebendazole, thiabendazole which serve as antihelmintics; omeprazole, lansoprazole, pantoprazole as proton pump inhibitors; astemizole as antihistaminic; enviradine as antivirals; candesarten cilexitil and telmisartan as antihypertensives [8]. 5-Methoxy-1H-benzo[d]imidazole-2(3H)-thione (5MBIT), is an important precursor required for the synthesis of omeprazole. 5MBIT can exist in tautomeric forms thione and thiol depending on the solvent conditions [9] (Fig. 1). Generally1H-benzo[d]imidazole-2(3H)-thione (BIT) exists predominantly in the thione form in the solid state and also in polar solvents [10], [11], [12], [13]. The proton always tends to be with electronegative nitrogen in such derivatives. The reason favouring the thione tautomeric form could be that the NH bond forms a stronger hydrogen bond with the solvent or another BIT molecule than the SH bond [12]. The crystal structure of 5MBIT has established thione form of the compound [14]. In the present study IR and 1H NMR spectral data suggested the presence of thione form in the solid state as well as in solution.
5MBIT in the solid form was procured from Sigma Aldrich (USA) with a stated purity of 99% and used as such for the studies. The Raman spectrum of 5MBIT was recorded in the region 4000-100 cm−1on Bruker RFS 27 FT-Raman spectrometer. The FT-IR spectrum of the compound as KBr pellet was recorded in the region 4000-400 cm−1 on Perkin Elmer Spectrum one FT-IR spectrometer. The UV-Vis spectra were recorded on Varian, Cary 5000 UV-Vis spectrophotometer. 1H and 13C NMR spectra were recorded in dmso-d6 solvent on AV500 - Bruker FT-NMR Spectrometer.
The entire quantum chemical calculations were carried out on Gaussian 03 program [15] using DFT (B3LYP) methods with 6–311++G (d, p) basis sets. Gauge-Independent Atomic Orbital (GIAO) 1H and 13C chemical shift calculations of the 5MBIT were done by the same method. Optimized geometrical parameters, fundamental vibrational frequencies, thermodynamical data were also worked out. IR and Raman spectra of 5MBIT were simulated. The vibrational frequencies calculated at B3LYP/6-311++G (d, p) level were scaled by 0.961 [16]. Calculations of the potential energy distribution (PED) were done with veda 4 program [17]. Electronic transitions in the UV-Vis spectral region were calculated by time dependent (TD) DFT method with B3LYP level and 6-311++G (d, p) basis set. HOMO and LUMO energies were determined. The Natural bond orbital (NBO) analysis, Fukui function and Molecular electrostatic potential (MEP) analysis of 5MBIT were done. Nonlinear optical properties such as dipole moment, anisotropy of polarizability and first order hyper polarizability were also calculated.
The Raman activities (Si) were calculated by Gaussian 03 program and suitably adjusted by the scaling procedure and subsequently converted to relative Raman intensities (Ii) using Equation (1) based on the basic theory of Raman scattering [18], [19].where is the exciting frequency (in cm−1), is the vibrational wave number of the ith normal mode, h, c and kb are universal constants, and f is the suitably chosen common scaling factor for intensities of all the peaks. For the plots of simulated IR and Raman spectra, pure Lorentzian band shapes were used with full width at half maximum of 10 cm−1.
Section snippets
Structural analysis
The X-ray crystal structure analysis of 5MBIT molecule was reported by Odabasoglu et al. [14]. In the present work the molecular structure along with numbering of atoms of 5MBIT (Fig. 2) was obtained from the analysis carried out on Gaussian 03. The optimized structural parameters calculated by DFT (B3LYP) with 6–311++G (d, p) basis set and the reported experimental values are listed in Table 1. A comparison between the experimental and the calculated values reveal that there are marginal
NMR spectra
The full geometry optimization of 5MBIT was performed using DFT (B3LYP)/6-311++G (d, p) method. Gauge-Independent Atomic Orbital (GIAO) 1H and 13C chemical shift calculations on 5MBIT were done by the same method. The isotropic shielding values of tetramethylsilane (TMS) were used to calculate the chemical shifts. The isotropic shielding values of TMS are 187.2 and 32.6 in 13C and 1H NMR spectra respectively. The chemical shifts were recorded in dmso-d6 using TMS as internal reference Fig. 9,
Conclusions
5-Methoxy-1H-benzo[d]imidazole-2(3H)-thione was theoretically optimized using B3LYP methods with 6–311++G (d,p) basis set. A comparison between the experimental and calculated structure showed a good correlation. The complete vibrational assignment with PED % was done. The vibrational modes were compared with the experimental IR and Raman data. The simulated experimental and theoretical spectra showed good relationship. The Nonlinear optical (NLO) properties were calculated theoretically. The
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