Bhawani Datt Joshi

Molecular structure, vibrational spectra and HOMO, LUMO analysis of yohimbine hydrochloride by density functional theory and ab initio Hartree-Fock calculations.

Yohimbine hydrochloride (YHCl) is an aphrodisiac and promoted for erectile dysfunction, weight loss and depression. The optimized geometry, total energy, potential energy surface and vibrational wavenumbers of yohimbine hydrochloride have been determined using ab initio, Hartree-Fock (HF) and density functional theory (DFT/B3LYP) method with 6-311++G(d,p) basis set. A complete vibrational assignment is provided for the observed Raman and IR spectra of YHCl. The UV absorption spectrum was examined in ethanol solvent and compared with the calculated one in gas phase as well as in solvent environment (polarizable continuum model, PCM) using TD-DFT/6-31G basis set. These methods are proposed as a tool to be applied in the structural characterization of YHCl. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) with frontier orbital gap are presented.

Study of polymorphism in imatinib mesylate: A quantum chemical approach using electronic and vibrational spectra

Imatinib mesylate, 4-(4-methyl-piperazin-1-ylmethyl)-N-u[4-methyl-3-(4-pyridin-3-yl)pyrimidine-2-ylamino)phenyl]benzamide methanesulfonate is a therapeutic drug that is approved for the treatment of chronic myelogeneous leukemia (CML) and gastrointestinal stromal tumors (GIST). It is known that imatinib mesylate exists in two polymorphic forms α and β. However, β-form is more stable than the α-form. In this work, we present a detailed vibrational spectroscopic investigation of β-form by using FT-IR and FT-Raman spectra. These data are supported by quantum mechanical calculations using DFT employing 6-311G(d,p) basis set, which allow us to characterize completely the vibrational spectra of this compound. The FT-IR spectrum of α-form has also been discussed. The importance of hydrogen-bond formation in the molecular packing arrangements of both forms has been examined with the vibrational shifts observed due to polymorphic changes. The red shift of the NH stretching bands in the infrared spectrum from the computed wavenumber indicates the weakening of the NH bond. The UV-vis spectroscopic studies along with the HOMO-LUMO analysis of both polymorphs (α and β) were performed and their chemical activity has been discussed. The TD-DFT method was used to calculate the electronic absorption spectra in the gas phase as well as in the solvent environment using IEF-PCM model and 6-31G basis set. Finally, the results obtained complements to the experimental findings.

Quantum chemical and experimental studies on the structure and vibrational spectra of an alkaloid--Corlumine.

The study concentrates on an important natural product, phthalide isoquinoline alkaloid Corlumine (COR) [(6R)-6-[(1S)-1,2,3,4-Tetrahydro-6,7-dimethoxy-2-methylisoquinolin-1-yl] furo [3,4-e]-1,3-benzodioxol-8(6H)-one] well known to exhibit spasmolytic and GABA antagonist activity. It was fully characterized by a variety of experimental methods including vibrational spectroscopy (IR and Raman), thermal analysis (DSC), UV and SEM. For a better interpretation and analysis of the results quantum chemical calculations employing DFT were also performed. TD-DFT was employed to elucidate electronic properties for both gaseous and solvent environment using IEF-PCM model. Graphical representation of HOMO and LUMO would provide a valuable insight into the nature of reactivity and some of the structural and physical properties of the title molecule. The structure-activity relationship have been interpreted by mapping electrostatic potential surface (MEP), which is valuable information for the quality control of medicines and drug-receptor interactions. Stability of the molecule arising from hyper conjugative interactions, charge delocalisation has been analyzed using natural bond orbital (NBO) analysis. Computation of thermodynamical properties would help to have a deep insight into the molecule for further applications.

A comparative computational study on molecular structure, NBO analysis, multiple interactions, chemical reactivity and first hyperpolarisability of imatinib mesylate polymorphs using DFT and QTAIM approach

Imatinib, a phenylaminopyrimidine compound is a therapeutic drug for treatment of chronic myelogeneous leukaemia and gastrointestinal stromal tumours. It is well known that imatinib mesylate (ImM) exists in two polymorphic forms α and β. In this work, a computational study on molecular properties of ImM polymorphs is presented using density functional theory, B3LYP functional and 6-311G(d,p) as basis set. Natural bond orbital analysis is carried out to investigate the various conjugative and hyperconjugative interactions within the molecule and their second-order stabilisation energy (E(2)). The local nucleophilic reactivity descriptors such as Fukui functions (), local softness () and electrophilicity indices () analyses are carried out to determine the reactive sites within the molecule. To determine strength and nature of intra- and intermolecular interactions, topological parameters as electron density (ED) (ρBCP), Laplacian of ED (▽2ρBCP) and total electron energy density (HBCP) at bond critical points have been analysed by ‘quantum theory of atoms in molecules’ in detail. The computed first hyperpolarisability (β0) for both forms of ImM molecule (10.927 and 10.354 × 10− 30 esu) suggests that the investigated molecule is an attractive object in future for non-linear optical properties. Molecular electrostatic potential surface of ImM has been mapped to predict the inhibitory activity and binding affinity with a panel of protein tyrosine kinases.