Molecular Structure, Hydrogen Bonding Interactions and Docking Simulations of Nicotinamide (Monomeric and Trimeric Models) by Using Spectroscopy and Theoretical Approach

The present work focuses on the structural properties, spectroscopic signatures, intermolecular hydrogen bonding interactions, chemical and biological activity of nicotinamide (NIC) based on its monomeric and trimeric models using density functional theory and vibrational spectroscopy. FT-IR and FT-Raman spectra were obtained using the double-side forward-backward acquisition mode under vacuum. UV-Vis absorption spectra were recorded in methanol and compared with the calculated values. Geometry optimization and vibrational wavenumbers were obtained with the aid of Gaussian 09 program packages. The structural analysis of NIC revealed that the trimeric model was in better agreement with the experimental values than the monomer due to the incorporation of nearest hydrogen bond interactions. Spectroscopic results showed that NH2 and C = O groups of NIC were involved in intermolecular interactions in the trimeric model. The natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses determined the presence, strength as well as nature of the hydrogen bonds were partially covalent. The lesser value of the HOMO-LUMO energy gap for the trimeric model indicated higher reactivity than monomer. Moreover, chemical reactivity was calculated using molecular electrostatic potential surface (MESP) and reactivity descriptors. The docking studies for NIC with several targets explored its biological activity.</p

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

<div><p>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 (<i>E</i>⁽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) (ρ_BCP), Laplacian of ED (▽²ρ_BCP) and total electron energy density (<i>H</i>_BCP) 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.</p></div