Investigation of Weak Molecular Interactions by Multinuclear NMR Spectroscopic Techniques and Theoretical Computations

Abstract

The work reported in this thesis contains extensive studies carried out to investigate the weak molecular interactions in small organic molecules by utility of multinuclear and multidimensional NMR spectroscopic techniques and multitude of Density Functional based theoretical computations. The chemical shift of NH protons in benzoyl urea and the solvent dilution experiments infer the existence of dimers. The DOSY NMR spectroscopy conforms the existence of dimer formation in benzoyl urea derivatives. The formation of dimers is attributed to intermolecular HB and supported by DMSO titration studies. Along with the intermolecular HB, we have also observed the formation of intramolecular HB in all the dimers. The different 1D and 2D NMR experiments revealed the existence of both inter- and intra- molecular HB in all the synthesized derivatives of benzoyl urea. The existence of benzoyl ureas as dimer has been further confirmed by the DFT based computational analysis. The computational studies have been carried out by using QTAIM, NBO, NCI, EDD plots and MESP plots. In QTAIM analysis, we have discussed about Koch and Popelier criteria required for HB. Most of the criteria are satisfied by both inter- and intra- molecular HBs. The NCI analysis also revealed existence of inter- and intramolecular HB in all the synthesized derivatives of benzoyl urea. The NBO studies provide the information about the strength of HBs in terms of orbital stabilization energy. The EDD plots showed the formation of inter- and intra- molecular interactions in terms of electron density shifts involved in bonding. MESP plots drawn revealed the electrostatic nature of HB in all the analysed dimers.