Experimental and theoretical studies of hydrogen bonding.

Abstract

The theoretical and experimental work in this thesis is primarily aimed at i) the quantification of the strengths of a number of hydrogen bonded systems, and ii) exploring the relationships that exist between the various physico-chemical properties determined in this study, which are related to the hydrogen bonding phenomenon. To this end a three part study of some hydrogen bonded systems has been undertaken. The study involves using a number of theoretical and experimental procedures, including a theoretical ab initio molecular orbital study, infrared spectroscopic determinations and a thermodynamic investigation involving measuring enthalpies and volumes of mixing and applying a theoretical model of interacting liquid mixtures. Conclusions based on ab initio molecular orbital theory, thermodynamic and infrared spectroscopic results conducted in this work include: i) the proton donating ability of the three hydrogen donor moieties studied in this work decreases in the order O-H > N-H ~ S-H, ii) the proton accepting competence of the three electron donor atoms considered in this work decreases in the order N > 0 > S in all cases except in the liquid phase systems involving dipropylamine and propane-1-thiol as proton donors, where the proton accepting ability of the atoms is in the opposite order i.e. S > 0 > N, and iii) a direct correlation exists between the shift in the A-H stretching wavenumber and the hydrogen bond interaction energy. . Although a number of factors influence the stability of the hydrogen bond, it was also tentatively concluded that in liquid phase systems involving weakly self-associated hydrogen bond donor molecules, the available surface area of the proton accepting atom becomes the dominant strength determining factor, otherwise factors such as basicity and electronegativity dominate.