This dissertation applies quantum chemical methods to study the structure,properties, and the mechanisms, for the formation of adenine in interstellar space and under pre-biotic conditions. The mechanism for the remarkable activation of inert dinitrogen at room temperature, using lithium clusters, has been reported. Further, computational methods are applied to design molecules with novel bonding patterns.Two major unsolved mechanistic puzzles are addressed. The first one is: how did biomolecules originate under prebiotic condition starting from abiotic precursors? The theoretical explorations (using Density functional and ab-initio calculations) of detailed,plausible, step by step mechanisms, for the chemical evolution of adenine (a biomolecule, which is one of the four fundamental nucleobases of DNA) under pre-biotic Earth (or interstellar medium) starting from abiotic starting materials, is reported in this dissertation. The pathway was unknown, but possible intermediates have been isolated. Experimental investigations are precluded by low yields and many side reactions. The thermodynamically feasible mechanism to explain how adenine might be built up from the combination of five hydrogen cyanide molecules has been reported.The second unsolved mechanistic puzzle addressed is: what is themechanism for activation of inert molecular nitrogen, by using lithium clusters? Activation and reduction of dinitrogen remains a fundamental challenge to synthetic as well as to theoretical chemistry. Lithium is exceptional among the main group elements in that it slowly reacts with N2 at room temperature, leading finally to (NLi3)x. This dissertation is aimed at understanding the molecular dinitrogen activation mechanism using model lithium clusters, and how this model study can be used to design mulimetallic catalysts, which will be able to activate as well as cleave N2.This dissertation further reports the theoretical design of molecules with novel bonding patterns, such as planar tetra-coordinated carbon (ptC) in transition metal clusters. The molecular orbitals, electronic structures, energies, and magnetic properties of CM4n+ species (where M represents isoelectronic combinations of Cu, Ni, Ag, and Pd, and n is the charge) have been studied to determine the plausible candidate for experimental detection of ptC in transition metal cluster.The extension of the concept of aromaticity, from 2 electron singlet species, to open-shell doublet systems with one delocalized electron, is discussed. The degree of one electron delocalization using energetic, geometric and magnetic criteria for a multitude of theoretically designed two and three dimensional cationic, anionic and neutral monoradical systems is reported in the dissertation.
