Theoretical studies of novel electronic effects in radicals and radical ions

Abstract

The thesis entitled "Theoretical Studies of Novel Electronic Effects in Radicals and Radical Ions" consists of five chapters. Chapter I is a general introduction to the various techniques that are commonly employed to study radicals and radical ions. Diverse experimental methods like mass spectrometry, electron transmission spectroscopy, etc., which are used for the generation and detection of radical ions in the gas phase on the one hand and electrochemical and matrix isolation studies involving radicals and radical ions in the condensed phase on the other are outlined. The computational approach is pointed out to be an attractive alternative to obtain detailed information about the structure and energetics of these transient species. The computational methodologies employed in the present investigation, viz., ab initio and semi-empirical molecular orbital theories, are described. Strategies required to reliably quantify electronic effects in open-shell systems are discussed. Chapter II involves a detailed account of the structure and energetics of a novel class of radical ions called distonic ions. Distonic ions are hydrogen-shifted isomers of classical radical ions, with their formal charge and radical centres located on two different atoms. The preference for radical ions to adopt these types of structures is remarkable if one recalls that the corresponding neutral molecules are not even minima on the potential energy surface in most cases. The stabilities of distonic ions relative to classical isomeric forms have been modelled by means of idealized reactions involving separation of charge and radical of smaller fragments. Ab initio energies at high levels of theory for these reactions indicate the existence of several distonic ions more stabilized than previously known organic distonic ions. This approach has led to the hypothesis that the tendency to adopt distonic structures should increase with the difference in electronegativities of the charge and radical-bearing atoms of the distonic isomer. For example, silanol radical cation has been shown to favour the distonic structure (SiH?OH•?) over the classical structure (SiH?OH?•) to a greater extent than methanol radical cation. Calculations carried out at ab initio and semi-empirical levels have revealed that the preference for distonic structures is, in general, more among silicon-based radical cations than their carbon-based analogues, thus confirming the original proposition. Radical anions too have been shown to prefer distonic-type structures. Going by the argument based on electronegativity difference, organometallic systems like CH?BH?, CH?BeH, etc., are the ones that have been chosen as model systems to study the tilt towards distonic structures on addition of an electron. Among the various radical anions that have been studied, methyl borane radical anion is the one that has been found most promising to adopt a distonic structure. The tendency to assume distonic structures on addition or removal of an electron has been proved to be quite general. Various phosphorus- and boron-based radical ions have been shown to favour hydrogen-rearranged structures. Experimentally accessible systems which are likely to adopt distonic structures have been studied. The investigations that have been carried out on radical ions have led to the interesting suggestion that some molecules would undergo hydrogen migration both on removal and addition of an electron. Chapter III deals with the estimation of the magnitude of the captodative effect in radical ions. Calculations carried out using ab initio and semi-empirical methods indicate that the effect is much more pronounced in radical cations and radical anions than in neutral radicals. Isodesmic reactions, including those which take into account the non-additivity effects in the corresponding even-electron systems, have been made use of for the estimation of the effect. The enhanced captodative stabilization in radical ions has been rationalized on the basis of qualitative molecular orbital theory. Implications for preferred mass spectral fragmentation pathways are pointed out. Chapter IV focuses attention on the competition between two important non-additivity effects operating in some disubstituted radicals, namely, the anomeric effect and the one due to extended ?-delocalization. Different conformations of such radicals which enjoy stabilization due to varying extents of these non-additivity effects have been examined. Many symmetrically disubstituted radicals, including dihydroxy and diamino methyl radicals, have been shown to adopt non-planar asymmetric structures which are seldom proposed for them. The results have wide implications in determining the course of stereospecific radical reactions in carbohydrate substrates. Attention has been drawn to the fallacy of using these disubstituted radicals as reference species for estimating other non-additivity effects. Chapter V deals with an MNDO study of organometallic complexes of the type Li(R)?, with R = C?H?, C?H? or C?H? for n = 1 or 2. Several structures proposed for these complexes have been examined and characterized by carrying out frequency calculations. Based on the results obtained for the various complexes, the complexation has been interpreted to be an interaction between Li? and the ligand radical anion. For the ligands ethylene and acetylene, the tris-complexes too have been studied. The preferred structures have been rationalized on the basis of Hückel and Möbius topologies of the ligand ?* orbitals.