THEORETICAL STUDIES ON STRUCTURE, ISOMERISM AND FLUXIONALITY IN SOME BORON COMPOUNDS

Abstract

Boron is the second most catenating element after carbon and it can form complex self bonded molecular networks. The chemistry of boron exhibits many unique features that distinguish it from any other element. Thus boron demonstrates exceptional ability in molecular, ionic, and solid state environments to form very stable compounds exhibiting structures based on icosahedral and other deltahedral units. In addition, boron forms a variety of very stable mononuclear tetrahedral as well as polynuclear cage anions including some of the most weakly coordinating anions currently known. Boron hydrides have long being of interest to theoretical and experimental chemists due to their electron deficiency and bonding pattern, which is unique to them. These electron deficient molecules are extensively used in chemistry. Carboranes were discovered in the search for high energy fuels during the middle 1950s. Geometrical structures of carboranes are similar to that of boranes, the difference being that some boron atoms in the polyhedral boranes are replaced by carbon atoms. In this work studies are carried on borane dianions and carboranes as these compounds which contain boron deltahedra display fluxionalism in some of their isomers and are characterized by unusual stability compared with the reactive and frequently unstable neutral boron hydrides. Diborane and related species are also studied, since its fluxional nature has been predicted byNMR studies. All calculations have been performed using the Gaussian 98W. The thesis is divided into the following chapters. The First chapter presents a general introduction and an overview of the bonding in boranes and the fluxionalism displayed by them. Different types of borane clusters and borane polyhedra have been discussed, emphasis being placed on the c/oso-boranes. The related carboranes are also discussed. A critical review of the available literature on computational studies on boranes is presented, comparison with relevant experiment is also made wherever possible. The Second chapter outlines the computational methods used. A brief introduction to the techniques of geometry prediction, using ab initio SCF and (i) Density Functional methods, and of characterization of stationary points on the potential energy surface are given. The Third chapter deals with the computational studieson the dianion B8H82" and isoelectronic carboranes. Geometry optimization of the different possible isomers ofB8H82" at Hartree-Fock and DFT/B3LYP level, using 6-31G, 6-31G**, 6-31++G**, and Dunnings basis sets both with and without polarization functions have been performed. Along with the earlier reported isomers some new stable structures are obtained which are reported along with their energies. Transition state calculations for conversion between the isomers of different but close energies have been performed, and the barriers are reported. A possible interchange mechanism among the isomers is described wherever a transition structure is obtained. All the possible closocarboranes related to B8H82" were studied, i.e., forms with formulae CB7H8", C2B6H8, C3B5H7. A dianion carborane series with the formula CB7H72~, C2B6H62~, C3B5H52~, C4B4H4 ", C5B3H3 ', C6B2H2 "C7BH "and C82~ were also studied, all the carboranes in this series are isoelectronic with B8H82\ This series involves successive substitution of a B-H unit with a carbon atom. The geometries were optimized at Hartree-Fock level using 6-3IG basis set. Frequency analyses were also performed to confirm whether the structures obtained were true minima on the PES. It is found that the cage structures become unstable relative to non-cage ones as the number of carbons increases in this series. Fluxionality among the isomers of CB7H8", C2B6H8, C3B5H7 was also studied. The energies of the transition state and the interconversion mechanism is also reported along with the energy barriers. The energies and the structures of all the stable isomers of the dianion series obtained are reported. The Fourth chapter of the thesis incorporates computational studies on the dianion BnHn2~ and related carboranes. All the possible structures of B,,Hn2" have been investigated, and the energies and structures are reported. Geometry optimization of all the polyhedral structures with molecularformula B,,Hn2" and their corresponding carboranes were carried out using 6-3IG* and D95V** basis sets at RHF and B3LYP levels. The nature of each stationary point was probed by analytical frequency calculations. Keeping the fluxional nature ofthe dianion in mind a possible transition structure was also optimized. A Cs structure has been identified as a transition state in the interconversion of the C2v stable states. Calculations have also been performed on four different types of carboranes of B,,Hn2" i.e., CB,0Hn" , (ii) C2B9Hn, C3B8H|0 and C4B7H9 which are isoelectronic with BuHn2". The energies and the structures of all these carboranes are also reported. The Fifth chapter of the thesis reports investigation on diborane and related species. Ab initio calculations have been performed with 6-3IG, 6-3IG*, 6-3IG**, and the D95V Dunning's basis sets both with and without polarization functions and with the cc-pVDZ basis set at Hartree-Fock, hybrid density functional theory with B3LYP functional and MP2 levels of theory. All structures were optimized using analytical gradients and harmonic frequencies were calculated using analytical second derivatives. Temperature dependent NMR spectra shows diborane to be fluxional, so a possible rearrangement mechanism was sought for diborane. Different types of transition states i.e., mono-bridged and tri-bridged were tried for diborane, but the only transition structure obtained corresponded to the dissociation of the diborane into two BH3 molecules. This studywas motivated by existing studies on A1BH6, in which case a tribridged transition state was found and it was suggested that in the case of diborane a similar transition state is involved. While the A1BH6 results were confirmed in our studies which extended existing work of Barone et al., a similar transition state could not be located for B2H6. However in related molecules like B2H5.NH2 where NH2 group replaces one of the bridge hydrogens a single bridged transition state could be characterized and a mechanism for proton interchange via this transition state is suggested. In the case of diborane in ether solution, an ether mediated mechanism for proton interchange looks plausible but no transition structure could be found. Possible structures of the related ionic species B2H5+ and protonated diborane (B2H7+) were also investigated to determine stable structures. In these cases transition structures for proton interchange could also be located.