Etude théorique en méthode de la fonctionnelle de la densité des composés mixtes aluminium-bore et carboranes

Abstract

This thesis describes a density functional theory (DFT) studies of electronic structures of real and hypothetical boranes, heteroboranes and metalaheteroboranes clusters, by the use of B3LYP functional, 6-311++G(d,p) basis in chapter two, three and four and LANL2DZ in chapter five with the Gaussian98 program. The first chapter is an introduction to the density functional theory (DFT), geometry of borane clusters and PSEP theory. The second chapter concerns the prediction of geometries, relative stabilities, electronic structures and bonding analysis of mixed AlmBn-mHn2- and CmBn-mHnm-2 (n = 6, 10, 12 and m = 1, 2) clusters, being compared to the BnHn2- ones. Therefore, the DFT results suggest that the replacing of boron by aluminium or carbon is governed by natural net charges following Gimar‟s and Williams‟s rules. The AlmBn-mHn2- structures are relatively distorted compared to those of BnHn2- and CmBn-mHnm-2. In AlmBn-mHn2- structures Al atoms prefer the adjacent sites, however for the C2Bn-2Hn cluster cages, the carbon atoms are positioned at diametrically opposed sites. The large HOMO–LUMO gaps show that the predicted clusters have chemical stabilities, principally, those of AlmBn-mHn2- ones, which are not experimentally isolated. Based on the polyhedron skeleton electron pair theory (PSEP), structural preferences in 10- and 12-vertex closo- and nido heteroboranes, GamBn-mHn2-, GemBn-mHnm-2 and AsmBn-mHn 2m-2 [n=10,12 and m =1,2] were investigated in chapter two. The different cluster stabilities were studied according to gimarc‟s and wiliams rules, our calculations showed that As monosubstituted deviate to these rules. The AsmBn-mHn2m-2 [n=10 and m =2] structures leads to nido type clusters however, GemBn-mHnm-2 [n=10 and m =2] give rise to closo isomers closnes in energy.The galaboranes exhibits smallest HOMO-LUMO gaps suggesting that they are with lower chemical stabilities, in accordance with the computed ionisation potential Substitution effects on AlB11H122- and 1,2-Al2B10H122- cluster cage throw replacing the hydrogen attached to aluminium atom by electrophiles such as CH3, C2H5, C3H7, NH2, NO2,OCH3 and Cl, was the subject of chapter three. The structural chemistry of 12 vertex metalaheteroboranes was studied in chapter 4 using LANL2DZ basis set. The tendency and degree of slip in these species could be attributed to orbital control. Vibrational frequency calculations were performed on all the optimized geometries to verify that these structures are local minima on the potential energy surface.