COMPARATIVE THEORETICAL-STUDY OF LEWIS ACID-BASE COMPLEXES OF BH3, BF3, BCL3, ALCL3, AND SO2

Quantum mechanical calculations at the MP2/TZ2P level of theory predic t geometries and bond energies of donor-acceptor complexes of the Lewi s acids BH3, BF3, BCl3, AlCl3, and SO2 which are in very good agreemen t with experimental gas-phase values. Strong donor-acceptor bonds are calculated for the boron complexes OC-BH3 (D-0(298) = 25.1 kcal/mol), H3N-BH3 (D-0(298)= 30.7 kcal/mol), Me(3)N-BH3 (D-0(298) = 41.1 kcal/mo l), H3N-BF3 (D-0(298) = 22.0 kcal/mol), Me(3)N-BF3 (D-0(298) = 32.9 kc al/mol), H3N-BCl3 (D-0(298) = 29.7 kcal/mol), and Me(3)N-BCl3 (D-0(298 ) = 40.5 kcal/mol). Weakly bound van der Waals complexes are predicted for OC-BF3 (D-0(298) = 4.7 kcal/mol), HCN-BF3 (D-0(298) = 7.2 kcal/mo l), MeCN-BF3 (D-0(298) = 9.1 kcal/mol), OC-BCl3 (D-0(298) = 4.0 kcal/m ol), and MeCN-BCl3 (D-0(298) = 6.4 kcal/mol). Intermediate dissociatio n energies are calculated for the BF, complexes with Me(2)O (D-0(298) = 17.3 kcal/mol), benzaldehyde (D-0(298) = 13.0 kcal/mol), and 2-methy lacrolein (D-0(298) = 12.8 kcal/mol). The strongest donor-acceptor bon d is calculated for Me(3)N-AlCl3, (D-0(298) = 49.3 kcal/ mol). A stron g bond is also predicted for EtCClO-AlCl3 (D-0(298) = 24.8 kcal/mol), while the complex Me(3)N-SO2 is more weakly bound (D-0(298) = 15.5 kca l/mol). The bond lengths of the Lewis acids are longer in the complexe s than in the isolated molecules. A good correlation is found between the calculated bond strengths of the BF3 complexes and the lengthening of the B-F bond. The NBO partitioning scheme suggests that there is n o correlation between the charge transfer and the bond strength. The t opological analysis of the electron density distribution shows that th e donor-acceptor bonds of the strongly bound boron complexes have sign ificant covalent contributions, while the weakly bound boron complexes are characterized by electrostatic interactions between the Lewis aci d and base. However, the nature of the strongly bound AlCl3 complexes is different from that of the strongly bound boron complexes. The stro ngest donor-acceptor bond calculated for Me(3)N-AlCl3 is characterized by electrostatic interactions and very little covalent contributions. The bond shortening of the donor acceptor bonds between the gas phase and the solid state is calculated to be mainly due to short-range dip ole-dipole interactions. The geometry-optimized dimer and tetramer of H3N-BH3 and the dimer of H3N-BF3 have significantly shorter B-N bonds than the monomer.