An ab initio and density functional theory investigation of the structuresand energetics of halide ion-alcohol complexes in the gas phase

The gas-phase clustering equilibria of halide ions to a homologous series o f alcohol molecules, X- + HOR reversible arrow X-(HOR) (X = F, Cl, Br, I; R = CH3, CH3CH2, (CH3)(2)CH, (CH3)(3)C), have been investigated using ab ini tio (MP2(full)) and density functional theory (B3LYP) computational methods . For both methods, extended basis sets, including diffuse and polarization functions for all atoms and anions, except I-, were used. For I- three dif ferent effective core potentials (ECP) were used to test their suitability for these systems. Comparing the Delta H(298)degrees and Delta S(298)degree s values obtained with various experimental data indicates that the MP2 and MP2//B3LYP methods perform best. Structural and spectroscopic features, as well as charge distributions, show interesting trends for the various X-(H OR) complexes, and the intrinsic contributions of the halide ions and the a lcohol molecules to these trends are discussed. Finally, two-dimensional po tential energy surface scans were performed for the X-(HOCH3) complexes at the MP2/6-311++G(d,p) level of theory. These surfaces reveal the asymmetric nature of the potential energy surface for the heavier halide ions, and th e "floppy" nature of all the halide ion adducts.