Ab initio study of structurally nonrigid molecules of LiSiX5 salts and LiPX5+ (X = F, H) ion-molecular complexes

Ab initio calculations of potential-energy surfaces are performed in the vi cinity of key structures of the molecules of lithium salts and ion-molecula r complexes with SiF5- and PF5 pentafluorides and hypothetical SiH5- and PH 5 hydrides. The correlated MP4SDTQ/6-31 G**//MP2/6-31G* + ZPE(MP2/(6-31G*) and MP2/6-31G*//MP2/6-31G* + ZPE(MP2/(6-31G*) approximations are used for h ydrides and fluorides, respectively. The dissociation energies, equilibrium geometric parameters, and relative energies of tri-, bi-, and monodentate structures and normal vibration frequencies and IR intensities of isomers a re determined. The energy barriers to the migration of the Li+ cation aroun d the AX(5) bipyramidal cores in salts and to the migration of the LiX diat omic molecules around the AX(4) tetrahedra in the weak van der Waals comple xes XLi . AX(4) are estimated. The trends in molecular characteristics in s eries of related compounds LAX(5) with various A and X atoms and similariti es and distinctions between hydrides and fluorides and those between neutra l molecules and ion-molecular complexes are analyzed. The shifts and splitt ings of vibrational frequencies and the geometric deformation and polarizat ion of the cores (cis and trans influence) in the field of the cation are d iscussed for various modes of their mutual orientation.