Ab Initio Study of the Isomerism, Structure, and Stability of Li[XMY_2] Molecules and Li[XM'Y_2]~+ Ions (M = C, Si; M' = N, P; X = F, Cl, H; Y =O, S)

摘要

Ab initio correlated MP4/6-31 (+)G**//MP2/6-31G*+ZPE(MP2/6-31G*) calculations of potential energy surfaces in the vicinity of the key structures were performed for planar trigonal dioxo and dithio halide and hydride anions XMY~-_2 = XCO~-_2, XsiO~-_2, XCS~-_2, and XsiS~-_2; isoelectronic molecules XM'Y_2 = XNO_2, XPO_2, XNS_2, and XPS_2; and their lithium salts Li[XMY_2] and ion-molecular complexes Li[XM'Y_2]~+ with substituents X = F, Cl, or H and Y = O or S. Equilibrium geometric parameters of isomers, vibrational frequencies and intensities, relative energies, and energies of decomposition into fragments were determined, and trends in their behavior in related series were traced. Differences between salt molecules and isoelectronic ion-molecular complexes, the character of polarization and deformation of XMY~-_2 anions and XM'Y_2 molecules in the field of the Li~+ cation at different coordination modes, and some aspects of the mutual influence of substituents upon replacement of light central atoms C and N with Si and P, F with Cl and H, O with S, etc. were analyzed. The possibility of identification of isomers by IR spectroscopy was discussed. Cases of energetically close "valence-bonded" salt structures Li~+ XMY~-_2 and "nonvalence-bonded" molecular complexes LiX centre dot MY_2 or LiYH centre dot MY and possible conversion between them (valence-nonvalence isomerism) were considered.