Ab initio study of isomerism of Li(2)AB(2) molecules and Li(2)AB(2)(+) ions with 16 valence electrons

Ab initio MP2/6-31G*//HF/6-31G* + ZPE(HF/6-31G*) and MP4SDTQ/6-31G*//MPu2/6 -31G* + ZPE(MP2/6-31G*) calculations of potential energy surfaces (PESs) of Li(2)AB(2) salt molecules (Li2BeO2, Li2MgO2. Li2BeS2, Li2MgS2, Li2CN2, Li2 SiN2, and Li2CP2) and Li(2)AB(2)(+) ions (Li2BO2+, Li2AlO2+, Li2BS2+, Li2Al S2+, Li2N3+, Li2PN2+, and Li2P3+) with 16 valence electrons were performed. For oxides and nitrides, the global minimum corresponds to the symmetrical linear D-infinityh structure; bent planar C-2v structure and nonplanar C-2 structure with the phi (LBA) angle about 90 degrees -110 degrees are more favorable for sulfides and phosphides. The second low-lying isomer C-s with the nonlinear NCN2- dianion was found for the Li2CN2 molecule; in this iso mer, one cation is coordinated to one and the second, to two nitrogen atoms . The most favorable isomer of the Li2CP2, molecule has the C-2v structure of a half-open butterfly, in which both cations are coordinated to the carb on atom and the angles phi (PCP) and phi (LiCLi) are about 170 degrees and 111 degrees, respectively. For the Li2P3+ ion, the bipyramidal isomer D-3h in the triplet state was localized, along with the bidentate C-2v isomer in the singlet state. The isomers are close to each other in energy. In the f ormer isomer, both cations are tridentately coordinated to the cyclic P-3(- ) anion. The equilibrium geometrical parameters, relative and decomposition energies of isomers, and frequencies and IR intensities of normal vibratio ns, which can be used to identify the isomers by IR spectroscopy, were calc ulated. The trends in the molecular properties in different series of Li2AB 2 molecules were analyzed, and compounds with the unusual coordination of t he anion and cations were predicted. It was found that metaborates and alum inates possess a very high affinity to the Li+ ion (approximate to 75-87 kc al/mol) and Li2BeO2 beryllates are highly stable toward decomposition into oxides (approximate to 140 kcal/mol). The deformation and polarization of A B(2)(-) and AB(2)(2-) anions and changes in their vibrational frequencies i n the field of two cations, depending on their coordination modes, are disc ussed.