Ab initio study of the isomerism, structure, and stability of dimeric saltmolecules (LiMgH3)(2) and (LiMgF3)(2) and their fragments

Ab initio QCI-SD(T)/6-31(+)G**//MP2/6-31G* + ZPE(MP2/6-31G*) and MP2/6-31()G*//HF/6-31G* + ZPE(HF/6-31G*) calculations of the potential energy surfac e (PES) in the vicinity of key structures of structurally nonrigid dimeric salt molecules (LiMgX3)(2), Mg2X5- binuclear anions, Mg2X62- dianions, and Mg2X4 molecules (X = H, F) were performed. Double-bridged structures b and 1 of the D-2h Symmetry were found to be the most favorable for Mg2X4 and Mg 2X62-, respectively, and triple-bridged bipyramidal configuration T, D-3h w as found to be most stable for the Mg2X5- anions. The most favorable struct ure of salt dimers is the nonrigid "skewed" structure of the ion triad (Li)(2) . Mg2X62- (Ia, C-2h) with the diborane-like Mg2X62- dianion and two Li f cations migrating above and below the four-membered [Mg(X-b)(2)Mg] ring. In addition to this structure, a number of isomers were located, including the ion triad (Li+)(2) . Mg2X62- (Ib) with the cations lying in the plane o f the [Mg(X-b)(2)Mg] ring, the ion pair (HLi2+) . Mg2X5- (IV) with the doub le-bridged Mg2X5- anion and the six-membered (MgX2Li2X) ring, structure II with two lithium bridges between the MgX3- anions and the eight-membered [M gX2Li2X2Mg] ring, and other structures. The equilibrium geometry parameters , relative and dissociation energies of isomers, and frequencies and intens ities of normal vibrations were calculated. The results obtained are compar ed with the data on similar beryllium compounds.