Ab initio study of isomerism, structure, and stability of dimeric molecules of beryllate salts (LiBeH3)(2) and (LiBeF3)(2) and their fragments

Ab initio calculations of potential-energy surfaces are performed in the vi cinity of key structures of nonrigid dimer molecules of beryllate and fluor oberyllate complex salts (LiBeX3)(2), binuclear Be2X5- anions and Be2X62- d ianions, and Be2X4 molecules (X = H, F) in terms of the QCI-SD(T)/6-31(+)G* *//MPu6-31G* + ZPE(MP2/6-31G* and MP2/6-31(+)G*//HF/6-31G* + ZPE(HF/6-31G*) approaches. In Be2X4 molecules and Be2X62- dianions, the D-2h structures b and 1 with two bridges are strongly preferable, while the T, D-3h three-br idged bipyramidal configuration is the most stable for the Be2H5- anion. In the case of Be2F5- anion, the M, B, and T structures with one, two, and th ree bridges, respectively, are close in energy so that, unlike the above sy stems, a nonrigid intramolecular rearrangement can occur, which is accompan ied by the exchange between bridging and terminal fluorine atoms. In the di meric salt molecules (LiBeX3)(2), the most stable configuration is a nonrig id structure of the (Li+)(2) . Be2X62- ion triad with the Be2X62- diborane- like dianion and the cations migrating easily over and under the [Be(X-b)(2 )Be] four-membered ring in the vicinity of the flat global minimum I, D-2h for hydride and Ia, C-2h for fluoride. In addition to this structure, a num ber of isomers are determined, including (Li+)(2) . Be2X62- (Ib) with the c ations in the plane of the ring, (HLi22+) . Be2X5- (IV) with the Be2X5- two -bridged anion and the [BeX2Li2X] six-membered ring, (BeX3) . (Li+)(2) . (B eX3-) (II) with two lithium bridges BeX3- anions and the [BeX2Li2X2Be] eigh t-membered ring, and some others. All of these rings are characterized by n onrigidity with respect to strong angular distortions at slight changes in energy. Equilibrium geometric parameters, relative energies and dissociatio n energies of isomers, and frequencies and IR intensities of normal vibrati onal modes are determined. Similarities and distinctions between hydrides a nd fluorides are considered, and distortions and polarization of anions und er the influence of cations are analyzed for various types of their mutual orientation.