AB-INITIO STUDY ON THE GEOMETRIES OF HXLIF (X=B, AL)

For the first timet the geometries of the two lacking electron fragmen ts HXLiF(X = B, Al) are investigated by ab initio method at RHF/6-31G and RHF/6-31G levels. The results indicate that the different basis s ets have trivial effect on the equilibrium geometries and their energy orders. Both HBLiF and HAlLiF have four types of equilibrium geomtrie s, [GRAPHICS] (1), [GRAPHICS] (2), the four-membered ring [GRAPHICS] ( 3) and the three membered ring [GRAPHICS] (4). The total energies of t he geometries for HBLiF decrease in the order of (1) > (3) > (4) > (2) . For HAlLiF, they decrease in the order of (1) > (2) > (4) > (3). To analyse characteristics and stabilities of the geometries the Mulliken bond orders have also been reported. In the geometry la (a denotes th e geometries of HBLiF, b those of HAlLiF), the bond order of BLi (0.13 8, at 6-31G) is much smaller than those of 2a (0.399) and 4a (0.330), while the bond length in la (0.2394 nm, 6-31G) bigger than those of 2a (0.2248) and 4a (0.2093), and stabilizing energy of la relative to th e free entities, HB and FLi, is also smaller than the latter two geome tries, which implies that the BLi bond of the geometry la is weaker th an that in 2a and 4a. The negative bond order of AlLi in 1b (-0.097/6- 31G, 0.000/6-31G) indicates that no real chemical bond is present bet ween the Al and Li atoms, the stabilizing energy of 1b ought to be att ributed to the van der Waals interaction. Therefore, among all the geo metries, it is preliminarily certain that 1a and 1b are the most unsta ble. About 2a and 2b, the bond orders of FLi are negative at the two l evels, the bond lengthes much bigger than the ordinary FLi bond. The b ond orders of the HX, XLi, FLi and HLi are positive, to some extent, w hich states that 3a and 3b are definitely four-membered ring. 4a and 4 b are types of three-membered ring, in which the bond orders of HX, XL i and FLi are positive. H2CLiF, H2SiLiF and HNLiF have also the stable three-membered ring geometries, the relative stabilities of the geome tries are to be further studied in terms of the isomerization barriers .