HXLiF（X＝B，Al）分子几何构型的从头算

Ab Initio Study on the Geometries of HXLiF(X=B, AI)

用从头算方法研究类硼烯ＨＢＬｉＦ和类铝烯ＨＡＩＬｉＦ的几何构型。它们均有四种平衡构型，分别为型（１）、型（２）、四元环型（３）和三元环型（４）。结果表明，在６－３１Ｇ和６－３１Ｇ水平上，类硼烯的能量降低顺序为（１）＞（３）＞（４）＞（２）；而类铝烯的能量则以（１）＞（２）＞（４）＞（３）顺序降低。分析各构型的特点，从Ｍｕｌｌｉｋｅｎ集居数和分子间相互作用的角度讨论其稳定性，并与Ｈ２ＣＬｉＦ、Ｈ２ＳｉＬｉＦ和ＨＮＬｉＦ进行了比较。

For the first time,the geometries of the two lacking electron fragments 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 have trivial effect on the equilibrium geometries and their energy orders.Both HBLiF and HAlLiF have four types of equilibrium geomtries,(1), (2), the four-membered ring (3)and the three membered ring (4).The total energies of the 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 the geometries of HBLiF, b those of HAlLiF) the bond order of BLi (0.138, at 6-31G) is much smaller than those of 2a (0. 399) and 4a(0.330), while the bond length in 1a (0. 2394 nm, 6-31G) bigger than those of 2a(0. 2248) and 4a (0.2093), and stabilizing energy of la relative to the free entities, HB and Fli,is also smaller than the latter two geometries, which implies that the BLi bond of the geometry la is weaker than 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 between the Al and Li atoms, the stabilizing energy of 1b ought to be attributed to the van der Waals interaction. Therefore, among all the geometries, it is preliminarily certain that la and 1b are the most unstable. About 2a and 2b, the bond orders of FLi are negative at the two levels, the bond lengthes much bigger than the ordinary FLi bond.The bond orders of the HX, Xli, FLi and HLi are positive, to some extent, which states that 3a and 3b are definitely four-membered ring.4a and 4b are types of three-membered ring, in which the bond orders of HX, XLi and FLi are positive. H2CLiF, H2SiLiF and HNLiF have also the stable three -membered ring geometries.the relative stabilities of the geometries are to be further studied in terms of the isomerization barriers.