Applications of Diatomics-in-Molecules Theory. IV. Dimers of LiH

Ellison's semiempirical diatomics-in-molecules method, applied to the study of dimeric LiH systems, yields predictions of two conformations stable with respect to isolated diatomic systems. A metastable D2h diamond structure (comparable to alkali-metal halide dimers studied by Berkowitz) with a Li–H separation of 3.36 Bohr units lies 13.8 kcal/mole lower in energy than the system Li2 + H2 and 28.4 kcal/mole lower than 2 LiH. A stable C2υ structure (analogous to previously predicted disphenoid Li4 molecules) with perpendicular diatomics Li2 and H2 separated by 2.66 Bohr units, lies 26.7 kcal/mole lower than Li2 + H2 and 41.3 kcal/mole lower than 2 LiH. Fundamental vibrational frequencies and certain characteristics of potential surfaces are presented for both stable species. The properties of four unstable linear dimers are examined.