Properties and reactivity of first and second row hydrides. Introductory remarks, isomerizations, and inversion barriers of the AH2, AH3, AH4 and related systems

Abstract

In this paper and in the following parts of this series chemical reactivity and physical characteristics of hydrides of the first (LiH, BeH2, BH3, CH4, NH3, H2O, HF) and second (NaH, MgH2, AlH3, SiH4, PH3, H2S, HCl) row atoms of the Periodic System are discussed in terms of nonempirical quantum chemical characteristics. Semiquantitative features and tendencies rather than quantitative aspects of reactivity are investigated, which permits one to use rather modest theoretical methods. Besides the members of the mentioned series also radicals, ions, and radical ions thereof will be treated. In this part isomerizations of ìunnaturalî ionic hydrides (cations derived from electron-deficient and anions derived from electron-rich hydrides, e.g., LiH(+). and LiH2(+) or HF(-)., H2F(-)) are studied. One or two van der Waals ions derived from all these systems represent thermodynamically rather stable forms. Relations between structure and inversion barriers have been studied for bent triatomic (C2v), pyramidal tetraatomic (C3v) and tetrahedral pentaatomic (Td) molecules and ions. Inversion barriers increase when passing from the first-row C2v and C3v hydrides to higher-row analogues; both decrease (e.g., CH4 → SiH4) as well as increase (e.g., BH4(-) → AlH4(-)) were calculated for transitions of Td hydrides.