Bonding and structure in the hydrides of groups III-VI deduced from photoionization studies

Abstract

We present an overview of recent photoionization studies of transient hydride species of Groups III–VI, and the conclusions drawn from these studies. The transient species investigated include BH 3, C 2H 3, SiH n( n = 1–3) PnH n (Pn = N, P, As; n = 1,2), and ChH (Ch = O, S, Se). Photodissociative ionization of certain stable species, e.g. B 2H 6, SiH 4, PnH 3 and ChH 2 inevitably become part of the investigation. From the various dissociative and adiabatic ionization thresholds, we determine a variety of useful thermochemical data, which either help to resolve controversies or provide new experimental information. These include (a) the dimerization energy of borane, (b) the C-H bond energy in ethylene, (c) the incremental bond energies in SiH n PnH n and ChH n , and (d) the 3 B 1− 1 A 1 splitting in SiH 2, NH 2 +, PH 2 + and AsH 2 +. These results provide experimental tests of recent ab initio calculations. They also reveal the limitations of the semiempirical Goddard-Harding model for PnH n and ChH n Perhaps the most interesting structural information is a consequence of the interpay between our experiments and ab initio calculations. Such comparisons lead to the conclusion that the most stable structure of B 2H 5 + is a triple hydrogen bridge, that the ground state of vinyl cation has a very different structure from that of vinyl radical, and is probably the non-classical bridged structure, while the excited triplet state has a more classical ethylene-like structure. Also, the ground state of SiH 4 + is highly distorted from a tetrahedral structure. En passant, the methods of preparation of the transient species and the dynamical behavior of the photoionization process in selected cases will be mentioned.