Compared electronic structure of negative ions M p C− n : I. Normal elements in Hückel theory

Abstract

Negative cluster ions M p C− n (M normal element, n < 10, p = 1–4) produced by various experimental techniques from carbides show in their emission intensities a very strong evenodd effect according to the parity of the carbon atom number n. This is in particular the case when M = N, F, Cl (p = 1), M = H, Al, Si, S (p = 1, 2) or M = B (p = 1–4). The largest intensities of M P C− p ions always take place for even n except in the cases of NC− n , B2C− n and Al2C− n , for which the maxima of emission occur for odd n. This oscillating behaviour corresponds to alternations in the stability of the clusters which are mainly due to the fact that, in Pitzer and Clementi model (linear chains in the sp hybridization within the framework of Huckel theory), the HOMO (highest occupied molecular orbital) of the clusters lies in a double degenerate π level band: a cluster with a complete HOMO is always more stable than a cluster with a nearly empty HOMO. This result involves that the total number of π electrons is the main factor governing the parity of the stability alternations. Accordingly, since the knowledge of the π electron number requires the determination of the σ electron number too, these alternations enable us to infer a very likely electronic structure of the ions.