Quantum mechanical valence study of a bond‐breaking–bond‐forming process in triatomic systems

Abstract

AbstractThe recently introduced set of the quadratic, two‐electron covalent and ionic valence indices is used to investigate the bond‐breaking–bond‐forming (BB‐BF) process in an atom exchange reaction between H2 and X (X = H, F—I) as well as in the O2—H system. Valence changes accompanying selected charge reorganizations are examined within the three‐orbital model and valence diagrams for symmetric transition states (TSs) are given. The UHF valence data for Li2O and CO2 and the H—H—X, O—O—H, and O—H—O (ABC) TSs (collinear and angular) are reported and compared to valence data in the separated fragments limits (SFL), AB and BC. The overall valence, ν(ABC), and the total (ionic plus covalent) diatomic valences, νAB and νBC, are used as measures of the overall bond‐order in a concerted BB–BF reaction, to test the postulate of the bond‐energy–bond‐order (BEBO) model. In collinear TSs of H2X, ν ≊ −1, i.e., one bonding electron pari, is found to be roughly preserved, whereas in the angular H2X and in collinear O—H—O TSs, the effect of increased valence at the saddle‐point is observed, relative to that of diatomic fragments (reactiants or products). For the angular O—O—H TS, a similar increase in | ν (ABC)| relative to both O2 and OH SFLs is detected; smaller changes relative to the O2 data are found in the collinear TS. This observation is in agreement with earlier predictions from the intersecting‐state model. The relative diatomic valences, ν/ν and ν/ν, are shown to conserve the overall relative bond multiplicity around 1 in both collinear and angular TSs of the H2—X systems. © 1994 John Wiley & Sons, Inc.