QUANTUM-MECHANICAL VALENCE STUDY OF A BOND-BREAKING BOND-FORMING PROCESS IN TRIATOMIC SYSTEMS

The recently introduced set of the quadratic, two-electron convalent a nd 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 accompa nying selected charge reorganizations are examined within the three-or bital 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 compare d to valence data in the separated fragments limits (SFL), AB and BC. The overall valence, V(ABC), and the total (ionic plus covalent) diato mic valences, V(AB) and V(BC), are used as measures of the overall bon d-order in a concerted BB-BF reaction, to test the postulate of the bo nd-energy-bond-order (BEBO) model. In collinear TSs of H2X, V congruen t-to - 1, i.e., one bonding electron pair, is found to be roughly pres erved, whereas in the angular H2X and in collinear O-H-O TSs, the effe ct of increased valence at the saddle-point is observed, relative to t hat of diatomic fragments (reactants or products). For the angular O-O -H TS, a similar increase in \V(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 prediction s from the intersecting-state model. The relative diatomic valences, V (AB)TS/V(AB)SFL and V(BC)TS/V(BC)SFL, are shown to conserve the overal l relative bond multiplicity around 1 in both collinear and angular TS s of the H2-X sytems. (C) 1994 John Wiley & Sons, Inc.