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

The 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 H 2 and X ( X = H, F—I) as well as in the O 2 —H system. Valence changes accompanying selected charge reorganizations are examined within the three‐orbital model and valence diagrams for symmetric transition states ( TS s) are given. The UHF valence data for Li 2 O and CO 2 and the H—H— X , O—O—H, and O—H—O ( ABC ) TS s (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, ν AB and ν BC , 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 TS s of H 2 X , ν ≊ −1, i.e., one bonding electron pari, is found to be roughly preserved, whereas in the angular H 2 X and in collinear O—H—O TS s, 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 O 2 and OH SFL s is detected; smaller changes relative to the O 2 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 TS s of the H 2 — X systems.