Thermochemical considerations and bond enthalpy ratios involving triatomic 16-valence electron neutrals and ions and some isoelectronically related pentaatomics

Following thermochemical studies and the accompanying analysis in the calorimetric literature, in earlier computational and conceptual studies we investigated the systematics of the mean bond enthalpy ratio (D degrees M-m(XY2))/D degrees(m) (XY) for 16-valence electron neutral and ionic triatomic species. For neutral species wherein X is a group 14 element and Y is from group 16, the ratio was ca. 0.8 rising to ca. 0.9 for the corresponding anions with X from group 13, and falling to ca. 0.7 for the cations with X from group 15. Good agreement between theory and experiment was found where admittedly the majority of the data for the latter is for the neutral species. In the current study at the G2 computational level (and G3 for selected systems), the related asymmetric valence isoelectronic triatomic species YXZ are discussed and this pattern calculationally preserved. The results are in concurrence with the one experimentally available datum for comparison, namely from the literature measured value for the enthalpy of formation of OCS. Other surprises and systematics are discussed as well for what otherwise might have been superficially considered to be simple species, for these and related pentaatonic species of the type HYXYH. (C) 2007 Elsevier B.V. All rights reserved.