No abstract

Simple-harmonic-oscillator calculations of the deformation energy necessary to transform molecules with n-electron systems into their Kekul6 structures are presented. They enable estimation of (i) relative stabilization energies and (ii) Kekul6-structure contributions; both are calculated directly from experimental molecular geometries. Correlation between HOSE (Harmonic Oscillator Stabilization Energy) values and the Hess & Schaad resonance energies for alternant unsaturated hydrocarbons is very good (r = 0.991, n = 22); for non-alternant species the correlation is worse (r = 0.937, n = 12) but still acceptable. A very good correlation exists too between percentage contributions of the Kekul6 structures calculated by use of the HOSE model and those calculated by use of the quantumchemical method of Randi6 (r = 0.985, n = 65). Analysis of errors shows that only the geometries of molecules estimated with e.s.d.'s for bond lengths <0.004A are sufficiently precise for successful application of the HOSE model. The HOSE model enabled the percentage contributions of the quinoid structure to be estimated for EDA complexes of N,N,N',N'-tetramethyl-p-phenylenediamine and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ); in both cases the percentage quinoid form obtained was in line with chemical expectations. For TCNQ salts a good correlation was found between the Flandrois-Chasseau• charge at TCNQ species and percentage contribution of the quinoid form calculated by use of the HOSE model (r = 0.992, n = 11). The HOSE model may serve as a convenient method of prediction and summarization of some chemical properties of molecules of n-electron compounds, directly from experimental geometry. AbstractLattice-energy calculations in the atom-atom potential approach have been performed for observed and isostructurally derived hypothetical forms of pheno-0108-7681/83/060739-04501.50 thiazine and phenoselenazine compounds. Energy minimizations with respect to cell constants and molecular rigid-body coordinates lead to absolute minima of energy surfaces in all cases. The experimental values of cell constants for the three

No abstract

The possibility of an excess electron binding to canonical isomers of xanthine in the gas phase was studied at the coupled-cluster level with single and double excitations using the 6-31++G** basis sets supplemented with the 4(sp)3d set of diffuse functions. It was found that xanthine should exist in the gas phase as one canonical tautomer while all the other tautomers are not likely to be detected experimentally because of their significant thermodynamic instability. On the other hand, all canonical tautomers (except one) were found to be capable of forming electronically stable anionic states of dipole-bound nature. However, the only thermodynamically stable anion (with vertical electron binding energy estimated to be 0.041 eV) based on xanthine tautomers is the one supported by the most stable neutral species.