Time-dependent, two-dimensional, numerical simulations of a transmitted detonation show reignition occuring by one of two mechanisms. The first mechanism involves the collision of triple points as they expand along a decaying shock front. In the second mechanism ignition results from the coalescence of a number of small, relatively high pressure regions left over from the decay of weakened transverse waves. The simulations were performed using an improved chemical kinetic model for stoichiometric H2-O2 mixtures. The initial conditions were a propagating, two-dimensional detonation resolved enough to show transverse wave structure. The calculations provide clarification of the reignition mechanism seen in previous H2-O2-Ar simulations, and again demonstrate that the transverse wave structure of the detonation front is critical to the reignition process.

Ab initio molecular orbital calculations with the minimal STO-3G basis set have been carried out for a series of meta-and para-substituted benzyl and anilide anions with substituents CH3, NH2, OH, OCH3, F, N02, CN, CHO, and CF3. The results are analyzed in terms of substituent interactions and compared with previously reported data for toluenes, anilines, phenols, and phenoxide anions. The results suggest that (a) substituted phenoxide anions are useful models for substituent effects in corresponding benzyl anions in the gas phase, (b) effects of substituents on acidities of toluenes, anilines, and phenols are largely due to substituent interactions in the anions and only to a smaller extent to interactions in the neutrals, (c) the sensitivity of acidity to substituent change increases slightly in the order anilines < phenols < toluenes due in part to substituent effects in the neutrals, and (d) differences in the solution behavior of substituted phenoxide and benzyl anions stem from the large differences in solvent effects for these two systems.

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A theoretical study of the ground electronic state of N4+, the dimer ion of N, , has shown, in agreement with other studies, that the Mrrller-Plesset perturbation expansion for open-shell molecular species is unstable and divergent when used with certain spincontaminated UHF reference wavefunctions. This failure is more serious than in the earlier studies, which noted that the breakdown occurred for the extended bond lengths descriptive of potential energy curves of diatomic systems. In this study the breakdown occurs for the potential energy surface of N4+ close to the global minimum structure, which is incorrectly determined as Corresponding to a trans-planar geometry. In comparison variational configuration interaction procedures represented by the CISD and QCISD methods are clearly superior in the case of N4+, with both yielding the correct linear geometry. The QCISD calculation provides a calculated antisymmetric vibration frequency which is in good agreement with, and hence confirms, the assignment of the neon-matrix infra-red spectrum attributed to the N4+ species. The search for a bound excited electronic state of N4+ was partly successful in that a configuration of rectangular geometry shows properties in fair agreement with experimental results. ~~

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