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Six exemplary molecules are treated for which the ground electronic state is dissociative whereas the corresponding cation is quite stable. Such molecules possess long-lived excited Rydberg states with a quasi-hydrogenic electron orbiting outside the cationic core. Electronic energies and spectroscopic transition moments are calculated by ab initio methods employing a floating spherical Slater orbital (FSSO) variational function. For H3, the results agree satisfactorily with other calculations. For the five second-row hydride molecules, strong transitions are predicted to occur in two regions: 5000-7000 Á and 9000-12000 Á. Comparison of our results for H3 and NH4 with the experimental spectra of Herzberg shows excellent agreement for some transitions, but large discrepancies in wavelength and especially in intensities appear for other transitions.

In experimental studies of vibrational relaxation, dissociation, or isomerization of molecular gases, it is common to use mixtures of the reacting gas with inert diluents. Rate constants for the reaction in pure reactant gas or pure diluent gas are then evaluated by extrapolation using the linear mixture rule (LMR): kLMR=∑ixiki, where xi is the mole fraction of gas i and ki is the rate constant for the reaction in pure component i. However, this rule is only obeyed rigorously for first order or pseudo-first-order processes having a single rate-determining step or which proceed at thermal equilibrium. We demonstrate theoretically that deviations from the linear mixture rule are always positive or zero (ktrue≥kLMR) and show with model calculations that the neglect of these deviations resulting from the use of the linear mixture rule may lead to large overestimates of the true pure component rate constants.

The ab initio cyclic periodic wave function (CPWF) approach is developed for the treatment of infinitely periodic systems. Using the full infinite Hamiltonian operator, as well as symmetrically identical basis set wave functions that preserve the translational symmetry of the electron density of the system, this approach can be applied at the Hartree–Fock level, or correlation can be directly included by the usual modes. In this approach, all many-body interactions are included, and no edge effects occur. Initial test calculations of the CPWF method at the ab initio Hartree–Fock level are performed on the chains of hydrogen fluoride molecules.