Complete sets of harmonic, semidiagonal cubic as well as diagonal cubic and quartic force constants are reported for the internal coordinates of terminal, ≣SiOH, and bridging, ≣SiOH·Al≣, surface hydroxyls on silica and zeolites. They are obtained by numerical differentiation of analytically calculated gradients of the energy (SCF approximation, 6-31 G* basis set). A GF vibrational analysis is performed and after making a nonlinear transformation of the force constants into normal coordinates the anharmonicity constants are evaluated by perturbation theory. Comparison is made with the D2OH+ ion and the DOH molecule. The calculated anharmonicities of the OH bonds in the systems studied are remarkably constant and vary between -76 and -84 cm-1, only in agreement with the values observed for DOH (-83 cm-1) and surface silanols, ≣SiOH (-90 ± 15 cm-1).

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For the C-H bond fission in CH 3 , the reaction path, the potential profile and the frequencies of the normal modes perpendicular to the reaction path are calculated by means of ab initio quantum chemical approaches (UHF, second-order Moller-Plesset perturbation theory). On the whole, these data confirm the assumptions made in the semiempirical statistical adiabatic channel model of Quack and Troe qualitatively; in particular, the decrease of the frequencies of reactant bending modes which transform into free rotations of the product is well reproduced by an exponential switching function with a common parameter. However, this single-parameter switching function cannot be used for all vibrations changing their eigenvalues. Furthermore, within the quantum chemical approaches employed, the Morse function with a constant parameter fl is a crude approximation to the potential energy profile. On the basis of these theoretically estimated data, adiabatic rovibronic energy terms are constructed and high-pressure thermal rate constants are calculated; the latter turn out to be predominantly determined by the bond dissociation energy. For comparison, high-pressure recombination rate constants are given.

A scheme for systematic reduction of the theoretical treatment of elementary reactions involving polyatomic molecules is described; it consists of (1) limitation to the energetically relevant regions of the nuclear configuration space (the reaction path and its near environs) and (2) restriction to the dynamically relevant subspace of the nuclear configuration space (the active modes). Starting from a generalized reaction path Hamiltonian of Nauts and Chapuisat allowing for the use of arbitrary curvilinear coordinates and several large-amplitude modes, the realization of the above-sketched scheme is discussed. A compilation of recent work along these lines, mostly based on the simplified Miller-Handy-Adams reaction path Hamiltonian, is given with particular emphasis on applications of a statistical adiabatic model.