Accurate quantum calculations of reaction probabilities have been carried out using Delves' polar coordinates for the collinear reactions C1 + XCI(v) -~CIX(v') + C1 (X= Mu, H, D). An extended London-EyringPolanyi-Sato potential energy surface with a linear symmetric barrier height of 35.77 kJ mol t has been used in the calculations. The diagonal v-~v reaction probabilities dominate over the off-diagonal v~v'#v reaction probabilities and show sinusoidal oscillations as a function of energy. Superimposed on these oscillations for X = H, D is a spectrum of narrow resonances. The positions of the resonances can be predicted very accurately from the solution of a vibrationally adiabatic (VA) single channel Schr6dinger equation provided the diagonal corrections to the VA potential are also included. The sinusoidal oscillations are analysed using VA semiclassical and quasiclassical theories. There is good agreement between the quantum, semiclassical and quasiclassi~al results at low energies, but differences appear at high energies where the VA assumption starts to break down. The VA approximation is more accurate the lighter the mass of the central atom, that is, its validity increases in the order D < H < Mu.

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The vapour phase absorption spectrum of nitrosomethane in the 600-72Onm region has been re-examined. The overall complexity of the spectrum is shown to be due to a conformational change from the eclipsed ground state to a staggered excited state. The prominent torsional features have been reassigned and are found to be consistent with a barrier of 500+ 100 cm-' to internal rotation in the excited state. Progressions of 390 and 1420 cm-1 have been assigned to the CNO bending and NO stretching modes in the excited state. Only one electronic absorption system with a Franck-Condon forbidden origin at 694.1 nm corresponding to the 'A" +-'A' (L, T*) transition has been observed .The geometry changes accompanying electronic excitation have been estimated using ab initio calculations for the 'A" state. A substantial lengthening of the CN and NO bonds, a widening of their bond angle and a change in the phase of the torsional barrier are predicted.* h = 1 hartree = 2.1948 x lo5 cm-l.

Experimental studies of the HF molecular spectrum have heretofore been unable to arrive at a suitably consistent theoretical assignment for the various measured band systems therein. To aid in this pursuit a series of ab initio C1 calculations in an AO basis containing 40 contracted gaussians has been carried out to an accuracy which is close to the full C1 limit as a result of the use of energy extrapolation techniques described elsewhere. In addition to obtaining generally quite good agreement with experimental spectroscopic constants including the dissociation energy, this treatment allows for a careful description of the change in composition of the HF ground state from ionic to covalent character as molecular stretching occurs, as well as a good representation of the upper B 1Z+ state with which it undergoes a strongly avoided crossing. The repulsive branch of the latter potential energy curve is shown to intersect the Rydberg 1Z+ manifold at relatively short bond distances, leading to a series of mixed valence-Rydberg states which are ultimately responsible for the large deviations from normal Rydberg series which are observed experimentally. Similar crossings of a---,~* and a~a* valence states with Rydberg species of 3'1/7 and/or 3Z+ symmetry are calculated to result in heavy mixing over only a relatively short range of bond distances, and thus do not produce the same magnitude of perturbations as do the 1Z+ states. Finally the calculated potential curve for the parent X2II ionic state for such Rydberg species also proves to be quite consistent with known structural data, giving independent evidence for the overall high level of accuracy of the theoretical treatment employed in the present work.

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A quasiclassical trajectory study of the heavy-light-heavy ClHCl and IHI reactions: Do threedimensional partial cross sections oscillate as a function of energy? J. Chem. Phys. 86, 5534 (1987); 10.1063/1.452525Collinear quantum mechanical treatment of the heavy−light−heavy mass combination: Cl+HBr→HCl+BrThe oscillatory reaction probability (as a function of energy) of collinear heavy + light-heavy systems (e.g., I + HI~IH + I) that has been seen in earlier quantum mechanical reactive scattering calculations is shown to be described quantitatively by a semiclassical WKB model. Because these reactions are highly vibrationally adiabatic they reduce to a two-state symmetric resonance system (analogous to symmetric charge transfer, e.g., H+ + H~H + H+) that involves only the phase shifts of the one-