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New quantum chemistry calculations (with a triple zeta plus polarization basis set, and a single and double configuration interaction) have been carried out to determine the equilibrium points and the transition state for the vinylidene (H2C=C:)→acetylene (HC≡CH) isomerization. A classical barrier height (i.e., with no zero point energy effects) of 6.3 kcal/mol is obtained, and application of the Davidson correction for unlinked clusters reduces this to 5.4 kcal/mol. Our best estimate is that the true classical barrier lies in the range 2–4 kcal/mol. The dynamics of the vinylidene/acetylene isomerization is described with the framework of the reaction path Hamiltonian. The lifetime of vinylidene (in its ground vibrational state) with respect to this process is calculated to be 0.24 to 4.6 ps for a classical barrier of 2 to 4 kcal/mol. This lifetime decreases by a factor of ∼2 if one quantum of the CH2 scissors mode of vinylidene is excited, but is predicted to increase somewhat if a quantum of the C–C stretch is excited. These results are all consistent with the recent experimental observation of vinylidene via photodetachment of C2H−2.

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Semiempirical molecular dynamics procedures are used to theoretically investigate the trajectories and quantum yields of the rhodopsin leads to bathorhodopsin and bathorhodopsin leads to rhodopsin photoisomerizations. The calculations are based on the semiclassical trajectory formalism and rhodopsin binding site model proposed by Birge and Hubbard (1980. J. Am. Chem. Soc. 102: 2195-2205). The rhodopsin leads to bathorhodopsin photoisomerization is predicted to occur in approximately 2.2 ps with a quantum yield of 0.62 in reasonable agreement with experiment (less than 6 ps, phi = 0.67). The bathorhodopsin leads to rhodopsin photoisomerization is predicted to occur in approximately 1.8 ps with a quantum yield of 0.48. The latter number is in good agreement with the observed quantum yield for cattle bathorhodopsin (phi = 0.5) but in poor agreement with the observed value for squid bathorhodopsin (phi = 0.36). Our calculations suggest that the observed photochemical preference of the chromophore in cattle bathorhodopsin to isomerize to form rhodopsin (phi = 0.5), instead of isorhodopsin (phi - 0.054), is associated with a significant out-of-plane distortion (9-17 degrees) of the 11,12-trans dihedral angle in the batho chromophore.

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The semiclassical perturbation (SCP) approximation of Miller and Smith is applied to the inelastic scattering of atoms from solid surfaces. Earlier applications of this approach have been to diffraction (i.e., elastic scattering) of He from LiF and to diffraction and rotationally inelastic scattering of H2 from LiF, and the present work is an extension which allows for motion of the surfaces atoms (and thus phonon inelasticity). Therefore, phenomena that involve energy transfer between the surface and gas molecules can be described. In the present work sticking probabilities are calculated for He–W(110) and Ne–W(110), and the results show that a simple model such as this one can be quite useful in predicting energy transfer in gas–solid scattering systems.