Chemical dynamics from the gas-phase to surfaces

The field of gas-phase chemical dynamics has developed superb experimental methods to probe the detailed outcome of gas-phase chemical reactions. These experiments inspired and benchmarked first principles dynamics simulations giving access to an atomic scale picture of the motions that underlie these reactions. This fruitful interplay of experiment and theory is the essence of a dynamical approach perfected on gas-phase reactions, the culmination of which is a standard model of chemical reactivity involving classical trajectories or quantum wave packets moving on a Born–Oppenheimer potential energy surface. Extending the dynamical approach to chemical reactions at surfaces presents challenges of complexity not found in gas-phase study as reactive processes often involve multiple steps, such as inelastic molecule-surface scattering and dissipation, leading to adsorption and subsequent thermal desorption and or bond breaking and making. This paper reviews progress toward understanding the elementary processes involved in surface chemistry using the dynamical approach

Spin-forbidden carbon–carbon bond formation in vibrationally excited α-CO

Fourier transform infrared spectroscopy of laser-irradiated cryogenic crystals shows that vibrational excitation of CO leads to the production of equal amounts of CO2 and C3O2. The reaction mechanism is explored using electronic structure calculations, demonstrating that the lowest-energy pathway involves a spin-forbidden reaction of (CO)2 yielding C(3P) + CO2. C(3P) then undergoes barrierless recombination with two other CO molecules forming C3O2. Calculated intersystem crossing rates support the spin-forbidden mechanism, showing subpicosecond spin-flipping time scales for a (CO)2 geometry that is energetically consistent with states accessed through vibrational energy pooling. This spin-flip occurs with an estimated ∼4% efficiency; on the singlet surface, (CO)2 reconverts back to CO monomers, releasing heat which induces CO desorption. The discovery that vibrational excitation of condensed-phase CO leads to spin-forbidden C−C bond formation may be important to the development of accurate models of interstellar chemistry