Quantum dynamics of complex-forming bimolecular reactions

Many gas-phase chemical reactions proceed via reaction intermediates, supported by potential wells. The characteristics of such complex-forming reactions differ drastically from those for direct reactions that involve barriers. For example, the reaction path for a complex-forming reaction is often barrierless, which results in weak and sometimes negative temperature dependence for its rate constant. The product angular and internal distributions of such reactions also bear clear signatures. Specifically, the angular distribution (i.e. differential cross-section) of a complex-forming reaction is often dominated by scattering in the forward and backward directions, and the product rotational state distribution usually peaks near the highest accessible rotational state, while vibrational state distribution often decays monotonically. While the quantum dynamics of direct reactions is well established, our understanding of complex-forming reactions is still far from complete. Given the importance of such reactions in interstellar, atmospheric and combustion chemistry, much research effort has recently been devoted to understand their dynamics. In this review, we survey the recent progress in the quantum dynamics of several prototypical complex-forming reactions, particularly those involving three or four atoms. We will focus on methodological advances in quantum scattering theory, quasi-classical trajectory methods and statistical models.