The statistical model of atom–diatom insertion reactions is combined with coupled-channel capture theory and used to calculate differential cross sections for the reactions of C(1D), N(2D), O(1D) and S(1D) with H2. In the case of C(1D) and S(1D), the resulting statistical differential cross sections are found to be in excellent agreement with the recent quantum reactive scattering calculations of Honvault and Launay. They are therefore also in good agreement with molecular beam experiments for the S(1D)+H2 reaction, in contrast to the results of earlier calculations based on a less rigorous statistical theory. However, because the exact quantum mechanical differential cross sections for N(2D) and O(1D) exhibit a slight forward–backward asymmetry, the agreement with the statistical model for these reactions is not quite so good. The difference between the two cases can be rationalized in terms of the greater exoergicities of the N(2D) and O(1D) reactions, which lead to broader resonances and hence to shorter lifetimes of the H2O and NH2 collision complexes than those of CH2 and H2S.

The study of insertion atom–diatom reactions is usually complicated by the existence of deep potential wells between reactants and products. The large number of bound and resonance states to be properly described makes exact quantum mechanical calculations extremely demanding in terms of numerical effort. In this type of collision, the process proceeds via the formation of an intermediate complex of finite lifetime. The application of statistical quantum methods provides a valid alternative to investigate the dynamics of the reaction. In this work, studies performed with a statistical quantum method recently developed by Rackham et al. on a variety of different reactions are extensively reviewed. The overall dynamics of the processes selected to test this statistical model range from complex-forming mechanisms to a competition between insertion and direct abstraction reaction pathways. This review includes studies of X + H₂ reactions, where X is one of the following electronically excited non-metallic atoms: C(¹ D), N(² D), O(¹ D) or S(¹ D). An ion–diatom collision, the H system (with its isotope variants), is also investigated. Finally, results of the statistical study of the H + O₂ reaction are discussed. In all cases, comparison with both exact quantum mechanical calculations and experimental measurements shows that the method provides valid and rigorous information about the underlying dynamics of the reactions under study.

A recently derived transmission-free absorbing potential is applied to the study of atom–diatom chemical reactions. This absorbing potential only depends on a single parameter—the width of the absorbing region—and its reflection properties are guaranteed to improve as this parameter is increased. Converged results can therefore be obtained very easily, as we illustrate with time-dependent wave packet calculations on the H+H2,F+H2, and H+O2 reactions.

Prestellar cores form from the contraction of cold gas and dust material in dark clouds before they collapse to form protostars. Several concurrent theories exist to describe this contraction but they are currently difficult to distinguish. One major difference is the timescale involved in forming the prestellar cores: some theories advocate nearly free-fall speed via, e.g., rapid turbulence decay, while others can accommodate much longer periods to let the gas accumulate via, e.g., ambipolar diffusion. To tell the difference between these theories, measuring the age of prestellar cores could greatly help. However, no reliable clock currently exists. We present a simple chemical clock based on the regulation of the deuteration by the abundance of ortho–H2 that slowly decays away from the ortho-para statistical ratio of 3 down to or less than 0.001. We use a chemical network fully coupled to a hydrodynamical model that follows the contraction of a cloud, starting from uniform density, and reaches a density profile typical of a prestellar core. We compute the N2D+/N2H+ ratio along the density profile. The disappearance of ortho-H2 is tied to the duration of the contraction and the N2D+/N2H+ ratio increases in the wake of the ortho-H2 abundance decrease. By adjusting the time of contraction, we obtain different deuteration profiles that we can compare to the observations. Our model can test fast contractions (from 104 to 106 cm-3 in ~0.5 My) and slow contractions (from 104 to 106 cm-3 in ~5 My). We have tested the sensitivity of the models to various initial conditions. The slow-contraction deuteration profile is approximately insensitive to these variations, while the fast-contraction deuteration profile shows significant variations. We found that, in all cases, the deuteration profile remains clearly distinguishable whether it comes from the fast collapse or the slow collapse. We also study the para-D2H+/ortho-H2D+ ratio and find that its variation is not monotonic, so it does not discriminate between models. Applying this model to L183 (=L134N), we find that the N2D+/N2H+ ratio would be higher than unity for evolutionary timescales of a few megayears independently of other parameters, such as cosmic ray ionization rate or grain size (within reasonable ranges). A good fit to the observations is only obtained for fast contraction (≤0.7 My from the beginning of the contraction and ≤4 My from the birth of the molecular cloud based on the need to keep a high ortho-H2 abundance when the contraction starts – ortho-H2/para-H2 ≥ 0.2 – to match the observations). This chemical clock therefore rules out slow contraction in L183 and steady-state chemical models, since steady state is clearly not reached here. This clock should be applied to other cores to help distinguish slow and fast contraction theories over a large sample of cases.