The real‐time dynamics of hydrogen‐atom‐transfer processes under collisionless conditions are studied using femtosecond depletion techniques. The experiments focus on the methyl salicylate system, which exhibits ultrafast hydrogen motion between two oxygen atoms due to molecular tautomerization, loosely referred to as intramolecular ‘‘proton’’ transfer. To test for tunneling and mass effects on the excited potential surface, we also studied deuterium and methyl‐group substitutions. We observe that the motion of the hydrogen, under collisionless conditions, takes place within 60 fs. At longer times, on the picosecond time scale, the hydrogen‐transferred form decays with a threshold of 15.5 kJ/mol; this decay behavior was observed up to a total vibrational energy of ∼7200 cm⁻¹. The observed dynamics provide the global nature of the motion, which takes into account bonding before and after the motion, and the evolution of the wave packet from the initial nonequilibrium state to the transferred form along the O–H—O reaction coordinate. The vibrational periods (2π/ω) of the relevant modes range from 13 fs (the OH stretch) to 190 fs (the low‐frequency distortion) and the motion involves (in part) these coordinates. The intramolecular vibrational‐energy redistribution dynamics at longer times are important to the hydrogen‐bond dissociation and to the nonradiative decay of the hydrogen‐transferred form.

First accurate quantum mechanical (QM) calculations of integral and differential cross sections for the $\mathrm{C}{(}^1\mathrm{D)+}{\mathrm{H}}_2\text{(v=0,j=0,1)}$ insertion reaction have been performed on a newly developed ab initio potential energy surface [B. Bussery-Honvault et al., J. Chem. Phys. 115, 10701 (2001)]. These results have been compared with those obtained with a quasi-classical trajectory (QCT) method. A Gaussian-weighted binning procedure to assign product quantum states in the QCT calculations yields vibrational branching ratios and rotational distributions in better agreement with the QM calculations than those obtained when the usual histogramatic binning method is employed. This is the first time that the Gaussian-weighted binning procedure is used for an insertion reaction.

Recent quasi-classical trajectory (QCT) calculations of the dynamics of some prototypic elementary reactions, from state-resolved differential cross-sections (DCS) to thermal rate constants, are reviewed. The reactions studied are H + H₂, F + H₂, Cl + H₂ and O(¹D) + H₂, for which reliable potential-energy surfaces (PES) are available. The QCT results are analysed in the light of the most recent quantum mechanical (QM) calculations and experimental findings. In general, QCT integral, differential reaction cross-sections and rate constants are found to be in good agreement with their QM and experimental counterparts, indicating that, for the systems considered, the motion of the nuclei during reactive encounters is largely classical and that quantum effects, such as tunnelling, play a relatively minor role in the overall dynamics. The importance of the zero-point energy of the transition state is highlighted as one of the most important deficiencies of the QC treatment. The need for precise QC and quantal simulations of the actual laboratory measurements, in order to identify experimental quantum effects clearly, is emphasized. Finally, the importance of the calculation and measurement of vector correlations in chemical reactions is stressed and some examples are presented.

Progress in the understanding of the H + H₂ reaction and its isotopic variants is reviewed with special attention to the achievements of the last decade. The detailed agreement between theory and experiment attained during this period is emphasized and major experimental and theoretical advances are highlighted. The excellent description of most experimental findings, from state-resolved cross sections to thermal rate constants, provided by the available quantum mechanical treatments, as well as the good overall behaviour of classical mechanics are underlined. Debated issues on short-lived complexes and delayed scattering, resonances and interferences, or geometric phase effects are extensively discussed. Finally, the state-of-the-art is summarized and prospects for future research on this prototypic system are presented.