Quest for IR-Pumped Reactions in Dihydrogen-Bonded Complexes

As possible substrates for one-photon infrared-pumped reaction (IRPR), the structures and decomposition paths of complexes between AlH 4 - and three proton donors, H 2 O, HF, and HCl, have been studied by ab initio methods. In each case only one transition state was found for the proton-transfer and H 2 -loss process. For each cluster, the geometry and energy characteristics of reactants, complex, transition state, and products were analyzed with [AlH 4 ···HCl] - emerging as the best IRPR candidate. The MP2//6-311++G**-calculated intrinsic reaction coordinate (IRC) confirmed the one-step proton transfer and H 2 loss with no intermediate. Classical trajectories were calculated on the ab initio potential-energy surface, beginning from a large number of initial conditions. With zero-point vibrational energies (ZPVE = 1/2 h ν i ) assigned to all normal modes, based on their calculated harmonic frequencies, ν i, one or more additional excitation quanta were added to modes associated with Cl−H and Al−H stretching. Proton transfer from HCl and loss of H 2 were calculated to occur on the femtosecond time scale when stretching modes involving the dihydrogen-bonded hydrogens were excited. However, many vibrational oscillations take place before H 2 release. Analysis of the dynamics in terms of the complex's normal modes indicates that the excitation in the reaction-relevant modes remains localized on a time scale >1 ps.