The reaction between HO and (H2O) n (n = 1, 3) clusters: reaction mechanisms and tunneling effects

Abstract

The reaction between the HO radical and (H2O)n (n = 1, 3) clusters has been investigated employing high-level quantum mechanical calculations using DFT-BH&HLYP, QCISD, and CCSD(T) theoretical approaches in connection with the 6-311 + G(2df,2p), aug-cc-pVTZ, and aug-cc-pVQZ basis sets. The rate constants have also been calculated and the tunneling effects have been studied by means of time–dependent wavepacket calculations, performed using the Quantum–Reaction Path Hamiltonian method. According to the findings of previously reported theoretical works, the reaction between HO and H2O begins with the formation of a pre-reactive complex that is formed before the transition state, the formation of a post-reactive complex, and the release of the products. The reaction between HO and (H2O)2 also begins with the formation of a pre-reactive complex, which dissociates into H2O…HO + H2O. The reaction between HO and (H2O)3 is much more complex. The hydroxyl radical adds to the water trimer, and then it occurs a geometrical rearrangement in the pre-reactive hydrogen-bonded complex region, before the transition state. The reaction between hydroxyl radical and water trimer is computed to be much faster than the reaction between hydroxyl radical and a single water molecule, and, in both cases, the tunneling effects are very important mainly at low temperatures. A prediction of the atmospheric concentration of the hydrogen-bonded complexes studied in this work is also reported.