REGULAR ARTICLE The reaction between HO and (H 2 O) n (n 5 1, 3) clusters: reaction mechanisms and tunneling effects Javier Gonzalez • Marc Caballero • Antoni Aguilar-Mogas • Miquel Torrent-Sucarrat • Ramon Crehuet • Albert Sole ´ • Xavier Gime ´nez • Santiago Olivella • Josep M. Bofill • Josep M. Anglada Received: 5 July 2010 / Accepted: 13 September 2010 / Published online: 30 September 2010 Ó Springer-Verlag 2010 Abstract The reaction between the HO radical and (H 2 O)n (n = 1, 3) clusters has been investigated employ- ing 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 con- stants have also been calculated and the tunneling effects have been studied by means of time–dependent wavepac- ket calculations, performed using the Quantum–Reaction Path Hamiltonian method. According to the findings of previously reported theoretical works, the reaction between HO and H 2 O 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 (H 2 O) 2 also begins with the formation of a pre-reactive complex, which dis- sociates into H 2 O…HO ? H 2 O. The reaction between HO and (H 2 O) 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 com- plex 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 tun- neling effects are very important mainly at low tempera- tures. A prediction of the atmospheric concentration of the hydrogen-bonded complexes studied in this work is also reported. Keywords Atmospheric chemistry  Hydroxyl radical  Water clusters  Reaction mechanism  Tunneling effects 1 Introduction Hydroxyl radical (HO) is a very important species in sev- eral fields of chemistry. In the earth atmosphere, it plays a central role in the degradation processes of air pollutants such as carbon monoxide or volatile organic compounds (VOCs) [1]. In atmospheric conditions with low NO x concentrations, hydroxyl radical can destroy one ozone molecule producing molecular oxygen and hydroperoxyl radical, which destroys a second ozone molecule yielding molecular oxygen and recycling the hydroxyl radical [1–6]. Moreover, it also interacts with atmospheric gas-phase water, and it can be taken up by atmospheric aerosols or water droplets, so that it can oxidize soluble tropospheric pollutants [7–9]. In biological systems, hydroxyl radical is also a powerful oxidant. It is one of the well-known Published as part of the special issue celebrating theoretical and computational chemistry in Spain. J. Gonzalez  M. Torrent-Sucarrat  R. Crehuet  S. Olivella  J. M. Anglada (&) Departament de Quı ´mica Biolo `gica i Modelitzacio ´ Molecular, Institut de Quı ´mica Avanc ¸ada de Catalunya, IQAC–CSIC, c/Jordi Girona, 18, 08034 Barcelona, Spain e-mail: anglada@iqac.csic.es M. Caballero  A. Aguilar-Mogas  A. Sole ´  X. Gime ´nez Departament de Quı ´mica Fı ´sica, Universitat de Barcelona, c/Martı ´ i Franque ´s, 1, 08028 Barcelona, Spain J. M. Bofill (&) Departament de Quı ´mica Orga `nica, Universitat de Barcelona, c/Martı ´ i Franque ´s, 1, 08028 Barcelona, Spain e-mail: jmbofill@ub.edu M. Caballero  A. Aguilar-Mogas  A. Sole ´  X. Gime ´nez  J. M. Bofill Institut de Quı ´mica Teo `rica i Computacional, Universitat de Barcelona (IQTCUB), c/Martı ´ i Franque `s, 1, 08028 Barcelona, Spain 123 Theor Chem Acc (2011) 128:579–592 DOI 10.1007/s00214-010-0824-5