The role of water on the ground state multiple-proton transfer of neutral and ionic p-Cresol(H2O)(NH3) complex

F. HERNÁNDEZ; M. C. CAPELLO; A. N. OLDANI; M. BERDAKIN; J. C. FERRERO; P. MAITRE; G. A. PINO

Tokyo

Conferencia; Gordon Research Conference on Molecular and Ionic Clusters; 2010

Gordon Research Conferences

From excited state lifetime determination and DFT calculations it was determined that the ground state structure of the p-CrOH-H2O-NH3 complex responsible for the REMPI (1+1) and LIF spectra signature is cyclic with the H2O as the hydrogen acceptor from the p-CrOH molecule and the NH3 molecule as a hydrogen acceptor from the H2O molecule and hydrogen donor to the p-CrOH molecule. Upon ionization of this complex, it is observed that the excess energy in the ionic state is mainly channeled into evaporation of the H2O or NH3 molecule, the former channel accounting for more than 90 % of the evaporation process. If the evaporation of H2O in the ionic state takes place directly upon vertical ionization, the daughter CrOH+-NH3 ion would be very unstable since NH3 acts as proton acceptor, been more basic than CrOH+. From DFT calculation it is shown that the first step upon vertical ionization is the opening of the intermolecular cycle leaving the H2O molecule as a bridge between the CrOH+ and NH3 moieties. This open structure evolves through a concerted double proton transfer mechanism (CrOH+® H2O ® NH3) with an energy barrier of 800 cm-1 and a further cycle closing. In this new cyclic structure, the NH4+ cation is the proton donor to the CrO radical. From this structure, the excess energy is dissipated by H2O evaporation. In the ground state, the concerted double proton transfer proceeds without the opening of the cyclic structure up to an intermediate ion-pair (CrO--NH4+-H2O) with an energy barrier of 7000 cm-1. This intermediate state is not stable and proceeds to a further proton transfer from the NH4+ cation to CrO- anion to produce the other isomeric cyclic structure with the NH3 as the hydrogen acceptor from the p-CrOH molecule and the H2O molecule as a hydrogen acceptor from the NH3 molecule and hydrogen donor to the p-CrOH molecule