H-bonded network rearrangements in the S0, S1 and D0 states of neutral and cationic p-cresol(H2O)(NH3) complexes

Abstract

The H-bonded network rearrangements in the S₀, S₁ and D₀ states of the neutral and cationic p-CreOH(H₂O)(NH₃) complexes were studied experimentally by means of (1 + 1)/(1 + 1′) REMPI (Resonantly Enhanced MultiPhoton Ionization) and time resolved LIF (Laser Induced Fluorescence) spectroscopies combined with DFT (Density Functional Theory) calculations at the B3LYP/6-311G++(d,p) level. A comparison of the rearrangement process of the H-bonded network in the three states is given. Two cyclic H-bonded isomers were found on the S₀ potential energy surface and the results indicate that the rearrangement in this state is unlikely at the temperature of the supersonic expansion due to the presence of a high-energy barrier (7503 cm⁻¹). On the other hand, the re-determination of the S₁ excited state lifetimes confirms that neither the H-bonded rearrangement nor the excited state hydrogen transfer (ESHT) reaction takes place in the S₁ state at the excitation energies of this work. Thus, it is concluded that the absorption of the second photon to reach the D₀ state takes place from the S₁ state of the cyclic-(OH–OH₂–NH₃) isomer. A preferential evaporation of H₂O upon vertical ionization of the cyclic-(OH–OH₂–NH₃) isomer is observed which is consistent with a statistical redistribution of the internal energy. Nevertheless, our theoretical calculations suggest that initial excitation of the H-bonded network rearrangement modes may also play a role to leave the H₂O molecule as a terminal moiety in a chain-(OH–NH₃–OH₂)⁺ isomer. The reaction pathway for the solvent rearrangement involves a double proton transfer process with a very low energy barrier (575 cm⁻¹) that is overcome at the vertical ionization energy of the complex.