Effect of the intermolecular hydrogen bond conformation on the structure and reactivity of the p-cresol(H₂O)(NH₃) van der Waals complex

The structure and reactivity of p-CrOH(NH₃)₂ and p-CrOH(H₂O)(NH₃) complexes were studied using mass-resolved one-colour resonance-enhanced multi-photon ionization and laser-induced fluorescence (LIF) spectroscopy together with DFT calculations.

At the excitation energy of this work, the S₁ state of p-CrOH(NH₃)₂ shows a sub-nanosecond lifetime, as determined by time-resolved LIF spectra, as a consequence of a hydrogen transfer process that results in NH₄(NH₃) as a reaction product.

Substitution of NH₃ by H₂O closes the reaction channel as evidenced by the absence of excited-state hydogen transfer (ESHT) reaction products, (H₃O(NH₃) or NH₄(H₂O)) and results in a dramatic effect on the S₁ lifetime of the p-CrOH(H₂O)(NH₃) complex which rises to (12 ± 2) ns.

According to density functional theory calculations, the most stable isomer of the p-CrOH(H₂O)(NH₃) complex is a cyclic structure, in which H₂O acts as the H acceptor of the phenolic OH group (c-OH–H₂O–NH₃). However, the ESHT process is energetically disallowed upon electronic excitation.