Fragmentation of hydrogen-bonded molecular clusters on photoionization

Abstract

Photofragmentation mechanisms for the near-threshold ionization of (H₂O)_m(Ar)_n, (CH₃NH₂)_1,2(H₂O)_n and (N-methylformamide)_1,2(H₂O)_n clusters have been investigated based on the molecular-beam vacuum ultraviolet photoionization mass spectroscopy with Ar (11.83 and 11.62 eV) and Kr (10.64 and 10.03 eV) resonance lamps. In the Ar-lamp ionization of (H₂O)_m(Ar)_n cluster, (H₂O)⁺_k(2 ⩽k⩽ 10) ions were observed in addition to (Ar)_1,2(H₂O)⁺_n(2 ⩽n⩽ 5) clusters. This suggests that excess energy dissipation among H₂O–Ar and/or Ar–Ar vibrational modes finally leads to dissociation of the bond(s), preventing the activation of the proton-transfer reaction in an H₂O⁺–H₂O hydrogen-bonding unit. In the Kr lamp ionization of methylamine hydrated clusters and N-methylformamide hydrated clusters, proton-transfer reactions were found to be negligible. Even at 11.83 eV, CH₃NH⁺₂(H₂O)_n and HNCH₃CHO⁺(H₂O)_n ions were dominant fragments. The origin of the protonated ions was attributed to the ionization of the CH₃NH₂–CH₃NH₂ or HNCH₃CHO–HNCH₃CHO dimer unit in the hydrated clusters, based on the solute concentration dependence of the ion signals. When an H₂O molecule was ionized directly, the hydrogen atom of an adjacent molecule was abstracted by H₂O⁺, forming an H₃O⁺ ion. An increase of the excess energy did not change the degree of fragmentation as much, but changed the fragmentation pathways to the production of protonated ions. Selective ionization of a solute molecule enabled us to specify the bond which was primarily dissociated. The fragmentation mechanisms were elucidated by identifying the character of the ionized solute molecule in the hydrogen bond: electron donor (base) or hydrogen donor (acid).