TIME-RESOLVED STUDIES OF PHENOL PROTON-TRANSFER IN CLUSTERS .3. SOLVENT STRUCTURE AND ION-PAIR FORMATION

Time-resolved resonance-ionization spectroscopy was used to measure properties of proton transfer in clusters. Ground electronic ion-pair states of phenol, stabilized in (NH3)n and [(CH3)3N]n solvent clusters, were detected by their long-lived absorptions at 355 nm (> 1 ns). The ionization signals PhOH+(NH3)n and H+(NH3), exhibit a strong enhancement at n = 4 and 5, corresponding to a stable, closed solvent coordination shell. The detection of negative ions from ion-pair photodissociation showed no evidence of solvent-separated ion pairs. Instead, only dissociative ionization of a contact ion pair was observed. The rate of excited-state proton transfer of phenol dimer in (NH3)n solvent clusters was measured. The phenol molecules in the dimer are inequivalent, only one of which undergoes facile proton transfer to the solvent (1/k congruent-to 50 ps versus greater-than-or-equal-to 500 ps for n = 6). Finally, solvent bonding structures were deduced from resonance-ionization mass spectra of PhOH in mixed solvent clusters (NH3)m(B)n [B = CH3OH or (CH3)3N].