Infrared photodissociation spectroscopy of Al⁺(CH ₃OH)_n (n = 1-4)

Infrared photodissociation spectra Of Al⁺(CH₃OH) _n (n = 1-4) and Al⁺(CH₃OH)_n-Ar (n = 1-3) were measured in the OH stretching region, 3000-3800 cm^-1. For n= 1 and 2, sharp absorption bands were observed in the free OH stretching region, all of which were well reproduced by the spectra calculated for the solvated-type geometry with no hydrogen bond. For n = 3 and 4, there were broad vibrational bands in the energy region of hydrogen-bonded OH stretching vibrations, 3000-3500 cm^-1. Energies of possible isomers for the Al⁺(CH₃-OH)_3,4 ions with hydrogen bonds were calculated in order to assign these bands. It was found that the third and fourth methanol molecules form hydrogen bonds with methanol molecules in the first solvation shell, rather than a direct bonding with the Al⁺ ion. For the Al⁺(CH₃OH)_n clusters with n = 1-4, we obtained no evidence of the insertion reaction, which occurs in Al ⁺(H₂O)_n. One possible explanation of the difference between these two systems is that the potential energy barriers between the solvated and inserted isomers in the Al⁺(CH ₃OH)_n system is too high to form the inserted-type isomers.