Infrared plus vacuum ultraviolet spectroscopy of neutral and ionic methanol monomers and clusters: new experimental results.

We present new observations of the infrared (IR) spectrum of neutral methanol and neutral and protonated methanol clusters employing IR plus vacuum ultraviolet (vuv) spectroscopic techniques. The tunable IR light covers the energy ranges of 2500-4500 cm(-1) and 5000-7500 cm(-1). The CH and OH fundamental stretch modes, the OH overtone mode, and combination bands are identified in the vibrational spectrum of supersonic expansion cooled methanol (2500-7500 cm(-1)). Cluster size selected IR plus vuv nonresonant infrared ion-dip infrared spectra of neutral methanol clusters, (CH(3)OH)(n) (n=2,[ellipsis (horizontal)],8), demonstrate that the methanol dimer has free and bonded OH stretch features, while clusters larger than the dimer display only hydrogen bonded OH stretch features. CH stretch mode spectra do not change with cluster size. These results suggest that all clusters larger than the dimer have a cyclic structure with OH groups involved in hydrogen bonding. CH groups are apparently not part of this cyclic binding network. Studies of protonated methanol cluster ions (CH(3)OH)(n)H(+) n=1,[ellipsis (horizontal)],7 are performed by size selected vuv plus IR photodissociation spectroscopy in the OH and CH stretch regions. Energies of the free and hydrogen bonded OH stretches exhibit blueshifts with increasing n, and these two modes converge to approximately 3670 and 3400 cm(-1) at cluster size n=7, respectively.