Sugars in the gas phase -: Part 2:: the spectroscopy and structure of jet-cooled phenyl β-D-galactopyranoside

The gas-phase conformation of the model glycoside, phenyl beta-D-galactopyranoside (phe-beta-D-gal) was examined using a combination of resonant two-photon ionization (R2PI) and resonant ion-dip infrared spectroscopy (RIDIRS) in tandem with electronic structure theory calculations. A single conformer, in which the hydroxy methyl is in a gauche + orientation, is predominant in the free-jet expansion. This conformer is the analogue of the lowest-energy conformer found in a previous study of phenyl beta-D-glucopyranoside. A minor second conformer has also been identified. Other weak bands in the R2PI spectrum have been attributed to hot bands though it is possible that other conformers are also present at low abundance. The dominant conformer identified in this work has the same (gauche +) orientation of the hydroxy methyl group as the major rotamer identified in solution by NMR spectroscopy.