Hydrated sugars in the gas phase:: Spectroscopy and conformation of singly hydrated phenyl β-D-glucopyranoside

The structural investigation of a hydrated monosaccharide, phenyl beta-D-glucopyranoside (PhebetaGlc), in the gas phase is presented. It is based upon ab initio computation coupled with the analysis of the resonant 2-photon ionization and IR ion-dip spectra of the singly hydrated clusters stabilized in a free jet expansion and follows an earlier investigation of the structure and conformations of the unhydrated sugar.(1) Despite the potentially large set of possible binding sites and conformations, only two singly hydrated complexes are formed in the free jet expansion. Tentative structural assignments are made on the basis of comparisons with those already established for related systems; comparisons between the observed O-H vibrational frequencies and those computed for structures optimized at the B3LYP/6-31+G(d) level of theory, and in light of the relative energies of these structures, calculated at the single-point MP2/6-311+G(d,p) level are made. A discrepancy between the latest revision of the Gaussian 98 package (revision A.11) and earlier revisions, which can lead to different computed structures when optimizing noncovalently bound molecular complexes, is discussed in an appendix.