Structural Information from Methyl Rotors:  Methyl Torsional Barriers in p-Hydroxy-p‘-Methyl-t-Stilbene and Its Water Complexes

The barriers to internal rotation for the methyl group in p-hydroxy-p‘-methyl-t-stilbene and its one- and two-water complexes are reported. The jet-cooled fluorescence excitation spectra of these species are also presented. The excitation spectrum of both p-hydroxy-t-stilbene and p-hydroxy-p‘-methyl-t-stilbene exhibits two origins separated by ∼270 cm-1, which are due to the syn and anti conformers of the hydroxy group. The frequency difference between the two conformers changes dramatically in the water complexes. Additionally, barriers to internal rotation show a strong water complex dependence, changing by as much as 25% with hydrogen bonding at the hydroxy group, a site 12 atoms away from the methyl rotor.