IR-UV ion-depletion and fluorescence spectroscopy of 2-phenylacetamide clusters: hydration of a primary amide

Fluorescence excitation, resonant two-photon ionisation (R2PI) and IR-UV ion-depletion spectroscopy were used to study the structures of 2-phenylacetamide (2PA) and its dimer and water clusters in their S-0, S-1 and ionic states. The n = 1-3 hydrates have 'daisy chain' structures in which the water molecules link the amide NH and CO sites. The dimer is symmetric, connected via two strong NH . . . OC hydrogen bonds. Hydrogen-bonded NH and OH modes of the 1:1 hydrates are strongly coupled, to the extent that the symmetric combination is almost IR forbidden. Examination and reinterpretation of previously published IR data on the 1:1 hydrates of formamide and 2-pyridone revealed similar coupling. Solvation affects the photophysics of 2PA as it does N-benzylformamide; the isolated 2PA molecule does not appear to fluoresce but its clusters do. Hydrogen bonding to the amide carbonyl group could account for this behaviour by raising the energy of the lowest amide (n,pi*) states above that of the fluorescent S-1 (pi,pi*) level.