Eg. Robertson et al., IR-UV ion-depletion and fluorescence spectroscopy of 2-phenylacetamide clusters: hydration of a primary amide, PHYS CHEM P, 3(5), 2001, pp. 786-795
Fluorescence excitation, resonant two-photon ionisation (R2PI) and IR-UV io
n-depletion spectroscopy were used to study the structures of 2-phenylaceta
mide (2PA) and its dimer and water clusters in their S-0, S-1 and ionic sta
tes. 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 pr
eviously 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 fluore
sce 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.