Fluorescence excitation, resonant two-photon ionization (R2PI) and IR-UV ion dip spectroscopy have been used to study conformers of N-benzylformamide (NBFA) and associated clusters including hydrates with up to n ) 3 water molecules. The most stable conformer has a trans arrangement of the HNCO atoms. It is distinguished from the cis conformer by a higher frequency for the NH stretch (3478 cm -1 , compared to 3443 cm -1 ) and lower frequency for the amide I overtone (3435 cm -1 , compared to 3465 cm -1 ). The cis conformer forms cyclic H-bonded structures with one or two water molecules, binding via strong H-bonds to the neighboring CdO and NH groups. With the addition of a third water molecule, the cyclic water trimer binds to both these groups in preference to a linear chain of three waters. For trans-NBFA, a single water binds to the carbonyl group and is further stabilized by dispersive CH‚‚‚O water interactions. Two water molecules bind to the NH group instead and form a bridge to the π-system of the aromatic ring. A heterodimer species is also observed, composed of cis-and trans-NBFA. It is stabilized by NH trans ‚‚‚OdC cis and NH cis ‚‚‚π trans H-bonds, which give rise to shifts in the NH stretch frequency of -102 and -28 cm -1 , respectively. Flexibility of the amide side chain plays a key role in promoting additional CH‚‚‚O water interactions in these clusters. When compared to the unsolvated conformers, some of the clusters exhibit considerable distortion in the dihedral angle τ 1 (C 2 C 1 C R N) and in τ 2 (C 1 C R NC), equivalent to the Ramachandran angle φ in proteins. Solvation also affects the photophysics of NBFA, as the clusters show normal fluorescence behavior while the S 1 states of the isolated molecules are affected by a competing, nonfluorescent decay process.

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 and S 0, S 1 ionic states. The n \ 1È3 hydrates have " daisy chain Ï structures in which the water molecules link the amide NH and CO sites. The dimer is symmetric, connected via two strong NHÉ É ÉOC 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 a †ects the photophysics of 2PA as it does N-benzylformamide ; the isolated 2PA molecule does not appear to Ñuoresce 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,p*) states above that of the Ñuorescent (p,p*) level. S 1