IR−UV Ion-Dip Spectroscopy of N-Benzylformamide Clusters: Stepwise Hydration of a Model Peptide

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 C O 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 ···O C 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 α N) and in τ 2 (C 1 C α 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.