ULTRAVIOLET RESONANCE RAMAN-SPECTROSCOPY OF FORMAMIDE - EVIDENCE FOR N-PI-STAR INTERFERENCES AND INTERMOLECULAR VIBRONIC COUPLING

Raman spectra of formamide in H2O and D2O have been examined at 15 excitation wavelengths between 192 and 520 nm. The excitation profiles are structured, even though the first allowed electronic transition (π-π*) is at 183 nm. This structure is interpreted as resulting from interference between the π-π* state and the quasi-forbidden n-π* state at ∼220 nm. This interpretation is supported by model calculations that qualitatively reproduce the structure in the profiles, including a deep trough for the 609-cm-1 N-C-O bending vibration. This trough is attributed to a large change in the N-C-O angle in the n-π* excited state. The ultraviolet resonance Raman spectra reveal two new bands, at 1742 and 797 cm-1, whose molar intensities are concentration-dependent. These bands are attributed to coupled C=O stretching and N-C-O bending vibrations in formamide aggregates. They show excitation profile peaks at 220 nm, attributable to vibronic coupling of the n-π* and π-π* states in the aggregate. © 1990 American Chemical Society.