Quantitative resonance Raman spectroscopy of N-acetylpyrrolidine in aqueous solution

The resonance Raman spectra of aqueous solutions of N-acetylpyrrolidine are determined at seven excitation frequencies from 40 660 to 53 130 cm^-1 spanning the first strong absorption band which is broad and diffuse. The resonance Raman spectra are dominated by the single amide II′-like vibration at 1485 cm^-1 and its overtones of up to five quanta. Absolute resonance Raman cross sections are determined for these fundamental and overtone transitions at each excitation wavelength by reference to an internal standard of sodium perchlorate. A quantitative analysis of these data and the broad absorption spectrum is made on the basis of a model for the electronic excitation that includes the effects of inhomogeneous broadening. The observation of only a single enhanced vibrational normal mode, with the assumption that there is no Duschinsky rotation upon electronic excitation, makes this a particularly simple case for detailed analysis. A reasonably good fit to the experimental data is obtained using standard assumptions of Lorentzian inhomogeneous broadening and A-term (Condon) Raman scattering. In this fitting procedure, the integrated absorption spectrum determines the transition dipole length. The total line width of 1100-1500 cm^-1 is found to be roughly equally divided between homogeneous and inhomogeneous contributions. Despite the simplicity of this case and the observation of several overtone transitions at some excitation wavelengths, the excited-state electronic origin, vibrational frequency, and the displacement of the equilibrium position along the single active mode with respect to the ground state are found to be very strongly correlated and are not individually well determined in this analysis.