Ultraviolet Raman spectroscopy of polyribouridylic acid: Excitation profile of the hypochromism induced by order–disorder transition

Raman spectra of polyribouridylic acid excited in the uv region, from 363 to 290 nm, are reported. The conformational changes of the polymer from random coil to ordered structure with stacked bases at high and low temperature, respectively, are reflected by important changes in the Raman line intensities; this Raman hypochromism is itself a function of the excitation wavelength—its profile has been determined and shows negative values in the region of 290 nm (near resonance), i.e., hypochromism becomes hyperchromism. Thus the knowledge of the hypochromism excitation profile is important in following order–disorder transition of a polymer using resonance Raman spectroscopy. Theoretical attempts are proposed for explanation, involving not only the relative variations of the molar extinction coefficient on the order‐disorder transition of the polymer, but also the damping factors of the vibronic levels. The theoretical curve is found to fit adequately the experimental data over the excitation range, using only the frequency of the O‐O transition of uracil and a vibronic linewidth of 2200 cm−1.