Low-Frequency Vibrations of all-trans-Retinal:  Far-Infrared and Raman Spectra and Density Functional Calculations

The infrared and Raman spectra of all-trans-retinal in the region of the low-frequency internal vibrations have been measured at room temeprature and at 15 K. The frequencies and infrared intensities of the normal modes have been calculated by means of the density functional approach using the B3-LYP exchange + correlation functional. The vibrational analysis allows a complete assignment of all the observed fundamentals, both on a frequency and on a intensity basis. The calculated vibrational modes of all-trans-retinal have been also correlated to those of the component moieties, 3,7-dimethyl-2,4,6,8-octatetraenal and 1,3,3-trimethylcyclohexene. This gives an idea of the degree of localization of the vibrational modes. It turns out that many of them are localized on the ring or on the chain fragment of the molecule. The chain torsional modes, which are of primary interest for the photoisomerization process, are correlated with torsional modes of bacteriorhodopsin.