STRUCTURE OF ALL-TRANS-RETINAL IN THE T-1 STATE AS DETERMINED BY RAMAN-SPECTROSCOPY - A SET OF CARBON-CARBON AND CARBON-OXYGEN STRETCHING FORCE-CONSTANTS DETERMINED BY THE NORMAL-COORDINATE ANALYSIS OF THE T-1 RAMAN LINES OF THE UNDEUTERATED AND VARIOUSLY DEUTERATED RETINALS

The S-0 and T-1 Raman spectra of all-trans-retinal and its deuterio derivatives including 10-, 14-, and 15-monodeuterio-, 7,8- and 11,12-dideuterio, and 9- and 13-(trideuteriomethyl)-all-trans-retinal were recorded, and a set of empirical assignments of the S-0 and T-1 Raman lines of all-trans-retinal were given. An attempt was made to determine a set of carbon-carbon and carbon-oxygen stretching force constants of the conjugated chain self-consistently with the bond lengths, assuming a stretching force constant-bond length relationship: First, a set of S-0-state force constants were determined by the normal coordinate analysis of the S-0 Raman data. Second, the pi bond order of each carbon-carbon and carbon-oxygen bond in undeca-2,4,6,8,10-pentaen-1-al was calculated by the Pariser-Parr-Pople method including the single- and double-excitation configurational interactions; the results predicted an inversion of the bond orders in the central part of the conjugated chain upon triplet excitation. However. a set of carbon-carbon and carbon-oxygen stretching force constants reflecting this bond-order inversion (set A) failed to explain, even after iterative adjustment, the observed T-1 Raman frequencies and their shifts upon deuterations. Third, the additional two sets were tried as the initial sets: set B assuming a 1.5 bond order for all the carbon-carbon bonds except for both peripherals and set C actually being the S-0-state force constants. After iterative adjustment, both sets changed toward another set which is in between sets B and C. Finally, the set of carbon-carbon and carbon-oxygen stretching force constants were determined starting from the S-0-state force constants (set C). The results indicated that the bond orders in the central part of the conjugated chain change. upon tripler excitation, toward their inversion (double bonds become more single-bond-like. and single bonds become more double-bond-like) but that the inversion of the bond orders never takes place.