The quantum yields of fluorescence (F~0.OO7) and phosphorescence (p~O.OOO04) indicate that excitation results in nearly complete internal conversion and/or photochemistry. Theoretical calculations have been made including the potential energy curves resulting from twisting around the central carboncarbon single bond as well as the terminal carbon-carbon double bond. It appears that photoisomerization cannot occur around the single bond in either the 7r, 7r* or n, ,..* states but could occur around the double bond in the lowest ,.., 7r* triplet state. In the latter case, a potential energy minimum exists near 90° and degeneracy occurs with the ground state. Based on the spectroscopic and theoretical considerations herein reported, an analysis is made of the published spectroscopic results of conjugated keto steroids.
The IR and U V absorption spectra as well as the Raman spectra of monomer and dimer of 2-pyridone were studied. The molecular weight of 2-pyridone in benzene and dimethyl sulfoxide was determined in order to examine the association. 2-Pyridone in nonpolar solvents shows two types of carbonyl stretching vibration bands due to the coexistence of the monomer and dimer. The highest wavelength UV absorption of 2-pyridone in isooctane (2,2,4-trimethylpentane) is ascribed to π-π* electronic transitions of the monomer and dimer. The dimer absorption band appears at a higher wavenumber than that of the monomer by ca. 830 cm−1. The dimer band corresponds to the π-π* transition from the ground 1Ag state to the excited 1Bu state. No π-π* transition absorption from the ground 1Ag state to the excited 1Ag state could be observed because of the g-g Laporte forbiddance in the transition under an assumption of the molecular C2h symmetry.
Detailed absorption spectral studies have been carried out for the all-trans-, 9-cis-, 11-cis-, and 13-cisretinals at 77 OK and room temperature. Particular emphasis was placed on determining the number of transitions and the sensitivity of band maxima and intensities of all transitions (both E and oscillator strength) to temperature. Theoretical calculations were done and transition energies and oscillator strengths predicted. The experimental and theoretical data are in good agreement particularly using 77OK absorption data. Based on the foregoing and potential energy curves arising from carbon-carbon single-and double-bond twisting, notable geometry changes are predicted to occur when cooling from 298 to 77 OK. Geometric considerations are discussed for each isomer and a comparison is made among all isomers. (21) M. S. DeGroot and J. Lamb, Proc. Roy. Soc., Ser. A , 242, 36 ( 1957).
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