Photochemical dimerization in orotic acid in aqueous solution has been studied as a function of oxygen concentration, orotic acid concentration and the wavelength of irradiation under two pH conditions. One dimer was detected. Analysis of the data enabled us to obtain a minimum estimate of the efficiency of intersystem crossing, ø ISC . from singlet to triplet levels. This efficiency increases by a factor of two with increasing energy of excitation over the wavelength range 240–302 nm, and by a similar factor between pH 3 and pH 1. The maximum value obtained was 0.15 for irradiation at 240 nm at pH 1. The relation between quantum yield and ø ISC is discussed and possible mechanisms to explain this variation of ø ISC with energy are proposed.
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Whillans et al. (1969) studied this question.
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