Extended Hückel calculations on the conformation and structure of thymine photodimers

Extended Hückel calculations have been performed for the structure of C₅-C₆ dihydrouracil and thymine and for the uracil and thymine photodimers. The results indicate, in agreement with experiment, that the most stable conformation of the dihydropyrimidines should be the half-chair one. The preferred conformations of the dimers should correspond to planar bases forming an angle of 120° with the cyclobutane ring. The most stable dimers, per se, appear to be the trans-anti and the trans-syn ones in the case of uracil and the cis-anti and the trans-anti ones in the case of thymine. Other factors than inherent stability must play a decisive role in the production of the dimers under specific conditions. When a semi-empirical reduction is carried out upon their absolute values the dipole moments predicted for the different conformers of the thymine dimer agree very satisfactorily with experiment.