Nucleobases, conical intersections pyrimidine

It is interesting that the biradical geometries of the purine bases, resulting from the twisting of the N3-C2 bond, are very similar to the geometries of the nucleobases at the S1(tttt )/S0 conical intersections located by higher levels of theory [17-20], There is also a close geometrical similarity between the biradical state of pyrimidine... 

For cytosine, in common with other pyrimidine nucleobases, photostability is the result of an ethylenic-type conical intersection associated with torsion around a C = C double bond. This interpretation remains - but the barrier is reduced - when the solvent is included approximately. 

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