A theoretical model for the ultrafast internal conversion of cytosine is presented, in which a state switch from the initially prepared state to the out-of-plane deformed excited state of biradical character controls the rate of the decay. This mechanism successfully accounts for the dramatically longer lifetimes of 5-fluorocytosine and -acetylcytosine relative to cytosine. The replacement of the C5 hydrogen atom by a methyl group is predicted to lead to a substantial, but not dramatic, increase in the lifetime, also consistent with experiment. It is this ability to correctly predict the substituent effects that distinguishes the present model from the previously proposed mechanisms.
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