Uracil canonical forms

Figure 2 Canonical forms of cytosine, uracil, and thymine...

Nucleobases, such as cytosine, thymine, and uracil are the most important pyrimidine derivatives. In a recent gas-phase study [64], three low-energy tautomers were identified on the basis of CCSD(T)/cc-pVQZ energy calculations andMP2/6-311++G(2d,2p) estimation ofthe vibrational frequencies. The authors emphasize the importance of the high-level estimation of the frequencies and the related thermal correction in order to obtain rehable AG within a 0.7 kcal mol free energy range of the tautomers at T = 450 K. The lowest-free-energy structure is the canonical hydroxy-amine form of cytosine. 

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