Solvent effects on relative stabilities and ~(14)N NMR shielding of cytosine tautomers; continuous set of gauge transformation calculation using polarizable continuum model

摘要

The structure and relative energies of the tautomers of cytosine in gas phase and in different solvents are predicted using MP2 and density functional theory methods. The order of stability for these tautomers is C3 < C1 > C2 > C4 > C5 >C6, calculated by MP2 and C1 > C3 > C2 > C4 > C5 > C6, calculated by B3LYP method. In wide range of solvent dielectrics, relative energy calculation is performed and in all solvents the oxo-amino form C1 is predicted as the most stable tautomer. An empirical equation also calculated for dependence of the relative energy of transition state between C1 and C3 to dielectric of solvents that have no hydrogen bonded to oxygen. Solvent induced effect on nitrogen NMR shielding of two dominant tautomers is calculated using density functional theory combined with polarizable continuum model and using the continuous set gauge transformation. Direct and indirect solvent effects on shielding are also calculated.