DFT Meets Wave-Function Composite Methods for Characterizing Cytosine Tautomers in the Gas Phase

A general strategy for the accurate computation of structuralandspectroscopic properties of biomolecule building blocks in the gasphase has been further improved and validated with a special referenceto tautomeric equilibria. The main improvements concern the use ofthe cc-pVTZ-F12 basis set in both DFT and CCSD(T)-F12 computations,the inclusion of core-valence correlation in geometry optimizationsby double hybrid functionals, and the use of the cc-pVQZ-F12 basisset for complete basis set extrapolation at the MP2-F12 level. Theresulting model chemistry is applied to the challenging problem ofcytosine tautomers in the gas phase. The results are in remarkableagreement with experiment concerning both rotational and vibrationalspectroscopic parameters and permit their unbiased interpretationin terms of structural and thermochemical features. Together withthe intrinsic interest of the studied molecule, the accuracy of theresults obtained at reasonable cost without any empirical parametersuggests that the proposed composite method can be profitably employedfor accurate investigations of other molecular bricks of life.

Automated summary

A general strategy for accurate computation of biomolecule building blocks in gas phase, with a special focus on tautomeric equilibria, has been improved with the use of cc-pVTZ-F12 basis set, inclusion of core-valence correlation, and complete basis set extrapolation at MP2-F12 level. The resulting model chemistry is applied to cytosine tautomers and shows remarkable agreement with experimental spectroscopic parameters, enabling unbiased interpretation in terms of structure and thermochemical features. This composite method provides accurate investigations of other molecular bricks of life at reasonable cost without any empirical parameter.