Abstract
The goal of this study is to give reliable and accurate thermochemical data for uracil, thymine, cytosine, and adenine. The gas-phase heats of formation of these compounds were determined with the diet-HEAT-F12 protocol, which uses explicitly correlated coupled-cluster calculations along with anharmonic vibrational, scalar relativistic, and diagonal Born-Oppenheimer corrections. The thermochemically relevant tautomers of cytosine were also investigated. To derive heats of formation in the gas and solid phases as well as sublimation enthalpies, the thermochemical network approach was utilized, i.e., the available literature data were collected, reviewed, and combined with our theoretical calculations. Solvation free energies were also determined with various methods and compared to experimental data.
Highlights
Nucleobases, the purine bases adenine and guanine and the pyrimidine bases thymine and cytosine, play essential roles in many biological processes
The thymine molecule has a large-amplitude low-frequency motion corresponding to the rotation of the −CH3 group, and this phenomenon calls for the correction of the rigid rotorharmonic oscillator (RRHO) approximation invoked in our study
We note that the δECCSDT contribution has even an effect on the stability order of the tautomers: without this term, cytosine 2b would be more stable than cytosine 1
Summary
Nucleobases, the purine bases adenine and guanine and the pyrimidine bases thymine and cytosine, play essential roles in many biological processes. They are the building blocks of nucleic acids, encoding the information of protein sequences needed for the living organism. The derivatives of adenine play important role in cellular respiration (ATP, NAD, FAD) Because of their biological significance, it is important to know how nucleobases formed before living organisms appeared on Earth.[1−3]. To have accurate energetics of the processes involving nucleobases and to improve our understanding of structure− property relationships, high-quality thermochemical data are crucial Despite these needs, the available experimental data are in disarray, especially the sublimation enthalpies. There is some uncertainty surrounding the combustion enthalpy values, which have been used to derive the heat of formation of the nucleobases
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