Relative stabilities of three low-energy tautomers of cytosine

A coupled cluster electron correlation study

Research output: Contribution to journalArticle

104 Citations (Scopus)

Abstract

High-level quantum chemical calculations have been carried out in an effort to reinvestigate the relative stabilities of the three lowest-lying tautomers of cytosine. Geometries were optimized at the CCSD/TZP level and electronic energies calculated at CCSD(T)/cc-pVTZ and vibrational frequencies at MP2/TZP. Comparative DFT calculations were also performed. From these data Gibbs free energies and equilibrium mole ratios were calculated. In agreement with most previous theoretical and experimental results, the amino-hydroxy tautomer 2b was found to be the most stable structure. As a new result, the amino-oxo form 1 and the imino-oxo form 3a have very nearly the same electronic energy, about 1.5-1.7 kcal/mol above 2b. The calculated ΔG values at standard temperature are ∼0.8 kcal/mol relative to 2b, again for both 1 and 3a. These results about the stability of the oxo form 1 are in quantitative agreement with experimental estimates in the literature, both from matrix isolation infrared and from molecular beam microwave spectroscopy. However, the calculated stability of the imino form is much higher than suggested by experiment. Our tentative reanalysis of the IR spectrum did not resolve this discrepancy. The widely used MP2 method gives significant deviations from the coupled cluster results and may be not accurate enough for determining close-lying energies of tautomers. Also, density functional theory gives qualitatively different results from traditional wave function methods.

Original languageEnglish
Pages (from-to)1381-1390
Number of pages10
JournalJournal of Physical Chemistry A
Volume106
Issue number7
DOIs
Publication statusPublished - Feb 21 2002

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Electron correlations
Cytosine
tautomers
Microwave spectroscopy
electrons
Molecular beams
Vibrational spectra
Gibbs free energy
Wave functions
electronics
Discrete Fourier transforms
molecular beams
Density functional theory
energy
isolation
wave functions
density functional theory
Infrared radiation
deviation
microwaves

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Relative stabilities of three low-energy tautomers of cytosine : A coupled cluster electron correlation study. / Fogarasi, G.

In: Journal of Physical Chemistry A, Vol. 106, No. 7, 21.02.2002, p. 1381-1390.

Research output: Contribution to journalArticle

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