Infrared spectra and tautomerism of 5-fluorocytosine, 5-bromocytosine and 5-iodocytosine. Matrix isolation and theoretical AB initio studies

Abstract

The infrared (IR) spectra of 5-fluorocytosine (5FC), 5-bromocytosine (5BrC) and 5-iodocytosine (5IC) all isolated in an argon matrix at 15 K are presented and discussed. Vibrational assignments are proposed based on ab initio quantum mechanical calculations (3–21G basis set) of the IR frequencies and intensities for the amino-oxo and amino-hydroxy tautomers of 5FC. The IR spectra prove that the amino-hydroxy tautomer strongly dominates in an inert matrix for all the 5-halogenated cytosines studied, in sharp contrast with the dominance of the amino-oxo form in polar media. From the experimental absorbances and theoretical molar absorption coefficients the equilibrium constants. K(h/o), and the difference in the energies of the amino-hydroxy and amino-oxo tautomers are estimated. The value of 5FC agrees very well with the energy difference calculated using a 6–31G** basis set (single point) and 3–21G optimized geometry, corrected for the difference in the zero-point vibrational energy.