ab initio calculations and ir studies of tautometric forms of uracil and cytosine and comparing results in different temperatures (25˚c, 37˚c and 40˚c).

in this paper,the molecular geometry for three tautomers of uracil and four tautomers of cytosine has been analyzed. vibrational ir spectra of the tautomers were   investigated at hf and b3lyp level using the ab initio 6-31g* and lanl2dz basis sets from the program package gaussian 98 (a.7 public domain version). the physico-chemical and biochemical properties of uracil and cytosine are one of the principal criteria for the selection of these compounds. calculated results were compared with the corresponding experimental data if available. the harmonic wave numbers for the main tautomers of uracil (u1) and cytosine (c2) will compare at the 25ºc, 37ºc, and 40ºc.