Comparative study on the vibrational IR spectra of cytosine and thiocytosine by various semi-empirical quantum mechanical methods

The vibrational IR spectra of two tautomers of cytosine (amino-hydroxy and amino-oxo forms) and thiocytosine (amino-thiol and amino-thione forms) and their dipole moments have been calculated using the MNDO, AM1 and PM3 methods and the results are compared with those calculated from ab initio and density functional theory (DFT) calculations. The ability of the semi-empirical methods to predict the vibrational frequencies have been probed.