The tautomeric equilibria of cytosine studied by NQR spectroscopy and HF, MP2 and DFT calculations

A study was undertaken to check if such parameters as NQR frequencies, quadrupole coupling constants and the asymmetry parameters determined from the N-14 NQR spectra can provide sufficient information to infer about the tautomeric form of a compound studied. The influence of the choice of functional on the N-14 NQR parameters calculated by ab initio and DFT methods was analysed. Moreover, the influence of the neighbouring molecules on the NQR parameters was checked for the assumed symmetric planar environment of the cytosine molecule in which it is hydrogen-bonded to four other cytosine molecules. Since the use of a finite basis set is always a source of uncertainties in the electric field gradient tensor components, the final calculations for a layer were performed in two different basis sets: middle and extended. (C) 2002 Elsevier Science B.V. All rights reserved.