The tautomeric equilibria of cytosine studied by NQR spectroscopy and HF, MP2 and DFT calculations J.N. Latosin ´ska a, * , M. Latosin ´ska a , J. Koput b a Institute of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan ´, Poland b Department of Chemistry, Adam Mickiewicz University, Poznan ´, Poland Received 23 October 2001; revised 27 February 2002; accepted 27 February 2002 Abstract A study was undertaken to check if such parameters as NQR frequencies, quadrupole coupling constants and the asymmetry parameters determined from the 14 N 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 14 N 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. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Cytosine; Ab initio; DFT; Tautomeric equilibria; NQR parameters 1. Introduction The purine and pyrimidine bases are known to occur in different tautomeric forms depending on the pH of the environment [1]. The appearance of less stable tautomeric forms may lead to the so-called point mutation, originating from changes in DNA taking place on the molecular level [2,3]. It is known that cytosine, similarly as adenine, can occur in the imine tautomeric form appearing as a result of the amine-imine tautomerisation. In order to explain the origin of different mutations, many authors have studied the electronic structure and relative energy differences among different tautomeric forms, by the quantum chemistry methods, both semiempirical and non-empirical [4]. Determination of a relative stab- ility of tautomers requires the knowledge of differ- ences in their total energies in the equilibrium geometries, determined under the assumption that the zero vibration energies are the same. A correct reproduction of such a subtle effect as the differences in the tautomers energy can be achieved only using the non-empirical methods with full optimisation of geometry. Moreover, the quality of reproduction of NQR parameters seems to be a very good test of various ab initio and DFT methods. Literature provides many works on the tautomery of cytosine, however, hitherto mainly the effect of geometry has been analysed. Recently the influence of the planarity of the heterocyclic ring on the tautomers 0022-2860/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S0022-2860(02)00123-0 Journal of Molecular Structure 648 (2003) 9–18 www.elsevier.com/locate/molstruc * Corresponding author. Tel.: þ 48-618295277; fax: þ 48- 618257758. E-mail address: jolanala@amu.edu.pl (J.N. Latosin ´ska).