1. J. Elguero, C. Marzin, A. R. Katritzky, and P. Linda, in The Tautomerism of Heterocycles (Academic, New York, 1976), pp. 502–551.
2. V. M. Bilobrov, Hydrogen Bond: Intramolecular Interactions (Naukova Dumka, Kiev, 1991) [in Russian].
3. G. N. Ten, T. G. Burova, and V. I. Baranov, Spectroscopic Studies of the Structure of Nucleic Acid Bases (Nauchnaya Kniga, Saratov, 2004) [in Russian].
4. G. N. Ten, T. G. Burova, and V. I. Baranov, Quantum-Chemical Studies of Prototropic Tautomerism of Nucleic Acid Bases (Nauchnaya Kniga, Saratov, 2008) [in Russian].
5. M. J. Nowak, L. Lapinski, and J. Fulara, Spectrochim. Acta. A 45, 229 (1989).