1. V. D. Yumatov, N. P. Erchak and É. Lukevits, Khim. Geterotsikl. Soedin., No. 9, 1188 (1993).
2. C. N. Rao and P. K. Basu. Physical Methods in Heterocyclic Chemistry, R. R. Gupta (ed.), Wiley, New York (1984), p. 231.
3. C. N. Rao, P. K. Basu and M. S. Hegde, Appl. Spectrosc. Rev., 15, 1 (1979).
4. V. I. Nefedov and V. I. Vovna. Electronic Structure of Organic and Organometallic Compounds [in Russian], Nauka, Moscow (1989), p. 23.
5. V. I. Vovna. Electronic Structure of Organic Compounds [in Russian], Nauka, Moscow (1991), p. 183.