Vibrational spectra of nitrogen-containing six-membered heterocyclic compounds

Abstract

Six-membered nitrogen-containing heterocyclic compounds play an important role in the active life of plant and animal organisms. Therefore, they have been studied for a long time by the IR spectroscopy methods and are the subject of many investigations [1-4]. All the theoretical investigations were based on a detailed study of the vibrational spectrum of benzene. The results of these studies showed that the replacement of carbon atoms in the benzene ring by nitrogen atoms leads to a change in the valence angles and disturbance of the equivalency of the bond lengths. However, in all compounds of this homologous series, the chemical bonds formed by nitrogen atoms are very similar in nature for all these compounds. Therefore, it was important to ascertain that the vibrational spectra of these structures can be described by one single set of molecular parameters. We interpreted the vibrational spectra on the basis of the geometrical data given in the fundamental work [2]. The following values were accepted for pyridine: r i = 0.14, r 2 = 0.138, r 3 = 0.136, r 7 = 0.108, r 8 = 0.108, r 9 = 0.109 nm; ~1,6 = 118~ ~1,2 = 118~ ~2,3 = 125~ ~3,4 = 114~ 61 8 = 121~ 62,9 = 120~ For pyrazine, the accepted geometrical structure was: r I = 6.134, r 2 = 0.139, r 7 = 0.1085 nm; ~i 6 = 116~ ~1,2 = 122~ 61,7 = 116~ The accepted geometry of the pyrimidine molecule'was: r I = 0.138, r 2 = 0.135, r 3 = 0.133, r 7 = 0.108, rs = 0.109, r 9 = 0.1085 nm; ~1,6 = 116~ ~1,2 = 122~ ~2,3 = 115~ ~3,4 = 127~ 62,a = 116~ The geometrical parameters of pyridazine had the values: 21 = 0.138, r 2 = 0.139, r 4 = 0.134, r s = 0.132, r7 = 0.109, r 8 = 0.108 nm; ~1,6 = 124~ ~1,2 = 116~ ~4,5 = 119 ~ , 66,7 = 114~ 61,s = 121~ The following geometrical structure was accepted for sym-triazine: r I = 0.134, r7 = 0.108 nm; ~1,6 = 126~ ~1,2 = 113o12 ' The designations of the parameters corrrespond to the equilibrium configurations of the models in Fig. i; the following designations have been accepted: r bond length, magnitude of the intracyclic CCC angle, 6 magnitude of the extracyclic CCH angle. The aim of the present investigation was also to verify, as in [5, 6], the possible application of the internal standard method, based on the data of one single set of molecular parameters, for structures of the whole homologous series. This, in turn, makes it possible to broaden the contents of the computer library. The distribution of copies of such computing libraries between groups of research workers allows the workers to acquire the experience with the accumulation and standardization of molecular models. The necessity of carrying out this work has already been discussed in [7, 8]. In order to compile a table of half-widths, the calculations had to be treated so that they were suitable for plotting a curve of the spectral distribution of the absorption coefficient.