A study of the intra-red spectra of cyanine and merocyanine dye intermediates—some heterocyclic quaternary amine iodides

Abstract

It has been shown that for heterocyclic quaternary amine iodides there are intense bands in the region 900-700 cm −1 which are a function of the out-of-plane bending vibrations of the number of adjacent hydrogens on the isolated carbocyclic and heterocyclic rings. For those heterocyclic quaternary salts containing four adjacent hydrogens on the carbocyclic ring, the strongest absorption occurs in the region 758–788 cm −1. When there are two adjacent hydrogens, the absorption mode shifts to a frequency near 810 cm −1, and with only one hydrogen on this ring, the band appears near 880 cm −1. For those quaternary salts containing three adjacent hydrogens on the heterocyclic ring, the band appears near 810 cm −1. With two adjacent hydrogens on the heterocyclic ring, the absorption shifts to a region 826–838 cm −1 and for only one hydrogen on this ring, the band appears in the region 868–887 cm −1. The isolated ▪ stretching mode in 2-methylthiazoline ethiodide absorbs strongly at 1621 cm −1. Conjugation with a carbon—carbon unsaturated group, as in 2,4-dimethylthiazole ethiodide, shifts this mode to a region near 1580 cm −1. When these same two double bonds (i.e. the ▪ and ▪ groups) are in resonant conjugation, as in the compound [2- bis (3-ethylthiazolinyl)] monomethine cyanine iodide, the absorption mode shifts to 1547 cm −1. When the ▪ and ▪ functional groups are present in conjugated aromatic rings, there is such a strong interaction between them that they completely lose their individual identity. As a result, a complex series of bands usually appear in the aromatic stretching region (1600-1400 cm −1). In making the assignments, the entire group of absorptions is considered as a whole in relation to the particular structural unit. Thus, for the quinolines the bands are designated as quinoline bands I, II, etc., for the benzothiazoles the bands are designated as benzothiazole bands, I, II, etc.