Structural dependence of CH bond absorptivities and consequences for FT-ir analysis of coals

Nine liquid products from processing of high- and low-rank coals, oil shale and crude oil were analysed by quantitative FT-i.r. spectroscopy to understand the effect of sample structure on the absorptivities for aliphatic and aromatic CH bonds. Variation in the structure of samples was additionally enlarged by comprehensive fractionation of the products by liquid chromatography. H-1 n.m.r. spectroscopy was used as a reference analytical tool for the determination of the absorptivities. Absorptivity for aliphatic CH bond stretching modes was found to depend linearly on hydrogen aromaticity of the hydrocarbon-type samples with heteroatom content less than or similar to 2wt%. The absorptivities of heteroatom-rich fractions were lower than those of hydrocarbons and did not show any dependence on hydrogen aromaticity. Their values were rather random in a broad interval, thus showing high but not predictable influence of molecular structure. Absorptivities for aromatic CH bond stretching and out-of-plane bending modes were found to be independent of structure. As a consequence for structural analysis of coal, the structural independence of absorptivity for aromatic CH bonds allows a relatively reliable assessment of aromatic hydrogen content in coals. On the other hand, evaluation of aliphatic hydrogen content can be greatly affected by the amount of chloroform-extractables in a coal, which contain a highly aliphatic material with a high absorptivity of aliphatic CH bonds, especially those in long methylene chains. For reliable structural analysis by FT-i.r., extraction of coal with chloroform is recommended to analyse the bulk of the coal substance. Copyright (C) 1996 Elsevier Science Ltd.