Even-odd alternation of the melting points of α,ω-disubstituted linear alkanes such as alkane-α,ω-diols, alkane-α,ω-dinitriles and α,ω-diaminoalkanes is well known. Melting points for compounds with an even number of carbons in their alkyl chains are systematically higher than those for compounds with an odd number of carbons. In order to clarify the origin of this alternation, near-infrared absorption spectra of linear alkane-α,ω-diols with 3 to 9 carbon atoms in their alkyl chains were measured in the liquid and solid states. The band due to the first overtone of the OH stretching mode was investigated. The temperature-dependent spectra of all alkane-α,ω-diols in their liquid states were found to be similar; no even-odd alternation was observed. In the solid state, however, spectra of alkane-α,ω-diols with even and odd numbers of carbon atoms differed greatly. Spectra of alkane-α,ω-diols with an odd number of carbon atoms in their solid states were similar to those in the liquid states, although the variation of spectra observed upon lowering the temperature of liquid seemed to continue when the liquids were frozen. In contrast, spectra of alkane-α,ω-diols with an even number of carbon atoms in their liquid and solid states were found to be quite different. New bands appeared upon freezing. The observed even-odd alternation of the spectra observed for alkane-α,ω-diols in their solid states is presumably caused by their even-odd alternation of crystal structures.
Keywords: Alkane-α,ω-diols; Even-odd alternation; Hydrogen bond; Near-infrared spectra; Solid state.
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