Abstract

We characterized near-infrared spectra of the CH<sub>2</sub> sequence in CH<sub>2</sub>X<sub>2</sub> (X = halogen), CH<sub>2</sub>ClCHCl<sub>2</sub>, and CH<sub>3</sub>(CH<sub>2</sub>)<sub>5</sub>CH<sub>3</sub>. Each near-infrared absorption in the region from 3500 to 10 000 cm<sup>−1</sup> is consistently assigned to one of the five different combination or overtone groups, in the order of increasing frequency, of the {[<i>v</i>(CH)]+[δ(CH)]} (A), {[<i>v</i>(CH)]+[2δ(CH)]} (B), [2<i>v</i>(CH)] (C), {[2<i>v</i>(CH)]+[δ(CH)]} (D), and [3<i>v</i>(CH)] (E) types, where <i>v</i>(CH) and δ(CH) denote the CH stretching and CH deformation normal modes, respectively. Each group has its own characteristic frequency zone. The bands of B, D, and E, which are second-order combinations or overtones, are weaker by 1/10–1/50 than those of A and C, which are first-order combinations or overtones. The near-infrared spectra of the CH<sub>2</sub> sequence show "window zones" of very weak or no absorptions. This suggests that we can perceive the characteristic near-infrared bands of a functional group through <i>the window zones</i>, and we give an example to demonstrate this. The first-order combination bands of type A only of CH<sub>2</sub>X<sub>2</sub> are reasonably assigned to a pair of the normal modes of <i>v</i>(CH) and δ(CH). From this we predict that the first-order combination bands should give structural information on the CH<sub>2</sub> chain, similar to the infrared fundamental bands.

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