Near-infrared combination and overtone bands of the CH2 sequence in CH2X2, CH2XCHX2, and CH3(CH2)5CH3 and their characteristic frequency zones

We characterized near-infrared spectra of the CH2 sequence in CH2X2 (X=halogen), CH2ClCHCl2, and CH3(CH2)5CH3. Each near-infrared absorption in the region from 3500 to 10,000 cm-1 is consistently assigned to one of the five different combination or overtone groups, in the order of increasing frequency, of the {v(CH)]+delta(CH)]} (A), {v(CH)]+2delta(CH)]} (B), 2v(CH)] (C), {2v(CH)]+delta(CH)]} (D), and 3v(CH)] (E) types, where v(CH) and delta(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 CH2 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 the window zones, and we give an example to demonstrate this. The first-order combination bands of type A only of CH2X2 are reasonably assigned to a pair of the normal modes of v(CH) and delta(CH). From this we predict that the first-order combination bands should give structural information on the CH2 chain, similar to the infrared fundamental bands.