On the CD₂ probe infrared method for determining polymethylene chain conformation

The rocking mode frequency of a CD₂ group substituted in a polymethylene chain is sensitive to conformation in the immediate vicinity of the CD₂ group. This sensitivity forms the basis of a commonly used infrared method for determining site-specific conformation in polymethylene systems. In the present work, the CD₂ probe method has been extended and quantified with the use of infrared data on model CD₂-substituted n-alkanes. The frequency of the CD₂ rocking band is determined primarily by the conformation of adjoining CC bonds, i.e., by tt, gt, and gg pairs. However, we have found that there are significant frequency shifts associated with other factors. These include the conformation of the next nearest CC bonds, both with the CD₂ positioned at the end and in the interior of the chain, and chain length. In addition, the ratio of the absorptivities of the tt to gt bands has been established. These results enable the method to provide new details about the conformation of the chains in polymethylene systems and reliable estimates of the concentrations of specific kinds of short conformational sequences.