Conformation-Sensitive Isolated C−D Stretching Vibrations in Alkoxyl Chains

The isolated C−D stretching vibrations have been studied for a number of selectively monodeuterated species of tetrahydropyran, methyl ethyl ether, methyl propyl ether, methyl butyl ether, methyl hexyl ether, and cyclohexane, and the dependence of the C−D stretching wavenumbers on the conformation of the CO−C(D)−CC, OC−C(D)−CC, and CC−C(D)−CC structures of the alkoxyl chain has been examined. The infrared spectra of three monodeuterated species of tetrahydropyran with a chair form gave definitive evidence for the conformational dependence of the C−D stretching wavenumbers. The observed wavenumbers of the C−D stretching vibrations for the monodeuterated compounds studied are correlated linearly to the C−D bond lengths calculated by the ab initio MO method using the basis set with polarization functions. The dependence of the C−D stretching wavenumbers on the conformational disposition of the C−D bond in the CO−C(D)−CC structure is more significant than that of the C−D bond in the OC−C(D)−CC or CC−C(D)−CC structure. For the CO−C(D)−CC structure, the stretching wavenumbers of the C−D bond trans to the DC−OC and DC−CC bonds are higher by about 90 cm -1 than those of the C−D bond gauche to the same bonds. C−D stretching vibrational spectroscopy has been shown to be one of the advantageous methods for determining the conformation of alkyl chains, which may contain any heterogenous atoms.