Vibrational spectra of crystalline dimethyl ethers

Abstract

Infrared and Raman spectra have been obtained of crystalline films of CH 3 16OCH 3, CD 3 16OCD 3, CD 3 16OCH 3, CD 3 16OCD 2H, and CH 3 16OCH 2D. Some new data are also reported for these species in the gas phase. The infrared spectra include also data for CH 3 18OCH 3 and for nearly all the above isotopic species in isotopic solid solutions. The observation for the first time of all the CH 3 deformation frequencies clarifies the role played by Fermi resonance in the CH stretching region. The interpretation of the latter is based on the CD stretching region, where Fermi resonance is plainly insignificant. Simple force field calculations together with the νCH bands of CD 3OCD 2H molecules show clearly that the CH bond in the plane of the skeleton is slightly stronger than those out of the plane. Doublet splittings seen in all the CH and CD bands are attributed not to methyl-methyl coupling, but to non-equivalent methyl groups in a site of symmetry lower than C 2v. This is substantiated by the extensive doublet splittings found amongst the vCO, rCH 3 and rCD 3 bands of H 3D 3 but not in the corresponding spectra of H 6 and D 6. Factor group splittings in ν s CO of H 6 and D 6 indicate the presence of at least four molecules in the unit cell. Some changes in the infrared spectra upon annealing indicate the occurrence of a metastable crystalline phase. Several hitherto unobserved rocking modes are located and the assignments for the H 6, D 6 and H 3D 3 species are essentially complete. An unusually high product ratio discrepancy in the solid state frequencies in the a 1 and a' species may be due to coupling between the skeletal bending mode and molecular translational motion.