The Rotation-Vibration Spectra of Allene and Allene-d4

The infrared absorption spectra of allene and allene-d4 vapor have been measured over the spectral range 250–5000 cm−1 with a prism spectrometer. The infrared data, together with previously obtained Raman spectra, enable a complete vibrational analysis that satisfies the product rule well for all vibrational species. The torsional vibration is located at 865 cm−1 in allene and 615 cm−1 in allene-d4. Fine structure was resolved for all perpendicular fundamental bands except that of the lowest frequency in both compounds. With help of the zeta-sum rule, a modified equation of H. H. Nielsen and the statistical weights of the rotational levels, the fine structure is analyzed to yield a complete set of zeta-values for both molecules and a moderately accurate value for the moment of inertia about the figure axis. The latter gives for the HCH angle the figure 113°±1° if one assumes a C–H distance of 1.071A.