The molecular structure of carbon suboxide, C3O2, has been studied by the sector-micro-photometer method of gas electron diffraction. The C=O and C=C distances (rg) have been determined to be 1.1632±0.0013 Å and 1.2894±0.0022 Å, respectively, where the uncertainties represent estimated limits of error. The observed mean amplitudes for three nonbonded atom pairs and their shrinkages are found to be much larger than those expected for ordinary molecules, with linear framework. A usual harmonic approximation up to the quadratic terms of displacements cannot account for the observed values in this molecule. This indicates that the πu bending vibration ν7 has a very low frequency (an effective frequency being about 68 cm−1) and that this vibration is essentially a bending motion of the valence angle of the central carbon atom, in agreement with recent spectroscopic studies. The effect of the large-amplitude vibration on mean amplitudes and shrinkages has been studied in detail by using a model, which assumes that atomic motions take curvilinear paths in the ν7 vibration with the valley in the potential surface running along the loci of fixed bond lengths. The potential function for this bending motion has been estimated by a least-squares analysis on the observed structural parameters. The potential function is given by V(Q7)=(23±5)Q74−(100±31)Q72 (cm−1), where the uncertainties representing random standard errors take the upper and lower signs in the same order and Q7= sin α⁄0.145 (amu1⁄2 Å). The potential rises steeply with increasing angle of bending, α=1⁄2[180°∠–(C=C=C)], and appears to have a small hump at α=0. This “quasi-linear” potential function is consistent with the recent experimental estimates by Seip et al. within experimental errors, but the double-minimum nature of the potential is subject to a further critical examination by other experimental methods.
The Chemical Society of Japan