Far infrared and Raman spectra of gaseous carbon suboxide and the potential function for the low frequency bending mode

Abstract

The far infrared spectrum of gaseous carbon suboxide under high resolution (0.3 cm⁻¹) has been investigated (20–80 cm⁻¹). Two series of absorption maxima were observed resulting from the anharmonicty of the low frequency bending vibration, ν₇. In one series the maxima, found to be relatively sharp, were assigned to P‐branch bandheads, and in the other the more diffuse peaks were assigned to Q branches for excited state transitions of ν₇. The Raman spectra of gaseous and solid carbon suboxide were obtained under moderately high resolution (1.0 cm⁻¹). Several lines were found to have striking fine structure. More than ten Q branches were observed in the Raman spectrum of the gas in the region of 2ν₇. The observed Q branches in the infrared and Raman spectra were assigned with the help of a potential function of the form $\mathrm{V(}{\mathrm{cm}}^{\text{−1}}{\text{)=0.728 ± 0.010\hspace{0.17em}q}}^4{\text{−6.40 ± 0.36\hspace{0.17em}q}}^2$ , where q is one of the reduced polar coordinates q and φ. This function leads to a barrier to the linear configuration of $\text{14 ± 2\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ with the lowest energy level at 19.7 cm⁻¹. Thus, the carbon suboxide molecule is linear in the ground vibrational state. The 0→ 1 transition was found to be at 22.2 cm⁻¹. The potential function transformed to the dimensioned form by using the reduced mass gives an angle of 10.2° at the shallow potential minimum. Many of the other Raman bands showed extensive fine structure due to ``hot band'' transitions associated with ν₇ and it was only possible to locate the ground state transition reliably by observing the corresponding frequency in the solid. The gas‐phase entropy of carbon suboxide, calculated with the help of the new assignment for ν₇ agrees reasonably well with the experimental third‐law entropy by there is indication that a small amount of additional entropy may be required from a solid‐state transition near 120 °K with $\text{Δ H ≃ 50\hspace{0.17em}}\mathrm{cal/mole}$ .