Bending Frequency of the C3 Molecule

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Abstract
Ab initio calculations have been carried out to determine a potential curve for the bending motion of C3. The work was in part motivated by the disagreement between theoretical and experimental values of the entropy of C3. Three basis sets were used, with (4s2p), (4s2p1d), and (4s3p1d) centered on each carbon atom. Both self‐consistent‐field (SCF) and configuration interaction (CI) (656 configurations) calculations were carried out with the smallest basis. The CI wavefunctions were obtained by a method which may be viewed as an extension of the pseudonatural orbital technique of Edmiston and Krauss. Using the smallest basis, both SCF and CI calculations yield ``normal'' bending frequencies, ∼320 cm−1. It is concluded that electron correlation has little effect on the bending frequency. The larger (4s2p1d) and (4s3p1d) basis SCF calculations yield much smaller bending frequencies, the latter being 69 cm−1, in good agreement with the unusually low experimental value of Gausset, Herzberg, Lagerqvist, and Rosen. The bending potential is predicted to be quite anharmonic. These results are discussed qualitatively in terms of a Walsh diagram and the importance of d orbitals by symmetry considerations.

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