Note on the vibrational spectrum of C4 and C5

The geometry and infrared spectrum of C5 have been computed ab initio using the MP2/6-31G* theoretical model. The inner and outer bond lengths are 1.291 and 1.300 Å, respectively. A very intense (1243 km mol−1) ∑+u band is predicted at 2193 cm−1, while another ∑+u mode at 1368 cm−1 is found to be much less intense (64 km mol−1). The results confirm the assignment of the experimental infrared (IR) frequencies of C4 and C5 suggested by Bernholdt et al. [J. Chem. Phys. 89, 3612 (1988)].