On the vibrational spectrum of C9, C11 and C13

The harmonic frequencies and infrared intensities of C9, C11 and C13 have been calculated using SCF and complete active space SCF (CASSCF) methods. The ordering of the harmonic frequencies in C9 is predicted wrongly unless at least the π HOMO and LUMO are included in the active space. Infrared intensities depend crucially on the size of the active space. For linear odd-numbered clusters C13 and larger, the computed SCF spectrum is qualitatively wrong. The recent observation of a band near 1809 cm−1 in the gas phase is explained using our CASSCF results on C13.