On the vibrational spectrum of C9, C11 and C13

Abstract

The harmonic frequencies and infrared intensities of C-9, C-11 and C-13 have been calculated using SCF and complete active space SCF (CASSCF) methods. The ordering of the harmonic frequencies in C-9 is predicted wrongly unless at least the pi 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 C-13 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 C-13.