ABSTRACT

Rotational spectra of C n O with n=2, 4, 6, and 8 have been observed by using a Fabry–Perot type Fourier‐transform microwave spectrometer cooperated with a pulsed discharge nozzle. The molecules have been generated by an electric discharge of carbon suboxide diluted in Ar, and adiabatically cooled to ≊2 K in a subsequent supersonic expansion. All the observed spectra for these species are characterized as linear molecules in the 3Σ− electronic ground state. Since all the three spin sublevels have been detected even in the free‐jet condition, the spin–spin coupling constants have been determined precisely as well as other spectroscopic constants. The coupling constants show rapid increase as n becomes larger, indicating smaller energy gaps between the excited 1Σ+ state and the 3Σ− ground state for the longer species. Along with the recent observation of singlet C n O (n=5, 7, and 9) [Ogata, Ohshima, and Endo, J. Am. Chem. Soc. (submitted)], the present study has established the existence of a complete set of the linear carbon‐chain series C n O up to n=9 in the gas phase. The effective C=C bond lengths evaluated from the rotational constants decrease gradually to a converging value of ≊1.28 Å as n becomes larger. No apparent quasilinearity has been observed in the centrifugal‐distortion constants of all the members, in contrast to the relevant series of the pure carbon clusters, C n , some of which (n=3 and 7) have shown substantial nonrigidity for the bending vibration.