Electronic spectra of the carbon chain anions C2n-1-H-1(-) (n=5-8) in the gas phase

The electronic absorption spectra of the monohydrogenated odd carbon chain anions C2n-1H- (n = 5-8) have been measured in the gas phase using resonant two-color electron detachment spectroscopy. As for the even series C2nH- ( n = 5-10) electronic transitions of two isomeric species are observed. Ab i nitio calculations on C9H- support the assignment of one band system to a ( 3)Sigma(-) <-- X(3)Sigma(-) transition of a linear molecule and the other C o a (1)A' <-- X(1)A' transition of a bent isomer. The wavelength of the ban d systems shows an approximately linear dependence on the chain length. Thi s plot is offset by 60 nm compared to the one for the linear C2nH- anions, while the wavelengths for the even and odd carbon series of the cumulene-li ke isomer fall on the same line. These trends can be rationalized in terms of their electronic structure.