The structure of the HCCCO radical. Rotational spectra and hyperfine structure of monosubstituted isotopomers

Abstract

Rotational spectra of six isotopomers of the propynonyl radical—HCCCO, DCCCO, H13CCCO, HC13CCO, HCC13CO, and HCCC18O—have been measured and analyzed, yielding the geometry and leading hyperfine constants of its 2A′ vibronic ground state. The radical is found to have an acetylenic carbon chain with the unpaired electron strongly localized on the carbon atom Cc, with the carbon atom positions labeled according to HCaCbCcO. The geometry, assumed to be fully trans, is given by the parameters θHCC = 168°, θCCC = 163°, θCCO = 136.5°, rHCa = 1.060 Å, rCaCb = 1.219 Å, rCbCc = 1.387 Å, and rCcO = 1.192 Å. The Fermi contact hyperfine constants are ac(H) = −11.593(41) MHz, ac(13Ca) = 35.8(1.4) MHz, ac(13Cb) = 166.2(3.8) MHz, and ac(13Cc) = 347.6(3.2) MHz, where dipolar terms in the proton and 13Ca hyperfine structure have been neglected. No evidence of other HCCCO isomers is found in the spectra, although ab initio calculations identify a second minimum at a cumulenic structure with the unpaired electron localized at atom Ca. Brief investigation of the formation mechanism for HCCCO from C2H2 and CO in our apparatus indicates that roughly half the HCCCO is formed without breaking the CO bond.