THE STRUCTURE OF THE HCCCO RADICAL - ROTATIONAL SPECTRA AND HYPERFINE-STRUCTURE OF MONOSUBSTITUTED ISOTOPOMERS

Rotational spectra of six isotopomers of the propynonyl radical-HCCCO, DCCCO, (HCCCO)-C-13, (HCCCO)-C-13, (HCCCO)-C-13, and (HCCCO)-O-18-hav e been measured and analyzed, yielding the geometry and leading hyperf ine constants of its (2)A' vibronic ground state. The radical is found to have an acetylenic carbon chain with the unpaired electron strongl y localized on the carbon atom C-c, with the carbon atom positions lab eled according to HCaCbCcO. The geometry, assumed to be fully trans, i s given by the parameters theta(HCC) = 168 degrees theta(CCC) = 163 de grees, theta(CCO) = 136.5 degrees, r(HCa) = 1.060 Angstrom, r(CaCb) = 1.219 Angstrom, r(CbCc) = 1.387 Angstrom, and r(CcO) = 1.192 Angstrom. The Fermi contact hyperfine constants are a(c)(H) = -11.593(41) MHz, a(c)(C-13(a)) = 35.8(1.4) MHz, a(c)(C-13(b)) = 166.2(3.8) MHz, and a(c )(C-13(c)) = 347.6(3.2) MHz, where dipolar terms in the proton and C-1 3(a) hyperfine structure have been neglected. No evidence of other HCC CO isomers is found in the spectra, although ab initio calculations id entify a second minimum at a cumulenic structure with the unpaired ele ctron localized at atom C-a. Brief investigation of the formation mech anism for HCCCO from C2H2 and CO in our apparatus indicates that rough ly half the HCCCO is formed without breaking the CO bond.