C-13 HYPERFINE-STRUCTURE OF THE CCCCH RADICAL

The fundamental (N = 1 - 0) rotational transitions of the ground (2) Sigma(+) electronic state of the four singly substituted C-13 isotopomers of CCCCH have been measured by pulsed-jet Fourier transform microwave spectroscopy. In each isotopomer this transition is split into many well-resolved hyperfine components owing to interaction between the electron spin and the molecular rotation, the proton spin, and the C-13 nuclear spin. Here, the hyperfine transition frequencies are analyzed with the higher rotational millimeter-wave frequencies described in the previous paper of McCarthy et al. to produce a precise set of rotational, centrifugal distortion, spin-rotation, and hyperfine coupling constants. In particular, the Fermi-contact interaction of the C-13 nucleus has been measured at each substituted position, yielding information on the distribution of the unpaired electron spin density along the carbon chain. The Fermi-contact constants, b(F)(C-13), of 396.8(6), 57.49(5), -9.54(2), and 18.56(4) MHz, for successive C-13 substitutions starting furthest from hydrogen indicate that the electronic structure is essentially acetylenic with alternating triple and single bonds. (C) 1995 American Institute of Physics.