Carbon-13 hyperfine structure of the CCCCH radical

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Abstract
The fundamental (N = 1 – 0) rotational transitions of the ground 2Σ+ electronic state of the four singly substituted 13C isotopomers of CCCCH have been measured by pulsed‐jet Fourier transformmicrowave 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 13C 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 13C 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 (13C), of 396.8(6), 57.49(5), −9.54(2), and 18.56(4) MHz, for successive 13C substitutions starting furthest from hydrogen indicate that the electronic structure is essentially acetylenic with alternating triple and single bonds.