Rotational spectrum and carbon-13 hyperfine structure of the C3H, C5H, C6H, and C7H radicals -: art. no. 174308

By means of Fourier transform microwave spectroscopy of a supersonic molecular beam, we have detected the singly substituted carbon-13 isotopic species of C5H, C6H, and C7H. Hyperfine structure in the rotational transitions of the lowest-energy fine structure component ((2)Pi(1/2) for C5H and C7H, and (2)Pi(3/2) for C6H) of each species was measured between 6 and 22 GHz, and precise rotational, centrifugal distortion, Lambda-doubling, and C-13 hyperfine coupling constants were determined. In addition, resolved hyperfine structure in the lowest rotational transition (J=3/2 -> 1/2) of the three C-13 isotopic species of C3H was measured by the same technique. By combining the centimeter-wave measurements here with previous millimeter-wave data, a complete set of C-13 hyperfine coupling constants were derived to high precision for each isotopic species. Experimental structures (r(0)) have been determined for C5H and the two longer carbon-chain radicals, and these are found to be in good agreement with the predictions of high-level coupled-cluster calculations. C3H, C5H, and C7H exhibit a clear alternation in the magnitude and sign of the C-13 hyperfine coupling constants along the carbon-chain backbone. Because the electron spin density is nominally zero at the central carbon atom of C3H, C5H, and C7H, and at alternating sets of carbon atoms of C5H and C7H, owing to spin polarization, almost all of the C-13 coupling constants at these atoms are small in magnitude and negative in sign. Spin-polarization effects are known to be important for the Fermi-contact (b(F)) term, but prior to the work here they have generally been neglected for the hyperfine terms a, c, and d. (c) 2005 American Institute of Physics.