STRUCTURE OF THE CCCN AND CCCCH RADICALS - ISOTOPIC-SUBSTITUTION AND AB-INITIO THEORY

The millimeter-wave rotational spectra of the C-13 isotopic species of the CCCCH and CCCN radicals and (CCCN)-N-15 were measured and the rot ational, centrifugal distortion, and spin-rotation constants determine d, as previously done for the normal isotopic species [Gottlieb et al. , Astrophys, J. 275, 916 (1983)]. Substitution (r(s)) structures were determined for both radicals. For CCCN, an equilibrium structure deriv ed by converting the experimental rotational constants to equilibrium constants using vibration-rotation coupling constants calculated ab in itio was compared with a large-scale coupled cluster RCCSD(T) calculat ion. The calculated vibration-rotation coupling constants and vibratio nal frequencies should aid future investigations of vibrationally exci ted CCCN. Less extensive RCCSD(T) calculations are reported here for C CCCH. The equilibrium geometries, excitation energies (T-e), and dipol e moments of the A(2) Pi excited electronic state in CCCN and CCCCH we re also calculated. We estimate that T-e=2400+/-50 cm(-1) in CCCN, but in CCCCH the excitation energy is very small (T-e=100+/-50 cm(-1)). O wing to a large Fermi contact interaction at the terminal carbon, hype rfine structure was resolved in (CCCCH)-C-13. Measurements of the fund amental N=0-->1 rotational transition of CCCCH with a Fourier transfor m spectrometer described in the accompanying paper by Chen et al., yie lded precise values of the Fermi contact and dipole-dipole hyperfine c oupling constants in all four C-13 species. The Fermi contact interact ion is approximately two times larger in CCCN, allowing a preliminary estimation of hyperfine coupling constant b(F) in (CCCN)-C-13 and (CCC N)-C-13 from the millimeter-wave rotational spectra. (C) 1995 American Institute of Physics.