Structure of the CCCN and CCCCH radicals: Isotopic substitution and ab initio theory

The millimeter-wave rotational spectra of the 13C isotopic species of the CCCCH and CCCN radicals and CCC15N were measured and the rotational, centrifugal distortion, and spin-rotation constants determined, as previously done for the normal isotopic species [Gottlieb et al., Astrophys. J. 275, 916 (1983)]. Substitution (rs) structures were determined for both radicals. For CCCN, an equilibrium structure derived by converting the experimental rotational constants to equilibrium constants using vibration–rotation coupling constants calculated ab initio was compared with a large-scale coupled cluster RCCSD(T) calculation. The calculated vibration–rotation coupling constants and vibrational frequencies should aid future investigations of vibrationally excited CCCN. Less extensive RCCSD(T) calculations are reported here for CCCCH. The equilibrium geometries, excitation energies (Te), and dipole moments of the A2Π excited electronic state in CCCN and CCCCH were also calculated. We estimate that Te=2400±50 cm−1 in CCCN, but in CCCCH the excitation energy is very small (Te=100±50 cm−1). Owing to a large Fermi contact interaction at the terminal carbon, hyperfine structure was resolved in 13CCCCH. Measurements of the fundamental N=0→1 rotational transition of CCCCH with a Fourier transform spectrometer described in the accompanying paper by Chen et al., yielded precise values of the Fermi contact and dipole–dipole hyperfine coupling constants in all four 13C species. The Fermi contact interaction is approximately two times larger in CCCN, allowing a preliminary estimation of hyperfine coupling constant bF in 13CCCN and C13CCN from the millimeter-wave rotational spectra.