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Making use of CEPA-1, CCSD and CCSD(T) calculations with relatively large basis sets and taking the major anharmonicity effects into account, predictions are made for the following 12 molecules which are of interest to interstellar cloud chemistry: HC;

The millimeter-wave rotational spectra of the 13 C isotopic species of the CCCCH and CCCN radicals and CCC 15 N 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 ͑1983͔͒. Substitution ͑r s ͒ 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 RCCSD͑T͒ calculation. The calculated vibration-rotation coupling constants and vibrational frequencies should aid future investigations of vibrationally excited CCCN. Less extensive RCCSD͑T͒ calculations are reported here for CCCCH. The equilibrium geometries, excitation energies ͑T e ͒, and dipole moments of the A 2 ⌸ excited electronic state in CCCN and CCCCH were also calculated. We estimate that T e ϭ2400Ϯ50 cm Ϫ1 in CCCN, but in CCCCH the excitation energy is very small (T e ϭ100Ϯ50 cm Ϫ1 ͒. Owing to a large Fermi contact interaction at the terminal carbon, hyperfine structure was resolved in 13 CCCCH. Measurements of the fundamental Nϭ0→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 13 C species. The Fermi contact interaction is approximately two times larger in CCCN, allowing a preliminary estimation of hyperfine coupling constant b F in 13 CCCN and C 13 CCN from the millimeter-wave rotational spectra.

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The vibrational structure of the first bands of the photoelectron (PE) spectra of the radicals SiH3, CF3, CH2CN and CH2NC has been calculated by means of the Coupled Electron Pair Approximation. Excellent agreement with experiment is obtained for SiH3. A long progression in the umbrella bending vibration is calculated for CF3. Furthermore, two combination tone series should be observable in the PE spectrum at higher resolution. The PE spectra of CH2CN and CH2NC are dominated by the adiabatic peaks. Adiabatic ionization potentials of 8.98±0.05 eV, 10.20±0.05 eV and 9.36±0.03 eV are predicted for CF3, CH2CN and CH2NC.