An a b i n i t i o study of the 24 electron radicals PF, SO, NCl, SF+, ClO+, SiF−, PO−, NS−, and CCl− in their X 3∑− electronic states

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Abstract
The potential energy functions (PEFs) of the X 3∑− states of PF, SO, NCl, SF+, ClO+, SiF−, PO−, NS−, and CCl− have been calculated by several ab initio methods using a basis set of 101 contracted Gaussian-type orbitals. The methods used included fourth order Mo/ller–Plesset perturbation theory with single, double, and quadruple substitutions (MP4SDQ), singles and doubles configuration interaction (CI-SD), version 1 of the coupled electron pair approximation (CEPA-1), and complete active space self-consistent field (CASSCF). The CI-SD PEFs were very similar to those calculated by CEPA-1 when a size-consistency correction was added to the CI-SD total energies, and these results were designated CI-SD(s). The results for the neutral radicals are compared to the available experimental data, and predictions of the rotational and vibrational spectra of the ions have been made. Dissociation energies have been calculated for all nine species at the MP4SDTQ (MP4SDQ including triple substitutions) level of approximation, which also yielded MP4SDTQ ionization potentials for SF and ClO and electron affinities of SiF, PO, NS, and CCl. All of the negative ions are predicted to be stable with respect to loss of an electron by at least 20 kcal/mol. Electric dipole moment functions have also been computed for each species by CI-SD, CEPA-1, and CASSCF. Molar integrated IR absorption intensities for the fundamental transitions are predicted to be (in cm2mol−1) 15 860 (PF, CEPA-1), 1890 (SO, CASSCF), 5995 (NCl, CEPA-1), 7235 (SF+, CEPA-1), 1205 (ClO+, CASSCF), 20 070 (SiF−, CEPA-1), 20 390 (PO−, CASSCF), 90.0 (NS−, CASSCF), and 22 600 (CCl−, CEPA-1).

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