Toward spectroscopic accuracy for open-shell systems: Molecular structure and hyperfine coupling constants of H2CN, H2CP, NH2, and PH2 as test cases

In the present paper, we investigate the molecular structure and hyperfine couplings of a series of sigma radicals containing first- and second-row atoms (H2CN, H2CP, NH2, PH2) for which accurate gas-phase microwave results are available. The presence of alpha- and, especially, beta-hydrogen atoms makes the evaluation of magnetic properties of these radicals particularly challenging. Geometrical parameters have been computed by the coupled-cluster ansatz in conjunction with hierarchical series of basis sets, thus accounting for extrapolation to the complete basis-set limit. Core correlation as well as higher excitations in the electronic-correlation treatment have also been taken into account. An analogous approach has been employed for evaluating hyperfine coupling constants with particular emphasis given to basis-set, correlation, and geometrical effects. The corresponding vibrational corrections, required for a meaningful comparison to experimental data, have also been investigated. The remarkable agreement with experiment confirms the reliability of the present computational approach, already validated for pi radicals, thus establishing the way for setting up a benchmark database for magnetic properties. (C) 2010 American Institute of Physics. [doi:10.1063/1.3503763]