Toward spectroscopic accuracy for open-shell systems: X2AB radicals as test cases

The structures, force fields and electro-magnetic properties of small molecules represent invaluable benchmarks for the most refined quantum mechanical (QM) approaches and an ideal playground where the accuracy of experimental and computational approaches rivals and synergically increases. At the same time, development and validation of the more approximate QM approaches, which are unavoidable for large systems, strongly benefit from the rich and variegate information issuing from sophisticate benchmark computations for the widest possible range of physico-chemical properties. On these grounds, very reliable extrapolation procedures have been developed and validated, which, however, mostly refer to closed-shell systems. The situation is more involved for open-shell systems due to both increased technical difficulties and to the paramount relevance of EPR spectroscopy, which requires, in turn, reliable computations of specific properties (especially isotropic hyperfine couplings) with peculiar method/basis set requirements [1]. At the same time, direct structural information are seldom available and assignments of vibrational frequencies are often not straightforward [2]. This prompted us to start a comprehensive research program on prototypical small organic X2AB radicals (with X=halogen atom, A=B,C,N and B=N,O) with the aim of extending the data base of highly accurate QM results to the structures, force fields and magnetic properties. All of these have been computed by the coupled cluster ansatz using a hierarchic series of basis sets and, in some cases, extrapolation procedures to reach the complete basis set limit. Methods rooted into the density functional theory have been used to estimate vibrational and environmental effects. The remarkable agreement with available experimental data confirms the reliability of the computational approach followed and suggests that our predictions for missing information should be highly reliable. References 1. Improta R., Barone V., Chem. Rev. (Washington, D. C.) 2004, 104, 1231; Al Derzi A.R., Fau S., Bartlett R., J. Phys. Chem. A 2006, 110, 4473; Barone V., Cimino P., Stendardo E., J. Comp. Theor. Chem. 2008, 4, 751. 2. Sattelmeyer K.W., Schaefer III H.F., J. Chem. Phys. 2002, 117, 7914; Barone V., Carbonniere P., Pouchan C., J. Chem. Phys. 2005, 122, 224308.