The hyperfine structures of molecules

A b initio quantum chemical studies of hyperfine structures (hfs) were initiated some 25 years ago, with the pioneering work of Meyer and others [127]. However, results from the early Hartee-Fock-based methods deviated considerably from experimentally determined hf parameters. It was not until the configuration interaction (Cl) techniques were fully developed for hfs calculations, that theoretical predictions of high accuracy were possible for atomic and molecular radicals [20]. This is mainly associated with the importance of electron correlation and with the development of new and fax larger basis sets. In later years, hfs calculations have also been carried out with great success using various levels of multiconfiguration SCF (MCSCF) [21], multireference Cl (MRCI) [22] and coupled cluster (CC) theory [23]. 

The success of the PP functional in calculating geometries and highly accurate hyperfine structures has also been shown by Barone et al, in their investigation of fluorinated vinyl radicals [31, a]. Other LCAO-DFT studies of hyperfine structures involve LDA (Xa) calculations on the VN system by Mattar and Doleman [31, b]. There, a deviation from experiment is clearly shown, which most likely can be cured by the gradient corrections. 

In Table 8, a sample of results from previous hfs calculations using the PP functional are presented [2,30,47-49], along with some recent results. Of the many hydrocarbons investigated thus far we include results obtained for the C3 species C3H5 (allyl radical) [2], c-C3H t (cyclopropane cation) and C3HJ, (propane cation). In Section 6.3, the hfs of the methane cation is also discussed in great detail. In the Table, we also include results for H20+, H2CO+, HCN-, FCN-, [30] N02 [30,48] and NH2 [47,49]. The systems also included in ref [30], but not discussed in the present review, are FC1-, Cl2, HCO and FCO. 

Isotropic and anisotropic hf couplings constants (in Gauss) of the OH and CN radicals. All calculations axe done with the (5,2 5,2)/IGLO-III basis sets ((5,1 5,1) for hydrogen), and the experimental bond lengths Ro =0.970 A and Rc =1.172 A[140]. 

The hyperfine structures of a set of neutral and charged radicals, as calculated at the PP/IGLO-III level. All geometries are optimized at the corresponding level. 

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