Hyperfine structure calculations for geometry determination

In this section, two examples will be given of how experimentally determined hfs can be used along with hfs calculations to determine a) which isomer is observed out of several possibilities, and b) the correct geometries in cases where the geometry optimization calculations are not definitive. 

In the first case, we will examine the hfs of the four isomers on the H2CN potential energy surface (PES) as well as the six different (HO)CNH and (HO)H CN conformers. The latter case deals with the very weakly bonded difluoride anion, F2, where DFT predicts a 0.2 A more elongated FF bond than does, e.g., MRSDCI. 

DFT computed hf parameters (in Gauss) and relative energies (kcal/mol) of the neutral radicals H2CN, cis-HCNH, irans-HCNH and CNH2. All calculations are done using the PP potential and the (5,2 5,2)/IGLO-III basis sets. 

The second example of conformational analysis deals with the six possible isomeric forms of HOCNH , where H is bonded to either C or N. These axe depicted in Fig. 11, along with their relative energies (kcal/mol) from 

Comparing the calculated isotropic hf parameters (Table 10), a number of interesting features are observed. The experimental hf couplings axe A,- (H) =54.4 and 0.92 G, A,JO(C) =20.5 G and A,SO(N) =10.2 G, respectively [164]. No 170 measurements axe, as yet, available. The overall hf pattern, and in particular the carbon couplings listed in Table 10, definitely rule out the four (HO)CNH conformers (III-VI) as the species observed. 

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