Combined Zero-Field and Hyperfine Couplings

As first pointed out by Minakata and Iwasaki [16] an asymmetry in the hyperfine pattern between the = 1 -o- 0 and ms = 0- -1 electronic transitions of a triplet state can be attributed to the influence of the nuclear Zeeman term. This makes it possible in principle to determine the relative signs of the hyperfine coupling and the zero-field splitting. An application to trimethylene methane is shown in Fig. 3.13. 

The asymmetry of the ESR spectrum in Fig. 3.13 arises because of the different values for the nuclear frequencies vi and v i, while vh is approximately equal to the nuclear frequency, or ca. 14 MHz. The hyperfine sphtting in frequency units 

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