Spin—Orbit Interactions and Their Effect on the Zero-Field Splitting of the Methylene Radical

It is proposed that certain matrix elements of the spin—orbit interactions may combine with the spin—spin interactions to cause zero-field splittings. The calculation is performed for the linear CH2 radical, where it is found that the contribution from spin—orbit coupling to the parameter D=0.1128 cm−1. This should be added to the spin—spin contribution of 0.9055 cm−1 to give a total value D=1.0173 cm−1. We conclude that the general belief that spin—orbit contributions to zero-field splitting are always negligible is questionable.