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The pairing theorem predicts that for alternant hydrocarbon radical ions, the spin densities at corresponding positions should be the same. Although proton hyperfine splittings do show significant differences, the 13C splittings appear to be more analogous, at least for the anthracene ions. The biphenylene ions are unusual in that one of the proton hyperfine splittings shows an anomalous variation. It is thus of interest to see whether this variation is due to a breakdown of the pairing theorem or to an anomalous variation in the sigma–pi parameter QCHH. Two of the three 13C splittings were obtained in both the negative and positive ions of biphenylene. Signs and assignments were obtained on the basis of linewidth variations. The 13C splittings at corresponding positions were found to be almost identical. This result seems to imply that the anomalous variation in the proton hyperfine splittings is due to a change in QCHH and not to a redistribution of the spin densities.

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