Calibration of a semi‐empirical procedure for predicting the ground‐state spin multiplicities of open‐shell molecules. Applications to new systems

A semi‐empirical configuration interaction (CI) algorithm is presented for predicting the ground‐state spin multiplicity of open‐shell systems. The computed energy gaps ΔE(LSHS) of model systems, Y‐phenyl‐X‐phenyl‐Y, where Y is a nitrene or nitroxide radical and X is an exchange coupling unit, are critically compared with the available experimental data. The current method has attenuated the deficiencies of the earlier algorithm proposed by Lahti and Ichimura and the predictions are qualitatively clearer, facilitating the classification of high‐ and low‐spin topologies for isomeric species. The relative effectiveness of different exchange couplers can be discerned semi‐quantitatively, and several new exchange couplers are described. The reasons for the qualitative failure of the earlier algorithm are discussed.