Calculations of the relative energies of the low‐lying electronic states of 2,7‐naphthoquinodimethane and 2,7‐naphthoquinone. Substitution of oxygen for CH2 is predicted to increase the singlet‐triplet energy difference (ΔEST)

(12/12)CASSCF and CASPT2 calculations have been performed on the triplet and two lowest‐lying singlet states of the 2,7‐naphthoquinodimethane (NQDM) and 2,7‐naphthoquinone (NQ) diradicals. Both diradicals are predicted to have triplet ground states but to differ in the ordering of the two lowest singlet states, with E(1A1) < E(1B2) in NQDM, but E(1A1) > E(1B2) in NQ. The reason for the difference in ordering of the singlet states in these two diradicals is discussed. The state reordering is shown to be responsible for the prediction that the substitution of the oxygens in NQ for the CH2 groups in NQDM increases, rather than decreases, the singlet‐triplet energy separation (ΔEST). CASPT2 calculations predict that, although 2,7‐naphthoquinodimethane (X = CH2) and 2,7‐naphthoquinone (X = O) both have triplet ground states, the ordering of the 2 lowest singlet states changes from E(1A1) E(1B2) for X = O. As a consequence, the substitution of X = O for X = CH2 is calculated to increase the energy difference (ΔEST) between the triplet and the lower of the 2 singlet states.