- A general class of -network hydrocarbons that includes the possibility of dangling -bonds at carbene centers in the -network are investigated theoretically. Several semiempirical approaches are treated: the simple valence-bond model, a modified molecular-orbital scheme, and complete configuration interaction calculations on Parisier-Parr-Pople-type models. Often these three approaches agree with one another in the prediction of ground-state spin S. as well as with experiments when available. We find that for alternant -networks a very simple rule seems to accurately predict the value of S. 1988, American Chemical Society. All rights reserved.