Substitution of C-H groups in 1,3,5-trimethylenebenzene (1,3,5-TMB) triradical by one, two or three N, P and As atoms is studied using CASSCF/CASPT2 computations with a ANO-RCC triple-zeta basis set. The computations showed that consistently with the previous results, the unpaired electrons in 1,3,5-TMB are highly delocalized, but our computations show that they become localized on the P and As atoms, with a more pronounced localization effect of As. All studied compounds have a quartet ground state, but the results showed in this Letter indicate that the quartet-doublet energy gap is reduced upon substitution. Localization of the unpaired electrons on the ring reduces the average distances between them, which induces a destabilization of the quartet states compared to the doublet. Therefore the position of the unpaired electrons and the doublet-quartet gap can be tuned by chemical methods, which opens the way to fine-tune the properties of the building blocks of organic magnets. (c) 2010 Elsevier B. V. All rights reserved.