A homologous series of di-TEMPO biradicals formed from two 4-amino-TEMPOs linked by -(CH2)n(-), (n = 2, 3, 4, 6) were investigated using the electron spin resonance ( ESR) spectroscopy. The compounds structures including electron and charge distribution were calculated by quantum chemical methods. The electron coupling between two nitroxyl biradicals was confirmed that is transmitted directly through-space. The through-bond transmission by flexible methylene chain was excluded. Higher coupling constants J and more effective spin - spin interaction was found for higher temperatures and longer separating chains. Thermodynamic parameters for transition from open to closed loop conformations were also estimated. (C) 2008 Elsevier B.V. All rights reserved.