Toward parameterization of spin-orbit coupling in triplet organic diradicals separated by a partially conjugated spacer

Ab initio computations were carried out to map singlet-triplet spin-orbit coupling (SOC) matrix elements onto the complete conformational space of four triplet organic 1,3 and 1,4 diradicals. The previously postulated conformational dependence of SOC in simple diradicals based on the “sine”, or “90-degree rule” was shown not to be satisfactory for quantitative estimates. Based on the analysis of localized pre-NHO contributions, a much more precise parametric equation is developed for 1,3-trimethylene. It is demonstrated that the case of the more complex 1,4diradicals of general structure·C-C-X-C or C-C-X-Z, where one of the two radical centers is “partially” conjugated through X (O or C=C), can be easily reduced to the parent trimethylene case, allowing for a generalized treatment of SOC matrix elements in triplet organic diradicals.