The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

This perspective describes research, carried out in the authors' labs over the past forty years, aimed at understanding, predicting, and measuring the singlet–triplet energy differences (ΔEST) in diradicals. A theory for qualitatively predicting the ground states of diradicals and the use of Negative Ion Photoelectron Spectroscopy (NIPES) for measuring ΔEST are described. The application of this theory, ab initio calculations, and NIPES to the prediction and measurement of ΔEST in a wide variety of organic diradicals is detailed. Among the diradicals that are discussed in this perspective are HN, CH3N, PhN, CH2, trimethylenemethane (TMM), oxyallyl (OXA), meta-benzoquinodimethane (MBQDM), meta-benzoquinone (MBQ), tetramethyleneethane (TME), 1,2,4,5-tetramethylenebenzene (TMB), and D8h cyclooctatetraene (COT). All of these diradicals have been studied in one and, in most cases, in both of the authors’ laboratories. The studies of OXA and D8h COT were, in fact, collaborations between the research groups of the authors. These two projects both took advantage of the ability of NIPES to provide information about transition states. Transition-state spectroscopy was used to measure the carbonyl stretching frequency in the singlet state of OXA and to establish that D8h COT violates the strictest version of Hund's rule.