The neutral diradical 5,5'-bis(1,3,2,4-dithiadiazolyl) [-], the first main group radical to exhibit a dramatic increase in paramagnetism on mechanical grinding

Reduction of [()2][As6]2 with triphenylantimony and tetrabutylammonium chloride produced the diradical 5,5'-bis(1,3,2,4-dithiadiazolyl) [-] in high yield as a black solid with widely ranging magnetic susceptibilities (e.g., 0.6 to 2.6 μB), which on oxidation with AsF5 regenerated [()2][AsF6]2. The identity of [-] was established from EPR, vibrational, and mass spectra. Ab initio molecular orbital [MPW1PW91/6-311G(2df)] calculations show the lowest energy structure to consist of two coplanar rings separated by a C—C single bond (1.444 A), reflected in the comparison of the vibrational spectra of the diradical with that of [()2][AsF6]2 and the calculated spectra. Confidence in the calculated [MPW1PW91/6-311G(2df)] structure of the diradical is supported by the excellent agreement between the calculated and X-ray single crystal structure geometries of []2 and [()2]2+ in [()2][AsF6]2. The molecular orbitals indicate the diradical is essentially disjoint, confirmed by a very small (0.07 kJ mol–1) GVB-PP(TC-SCF...