Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene.

Tetrakis(trimethylsilyl)cyclobuta-1,3-diene (1) was subjected to a temperature-dependent EPR study to allow the first spectroscopic observation of a triplet diradical state of a cyclobutadiene (2). From the temperature dependent EPR absorption area we derive a singlet→triplet (1→2) energy gap, EST , of 13.9 kcal mol-1 , in agreement with calculated values. The zero-field splitting parameters D=0.171 cm-1 , E=0 cm-1 are accurately reproduced by DFT calculations. The triplet diradical 2 is thermally accessible at moderate temperatures. It is not an intermediate in the thermal cycloreversion of cyclobutadiene to two acetylene molecules.