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, E , of 13.9 kcal mol , in agreement with calculated values. The zero-field splitting parameters D=0.171 cm , E=0 cm 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.
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| http://dx.doi.org/10.1002/anie.201705228 | DOI Listing |
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