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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.

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Kostenko, A.; Tumanskii, B.; Kobayashi, Y.; Nakamoto, M.; Sekiguchi, A.; Apeloig, Y., Angew. Chem. Int. Ed. 2017, 56, 10183-10187Contributed by Steven BacharachReposted from Computational Organic Chemistry with permissionCyclobutadiene has long fascinated organic...