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Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene
Author(s) -
Kostenko Arseni,
Tumanskii Boris,
Kobayashi Yuzuru,
Nakamoto Masaaki,
Sekiguchi Akira,
Apeloig Yitzhak
Publication year - 2017
Publication title -
angewandte chemie
Language(s) - English
Resource type - Journals
eISSN - 1521-3757
pISSN - 0044-8249
DOI - 10.1002/ange.201705228
Subject(s) - diradical , cyclobutadiene , chemistry , electron paramagnetic resonance , triplet state , singlet state , photochemistry , acetylene , antiaromaticity , computational chemistry , molecule , excited state , atomic physics , nuclear magnetic resonance , physics , organic chemistry , aromaticity
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 ST , 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.