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Thermodynamic Aspects of Thermal Decomposition and Charge-Transfer-Induced Chemiluminescent Decomposition for Bicyclic Dioxetanes Bearing a 4-(Benzothiazol-2-yl)-3-hydroxyphenyl Moiety

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posted on 2010-06-04, 00:00 authored by Masatoshi Tanimura, Nobuko Watanabe, Hisako K. Ijuin, Masakatsu Matsumoto
Uncatalyzed thermal decomposition (TD) and charge-transfer-induced decomposition (CTID) of dioxetanes were investigated to determine their thermodynamic characteristics. The dioxetanes examined were a series of 1-[4-(benzothiazol-2-yl)-3-hydroxyphenyl]-4,4-dimethyl-2,6,7-trioxabicyclo[3.2.0]heptanes, 1ad, bearing an alkyl substituent R, e.g., methyl, ethyl, isopropyl, or tert-butyl, at the 5-position and a parent dioxetane 1e with R = H. X-ray single crystallographic analysis was achieved for 1ad. Both free energies of activation, ΔGTD and ΔGCTID, increased in the order 1a1b < 1c < 1d. The free energy difference ΔΔG = ΔGTD − ΔGCTID was ca. 27 kJ mol−1 regardless of the substituent R. However, the use of Taft’s dual-substituent parameter suggested that CTID was more sensitive to the polarity of the substituent R than TD. The entropy term for CTID, ΔSCTID, decreased from zero to a large negative value in the order of tert-butyl, isopropyl, ethyl, and methyl, whereas ΔSTD did not show a similar tendency.

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