The activation parameters [βG (200 K), βH , and βS ] for compounds, together with those for related compounds previously reported in the literature, show that conjugating the degenerate Cope rearrangements of barbaralane (1a) and several semibullvalenes (3a, 7a, 8a), and for those of a subsituents (e. g. aryl, CN, CO 2 R) in the 2,6 positions lower the barriers of the degenerate Cope rearrangement, whereas number of their 3,7-and 2,6-disubstituted derivatives, have been determined by variable-temperature carbon-13 NMR substituents that are electron-withdrawing through the inductive effect (e. g. Cl, CF 3 , SO 2 Ar) retard the reaction. spectroscopy at 126 and 151 MHz. the disubstituted compounds studied include the barbaralanes 1c,d,e,h,j,k, Substituents in the 3,7 positons have little effect or are rateretarding. The presence of 1,5-tri-and -tetramethylene and 2e, h, j, and the semibullvalenes 3d, e, h, 4e, h, i, l, m, and 5hΟͺ8h. Careful attention has been given to the bridges in semibullvalenes accelerates the rearrangements, the effect being larger in the case of the former. measurement of temperatures. The data for these