To obtain a more reliable temperature dependence of the kinetic deuterium isotope effect in a 1,5-hydrogen shift, the title compound has been designed to repress compromising side reactions, arguably arising from second-order ene-ene unions. The values of k(H)/k(D), 4.58-5.15, in the range 185.8-153.8 degrees C, on extrapolation by the derived Arrhenius equation, lead to 8-14 at 25 degrees C. Another goal is the evaluation of the effect on rates of the free rotation available to the phenyl groups in 2,4-diphenyl-1,3(Z)-pentadiene compared to the constraint to coplanarity of the phenyl groups in the title compound. The effect is insignificant. Incidental to the presence of the sterically effective methyl groups in the title compound, participation of the hydrogen atoms of an aromatic methyl group as part of the 1,3(Z)-pentadienyl system has been uncovered in 6,8-dimethyl-1-methylene-1,2,3,4-tetrahydronaphthalene. The rate of the rearrangement is too slow to compromise the kinetics of the hydrogen shifts in the title compound. A reassessment of the conventional procedure for estimating uncertainties in Arrhenius parameters based on a small number of rate constants has led to the proposal of an alternative procedure based on the statistically more significant uncertainties associated with the individual rate constants.