Two sets of syn-sesquinorbornene disulfones have been prepared that carry either one or two deuterium atoms ct to the sulfonyl substituents. The rates of dyotropic rearrangement were measured in these systems in order to assess the deuterium isotope effects associated with single and double H/D transfer. The temperature dependence of the deuterium isotope effects in the biscyclopropane series 3b-5b [k(HH)/k(HD) = 5.4 (0 degrees C) and 2.9 (100 degrees C); k(HH)/k(DD) = 33 (0 degrees C) and 8.5 (100 degrees C)] is not as steep as that when only one cyclopropane ring is present as in 3a-5a [k(HH)/k(HD) = 3.8 (0 degrees C) and 2.1 (100 degrees C); k(HH)/k(DD) = 200 (0 degrees C) and 11.2 (100 degrees C)]. Only in the b series is the Rule of Geometric Mean obeyed. In an effort to gain detailed insight into the relationship between strain energy changes and rates of these reactions, the isomerization rates for a significant number of syn-sesquinorbornene disulfones were first modeled by means of the empirical MM3 force field. Good agreement was found based upon a concerted model. Transition structures for seven concerted dyotropic hydrogen transfers of syn-sesquinorbornene analogs were located with ab initio 3-21G calculations. The potential energy paths for concerted and stepwise hydrogen transfer were also evaluated at the CASSCF level of theory using minimum STO-3G and 3-21G basis sets. The concerted path is predicted to be favored in the absence of tunneling as in the b series. To take the tunneling effect into consideration, a more detailed dynamic treatment of the one- or two-dimensional barrier issue was next implemented, This model supports-tunneling by a stepwise mechanism for both 3a-5a and 3b-5b, but the empirical energy parameters do not agree well with the best CASSCF calculations. Elaborate direct dynamics calculations on model compounds identical with 3a-5a except for the phenylsulfonyl groups were Carried out. Although they yielded results consistent with the two-oscillator calculations, the resulting activation energies are too low and leave room for some uncertainty about the mechanism, especially for compounds 3b-5b.