Intramolecular C-13 kinetic isotope effects were determined for the dimerization of cyclopentadiene. Substantial isotope effects were observed in three positions, despite the C, symmetry of the cycloaddition transition state and the absence of dynamical bottlenecks after this transition state. The observed isotope effects were predicted well from trajectory studies by extrapolating the outcomes of trajectories incorporating superheavy isotopes of carbon, ranging from C-20 to C-140. Trajectory studies suggest that the isotope effects are unrelated to zero-point energy or the geometrical and momentum properties of the transition state. However, steepest-descent paths in mass-weighted coordinates correctly predict the direction of the isotope effects, supporting a novel origin in Newton's second law of motion.