Substituting a deuterium isotope (2H) for a normal protium hydrogen isotope can slow the rate of a chemical reaction if this atom is involved in the reaction mode. The magnitude of the effect, referred to as a kinetic isotope effect or KIE is normally 2-7, but higher values of 20 or even more♥ are sometimes observed due to a phenomenon known as proton tunnelling. So a recent report[1] of a 1H/2H of ~2440 for the following palladium catalysed reaction caught my eye:
References
- Q. Wu, P. Liu, X. Zhang, C. Fan, Z. Chen, R. Qin, Y.Q. Gao, Y. Zhao, and N. Zheng, "Catalytic Hydrogenation Dominated by Concerted Hydrogen Tunneling at Room Temperature", ACS Central Science, vol. 11, pp. 2180-2187, 2025. https://doi.org/10.1021/acscentsci.5c00943