In some types of hydrogen abstraction reactions, the effect of changing the solvent is low. The reaction between ascorbate and 2,2,6,6-tetramethylpiperidine-1-oxyl radical was studied experimentally . The mechanism was best explained by proton-coupled electron transfer (PCET) (Figure ), where an electron and proton are transferred simultaneously but between different sets of orbitals. This mechanism differs from SPLET, as the electron and proton are transferred in a single elementary step rather than sequentially. The solvent was varied between water and mixtures of water and dioxane, decreasing its polarity. The quantity investigated was the kinetic isotope effect (KIE), which is defined as the ratio of the rates of hydrogen abstraction in water Figure : Example of the PCET mechanism for the hydrogen abstraction reaction between ascorbate and TEMPO radical . Reprinted with permission from Sajenko, I., Pilepić, V., Brala, C. J., & Ursić, S. (2010). Solvent dependence of the kinetic isotope effect in the reaction of ascorbate with the 2,2,6,6-tetramethylpiperidine-1-oxyl radical: tunnelling in a small molecule reaction. The Journal of Physical Chemistry A, 114 (10), 3423-3430. . Copyright 2010 American Chemical Society. and in D 2 O. KIE was found to only slightly increase with decreasing solvent polarity. Interestingly, hydrogen tunneling was suspected to take place in all solvents studied, because the experimental KIEs were larger than expected.