The latter is a relatively small imaginary frequency for proton transfer as previous studies on hydrogen transfer transition states showed typical values of >i1500 cm -1 . 31 A small imaginary frequency implicates a broad potential energy profile and generally indicates a low kinetic isotope effect for the replacement of H by D atoms. As such, the transition state geometries implicate that the reaction will not proceed with a large kinetic isotope effect in agreement with experimental observation. The singlet spin state was also tested but found to be higher in free energy for all structures, see Supporting Information.