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A kinetic isotope effect is a change in a reaction rate constant caused by isotopic substitution. Solvent isotope effects can constrain proton-transfer pathways, but coupling and kinetic masking prevent interpreting every rate ratio as one isolated bond-breaking event.

Protonation converts OH into a water leaving group; ionization then gives the diarylmethyl cation. The labeled C-H/D bond is retained, so $k_H/k_D=1.18$ is a normal alpha-secondary kinetic isotope effect, not a primary C-H cleavage effect.

At the labeled carbon, tetrahedral geometry becomes approximately trigonal planar. The associated change in bending vibrations and zero-point energies favors the H isotopologue in a substantially carbocation-like transition state. The two anisyl rings stabilize that cation and make this ionization pathway plausible.

A plausible ionic catalytic cycle is reversible thiocyanate addition to the activated maleic-acid double bond, with proton transfers, followed by rotation about the resulting C-C single bond and elimination that regenerates thiocyanate and forms the less crowded trans alkene.

The retained vinylic labels pass from approximately sp${}^2$ toward sp${}^3$ at the addition stage. Stiffer bending modes in that direction give an inverse secondary kinetic isotope effect, consistent with $0.86$. The cycle changes alkene geometry without requiring cleavage of either original labeled C-H bond. Since two positions are labeled, $0.86$ is a combined effect; equal independent contributions would each be $\sqrt{0.86}=0.927$.

The isotope effect supports an addition/elimination route rather than simple rotation of an intact double bond; it does not, alone, identify every proton-transfer order.