Based on the CV results above, we propose the following mechanism for the product selectivity change during AC electrolysis: due to the slow deprotonation of 3, a portion of 3 formed in the positive half-cycle of AC waveform cannot be immediately deprotonated and thus stays intact until the subsequent negative half-cycle. In the reducing environment of the negative half-cycle, deprotonation continues but the further oxidation of the deprotonated product 4 to 5 is prohibited, thereby shifting the product selectivity toward 1a (Figure ). According to the CV data, 3 can be hardly deprotonated to 4 and then oxidized to 5 within ~45 ms. Thus, the AC frequency that provides tox of 45 ms (i.e., fpred = 11 Hz, see Figure ) should result in the highest selectivity toward 1a. The fpred of 11 Hz from the CV measurements is in excellent agreement with the experimentally observed optimal frequency (fexp) of 10 Hz. At even higher frequencies such as 20 or 30 Hz, the deprotonation of 3 might not finish in one AC period and thus is carried over to the following AC period, resulting in deprotonation in an The intriguing agreement between the CV-derived frequency (fpred) and fexp led us to think if we can predict fexp for other amine substrates from their CVs without running the time-consuming trial-and-error reaction optimization. To test this idea, we collected the CVs of a variety of amines, including N-aryl pyrrolidines bearing electron-donating and withdrawing groups (7-18), N-aryl piperidine (19), Naryl tetrahydroisoquinoline (20), and aliphatic amine (32). These amines showed similar scan-rate dependent CVs as 1 (Figures S14-27 and S39): the ratio between i1 and io (|i1/io|) generally decreases with increasing scan rate, suggesting the slow deprotonation of amine cation radicals by NaOAc is true for all tertiary amines. We determined fpred from the scan rate that produced |i1/io| that is most close to one (Figure ). In parallel, we experimentally identified fexp via conventional reaction optimization (i.e., running reactions at different AC frequencies and determining the selectivity from crude NMR, Figure ). Figure shows the excellent agreement between fexp and fpred for various amines, confirming the predictive power of the simple electrochemical descriptor (i.e., |i1/io|) in finding the optimal frequency for selective arylation over cyanation. We also attempted to understand the dispersed fexp values (by two orders of magnitude) for different amines from the calculated pKa values of their corresponding cation radicals (Figure , Table ). However, we did not find any obvious correlation between fexp and pKa (Figure ), suggesting that the deprotonation rate is not solely determined by the acidity of these cation radical intermediates. We further varied the cyanoaromatic coupling partners, including 1,2-dicyanobenzene To confirm the one-electron oxidation of amines to αamine radicals at the optimal frequency, we performed the radical trapping experiment on amine 1 and 20. 5 equiv of TEMPO was added to the reaction mixture under the optimal AC electrolysis conditions. We did not observe the formation of arylation product 1a or 20a. However, the formation TEMPO-adduct of 1 and 20 (Figures ) indicates that this reaction proceeds via α-amine radical pathway.