Fractional charge density functional theory elucidates electro-inductive and electric field effects at electrochemical interfaces

The application of external voltages to self-assembled monolayers formed on electrodes allows for dynamic tuning of the properties of the constituent molecules. The electrode and the applied voltage give rise to two main effects on the molecules, the electro-inductive and electric field effects, both of which significantly alter their chemical properties and reactivity. We present fractional charge density functional theory coupled with a model electrode and a continuum model solvent (FC-DFT+) to offer quantitative insights into these effects. FC-DFT+ accurately predicts the slope of C ≡ N frequency vs. voltage curve, its frequency-flattening behavior, and ionic strength dependence of the slope. We show that the frequency flattening arises from electro-inductive effects, particularly when the electron count encounters the HOMO–LUMO gap. Further analysis reveals that electro-inductive effects dominate in the nitrile frequency shift, while electric field effects play a smaller but opposing role. FC-DFT+ also confirms that electro-inductive effects being responsible for the flattening behavior of NO 2 stretching frequency by 4-nitrobenzenethiol. In contrast, we show that electric field effects govern the Lewis adduct formation by 4-mercaptopyridine. Our study demonstrates the versatility of FC-DFT+ in predicting both electro-inductive and electric field effects, which should guide a broad spectrum of chemical applications at electrochemical interfaces.