Fig. 2. Effect of an electric field on the Diels-Alder reaction.

(A) Bias voltage–dependent experiments under 100 to 600 mV at 393 K. The right side is the zoom-in picture of the concerted reaction process. The left side is the statistical histogram of the highest conductivity state [zwitterionic intermediate (ZI)] under 300 to 600 mV. (B) Statistical histograms and reaction mechanism under 100 mV and the corresponding attribution of the six conductance states obtained from Gaussian fittings of I-t measurements. (C) Transmission spectra of six species, where the dominated transmission orbitals (p-HOMOs) are displayed. (D) Quantitative analysis of the EEF effect, showing that the calculated Gibbs free energies of each species (left) and transition states (right) change with the given electric field strength. This indicates that the strong EEF can overcome the huge disadvantage of the stepwise mechanism. (E) Gibbs free energies for the concerted and stepwise pathways at −2.57 V/nm.