To quantify the impact of EEFs on the DA reaction, we conducted a series of quantum-mechanical calculations where we first applied a uniform static and homogeneous EEF, F, to both the RC and TS along the X, Y, and Z directions, as defined in Figure . Fz is aligned along the reaction axis, corresponding to the direction along which the new C-C bonds are forming, whereas FX/FY are aligned perpendicularly to the reaction axis. Molecular structures were optimized under the action of the applied EEF and the resulting energetics are listed in Table . It is observed that only applying a positive FZ lowers the energy barrier, whereas the application of FX and FY raises the height of the barrier. This is consistent with the "reaction-axis rule" proposed by Shaik, which states that the application of an EEF along the reaction axis (FZ here) lowers the barrier, whilst FX/FY induce selectivity between endo-and exo-cycloadducts. Since we are interested in ascertaining the catalytic role of the EEF, we will focus on the results emerging from the application of FZ only. Figure shows that applying Fz in the positive direction lowers the barrier, whereas in the negative direction it raises the barrier, evidencing that EEFs on the z-axis have either significant catalytic or inhibitory effects on the DA reaction in both gas and aqueous phases. Tracking the energy change of the stationary points (Figure ) indicates that a positive Fz stabilizes the TS more than the RC, and this different stabilization leads to the barrier decrease. The different response of TS and RC to the EEF aligns with the fact that the Z-component of the dipole moment, µZ, is larger for the TS (2.26 D in gas, 3.03 D in aqueous phase) than for the RC (-0.36 D in gas, -0.54 D in aqueous phase). It is known that in the presence of a uniform EEF FZ, the change in free energy G of a molecular system is approximately ΔG = -μZ,0FZ. Hence, the barrier change subjected to the EEF is ΔΔG ‡ = ΔμZ,0 ‡ FZ. As shown in Figure , the barrier decrease in presence of FZ is faster in the aqueous than in the gas phase. This can be explained by the different ΔμZ,0 values: 2.62 D in the gas phase and 3.57 D in the liquid.