Table 3. QM/MM Free Energy Barriers (in kilocalories per mole) for the Nucleophilic Attack at Each of the Epoxide Carbons, C(1) and C(2), from the Extended Umbrella Sampling MD Simulations of the t-SO, t-DPPO(1), and t-DPPO(2) Model Systems.
| ΔG⧧(P) |
||||||
|---|---|---|---|---|---|---|
| t-SO |
t-DPPO(1) |
t-DPPO(2) |
||||
| QM method | C(1) | C(2) | C(1) | C(2) | C(1) | C(2) |
| AM1a | 22.7 | 30.1 | 19.0 | 25.0 | 23.4 | 30.7 |
| B3LYP/6-31G(d) | 7.3 | 16.6 | 9.7 | 11.0 | 6.8 | 13.9 |
| SCS-MP2/(aug)-cc-pVTZ | 12.3 | 19.9 | 12.8 | 15.8 | 13.2 | 21.8 |
a
Energy barriers calculated from extended umbrella sampling simulations with His523 protonated at the Nε position.