Table 2. Free Energy Calculations for Isomersa.

| R | exo (SM) | C2 | Cs | difference |
|---|---|---|---|---|
| H | 0.0 | –8.0 | –5.1 | 2.9 |
| Me | 0.0 | –7.1 | –3.8 | 3.3 |
| Et | 0.0 | –9.0 | –5.2 | 3.8 |
| Pr | 0.0 | –8.7 | –4.6 | 4.1 |
| Ph | 0.0 | –9.2 | –6.4 | 2.8 |
Gas-phase calculations were carried out using M06-2X/6-311+G(d,p)//B3LYP/6-31G(d) and are quoted in kcal/mol.