Figure 4 Transition structures from ab initio calculations
Using the original force field, nearly every boat B structure collapsed to boat A. Addition of the constraint has no effect on the chair and boat A transition structures, because the formyl hydrogen and the oxygen of the enol-borinate are more than 2.9 Å apart. However, it stabilised the boat B structure, and gave reasonable values for the geometries and the energies for all structures except for E. The transition state E is the only one for which the formyl hydrogen and the oxygen of the enol-borinate are too far apart for the constraint to have an effect. It is clear, therefore, that there is something else missing from the force field in this case
This table leads to confidence that the additional constraint should greatly improve the force fields ability to model the third transition structure Boat B
| ab initio results | force field results | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| chair | boat A | boat B | chair | boat A | boat B | boat B constrained | |||
| ethanal + formaldehyde | 7.41 | 1.84 | 9.12 | 14.24 | 8.21 | collapse to boat A | 20.66 | ||
| A acetone + formaldehyde | -6.07 | -7.82 | -0.84 | 17.38 | 15.26 | collapse to boat A | 28.94 | ||
| B Z-propanal + formaldehyde | 0.000 | 1.67 | note | 25.37 | 42.12 | note | note | ||
| C E-propanal + formaldehyde | 7.489 | 7.45 | 13.81 | 17.23 | 16.00 | collapse to boat A | 28.52 | ||
| D ethanal + ethanal (trans) | 18.74 | 8.75 | 16.23 | 20.35 | 9.66 | collapse to boat A | 23.43 | ||
| E ethanal + ethanal (cis) | 26.11 | 15.61 | 21.30 | 26.66 | 15.74 | collapse to boat A | collapse to boat A | ||
| F | 9.63 | 16.77 | 13.58 | 13.14 | 11.23 | collapse to boat A | 21.32 | ||
| G | 4.39 | 4.35 | 15.774 | 12.59 | 7.30 | 17.26 | 18.88 |