Expt. 6 - Substituent & Solvent Effects on Tautomeric Equilibria
Effect of Substituent on Conformational Energies
|
Tautomer |
AM1 (kcal/mol) |
AM1 Relative (kcal/mol) |
|
1a (R=H) |
39.878 |
0.0 |
|
1b (R=H) |
50.510 |
10.632 |
|
1c (R=H) |
48.575 |
8.697 |
|
1a (R=Cl) |
46.835 |
20.322 |
|
1b (R=Cl) |
27.951 |
1.438 |
|
1c (R=Cl) |
26.513 |
0.0 |
For R=H, AM1 predicts pyrrole, 1a, to be the most stable. For R=Cl, AM1 predicts the azadiene tautomer, 1c, to be the most stable.
Effect of Solvation on Conformational Energies
|
Tautomer |
AM1 (kcal/mol) |
AM1 Relative (kcal/mol) |
Dipole Moment (Debye) |
AM1 /SM2 (kcal/mol) |
AM1 /SM2 Relative (kcal/mol) |
|
2a (cis) |
-11.769 |
0.0 |
1.377 |
-20.310 |
1.764 |
|
2a (trans) |
-7.113 |
4.656 |
3.229 |
-15.957 |
6.117 |
|
2b |
-11.305 |
0.464 |
3.921 |
-22.074 |
0.0 |
Note the large difference in the relative energies and dipole moments of the cis & trans isomers of 2a. In the gas phase, AM1 predicts 2a (cis) to be more stable than 2b. In solution (water), AM1/SM2 predicts the opposite. However, both models predict the cis conformer of 2a to be more stable than the trans.