ORGN 776 |
| Secondary deuterium isotope effects (IEs) on acidities have been accurately measured by an NMR titration method applicable to a mixture of isotopologues. Deuteration definitely decreases the acidity of 1-8. For aliphatic acids the IEs decrease as the site of deuteration becomes more distant from the OH, but, surprisingly, IEs in phenol and benzoic acid do not decrease as deuterium moves from ortho to meta to para. The experimental data are supported by ab initio computations which, however, overestimate the IEs. The discrepancy is not due to solvation. The IEs originate in isotope-sensitive vibrations whose frequencies and zero-point energies are lowered on deprotonation. In the simplest case, formate (9), the key vibration can be recognized as the CH stretch, which is weakened by delocalization of the oxygen lone pairs. The ability of the frequency analysis to account for the IEs is evidence against an inductive origin.
|
|
Physical Organic Chemistry: Calculations, Mechanisms, Photochemistry, and High Energy Species
8:00 AM-12:00 PM, Thursday, March 29, 2007 McCormick Place Lakeside -- Room E451A/B, Level 4, Oral
Division of Organic Chemistry |
