ORGN 397 |
| Understanding the influences which determine conformational preferences of small organic molecules is an ongoing challenge to physical organic chemistry. To learn more about them, we report here a new conformational system of 1,2-substituted ethanes, represented by the amine oxide (1), as a function of pH with the aid of NMR spectroscopy. The conformational preferences were determined by comparing measured vicinal proton-proton coupling constants of the methylene proton resonances with estimated couplings for the individual conformers obtained by the Altona procedure. The trans conformation (1a) is preferred to an extent of ~75% in the pH ranges of 0-2 and 6-10. In contrast, the gauche conformation (1b, 1c) is favored essentially exclusively between pH 2-6, which is a strong indication of N-O−---H-O-N bonding. Two pKa's were found of about 2.0 and 6.1. A possible observable rotational barrier was sought, but at -100 ºC in C2D5OD, only partial line broadening was observed.
|
|
Physical Organic Chemistry: Calculations, Mechanisms, Photochemistry, and High Energy Species
8:00 AM-11:40 AM, Tuesday, August 21, 2007 BCEC -- 254 A/B, Oral
Division of Organic Chemistry |
