ORGN 40 |
| Delocalization of a lone pair antiperiplanar to a CH bond weakens that bond, lengthens it, and lowers its stretching frequency. The resultant lowering of zero-point energy is then responsible for the greater basicity of amines substituted with b deuterium, since the zero-point energy increases on N-protonation, especially for CH compared to CD. This was shown experimentally through a highly accurate NMR titration method. It was further shown that deuterium is most effective when it is antiperiplanar to the nitrogen lone pair, as in 1. Now it is shown that synperiplanar deuterium, as in pyrrolizidine-8-d (2) and 2-alkyl-2-azanorbornanes-3,3-d2 (3, R = CH3, CH2Ph), is also effective, but less so. This behavior is supported by DFT calculations on CH bond lengths and CD stretching frequencies in conformations of CH3NH2 or DCH2NH2. Delocalization of an antiperiplanar lone pair is also thought to be responsible for the reduction (Perlin effect) of the one-bond NMR coupling constant 1JCH of the axial CH in oxane (4). DFT calculations on the dependence of 1JCH on the dihedral angle ΤHCOC in conformations of ethers reproduce this effect. However, 1JCH is calculated to be maximum at 180, minimum at 0, and intermediate at 90, whereas delocalization of the pure-p lone pair is maximum at 90 and minimum at 0 and 180. Fourier analysis confirms that 1JCH and delocalization show different dependences on ΤHCOC. Therefore 1JCH is not determined by lone-pair delocalization! Instead it is proposed that 1JCH is determined by a dipolar interaction that acts via the electron density and the hybridization.
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James Flack Norris Award in Physical Organic Chemistry
1:00 PM-5:00 PM, Sunday, 13 March 2005 Convention Center -- Ballroom 20 C-D, Oral
Division of Organic Chemistry |
