Spectrum of the Month
February, 1998

This coincidence showed that the triplets were really two doublets of doublets where lines accidentally overlapped. This overlap was removed by the response of the chemical shift and J-coupling to the magnetic field strengths. In different magnetic fi lds the isotropic chemical shift stays the same when measured in ppm, but changes, when measured in Hertz. The J-coupling, on the other hand, is field independent and therefore remains constant when measured in Hertz, but seemingly changes when measured n ppm. Note how the two spectra change in width relative to each other, when plotted in ppm or Hz.

To obtain such a nice triplet from an accidental overlap of lines several conditions must be met:

1. The J-coupling constants of the two compounds need to be identical. In Jeff's case this is true as a result of symmetry: His compound contains two diastereomers, where the optical center at the phosphorous is reflected in the CH₂-group. Th J-coupling constants are the same because the local reflection symmetry. The isotropic chemical shifts of the diastereomers, however, differ slightly.
2. The intensities of the two contributions have to have a ratio of 1:1. This is approximately true for the spectrum taken in the Gem300. In the spectrum taken on the I400 the ratio of the diastereomers was 1.3:1. This fact makes the assignment of the l nes easy.