In P NMR, Due to the higher electronegativity of the O atom than the S atom, and higher pi-characteristic of the P=O bond than the P=S bond, it is expected that the signal of P atom in P=O group appear at higher chemical shifts than P=S. But, experimentally this trend is in converse. The P=O signal appears at about 30-50 ppm lower than P=S. For example, for compounds P(=O)(NHBu)3 and P(=S)(NHBu)3 the P signals appear at 25 ppm and 60 ppm, respectively. How this trend can be justified?
The chemical shift of 31P is dominated by the anisotropy effect of the P=O or P=S double bind. The phosphorous atom is in the deshielding area. The effect increases with distance from the center before decreasing at very large distances. The P=S bond is longer than the P=O bond. The same effect can be seen for C=O vs C=S bonds.
Dear Dr. Anklin
Thank you for your helpful response. This solved my problem.
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