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False AA'X Spin-Spin Coupling Systems in 13C NMR: Examples Involving Phosphorus and a 20-Year-Old Mystery in Off-Resonance Decoupling
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Abstract
It is common in both NMR textbooks and journal articles to loosely describe certain ABX spin systems, in which the nuclei in the AB pair are rendered chemically inequivalent by proximity of one to 12C and the other to 13C, as AA'X spin systems. While the chemical shifts normally are indistinguishable and so for all practical purposes such systems can be analyzed as AA'X, there is a danger that when the chemical shifts are not the same, one may fail to recognize that fact due to the assumption that the isotopomer shift is always insignificant. Examples of such false AA'X spectra are described for diphosphorus organic and organometallic compounds, in which DeltavPP' = 2.5 - 4 Hz using a 400 MHz (1H) NMR spectrometer; at lower field strength the ABX splitting might be expected to disappear due to the reduction in DeltavPP'. An example involving 13C off-resonance decoupling in the NMR spectra of maleic anhydride was shown to depend on decoupler power: at low power, the system is ABX, while at the high power normally used it is AA'X. The chemical inequivalence of the "AA'" pair in this 1H213C system is not due to an isotopic shift, but rather to the off-resonance irradiation itself, and the results explain a 20-year-old discrepancy in published spectra.
Keywords (Audience):
Upper-Division UndergraduateKeywords (Domain):
Organic ChemistryKeywords (Subject):
CarbonCiting Articles
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- Received: August 03, 2009
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