³¹Phosphorus NMR

The 1D ³¹Phosphorus NMR experiment is much less sensitive than Proton (¹H) but more sensitive than ¹³Carbon. ³¹Phosphorus is a medium sensitivity nucleus that yields sharp lines (fig. 1) and has a wide chemical shift range. It is usually acquired with ¹Hdecoupling (fig. 2) means that spin-spin couplings are seldom observed. This greatly simplifies the spectrum and makes it less crowded. Where there are one-bond ³¹P-¹H couplings present then the decoupling power needs to be at lest twice that needed for ¹³C because of the large coupling constant.

Fig. 1. Typical ³¹P-NMR spectrum of a mixture of organic phosphates

Integration is inaccurate (almost useless when there is one-bond coupling to ¹H) in a regular decoupled ³¹P NMR spectrum because of uneven NOE enhancement of the signals by decoupling and long longitudinal relaxation times (T₁'s). Quantitative spectra may be obtained by inverse gated decoupling.

Gated decoupling may be used in order to observe proton couplings (fig. 4). If coupling constants are required then a phosphorus coupled proton spectrum (fig. 5) is much more sensitive than the phosphorus spectrum although the reduced chemical shift range may cause overlapping that make analysis more difficult. One-bond couplings are typically 600 to 700 Hz, two-bond 20 to 30 Hz, three-bond 5 to 10 Hz and four-bond <1 Hz. Couplings may be observed with other nuclei such as ¹⁹Fluorine.

Fig. 2. ³¹P-NMR spectrum of diethylphosphite with ¹H decoupling

Fig. 3. Molecular structure of deithylphosphite

Fig. 4. spectrum of diethylphosphite showing one-bond and three-bond coupling to ¹H

Fig. 5. ¹H-NMR spectrum of diethyl phosphite showing coupling to ³¹P

A typical analysis of a ³¹P NMR spectrum consists of matching expected chemical shifts to the expected moieties. Each type of signal has a characteristic chemical shift range (fig. 6).

Fig. 6. Chemical shift ranges of phosphorus according to their chemical environment

Choose the structure that most closely represents the phosphorus in question. R = alkyl or H.

The heteronuclear coupling patterns between phosphorus and proton can be used to assign the proton spectrum (figs. 7, 8).

Fig. 7. ¹H-NMR spectrum of sphingomyelin showing the effects of ³¹P coupling

Fig. 8. ³¹P-NMR spectrum of sphingomyelin showing the effects of ¹H coupling

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Properties of ³¹P

(Click here for explanation)

Property	Value
Spin	½
Natural abundance	100%
Chemical shift range	430 ppm, from -180 to 250
Frequency ratio (Ξ)	40.480742%
Reference compound	85% H3PO4 in H2O = 0 ppm
Linewidth of reference	1 Hz
T1 of reference	0.5 s
Receptivity rel. to 1H at natural abundance	6.63 × 10-3
Receptivity rel. to 1H when enriched	6.63 × 10-3
Receptivity rel. to 13C at natural abundance	37.7
Receptivity rel. to 13C when enriched	37.7

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PAGE CREATED BY DR ANTHONY MELVIN CRASTO M.SC, Ph.D (ORGANIC CHEMISTRY,24+ years experience in the field of research and development, currently with Glenmark-Generics Ltd, Navi Mumbai,  India

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