Phosphorus-31: The bearer of light

Phosphorus is, in my completely biased opinion, one of the most interesting elements out there.

Why? Well, so so soooo many reasons!

1) It’s colorful. See the naturally occurring isotopes of phosphorus:^{[1, 2]} white (P₄ – used in incendiary bombs, flares and smoke grenades), red (an amorphous linkage network of P₄), violet (different crystalline form – it’s an extensive network of phosphorus norbornane and deltacyclane structures in 3D polymer), and black (graphite like rhombohedral structure).

Figure 1. Some allotropes of Phosphorus.

2) It, like carbon, has a propensity to catenate. I mean, sure, P-P bonds are longer and more labile than C-C, but regardless this leads to some pretty cool catena-phosphorus analogues of carbon compounds!

3) In fact, it has been speculated that it is phosphorus, not the heavier group 14 congeners (Si, Ge, Sn, Pb), that is the closest element to carbon. Not only do C and P have similar electronegativity and reactivity but there is also an isolobal relationship between the prototypical component of organic chemistry (i.e., CR₄) and phosphine (PR₃), phosphide (PR₂−) and phosphonium (PR₄+).

4) Phosphorus has many potential bonding modes,^[3] and not only can it support positive and negative charges, but it can expand its octet. This means phosphorus is involved in a ton of interesting and diverse chemistry (e.g., biomolecular processes, soaps, surfactants, fertilizers, pesticides, nerve gases, organometallic compounds).

5) It is a good – no, strike that – GREAT, NMR nucleus! For starters, ³¹P is I = ½ and 100% abundant. Moreover, the wide range of bonding environments lead to a wide spectral width (ranging from −300 to +250 ppm) and resultantly really indicative chemical shifts. Cliff notes – phosphorus is an excellent NMR handle to monitor reactions. It also has an okay gyromagnetic ratio (~40% of 1H, so about 25 MHz on the NMReady-60PRO) but typically has unreliable integrations.

The Nanalysis 60PRO benchtop NMR can acquire both, coupled ³¹P and decoupled ³¹P{¹H} NMR spectra. I’ve included an example of 0.5 M tetramethylphosphonium chloride in D₂O. You can see a nice doublet in the 4 scan ¹H NMR spectrum owing to both 12 equivalent methyl groups split by the central phosphorus atom. The 16 scan ³¹P{¹H} is a singlet and the 64 scan ³¹P is a tredecatet (with 1 : 12 : 66 : 220 : 495 : 792 : 924 : 792 : 495 : 220 : 66: 12 :1 intensities) where the peaks on the side are not well resolved.

Figure 2. ¹H NMR (left) and ³¹P and ³¹P{¹H} (right) spectra of tetramethylphoshonium chloride.

If you have any other questions or want to see how your phosphorus samples look at 25 MHz, please don’t hesitate to contact us!

[1] Corbridge, D. E. C. The Structural Chemistry of Phosphorus; Elsevier: New York, 1974.
[2] Palenik, G. J.; Donohue, J. Acta Cryst. 1962, 15, 564.
[3] Burford, N.; Ragogna, P. J. Dalton Trans. 2002, 4307-4315.