NMR active nuclei and isotopes as tracers

   Despite its low abundance , ¹⁷O has been used in studies of, for example, hydrated ions in aqueous solution and polyoxometallates. The isotope ¹⁸O is present to an extent of 0.2% in naturally occurring oxygen and is commonly used as a (nonradioactive) tracer for the element. The usual tracer for sulfur is ³⁵S, which is made by an (n,p) reaction on ³⁵Cl; ³⁵S is a b-emitter with t_1/2 = 87 days.

Worked example

NMR spectroscopy using ⁷⁷Se and ¹²⁵Te nuclei

The solution ¹²⁵Te NMR spectrum of Te(cyclo-C₆H₁₁)₂ at 298K shows one broad signal. On increasing the temperature to 353 K, the signal sharpens. On cooling to 183 K, the signal splits into three signals at δ 601, 503 and 381 with relative integrals of 25 :14 : 1. Rationalize these data.

Te(cyclo-C₆H₁₁)₂ contains only one Te environment, but the Te atom can be in either an equatorial or axial position of the cyclohexyl ring. This leads to three possible conformers:

  On steric grounds, the most favoured is the equatorial, equatorial conformer, and the least favoured is the axial, axial conformer. Signals at δ 601, 503 and 381 in the lowtemperature spectrum can be assigned to the equatorial, equatorial, axial,equatorial and axial,axial conformers respectively.   

   At higher temperatures, the cyclohexyl rings undergo ring inversion (ring-flipping), causing the Te atom to switch between axial and equatorial positions. This interconverts the three conformers of Te(cyclo-C₆H₁₁)₂. At 353 K, the interconversion is faster than the NMR timescale and one signal is observed (its chemical shift is the weighted average of the three signals observed at 183 K). On cooling from 353 to 298 K, the signal broadens, before splitting at lower temperatures.