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Date: 30-3-2019
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Date: 31-1-2018
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Date: 29-11-2018
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Group 18 : NMR active nuclei
In the NMR spectroscopic characterization of Xecontaining compounds, use is made of 129Xe, with a natural abundance of 26.4% and I = 1/2. Although direct observation of 129Xe is possible, the observation of satellite peaks in, for example, 19F NMR spectra of xenon fluorides is a valuable diagnostic tool as we illustrated for [XeF5]- .
Worked example : NMR spectroscopy of xenon-containing compounds
Reaction of XeF4 and C6F5BF2 at 218K yields [C6F5XeF2][BF4]. (a) Use VSEPR theory to suggest a structure for [C6F5XeF2]+. (b) The 129Xe NMR spectrum of [C6F5XeF2][BF4] consists of a triplet (J = 3892 Hz), and the 19F NMR spectrum shows a three-line signal (relative intensities ≈ 1 : 5.6 : 1), three multiplets and a singlet. The relative integrals of the five signals are 2 :2 :1 :2 : 4. Rationalize these data.
(a) Xe has eight valence electrons. The positive charge can be formally localized on Xe,
leaving seven valence electrons. Each F atom provides one electron to the valence shell of Xe. The C6F5 group is bonded through carbon to Xe and provides one electron to the valence shell of Xe.
Total number of electrons in the valence shell of Xe=10 The parent shape for [C6F5XeF2]+ is a trigonal bipyramid with the two lone pairs in the equatorial plane to minimize lone pair–lone pair repulsions. For steric reasons, the C6F5 group is expected to lie in the equatorial plane with the plane of the aryl ring orthogonal to the plane containing the XeF2 unit. The expected structure is T-shaped:
(b) The triplet in the 129Xe NMR spectrum of [C6F5XeF2][BF4] shows a large coupling constant (3892 Hz) and arises from coupling between 129Xe and the two equivalent, directly bonded 19F nuclei.
There are four F environments in [C6F5XeF2]+ (ortho, meta and para-F atoms in the aryl group and the two equivalent F atoms bonded to Xe, with a ratio 2:2:1:2, respectively. The signals for the aryl F atoms appear as multiplets because of 19F–19F coupling between non-equivalent F atoms. There are four equivalent F atoms in the [BF4]- ion leading to a singlet; coupling to 11B is not observed. Only the directly bonded 19F nuclei couple to 129Xe (I = 1 2, 26.4%). The signal
in the 19F NMR spectrum assigned to these F atoms appears as a singlet with satellites for the 26.4% of the 19F bonded to spin-active 129Xe. The relative intensities 1 :5.6 :1 correspond to 26.4% of the signal split into a doublet.
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