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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