Created by Margret J. Geselbracht, Reed College (mgeselbr@reed.edu) and posted on VIPEr
(www.ionicviper.org) on March 19, 2010. Copyright Margret J. Geselbracht 2010. This work is licensed
under the Creative Commons Attribution Non-commercial Share Alike License. To view a copy of this
license visit http://creativecommons.org/about/license/.
MULTINUCLEAR NMR OF XENON OXYFLUORIDES
1. (a) Using VSEPR, predict the structures and identify the shapes of the following molecules.
(b) Make predictions of bond angles (that is, either state the ideal angle or whether the angle
is < ideal or > ideal angle). (c) Predict what the 19F NMR spectrum would look like for each
of these molecules assuming a static structure. (d) 129Xe is also NMR-active (I = 1/2),
although the natural abundance is only 26.4%. Predict what the 129Xe NMR spectra would
look like for each of these molecules.
XeO2F2
(a) The shape of this molecule is sawhorse with the structure as drawn
on the left.
F
Xe
O
O
F
(b) The lone pair will distort the molecule and compress the bond angles
to values smaller than the ideal angles. So, the OXeO bond angle will
be slightly less than 120˚, the FXeF bond angle will be less than 180˚,
and the FXeO bond angles will be less than 90˚.
(c) Both F atoms are chemically equivalent and should lead to a single resonance in the
19
F NMR spectrum. On either side of the main peak however, there will be small
satellites, a doublet formed by 19F–129Xe coupling in 26% of the molecules. (d) The
129
Xe spectrum should consist of a triplet caused by 129Xe–19F coupling to 2 equivalent F
atoms.
XeOF4
O
F
Xe
(a) The shape of this molecule is square pyramidal with an axial oxygen atom.
F
F
(c) All four F atoms are chemically equivalent and should lead to a single
resonance in the 19F NMR spectrum. On either side of the main peak
however, there will be small satellites, a doublet formed by 19F–129Xe
coupling in 26% of the molecules. (d) The 129Xe spectrum should consist of a
quintet caused by 129Xe–19F coupling to 4 equivalent F atoms.
F
[XeO2F]+
Xe
F
(b) The lone pair will displace the four fluorine atoms towards the oxygen
atoms such that the FXeO bond angles are less than 90˚.
(a) The shape of this molecule is trigonal pyramidal.
O
O
(b) The lone pair would repel the three bonding pairs and compress the bond
angles such that the FXeO and OXeO bond angles are less than 109˚. The
OXeO bond angle is probably slightly larger than the FXeO bond angle as
there will be more bonding pair-bonding pair repulsion between the Xe–O
bonds than between the Xe–F and a Xe–O bond.
(c) The 19F NMR spectrum consists of a single resonance for the lone fluorine with small
129
Xe satellites (a doublet centered around the central resonance). (d) The 129Xe spectrum
should consist of a doublet caused by 129Xe–19F coupling to the lone F atom.
Created by Margret J. Geselbracht, Reed College (mgeselbr@reed.edu) and posted on VIPEr
(www.ionicviper.org) on March 19, 2010. Copyright Margret J. Geselbracht 2010. This work is licensed
under the Creative Commons Attribution Non-commercial Share Alike License. To view a copy of this
license visit http://creativecommons.org/about/license/.
[XeOF3]+
(a) The shape of this molecule is sawhorse with the structure as drawn on
the left.
F
O
Xe
F
F
(b) The lone pair will distort the molecule and compress the bond angles to
values smaller than the ideal angles. So, the FeqXeO bond angle will be less
than 120˚, the FaxXeFax bond angle will be less than 180˚, and the FaxXeO
and FaxXeFeq bond angles will be less than 90˚.
(c) To see the 19F NMR data, look at Figure 2 from R. J. Gillespie and G. J. Schrobilgen; Inorg.
Chem. 1974, 13, 2370-2374. The equatorial F produces a triplet with 129Xe triplet satellites on
either side of the main resonance, and the axial F produces a doublet with 129Xe doublet satellites
on either side of the main resonance.
Note that the authors of this paper point out that the NMR data is consistent with the correct
structure predicted by VSEPR theory, but that this data would also be consistent with the option
where the oxygen is axial and there are two equatorial fluorines and one axial fluorine.
(d) The 129Xe NMR spectrum will be somewhat complicated. There will be 129Xe–19F coupling
to the single equatorial fluorine and to the 2 identical axial fluorine atoms. Presumably, these
coupling constants will be different. It turns out the coupling constant to the equatorial fluorine
is larger (although that is not something we would know how to predict). We can use this to
think about how the spectrum arises. Consider first turning on the stronger coupling interaction
to the equatorial fluorine, which will split the 129Xe resonance into a doublet. Next we turn on
the 129Xe–19F coupling to the 2 identical axial fluorine atoms. This will split each component of
the double further, into a triplet. Thus, the overall spectrum could be described as a doublet of
triplets. I tried to find a picture of this, but was unlucky and only found a brief summary of the
NMR parameters in a large table in one paper.
[XeO6]4O
O
Xe
O
O
(a) The shape of this molecule is octahedral.
O
O
(b) The bond angles are all the ideal values of 90˚ and 180˚.
(c) There will be nothing observed in the 19F NMR spectrum since there are no
fluorine atoms! If one isotopically labeled the molecules with 17O, you could
do NMR…
(d) The 129Xe resonance should be a single peak.