Single-Crystal NMR Characterization of Halogen Bonds

Oxygen-17-enriched
triphenylphosphine oxide and three of its halogen-bonded
cocrystals featuring 1,4-diiodotetrafluorobenzene and 1,3,5-trifluoro-2,4,6-triiodobenzene
as halogen bond donors have been characterized by 31P and 17O single-crystal NMR spectroscopy. Single-crystal NMR allows
for the measurement of not only the magnitudes of various NMR interaction
tensors, but also their orientations relative to the crystal lattice
and therefore relative to the halogen bonds themselves. 31P chemical shift tensors, 17O chemical shift tensors, 17O quadrupolar coupling tensors, and 31P–17O indirect nuclear spin–spin (J) coupling
tensors are reported here for PO···I halogen
bonds. The angular deviations in the directions of the pseudo-unique
components of the 31P chemical shift tensors, the 17O chemical shift tensors, and the 17O quadrupolar
coupling tensors from the direction of the oxygen–iodine halogen
bond correlate with the deviations in linearity of the PO···I
halogen bond. There is also a clear decrease in anisotropy and an
increase in asymmetry of the J(31P,17O) coupling tensors attributable to the formation of iodine–oxygen
halogen bonds. The small but quantifiable changes in the tensors are
consistent with the weak nature of these halogen bonds relative to
the PO motif. Overall, this work establishes single-crystal
NMR as a novel probe of halogen bonds in solids. Analysis of the results
has provided insights into the correlations between the magnitude
and orientation of various NMR interaction tensors and the local geometry
of the halogen bond. Gauge-including projector-augmented wave computations
corroborate the experimental findings.