Question

(a) Assuming static structures and no observed coupling to the central atom, what would you expect to see in the \(^{19} \mathrm{F}\) NMR spectra of \(\mathrm{BrF}_{5}\) and \(\left[\mathrm{IF}_{6}\right]^{+} ?\) (b) Do you expect these spectra to be temperature-dependent?

Short answer

BrF5 shows two NMR signals; [IF6]+ shows one. Neither spectrum is temperature-dependent.

Step-by-step solution

Determine the Molecular Geometry of BrF5

BrF5 (bromine pentafluoride) has a square pyramidal shape with bromine at the center and five fluorine atoms surrounding it. In this geometry, there is one axial fluorine and four equatorial fluorine atoms.

Predict NMR Equivalence for BrF5

In the square pyramidal shape, the axial fluorine atom is distinguishable from the equatorial fluorines. Therefore, in the NMR spectra, BrF5 is expected to show two distinct signals: one for the axial fluorine and one for the four equivalent equatorial fluorines.

Determine the Molecular Geometry of [IF6]+

The structure of \([ \text{IF}_6 ]^+\) is octahedral. In this shape, the iodine atom is central with all six fluorine atoms symmetrically surrounding it, making all fluorine atoms equivalent.

Predict NMR Equivalence for [IF6]+

In the octahedral geometry, all six fluorine atoms are equivalent due to symmetry. Thus, \([ \text{IF}_6 ]^+\) will show only one signal in the NMR spectrum.

Analyze Temperature Dependence for BrF5 and [IF6]+

For BrF5, due to its rigidity in a square pyramidal structure at room temperature, the NMR spectra are unlikely to show significant temperature dependence. Similarly, \([ \text{IF}_6 ]^+\) is symmetrical and rigid as an octahedral structure, suggesting minimal to no temperature dependence in its NMR spectra.

Key concepts

BrF5 molecular geometry

The BrF5 molecule, or bromine pentafluoride, has a molecular geometry known as square pyramidal. This shape is created due to the arrangement of electrons around the central bromine atom. To be specific, BrF5 features one lone pair of electrons that contributes to its overall geometry. Here, bromine is surrounded by five fluorine atoms: one occupies an axial position and the other four occupy equatorial positions.

The unique layout of this configuration influences the molecule’s properties and interactions. Due to this structure:

• The axial fluorine atom is different from the four equatorial fluorine atoms. This distinction is important when examining the molecule’s behavior in various spectroscopic techniques, particularly NMR.
• It is arranged asymmetrically because it is not completely symmetric like a regular octahedral or tetrahedral shape, which can affect its electronic distribution and reactivity.

IF6+ ion symmetry

The \[\text{IF}_6\]^+ ion exhibits an octahedral geometry. In this symmetric structure, iodine acts as the central atom. It is symmetrically surrounded by six fluorine atoms, creating a uniform shape that is equidistant at all corners.

This symmetrical arrangement means:

• All fluorine atoms are equivalent. They share the same environment with respect to the central iodine atom.
• This high degree of symmetry leads to minimal electron pair repulsion among the fluorine atoms themselves, as they are placed at maximum distance from one another.
• The symmetry enhances the uniformity of the ion’s chemical and physical properties, making them easier to predict and analyze.

In terms of its application in spectroscopy, this symmetry has notable implications.

fluorine NMR signals

NMR spectroscopy provides insightful information about the environment of fluorine atoms in molecules like BrF5 and \[\text{IF}_6\]^+. This technique relies on detecting the magnetic fields around nuclear spins of fluorine, which are influenced by their nearby environment.

In BrF5, due to the square pyramidal structure:

• There are two distinct signals: one from the axial fluorine atom and a separate signal from the four equivalent equatorial fluorine atoms. This differentiation is due to the unique positions these atoms occupy in the molecular geometry.

For \[\text{IF}_6\]^+, the results are simpler:

• All six fluorine atoms are in identical environments due to the octahedral symmetry. Thus, they produce a single signal in the NMR spectrum.

Because these structures are quite rigid, especially at room temperature, the NMR signals for both BrF5 and \[\text{IF}_6\]^+ are usually steady and exhibit minimal temperature dependence. This is crucial for accurate analyses and predictions in structural chemistry.