Problem 18
Why is a coupling constant measured in \(\mathrm{Hz}\) and is not recorded as a chemical shift difference? Long-range couplings are often observed between \(^{31} \mathrm{P}\) and \(^{19} \mathrm{F}\) nuclei, between \(^{31} \mathrm{P}\) and \(^{1} \mathrm{H}\) nuclei, but not between remote non-equivalent \(^{1} \mathrm{H}\) nuclei. What does this tell you about the relative magnitudes of values of \(J_{\mathrm{PF}}, J_{\mathrm{PH}}\) and \(J_{\mathrm{HH}}\) for the respective pairs of nuclei when they are directly attached?
Problem 19
Rationalize the fact that the \(^{13} C\) NMR spectrum of \(\mathrm{CF}_{3} \mathrm{CO}_{2} \mathrm{H}\) consists of two binomial quartets with coupling constants of 44 and \(284 \mathrm{Hz}\) respectively.
Problem 24
(a) Predict the structure of \(\mathrm{SF}_{4}\) using the VSEPR model. (b) Account for the fact that at \(298 \mathrm{K}\) and in solution the \(^{19} \mathrm{F}\) NMR spectrum of \(\mathrm{SF}_{4}\) exhibits a singlet but that at \(175 \mathrm{K}\), two equal-intensity triplets are observed.
Problem 25
The \(^{19} \mathrm{F}\) NM \(\mathrm{R}\) spectrum of each of the following molecules exhibits one signal. For which species is this observation consistent with a static molecular structure as predicted by the VSEPR model: (a) \(\mathrm{SiF}_{4}\) i; (b) \(P F_{5} ;\) (c) \(\mathrm{SF}_{6}\) (d) \(\operatorname{SOF}_{2}\) (e) \(\mathrm{CF}_{4} ?\)
Problem 28
Account for the fact that the \(^{29}\) Si NMR spectrum of a mixture of \(\mathrm{SiCl}_{4}\) and \(\mathrm{SiBr}_{4}\) that has been standing for \(40 \mathrm{h}\) contains five singlets which include those assigned to \(\mathrm{SiCl}_{4}\) \((\delta-19 \mathrm{ppm})\) and \(\mathrm{SiBr}_{4}(\delta-90 \mathrm{ppm})\)
Problem 34
MeCN solutions of \(\mathrm{NbCl}_{5}\) and \(\mathrm{HF}\) contain a mixture of octahedral \(\left[\mathrm{NbF}_{6}\right]^{-},\left[\mathrm{NbF}_{5} \mathrm{Cl}\right]^{-},\left[\mathrm{NbF}_{4} \mathrm{Cl}_{2}\right]^{-}\) \(\left[\mathrm{NbF}_{3} \mathrm{Cl}_{3}\right]^{-}\) and \(\left[\mathrm{NbF}_{2} \mathrm{Cl}_{4}\right]^{-}\). Predict the number and coupling patterns of the signals in the \(^{19} \mathrm{F}\) NMR spectrum of each separate component in this mixture, taking into account possible isomers. (Assume static structures and no coupling to \(^{193} \mathrm{Nb}\).)
Problem 35
(a) Explain why the \(^{19} \mathrm{F}\) NMR spectrum of \(\left[\mathrm{PF}_{6}\right]\) appears as a doublet. (b) The \(^{31} \mathrm{P}\left\\{^{1} \mathrm{H}\right\\}\) NMR spectrum of trans\(=\left[\mathrm{PtI}_{2}\left(\mathrm{PEt}_{3}\right)_{2}\right]\) (3.10) shows a three-line pattern, the lines in which have relative integrals of \(\approx 1: 4: 1 .\) What is the origin of this pattern?
Problem 37
The \(^{19} \mathrm{F}\) NMR spectrum of the octahedral ion \(\left[\mathrm{PF}_{5} \mathrm{Me}\right]\) shows two signals \((\delta-45.8 \text { and }-57.6 \mathrm{ppm}) .\) Why are two signals observed? From these signals, three coupling constants can be measured: \(J_{\mathrm{PF}}=829 \mathrm{Hz}, J_{\mathrm{PF}}=680 \mathrm{Hz}\) and \(J_{\mathrm{FF}}=35 \mathrm{Hz}\). Explain the origins of these coupling constants.
