Question

The structure of PF3(C6H5)2 is trigonal bipyramidal, with one equatorial and two axial F atoms which interchange positions when heated. Describe the low- and high- temperature 31P and 19F NMR spectra.

Answer 1

Answer:

For 31 P NMR spectra

low temperature

there is two types of 19f seen in low temperature and they are

• one at equitorial
• one at axial

therefore in low temperature the 31p couples with the two types of 19F seen ( $b_{f} and c_{f}$to form a triplet and this couples more with $a_{f}$ to form a doublet. i.e. one (1) peak

High temperature

At High temperature The exchange is fast here therefore the 31p spectra sees all 19p at once and in the same environment leading to the formation of one (1) peak

For 19 P NMR spectra

low temperature

In low temperature $a_{f}, b_{f} , c_{f}$ is fixed  and the environment where $b_{f} and c _{f}$ is the same hence a peak is formed and another peak is formed by $a_{f}$ that makes the number of peaks = 2 peaks

High temperature

In high temperature $a_{f}, b_{f} , c_{f}$  exchange very fast therefore one peak is formed for all, since the fast exchanges makes NMR machine to take an average and produce just one peak for all

Explanation:

For 31 P NMR spectra

low temperature

there is two types of 19f seen in low temperature and they are

• one at equitorial
• one at axial

therefore in low temperature the 31p couples with the two types of 19F seen ( $b_{f} and c_{f}$to form a triplet and this couples more with $a_{f}$ to form a doublet. i.e. one (1) peak

High temperature

At High temperature The exchange is fast here therefore the 31p spectra sees all 19p at once and in the same environment leading to the formation of one (1) peak

For 19 P NMR spectra

low temperature

In low temperature $a_{f}, b_{f} , c_{f}$ is fixed  and the environment where $b_{f} and c _{f}$ is the same hence a peak is formed and another peak is formed by $a_{f}$ that makes the number of peaks = 2 peaks

High temperature

In high temperature $a_{f}, b_{f} , c_{f}$  exchange very fast therefore one peak is formed for all, since the fast exchanges makes NMR machine to take an average and produce just one peak for all