Question

4. Below is a plot of the variable-temperature 31P{H} NMR spectrum of the compound shown below. a) What are two possible structures for the species that give rise to the two signals? b) Why do the signals coalesce at elevated temperatures? HN. | 0 - Ph. HN-P-O-ěron Me 363K 353K 343K 333K 323K 313K 303K 293K 5.0 5 5 .0 55.0 5.5

Answer 1

These are the two possible structures, 1 TBP structure and 1 Square Pyramidal structure.

N Мо HN Rmn TBP ware Tyranidad

The two structures of the Molecule are in Rapid Equilibrium. This Phenomenon is called Fluxionality. At Elevated Temperatures, the Energy Barrier of Conversation between the two structures is easily overcome. But, at low temperature, the Molecule doesn't have enough energy to overcome the Energy Barrier, thus the 2 structures are frozen at low temperature and give rise to 2 different signals, while at Elevated Temperatures, due to Fluxionality, the two structures can't be recognized differently and as a result, Only 1 signal is observed.