Question

In H2O, the 31P NMR spectrum of the compound gives 2 lines shifted from each other by about 1ppm, corresponding to the hydrate and ketone in the ratio of 60:40.

You want to measure the rates of interconversion between them in the range of 0.1-3.0 per second using a cross relaxation method. Both the hydrate and ketone have T1's of 5.0 second and linewidths, delta v1/2 of 2 hz. The experiments are performed in a field such that the 31P resonant frequency is 125.000 Mhz.

a.) How will the system respond to a non selective inversion recovery sequence?

b.) How can you generate a state in which the hydrate line has been inverted and the ketone line has been de-excited using 2 non-selective pulses similar to those used for jump-return water supression?

c.) How will the system recover if you impose the conditions of b. on the system at t=0.

d.) Describe how you will measure the chemical interconversion rate from the data in c.

Answer 1

In the inversion recovery experiment, commonly used to measure T₁ values, the initial magnetization is inverted, , and so the recovery follows a subsequent 90 degree pulse shows the recovery depending on the time gap between inversion and 90 degree pulse.

In a standard inversion recovery experiment since both have similar T1 they will show similar intergral for the signal.

The jump-return sequence for water supression is a sequence 90x-delay-90(-x). If the magnetisation is on-resonance no rotation occurs during the delay and it effectively sees a 0 deg exciation (jump and return) pulse -> no signal is detected. If the delay is set in a way that magnetisation of interest with frequency f rotates by 90 deg this magnetisation does no "see" the 2nd pulse, as it is aligned with this pulse. This magnetisation sees a effective 90deg pulse and the full signal is observed.

So keeping your resonance on the ketone the ketone is de-excited and the hydrate line will be inverted with a 180 pulse after a 90 degree excitation on the ketone.

The system will show only the signal for the hydrate at t=0 in the jump-return sequence with the excitation line on the ketone.

changing the t and getting total data for the hydrate we can plot t versus siganl and get the interconversion rate with this plot when the siganl gets to zero.