NMR spin dynamics

Anonymous
timer Asked: Dec 12th, 2017

Question description

Question 1.

Consider an idealized symmetrical two-site exchange process, with kinetics described by the Arrhenius equation over a wide range in temperature. The pre-exponential constant is A = 1014 s-1 and the activation energy is Eact = 60 kJ mol-1. The exchange process causes a change in the 13C chemical shift at a certain molecular site to change by 30 ppm, while the 1H chemical shift at a different molecular site changes by 4 ppm. All the parameters are assumed to be temperature independent.

(i) At what magnetic field is the coalescence in the 13C line shapes observed if the sample temperature is T = 300 K? Hint: coalescence occurs at point where 𝑘 = |$%&$'| , where k is

exchange rate and dA, dB are chemical shift in environment A and B.
(ii) At what magnetic field is the coalescence in the 1H line shapes observed if the sample temperature is T = 300 K?
(iii) At what temperature is the coalescence in the 1H line shapes observed if the proton Larmor frequency is ω0/2π = −300 MHz?
(iv) Around which temperature does one expect the exchange process to become a mechanism for proton spin–lattice relaxation if the magnetic field is B0 = 14 T?


Question from Excercise 19.1 in Spin dynamics: Basics of Nuclear MAgenic Resonsance by MH Levitt

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