Exploration of Dynamic Exchange using Variable Field NMR

Abstract

Activation parameters (Δ𝐻‡, Δ𝑆‡ and Δ𝐺‡) for reversible chemical exchange processes on the 𝑚𝑠 to 𝑠 timescale are often calculated using Eyring analysis of rate constants determined by lineshape analysis of variable temperature NMR spectra. This approach typically gives estimates of Δ𝐻‡ and Δ𝑆‡ with large uncertainties if coalescence of the exchanging NMR peaks cannot be accessed. Chemical-shift scaling has the potential to mitigate this by artificially manipulating the field strength of an NMR spectrometer, scaling down chemical shifts, and lowering the coalescence temperature. In this work, an improved chemical-shift scaling pulse sequence has been tested on both non-exchanging and exchanging systems at 500 MHz and the resulting spectra were an excellent match to simulated and measured spectra between 10 and 400 MHz. Rate constant uncertainties were estimated for the amide N–CO rotation of N,N-dibenzylnaphthamide in CDCl3 from 25 to 50 °C using chemical-shift scaled spectra and variable temperature N

Date of Award	1 Oct 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Jonathan Clayden (Supervisor) & Craig P Butts (Supervisor)