DOSY NMR Experiments at Variable Temperature. A Thorough Analysis of Optimal Conditions for Reliable Results

Abstract

Multinuclear (1H, 31P, and 19F) DOSY technique has been used under variable temperature conditions, in order to establish the optimal conditions for obtaining reliable data at variable temperatures, while keeping experimental settings as simple as possible (e.g., using standard NMR tubes). For this goal, we have used the methodology developed in 2003 by Martínez-Viviente and Pregosin, but using more modern pulse sequences specially designed to avoid the effects of convection upon DOSY experiments. We have written new pulse sequences (modifying existing standard ones) in order to use proton/heteronuclear decoupling (in this last case, with the possibility also of adiabatic decoupling). We have performed extensive measurements, covering all possible variables: type of instrument/probe, solvent volatility, calibration of the instrument gradients, spinning/non-spinning of the sample, proton decoupling, heteronuclear decoupling (standard or adiabatic), and so on. The ensemble of all these measurements has allowed us to design a protocol for simple measurement of DOSY at variable temperature in a standard NMR lab, without any need of special instruments/probes or glassware.