Simple Parameters and Data Processing for Better Signal-to-Noise and Temporal Resolution in In Situ 1D NMR Reaction Monitoring

Abstract

In situ 1D NMR spectroscopic reaction monitoring allows detailed investigation of chemical kinetics and mechanism. Concentration versus time data are derived from a time series of NMR spectra. Each spectrum in the series is obtained by Fourier transform of the corresponding FID. When the spectrometer outputs FIDs recorded from multiple scans, the spectra benefit from an increase in signal-to-noise (S/N). However, this reduces the number of FIDs and, thus, kinetic data points. We report a simple alternative in which the same number of scans is acquired by the spectrometer, but each scan is saved independently. Signal averaging is then conducted by postacquisition processing. This leads to an increase in both the S/N and the number of kinetic data points and can avoid “overaveraging” effects. The entire series of single-scan FIDs spanning the reaction lifetime can be summed to yield a “total reaction spectrum” in which intermediates can be identified. The method can be applied in coherence with phase cycling to minimize spectral distortion during solvent signal suppression. Overall, the approach simplifies the preacquisition parameters to the estimation of the reaction duration and T₁^max and then the selection of the pulse angle, θ, and scan repetition time, τ_R, without the need to set the signal averaging before the experiment.