Overhauser dynamic nuclear polarization (ODNP)-enhanced two-dimensional proton NMR spectroscopy at low magnetic fields

Abstract

The majority of low-field Overhauser dynamic nuclear polarization (ODNP) experiments reported so far have been 1D NMR experiments to study molecular dynamics and in particular hydration dynamics. In this work, we demonstrate the application of ODNP-enhanced 2D J-resolved (JRES) spectroscopy to improve spectral resolution beyond the limit imposed by the line broadening introduced by the paramagnetic polarizing agent. Using this approach, we are able to separate the overlapping multiplets of ethyl crotonate into a second dimension and clearly identify each chemical site individually. Crucial to these experiments is interleaved spectral referencing, a method introduced to compensate for temperature-induced field drifts over the course of the NMR acquisition. This method does not require additional hardware such as a field-frequency lock, which is especially challenging when designing compact systems.

Figure 1

Molecular structure of ethyl crotonate and proton labels used throughout this work.

Figure 2

(a) One-dimensional ODNP-enhanced ethyl crotonate reference spectra with 10 mM TEMPONE acquired after each completed phase cycle step of JRES experiment. Spectra are vertically offset for clarity. (b) Chemical shift of the peak with the maximum intensity and fit (fourth-order polynomial) for experimental drift correction. (c) Residual of the fit.

Figure 3

One-dimensional ODNP-enhanced NMR spectra (128 averages) of ethyl crotonate with 10 mM TEMPONE using different apodization functions. Top: 1 Hz Lorentzian line broadening. Middle: Lorentz–Gauss transformation using 4 Hz line broadening for both Gaussian and Lorentzian linewidths. Bottom: resolution enhancement after Traficante and Nemeth (1987) and Traficante and Rajabzadeh (2000) using $T_{\mathrm{2}}^{\ast }$ of 0.2 s.

Figure 4

(a) ODNP-enhanced ${}^{\mathrm{1}}$ H JRES spectrum of neat ethyl crotonate with 10 mM TEMPONE. The dashed boxes indicate areas showing strong coupling effects. (b) Slices of JRES spectrum shown in panel (a) for each proton in ethyl crotonate. The expected multiplet pattern is indicated by vertical lines with corresponding tick marks. Spectra are offset for clarity. Features marked with an asterisk are artifacts of the JRES experiment.

Figure 5

NMR spectra of ethyl crotonate. Top: skyline projection of 2D JRES experiment. The locations of strong-coupling artifacts are indicated by asterisks. Bottom: 1D ${}^{\mathrm{1}}$ H NMR spectrum.