Characterization of Strongly Hyperfine-split Protons by DNP

TL;DR

This study uses reverse DNP and band-selective proton pulses to directly probe protons in the direct DNP region around the Ox063 trityl radical. The authors demonstrate an electron-detected proton spectrum that originates from protons in the immediate vicinity of the electron, and they quantify a spin-diffusion barrier that constrains polarization transfer to bulk protons to a radius of about $5.4$–$6.8\ \text{Å}$. They show that proton spin diffusion to the bulk occurs predominantly within a spectral window centered near $14.85\ \text{MHz}$ and that electron diffusion leaves a measurable imprint on the proton spectrum, consistent with a simple three-spin ($\text{ee}-\text{p}$) diffusion model with a diffusion rate constant around $\sim$ $100\ \mu\text{s}$. Electron decoupling sharpens the proton line, and hole-burning experiments reveal electron-electron diffusion processes between two electron spins and one proton. These findings illuminate which proximate protons participate in DNP transfer and offer guidance for designing radicals with optimized spin-diffusion pathways, with potential implications for ENDOR and broader DNP applications; the approach can be extended to radicals with broader EPR spectra.

Abstract

Dynamic nuclear polarization experiments use microwave irradiation to transfer the larger electron polarization to nuclear spins of interest, and thus enhance the NMR transitions above thermal equilibrium. How the polarization transfer from the electron spin to the nuclear spins in such experiments proceeds and which nuclear spins close to an unpaired electron get polarized and contribute through spin diffusion to the observable bulk nuclear magnetization is not fully understood. We address these questions by combining reverse DNP and band-selective inversion pulses on nuclear spins. We report the electron-detected NMR spectrum of proton spins involved in the direct DNP process in Ox063 trityl samples with protonated and deuterated solvents and variable radical concentrations. We also determine the spin-diffusion barrier surrounding trityl and find that proton spin diffusion is quenched for hyperfine coulings exceding ~250 kHz. This corresponds to a radius of the spin diffusion barrier in the range from 5.4 to 6.8 Å. Burning a hole into the NMR spectrum of proton spins involved in the direct DNP step reveals an electron-electron spin diffusion process imprinted on the proton spectrum. We explain this diffusion process using a three-spin system consisting of two electron spins and one proton and quantify the electron spin diffusion rate constant.

Figures (7)

• Figure 1: Pulse sequences used in the realm of this work. A) is the pulse sequence to measure the electron-detected proton spectra, B) the pulse sequence to monitor the spin diffusion towards the bulk and C) to burn a hole into the proton spectrum. For all experiments a saturation train on the proton spins at the end of each sequence was applied with a carrier frequency equal to the blue nutation pulses and $\phi=100^{\circ}$.
• Figure 2: Electron-detected proton spectrum for 5 mM trityl in gly-d$_8$:D$_2$O:H$_2$O (6:3:1) using the pulse sequence of Fig. \ref{['fig:RevDNPPulseSeq']} A) with $t_{\mathrm{del}}$ = 15 $\mu$s and $t_{\mathrm{SL}}$ = 4000 ns. The variable-amplitude Gauss pulse was placed in the middle of the delay $t_{\mathrm{del}}$. A) Amplitude nutation traces upon application of a band-selective Gauss pulse of 10 $\mu$s length. The relative amplitude $a$ of this Gauss pulse is varied from 0 to 40. The first minimum of the amplitude nutation trace is at a relative amplitude of $25$ corresponding to a Rabi frequency of $\sim$ 100 kHz. The carrier frequency of the amplitude nutation trace was varied from 14.14 to 15.10 MHz throughout different experiments. The minima of the amplitude nutation traces were extracted and plotted against the frequency to obtain the electron-detected proton spectrum. Comparison with the thermal equilibrium signal of the bulk protons using regular NMR detection is shown in red. B) Amplitude nutation traces upon application of the same band-selective Gauss pulse as in A) with simultaneous electron decoupling during that pulse.
• Figure 3: Electron-detected proton spectrum for 5 mM trityl Ox063 in gly-d$_8$:D$_2$O:H$_2$O (6:3:1) black, gly-d$_8$:D$_2$O (6:4) red, gly-d$_8$:H$_2$O (6:4) green and for 100 $\mu$M trityl Ox063 in gly-d$_8$:D$_2$O:H$_2$O (6:3:1) light-blue using the pulse sequence of Fig. \ref{['fig:RevDNPPulseSeq']} A). The variable-amplitude Gauss pulse was placed in the middle of the delay $t_{\mathrm{del}}$ = 15 $\mu$s and $t_{\mathrm{SL}}$ = 4000 ns. Panels A) and B) show the electron-detected proton spectra without and with electron decoupling, respectively. Within experimental error between the four different measurement sessions the electron-detected proton spectra are identical. Please note that the data in black were already shown in Fig. \ref{['res:fig:CompNutSpectrum_N']}.
• Figure 4: Measurement of spin diffusion from protons nearby an electron spin to bulk protons using the pulse sequence of Fig. \ref{['fig:RevDNPPulseSeq']} B). A) and B) Recorded amplitude nutation traces for $t_{\mathrm{del}}$ ranging from 15 $\mu$s to 3000 $\mu$s by varying the amplitude $a_2$ of the Gauss pulse. The sample was 5 mM trityl in gly-d$_8$:H$_2$O (6:4) and $t_{\mathrm{SL}}$ = 4000 ns. In A) the carrier frequency of the Gauss pulse was $\nu_{\mathrm{rf,2}}=14.77$ MHz and in B) 14.89 MHz, with the latter corresponding to the center of the proton spectrum. The data points to plot the proton spectra were extracted along the green dashed line. C) Proton spectra for different delays $t_{\mathrm{del}}$. The spin diffusion towards the bulk is more pronounced in the center of the spectrum. The reddish dashed lines indicate the spectral positions where the data points for the traces in D) were extracted. D) Diffusion traces extracted from C) for different frequencies $\nu_{\mathrm{rf,2}}$. The diffusion to the bulk is fastest for $\nu_{\mathrm{rf,2}}=14.89$ MHz corresponding to the center of the spectrum and gets slower moving away from the center. The black solid line indicates a measurement in the absence of any Gauss pulse i.e. $a_2 = 0$.
• Figure 5: Hole burning spectra for delays $\Delta t$ between the two Gauss pulses varying from 5 to 750 $\mu$s, $t_\mathrm{del}=$ 800 $\mu$s and $t_\mathrm{SL}=$ 4000 ns. The hole was burned at $\nu_{\mathrm{rf,1}}=14.69$ MHz. The black dashed lines are obtained by using the data points with a$_1$ = 0 i.e. in the absence of any hole burning pulse. The sample in subplot A) is 5 mM trityl in gly-d$_8$:D$_2$O:H$_2$O (6:3:1), in subplot B) 5 mM trityl in gly-d$_8$:D$_2$O (6:4), in subplot C) 5 mM trityl in gly-d$_8$:H$_2$O (6:4) and in subplot D) 100 $\mu$M trityl in gly-d$_8$:D$_2$O:H$_2$O (6:3:1).