Abstract
The dynamic nuclear polarization (DNP) efficiency is critically dependent on the properties of the radical, solvent, and solute constituting the sample system. In this study, we focused on the three spin e-e-n cross effect (CE)'s influence on the nuclear longitudinal relaxation time constant T1n, the build-up time constants of nuclear magnetic resonance (NMR) signal, TDNP and DNP-enhancement of NMR signal. The dipolar interaction strength between the electron spins driving the e-e-n process was systematically modulated using mono-, di-, tri-, and dendritic-nitroxide radicals, while maintaining a constant global electron spin concentration of 10 mM. Experimental results showed that an increase in electron spin clustering led to an increased electron spin depolarization, as mapped by electron double resonance (ELDOR), and a dramatically shortened T1n and TDNP time constants under static and magic angle spinning (MAS) conditions. A theoretical analysis reveals that strong e-e interactions, caused by electron spin clustering, increase the CE rate. The three spin e-e-n CE is a hitherto little recognized mechanism for shortening T1n and TDNP in solid-state NMR experiments at cryogenic temperatures, and offers a design principle to enhance the effective CE DNP enhancement per unit time. Fast CE rates will benefit DNP at liquid helium temperatures, or at higher magnetic fields and pulsed DNP, where slow e-e-n polarization transfer rate is a key bottleneck to achieving maximal DNP performance.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 27646-27657 |
| Number of pages | 12 |
| Journal | Physical Chemistry Chemical Physics |
| Volume | 20 |
| Issue number | 43 |
| DOIs | |
| State | Published - 2018 |
Funding
The authors would like to thank Timothy Keller for his assistance acquiring and analyzing the Q-band DEER data and Madhur Srivastava and Jack Freed for their assistance with the denoising analysis of the DEER data under ACERT grant NIH/NIGMS P41GM103521. This research was supported by the National Science Foundation (CHE #1505038, MCB #1617025, and MCB #1244651 to SH), the National Institute of Health (Grants R21GM103477 to SH and R01EB-019950-01A1 to AR), and the Binational Science Foundation (Grant #2014149 to SH). The content of this work is solely the responsibility of the authors and does not necessarily represent the official views of the NSF, NIH, or BSF. The MAS-DNP instrument acquired with the support of an NSF MRI (Grant #1429710) is part of the shared facilities of the UCSB MRSEC (NSF DMR 1720256), a member of the Materials Research Facilities Network (www.mrfn.org).
ASJC Scopus subject areas
- General Physics and Astronomy
- Physical and Theoretical Chemistry