Solid Effect Dynamic Nuclear Polarization Enhancement of >500 at 9.4 T

Abstract

Efficient polarizing agents for dynamic nuclear polarization (DNP) enhanced magic angle spinning (MAS) NMR spectroscopy are of high current interest due to the potential to significantly boost NMR sensitivity. While most efforts have centered on cross-effect (CE) or Overhauser effect (OE) mechanisms, yielding enhancement factors up to ∼300 at 9.4 T, radicals yielding solid effect (SE) DNP have seen less development. Here we model the comparative performance of OE and SE mechanisms and then measure ¹H enhancement factors up to 500 from 1,3-bisdiphenylene-2-phenylallyl (BDPA) in an ortho-terphenyl (OTP) matrix at 9.4 T, 100 K, achieved via increased microwave power across the sample volume. The measured SE and OE performances are in good agreement with the predictions. We note that both the experimental and theoretical analyses indicate that SE DNP remains saturation limited, particularly at elevated temperatures, and we envisage that further improvements in microwave power will further increase SE DNP enhancement factors.