title: Slow Molecular Motions in Ionic Liquids Probed by Cross-Relaxation of Nuclear Spins During Overhauser Dynamic Nuclear Polarization.
abstract: Solution-state Overhauser dynamic nuclear polarization (ODNP) at moderate fields, performed by saturating the electron spin resonance (ESR) of a free radical added to the sample of interest, is well known to lead to significant NMR signal enhancements in the steady state, owing to electron-nuclear cross-relaxation. Here it is shown that under conditions which limit radical access to the molecules of interest, the time course of establishment of ODNP can provide a unique window into internuclear cross-relaxation, and reflects relatively slow molecular motions. This behavior, modeled mathematically by a three-spin version of the Solomon equations (one unpaired electron and two nuclear spins), is demonstrated experimentally on the19F/1H system in ionic liquids. Bulky radicals in these viscous environments turn out to be just the right setting to exploit these effects. Compared to standard nuclear Overhauser effect (NOE) work, the present experiment offers significant improvement in dynamic range and sensitivity, retains usable chemical shift information, and reports on molecular motions in the sub-megahertz (MHz) to tens of MHz range-motions which are not accessed at high fields.
authors: MRI-MRS Centre and Department of Chemistry, Indian Institute of Technology Madras, Chennai, 600036, Tamil Nadu, India.
authors: Abhishek
authors: A
authors: Banerjee
authors: MRI-MRS Centre and Department of Chemistry, Indian Institute of Technology Madras, Chennai, 600036, Tamil Nadu, India.
authors: Arnab
authors: A
authors: Dey
authors: MRI-MRS Centre and Department of Chemistry, Indian Institute of Technology Madras, Chennai, 600036, Tamil Nadu, India.
authors: Narayanan
authors: N
authors: Chandrakumar
keywords: NMR spectroscopy; Overhauser DNP; cross relaxation; ionic liquids; slow molecular dynamics