NMR Intermolecular Dipolar Cross-Relaxation in Nanoconfined Fluids

Abstract

Spin relaxation, a defining mechanism of nuclear magnetic resonance (NMR), has been a prime method for determining three-dimensional molecular structures and their dynamics in solution. It also plays key roles for contrast enhancement in magnetic resonance imaging (MRI). In bulk solutions, rapid Brownian molecular diffusion modulates dipolar interactions between a spin pair from different molecules, resulting in very weak intermolecular relaxations. We show that in fluids confined in nanospace or nanopores (nanoconfined fluids) the correlation of dipolar coupling between spin pairs of different molecules is greatly enhanced by the nanopore constraint boundaries on the molecular diffusion, giving rise to an enhanced correlation for the spin pair. As a result, the intermolecular dipolar interaction behaves cooperatively, which leads to a large intermolecular dipolar relaxation rate and opposite in sign to the bulk solution. We found that the classical NMR relaxation theory fails to capture these observations in a nanoconfined fluid environment. Hence, we developed a formal theory and experimentally confirmed that enhanced correlation and cooperated relaxation are ubiquitous in nanoconfined fluids. The newly discovered phenomenon and the developed NMR method reveal new applications in a broad range of synthesized and naturally occurring materials in the field of nanofluidics to study molecular dynamics and structure as well as for MRI image enhancement.

Nuclear Magnetic Resonance Dipolar Cross-Relaxation Interaction between Nanoconfined Fluids and Matrix Solids

“…The drastically different behaviors between materials at the nano scale and ones at the bulk scale are well documented and can be related to the boundary conditions at nanometer or bulk scales . We have observed that intermolecular cross-relaxation in nanofluidics were enhanced by two orders of magnitude as a result of nanoconfinement on the molecular translational diffusion . Here, we further show how nanoconfinement of the fluids enhances the dipolar cross-relaxation between the fluid and the matrix solids.…”

“…In the 2D NOESY spectrum of Figure b, the cross peaks between CH 3 OD and MCM41-C1 are positive, which is the opposite result that would be generally observed in a bulk solution where the cross peaks are negative from positive intermolecular cross-relaxation rates . In addition, the cross peaks in Figure b are strong in comparison to the diagonal peaks, in contrast to the generally weak cross peaks of intermolecular NOEs in bulk fluids with fast molecular tumbling. , …”

“…48 In addition, the cross peaks in Figure 1b are strong in comparison to the diagonal peaks, in contrast to the generally weak cross peaks of intermolecular NOEs in bulk fluids with fast molecular tumbling. 32,48 We used a selective NOE experiment 51,52 to measure the intermolecular cross-relaxation rates between the methanol proton and solid methyl proton for higher accuracy than the 2D method. The pulse sequence with a detailed description of the NMR signals acquired in the selective NOE experiment is shown in Figure S4 in the SI.…”

NMR Dipolar Cross-Relaxation Interaction Between Nanoconfined Fluids and Matrix Solids

“…In the 2D NOESY spectra of Figure 1b, the cross peaks between CH3OD and MCM41-C1 are positive, which is the opposite results that would be generally observed in a bulk solution where the cross peaks are negative from positive intermolecular cross-relaxation rates 43 . In addition, the cross peaks in Figure 1b are strong in comparison to the diagonal peaks, in contrast to the generally weak cross peaks of intermolecular NOEs in bulk fluids with fast molecular tumbling 30,43 .…”

“…We have observed that intermolecular cross-relaxation in nanofluidics were enhanced by two order of magnitudes as a result of nanoconfinement on the molecular translational diffusion 30 . We establish a formal theory and experimentally show how nanoconfinement of the fluids enhances the dipolar cross-relaxation between the fluid and the matrix solids.…”