Orientation-Dependent Proton Relaxation of Water Molecules Trapped in Solids: Crystallites with Long-Lived Magnetization

Abstract

The longitudinal spin-lattice relaxation properties of water molecules trapped in a static powdered polycrystalline sample of barium chlorate monohydrate are investigated by means of solid-state 1 H NMR spectroscopy. Different portions of the inhomogeneous Pake pattern that are associated with crystallites at different orientations with respect to the external magnetic field show either a mono-or a bi-exponential recovery. At high field (9.4 T), the chemical shift anisotropy is the main interaction that is responsible for the inhomogeneity of the relaxation rates. A theoretical description of rapid two-site hopping about the H-O-H bisector in the framework of Liouville space agrees very well with the experimental evidence. Numerical simulations predict a distribution of mono-exponential time constants associated with individual single-crystal orientations. Overlapping signals give rise to bi-exponential recovery. This is confirmed experimentally by 1 H NMR spectra of static single crystals.

Spin Thermometry: A Straightforward Measure of Millikelvin Deuterium Spin Temperatures Achieved by Dynamic Nuclear Polarization

“…On the other hand, well-resolved dipolar Pake patterns can be observed in 1 H NMR of water molecules that are isolated in suitable matrices such as crystals of barium chlorate monohydrate. [25][26][27][28] As we shall discuss in a separate paper, the spin temperature of the protons in such isolated water molecules can be lowered by DNP after suspending the crystallites in a solution of radicals prior to freezing.…”

“…However, it appears difficult to determine the precise spin temperature from the analysis of the asymmetry of such broad lines. ,− Marohn et al detected asymmetric quadrupolar satellites in optically detected 71 Ga NMR and could assess the polarization level of its nuclear spin S = 3/2 from the asymmetry of the satellites. On the other hand, well-resolved dipolar Pake patterns can be observed in 1 H NMR spectra of water molecules that are isolated in suitable matrices such as crystals of barium chlorate monohydrate. − As we shall discuss in a separate paper, the spin temperature of the protons in such isolated water molecules can be lowered by DNP after suspending the crystallites in a solution of radicals prior to freezing.…”

“…Although such small-angle quadrupolar echoes do not achieve complete refocusing of either linear or quadratic inhomogeneities, which is best achieved by using a refocusing pulse , with β = 90°, they nonetheless result in reasonably undistorted line shapes that are much easier to interpret than those produced by a pulse-acquire approach . Furthermore and more importantly, echoes allow one to remove possible background signals, which are particularly detrimental when observing 1 H, and remove distortions due to acoustic ringing of the coil. ,,, The use of spin echoes has proven successful for the acquisition of undistorted inhomogeneous line shapes in static solid samples, even when only very weak rf field strengths are available . The black spectrum in Figure was acquired without microwave irradiation at T spin = T sample = 1.3 K (in liquid helium at 158 Pa) and results in a symmetric quadrupolar pattern.…”

Frozen water NMR lineshape analysis enables absolute polarization quantification

“…To improve the performance of this approach, the two Pake horns would have to be observed with much narrower linewidths. Isolated water molecules, such as those of H 2 O@C 60 25 or water-impregnated barium chlorate monohydrate 46,47 provide a choice of possible candidate systems.…”

“…(ii) The orientation dependence of the longitudinal relaxation time constant T 1 , which has been explored for cases such as solid hydrates. 46,47 Since the level of 1 H spin polarization depends on T 1 , the orientational dependence of T 1 may lead to an orientation-dependent polarization level, which could potentially distort the lineshape of the observed 1 H NMR spectra. However, it can be safely stated that these effects, even if present, are not sufficiently large as to be observed in the presence of the dominating dead time-induced spectral distortion at high levels of 1 H spin polarization;…”