Traditional nuclear magnetic resonance (NMR) spectroscopy relies on the versatile chemical information conveyed by spectra. To complement conventional NMR, Laplace NMR explores diffusion and relaxation phenomena to reveal details on molecular motions. Under a broad concept of ultrafast multidimensional Laplace NMR, here we introduce an ultrafast diffusion-relaxation correlation experiment enhancing the resolution and information content of corresponding 1D experiments as well as reducing the experiment time by one to two orders of magnitude or more as compared with its conventional 2D counterpart. We demonstrate that the method allows one to distinguish identical molecules in different physical environments and provides chemical resolution missing in NMR spectra. Although the sensitivity of the new method is reduced due to spatial encoding, the single-scan approach enables one to use hyperpolarized substances to boost the sensitivity by several orders of magnitude, significantly enhancing the overall sensitivity of multidimensional Laplace NMR.
Nuclear spin polarization can be significantly increased through the
process of hyperpolarization, leading to an increase in the sensitivity of
nuclear magnetic resonance (NMR) experiments by 4–8 orders of magnitude.
Hyperpolarized gases, unlike liquids and solids, can be more readily separated
and purified from the compounds used to mediate the hyperpolarization processes.
These pure hyperpolarized gases enabled many novel MRI applications including
the visualization of void spaces, imaging of lung function, and remote
detection. Additionally, hyperpolarized gases can be dissolved in liquids and
can be used as sensitive molecular probes and reporters. This mini-review covers
the fundamentals of the preparation of hyperpolarized gases and focuses on
selected applications of interest to biomedicine and materials science.
Distributions of nuclear magnetic resonance (NMR) relaxation times provide detailed information about the moisture absorbed in wood. In this work, T2*, T2, and T1 distributions were recorded from fresh sapwood and heartwood samples of pine (Pinus sylvestris) and spruce (Picea abies) at various temperatures. Below the melting point of bulk water, free water is frozen and its signal disappears from the distributions. Then, the low-temperature distributions of the unfrozen bound water contain more information about its components, because the large free water peaks hiding some smaller bound water peaks are absent and the exchange between free and bound water is prevented. Comparison of the total moisture signal integrals above and below the bulk melting point enables the determination of fiber saturation point (FSP), which, in this context, denotes the total water capacity of cell wall. T2*, T2, and T1 distributions offer different kinds of information about moisture components. All the peaks in the distributions were assigned, and it was demonstrated that the accessible hydroxyl site content and the amount of micropores can be estimated based on the peak integrals.
Thermal modification is an environmentally friendly method to increase the lifetime and improve the properties of timber. In this work, we investigate absorption of moisture in thermally modified pine wood (Pinus sylvestris) immersed in water using various nuclear magnetic resonance (NMR) methods. Magnetic resonance images (MRI) visualize the spatial distribution of absorbed free water. Spin−echo spectra measured both below and above 0 °C reveal that thermal modification partially blocks the access of water to cell walls; even modification at 180 °C slightly reduces the amount of bound water, and the amount decreases about 80% in the case of the sample modified at 240 °C. The spectra and MRI show that, above the modification temperature of 200 °C, the amount of free water decreases, indicating that high modification temperature tends to close the pits connecting the wood cells. T 2 relaxation time distributions measured using the Carr−PurcellMeiboom−Gill sequence show four components, two associated with bound water and two with free water. NMR cryoporometry measurements indicate that the bound water sites are mostly below 2.5 nm in size. A unique combined NMR cryoporometry and relaxometry analysis showed that the size of cell wall micropores is between 1.5 and 4.5 nm, and thermal modification significantly hinders the access of water to the pores.
We report (129)Xe NMR experiments showing that a Fe4L6 metallosupramolecular cage can encapsulate xenon in water with a binding constant of 16 M(-1). The observations pave the way for exploiting metallosupramolecular cages as economical means to extract rare gases as well as (129)Xe NMR-based bio-, pH, and temperature sensors. Xe in the Fe4L6 cage has an unusual chemical shift downfield from free Xe in water. The exchange rate between the encapsulated and free Xe was determined to be about 10 Hz, potentially allowing signal amplification via chemical exchange saturation transfer. Computational treatment showed that dynamical effects of Xe motion as well as relativistic effects have significant contributions to the chemical shift of Xe in the cage and enabled the replication of the observed linear temperature dependence of the shift.
To date, only metal-containing hydrogenation catalysts have been utilized for producing substantial NMR signal enhancements by means of parahydrogen-induced polarization (PHIP). Herein, we show that metal-free compounds known as molecular tweezers are useful in this respect. It is shown that ansa-aminoborane tweezers QCAT provided (20-30)-fold signal enhancements of parahydrogen-originating hydrogens in (1)H NMR spectra. Nuclear polarization transfer from the polarized hydrogens to (11)B nuclei leads to a 10-fold enhancement in the (11)B NMR spectrum. Moreover, our results indicate that dihydrogen activation by QCAT and CAT tweezers is carried out in a pairwise manner, and PHIP can be used for understanding the activation mechanism in metal-free catalytic systems in general.
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