51 research outputs found
Generating and sustaining long-lived spin states in 15N,15N′-azobenzene
Long-Lived spin States (LLSs) hold a great promise for sustaining non-thermal spin order and investigating various slow processes by Nuclear Magnetic Resonance (NMR) spectroscopy. Of special interest for such application are molecules containing nearly equivalent magnetic nuclei, which possess LLSs even at high magnetic fields. In this work, we report an LLS in trans-15N,15N′-azobenzene. The singlet state of the 15N spin pair exhibits a long-lived character. We solve the challenging problem of generating and detecting this LLS and further increase the LLS population by converting the much higher magnetization of protons into the 15N singlet spin order. As far as the longevity of this spin order is concerned, various schemes have been tested for sustaining the LLS. Lifetimes of 17 minutes have been achieved at 16.4 T, a value about 250 times longer than the longitudinal relaxation time of 15N in this magnetic field. We believe that such extended relaxation times, along with the photochromic properties of azobenzene, which changes conformation upon light irradiation and can be hyperpolarized by using parahydrogen, are promising for designing new experiments with photo-switchable long-lived hyperpolarization
Transfer of Parahydrogen Induced Polarization in Scalar Coupled Systems at Variable Magnetic Field
Para-Hydrogen Induced Polarization (PHIP) experiments were performed in
coupled multispin systems at variable magnetic fields. We studied the magnetic
field dependence of PHIP in styrene, which is the product of hydrogenation of
phenylacetylene. At low magnetic fields where the spins are coupled strongly
by scalar interaction efficient polarization transfer among the interacting
protons takes place. The experimentally observed spectra are in good agreement
with the simulation, which takes into account eight coupled spins. We also
demonstrate effects of nuclear spin level anti-crossings on the PHIP pattern.
It is shown that rapid passage through the level anti-crossing enables highly
efficient polarization transfer between specific spin orders. In addition, we
studied PHIP transfer to 13C and 19F hetero-nuclei. It is shown that hetero-
nuclei can be efficiently polarized in a wide field range; in particular, for
polarizing them it is not necessary to go to ultra-low fields, which provide
their strong coupling to protons. The resulting polarization is of the
multiplet type and gives strong enhancements of the individual NMR lines. In
general, variation of the magnetic field gives the opportunity for
manipulating PHIP patterns and transferring polarization to target spins of
choice
Exploiting adiabatically switched RF-field for manipulating spin hyperpolarization induced by parahydrogen
A method for precise manipulation of non-thermal nuclear spin polarization by
switching a RF-field is presented. The method harnesses adiabatic correlation
of spin states in the rotating frame. A detailed theory behind the technique
is outlined; examples of two-spin and three-spin systems prepared in a non-
equilibrium state by Para-Hydrogen Induced Polarization (PHIP) are considered.
We demonstrate that the method is suitable for converting the initial
multiplet polarization of spins into net polarization: compensation of
positive and negative lines in nuclear magnetic resonance spectra, which is
detrimental when the spectral resolution is low, is avoided. Such a conversion
is performed for real two-spin and three-spin systems polarized by means of
PHIP. Potential applications of the presented technique are discussed for
manipulating PHIP and its recent modification termed signal amplification by
reversible exchange as well as for preparing and observing long-lived spin
states
Manipulating spin hyper-polarization by means of adiabatic switching of a spin-locking RF-field
We propose a technique for transferring the multiplet spin polarization (CIDNP
or PHIP, or one created by any other method), which is the mutual entanglement
of spins, into net hyper-polarization with respect to the direction of a high
magnetic field by slowly (adiabatically) switching-off a strong external RF-
field with a specially selected frequency. The net hyper-polarized molecules
can then be used in NMR spectroscopy or imaging for strong signal enhancement
cis Versus trans-Azobenzene: Precise Determination of NMR Parameters and Analysis of Long-Lived States of 15N Spin Pairs
We provide a detailed evaluation of nuclear magnetic resonance (NMR)
parameters of the cis- and trans-isomers of azobenzene (AB). For determining
the NMR parameters, such as proton–proton and proton–nitrogen J-couplings and
chemical shifts, we compared NMR spectra of three different isotopomers of AB:
the doubly 15N labeled azobenzene, 15N,15N′-AB, and two partially deuterated
AB isotopomers with a single 15N atom. For the total lineshape analysis of NMR
spectra, we used the recently developed ANATOLIA software package. The
determined NMR parameters allowed us to optimize experiments for investigating
singlet long-lived spin states (LLSs) of 15N spin pairs and to measure LLS
lifetimes in cis-AB and trans-AB. Magnetization-to-singlet-to-magnetization
conversion has been performed using the SLIC and APSOC techniques, providing a
degree of conversion up to 17 and 24% of the initial magnetization,
respectively. Our approach is useful for optimizing the performance of
experiments with singlet LLSs; such LLSs can be exploited for preserving spin
hyperpolarization, for probing slow molecular dynamics, slow chemical
processes and also slow transport processes
Recent advances in the application of parahydrogen in catalysis and biochemistry
Nuclear Magnetic Resonance (NMR) spectroscopy and Magnetic Resonance Imaging (MRI) are analytical and diagnostic tools that are essential for a very broad field of applications, ranging from chemical analytics, to non-destructive testing of materials and the investigation of molecular dynamics, to in vivo medical diagnostics and drug research. One of the major challenges in their application to many problems is the inherent low sensitivity of magnetic resonance, which results from the small energy-differences of the nuclear spin-states. At thermal equilibrium at room temperature the normalized population difference of the spin-states, called the Boltzmann polarization, is only on the order of 10⁻⁵. Parahydrogen induced polarization (PHIP) is an efficient and cost-effective hyperpolarization method, which has widespread applications in Chemistry, Physics, Biochemistry, Biophysics, and Medical Imaging. PHIP creates its signal-enhancements by means of a reversible (SABRE) or irreversible (classic PHIP) chemical reaction between the parahydrogen, a catalyst, and a substrate. Here, we first give a short overview about parahydrogen-based hyperpolarization techniques and then review the current literature on method developments and applications of various flavors of the PHIP experiment
Spin dynamics in experiments on orthodeuterium induced polarization (ODIP)
A comprehensive description of the spin dynamics underlying the formation of Ortho-Deuterium Induced Polarization (ODIP) is presented. ODIP can serve as a tool for enhancing Nuclear Magnetic Resonance (NMR) signals of ²H nuclei, being important probes of molecular structure and dynamics. To produce ODIP, in the first step, the D₂ gas is brought to thermal equilibrium at low temperature, here 30 K, so that the ortho-component, corresponding to the total spin of the ²H nuclei equal to 0 and 2, is enriched, here to 92%. In the second step, the orthodeuterium molecule is attached to a substrate molecule using a suitable hydrogenation catalyst such that the symmetry of the two ²H nuclei is broken. As a result, the non-thermal spin order of orthodeuterium is converted into enhancement of observable NMR signals. In this work, we perform a theoretical study of ODIP and calculate the shape of ODIP spectra and their dependence on the magnetization flip angle. These results are compared with experiments performed for a number of substrates; good agreement between experimental and calculated ODIP spectra is found. We also discuss the performance of NMR techniques for converting anti-phase ODIP spectral patterns into in-phase patterns, which are more suitable for signal detection and for transferring ODIP to heteronuclei, here to ¹³C spins. Experimental procedures reported here allowed us to reach signal enhancement factors of more than 1000 for ²H nuclei in the liquid phase. These results are useful for extending the scope of spin hyperpolarization to the widely used ²H nuclei
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