67 research outputs found
From Free Radicals and Spin-Chemistry Over Spin-Dynamics and Hyperpolarization to Biology and Materials Science
This special issue of ZPC is dedicated to the 80th anniversary of Kev Salikhov, a prominent scientist who has made seminal contributions to spin chemistry, chemical kinetics, spin hyperpolarization and magnetic resonance. Altogether there are 41 scientific papers; for this reason, the guest editors have decided to publish three subsequent issues. Each issue is focused on a particular field, which is closely related to research interests of Salikhov or was even seeded by his work. As the following papers clearly demonstrate, the initial seed of Salikhov fell on a very fruitful soil and has grown to a huge and impressive scientific tree with branches ranging from fundamental science to applications in biochemistry and materials science
Non-thermal nuclear magnetic resonance quantum computing using hyperpolarized Xenon
Current experiments in liquid-state nuclear magnetic resonance quantum
computing are limited by low initial polarization. To address this problem, we
have investigated the use of optical pumping techniques to enhance the
polarization of a 2-qubit NMR quantum computer (13C and 1H in 13CHCl3). To
efficiently use the increased polarization, we have generalized the procedure
for effective pure state preparation. With this new, more flexible scheme, an
effective pure state was prepared with polarization-enhancement of a factor of
10 compared to the thermal state. An implementation of Grover's quantum search
algorithm was demonstrated using this new technique.Comment: 4 pages, 3 figures. Submitted for publicatio
analytical solutions for spectral patterns and their field dependence
We have obtained analytical solutions for Para-Hydrogen Induced Polarization
(PHIP) for several types of coupled spin systems, namely, for AB-, ABX-,
AA´A´´- and A2B-systems. Scalar spin-spin interactions were considered the
factor, that determines the PHIP spectral pattern; it is the variation of the
spin coupling regime (from strong coupling at low field to weak coupling at
high field), which is responsible for the PHIP magnetic field dependence. The
field dependence of polarization was considered in detail, general
peculiarities of PHIP were found, PHIP patterns were calculated for the
systems mentioned. Special attention was paid to the effects of field
switching on PHIP
Coherent manipulation of non-thermal spin order in optical nuclear polarization experiments
Time resolved measurements of Optical Nuclear Polarization (ONP) have been
performed on hyperpolarized triplet states in molecular crystals created by
light excitation. Transfer of the initial electron polarization to nuclear
spins has been studied in the presence of radiofrequency excitation; the
experiments have been performed with different pulse sequences using different
doped molecular systems. The experimental results clearly demonstrate the
dominant role of coherent mechanisms of spin order transfer, which manifest
themselves in well pronounced oscillations. These oscillations are of two
types, precessions and nutations, having characteristic frequencies, which are
the same for the different molecular systems and the pulse sequences applied.
Hence, precessions and nutations constitute a general feature of polarization
transfer in ONP experiments. In general, coherent manipulation of spin order
transfer creates a powerful resource for improving the performance of the ONP
method, which paves the way to strong signal enhancement in nuclear magnetic
resonance
Coherent transfer of nuclear spin polarization in field-cycling NMR experiments
Coherent polarization transfer effects in a coupled spin network have been
studied over a wide field range. The transfer mechanism is based on exciting
zero-quantum coherences between the nuclear spin states by means of non-
adiabatic field jump from high to low magnetic field. Subsequent evolution of
these coherences enables conversion of spin order in the system, which is
monitored after field jump back to high field. Such processes are most
efficient when the spin system passes through an avoided level crossing during
the field variation. The polarization transfer effects have been demonstrated
for N-acetyl histidine, which has five scalar coupled protons; the initial
spin order has been prepared by applying RF-pulses at high magnetic field. The
observed oscillatory transfer kinetics is taken as a clear indication of a
coherent mechanism; level crossing effects have also been demonstrated. The
experimental data are in very good agreement with the theoretical model of
coherent polarization transfer. The method suggested is also valid for other
types of initial polarization in the spin system, most notably, for spin
hyperpolarization
Spin mixing at level anti-crossings in the rotating frame makes high-field SABRE feasible
A new technique is proposed to carry out Signal Amplification By Reversible
Exchange (SABRE) experiments at high magnetic fields. SABRE is a method, which
utilizes spin order transfer from para-hydrogen to the spins of a substrate in
transient complexes using suitable catalysts. Such a transfer of spin order is
efficient at low magnetic fields, notably, in the Level Anti-Crossing (LAC)
regions. Here it is demonstrated that LAC conditions can also be fulfilled at
high fields in the rotating reference frame under the action of an RF-field.
Spin mixing at LACs allows one to polarize substrates at high fields as well;
the achievable NMR enhancements are around 360 for the ortho-protons of
partially deuterated pyridine used as a substrate and around 700 for H2 and
substrate in the active complex with the catalyst. High-field SABRE effects
have also been found for several other molecules containing a nitrogen atom in
the aromatic ring
High resolution NMR study of T1 magnetic relaxation dispersion. IV. Proton relaxation in amino acids and Met-enkephalin pentapeptide
Nuclear Magnetic Relaxation Dispersion (NMRD) of protons was studied in the
pentapeptide Met-enkephalin and the amino acids, which constitute it.
Experiments were run by using high-resolution Nuclear Magnetic Resonance (NMR)
in combination with fast field-cycling, thus enabling measuring NMRD curves
for all individual protons. As in earlier works, Papers I–III, pronounced
effects of intramolecular scalar spin-spin interactions, J-couplings, on spin
relaxation were found. Notably, at low fields J-couplings tend to equalize the
apparent relaxation rates within networks of coupled protons. In Met-
enkephalin, in contrast to the free amino acids, there is a sharp increase in
the proton T1-relaxation times at high fields due to the changes in the regime
of molecular motion. The experimental data are in good agreement with theory.
From modelling the relaxation experiments we were able to determine motional
correlation times of different residues in Met-enkephalin with atomic
resolution. This allows us to draw conclusions about preferential conformation
of the pentapeptide in solution, which is also in agreement with data from
two-dimensional NMR experiments (rotating frame Overhauser effect
spectroscopy). Altogether, our study demonstrates that high-resolution NMR
studies of magnetic field-dependent relaxation allow one to probe molecular
mobility in biomolecules with atomic resolution
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
Towards a common description of liquid-state and solid-state cases
Chemically Induced Dynamic Nuclear Polarization (CIDNP) is an efficient method
of creating non-equilibrium polarization of nuclear spins by using chemical
reactions, which have radical pairs as intermediates. The CIDNP effect
originates from (i) electron spin-selective recombination of radical pairs and
(ii) the dependence of the inter-system crossing rate in radical pairs on the
state of magnetic nuclei. The CIDNP effect can be investigated by using
Nuclear Magnetic Resonance(NMR) methods. The gain from CIDNP is then two-fold:
it allows one to obtain considerable amplification of NMR signals; in
addition, it provides a very useful tool for investigating elusive radicals
and radical pairs. While the mechanisms of the CIDNP effect in liquids are
well established and understood, detailed analysis of solid-state CIDNP
mechanisms still remains challenging; likewise a common theoretical frame for
the description of CIDNP in both solids and liquids is missing. Difficulties
in understanding the spin dynamics that lead to the CIDNP effect in the solid-
state case are caused by the anisotropy of spin interactions, which increase
the complexity of spin evolution. In this work, we propose to analyze CIDNP in
terms of level crossing phenomena, namely, to attribute features in the CIDNP
magnetic field dependence to Level Crossings (LCs) and Level Anti-Crossings
(LACs) in a radical pair. This approach allows one to describe liquid-state
CIDNP; the same holds for the solid-state case where anisotropic interactions
play a significant role in CIDNP formation. In solids, features arise
predominantly from LACs, since in most cases anisotropic couplings result in
perturbations, which turn LCs into LACs. We have interpreted the CIDNP
mechanisms in terms of the LC/LAC concept. This consideration allows one to
find analytical expressions for a wide magnetic field range, where several
different mechanisms are operative; furthermore, the LAC description gives a
way to determine CIDNP sign rules. Thus, LCs/LACs provide a consistent
description of CIDNP in both liquids and solids with the prospect of
exploiting it for the analysis of short-lived radicals and for optimizing the
polarization level
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