Mapping C hyperfine couplings and exchange interactions in short-lived charge separated states of rigid donor-bridge-acceptor dyads.

A detailed experimental study on reversible photo-induced intramolecular charge separation is presented based on nuclear magnetic resonance detection of chemically induced dynamic nuclear polarization. From variation of such polarization with the external magnetic field, the coupling constants of isotropic and anisotropic hyperfine interactions at individual C sites are measured in the short-lived charge separated state of dyad molecules composed of donor-bridge-acceptor parts. The objects of study were rigid donor-bridge-acceptor dyads, consisting of triarylamine as a donor, naphthalene diimide as an acceptor, and a meta-conjugated diethynylbenzene fragment as a bridge.

Hyperpolarization of cis- N -Azobenzene by Parahydrogen at Ultralow Magnetic Fields*.

The development of nuclear spins hyperpolarization, and the search for molecules that can be efficiently hyperpolarized is an active area in nuclear magnetic resonance. In this work we present a detailed study of SABRE SHEATH (signal amplification by reversible exchange in shield enabled alignment transfer to heteronuclei) experiments on N -azobenzene. In SABRE SHEATH experiments the nuclear spins of the target are hyperpolarized through transfer of spin polarization from parahydrogen at ultralow fields during a reversible chemical process.

Exchange interaction in short-lived flavine adenine dinucleotide biradical in aqueous solution revisited by CIDNP (chemically induced dynamic nuclear polarization) and nuclear magnetic relaxation dispersion.

Flavin adenine dinucleotide (FAD) is an important cofactor in many light-sensitive enzymes. The role of the adenine moiety of FAD in light-induced electron transfer was obscured, because it involves an adenine radical, which is short-lived with a weak chromophore. However, an intramolecular electron transfer from adenine to flavin was revealed several years ago by Robert Kaptein by using chemically induced dynamic nuclear polarization (CIDNP).

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 H nuclei, being important probes of molecular structure and dynamics. To produce ODIP, in the first step, the D gas is brought to thermal equilibrium at low temperature, here 30 K, so that the ortho-component, corresponding to the total spin of the H nuclei equal to 0 and 2, is enriched, here to 92%.

Author Correction: Generating and sustaining long-lived spin states in N,N'-azobenzene.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Positive electronic exchange interaction and predominance of minor triplet channel in CIDNP formation in short lived charge separated states of D-X-A dyads.

Previous transient absorption measurements using the magnetically affected reaction yield (MARY) technique for a series of rigidly linked electron donor/electron acceptor dyads (D-X-A) consisting of a triarylamine donor, a naphthalene diimide acceptor, and a meta-conjugated diethynylbenzene unit as a bridge had revealed the presence of electronic exchange interaction, J, in the photoexcited charge separated (CS) state. Here, we present results obtained by photochemically induced dynamic nuclear polarization (photo-CIDNP) that allows for determining the sign of J. By variation of the magnetic field from 1 mT to 9.

Generating and sustaining long-lived spin states in N,N'-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-N,N'-azobenzene.

Magnetic field and orientation dependence of solid-state CIDNP.

The magnetic field dependence of Chemically Induced Dynamic Nuclear Polarization (CIDNP) in solid-state systems is analyzed theoretically with the aim to explain the puzzling sign change of polarization found at low fields [D. Gräsing et al., Sci.

Field-cycling NMR experiments in an ultra-wide magnetic field range: relaxation and coherent polarization transfer.

An experimental method is described allowing fast field-cycling Nuclear Magnetic Resonance (NMR) experiments over a wide range of magnetic fields from 5 nT to 10 T. The method makes use of a hybrid technique: the high field range is covered by positioning the sample in the inhomogeneous stray field of the NMR spectrometer magnet. For fields below 2 mT a magnetic shield is mounted on top of the spectrometer; inside the shield the magnetic field is controlled by a specially designed coil system.

Versus -Azobenzene: Precise Determination of NMR Parameters and Analysis of Long-Lived States of N Spin Pairs.

We provide a detailed evaluation of nuclear magnetic resonance (NMR) parameters of the - and -isomers of azobenzene (AB). For determining the NMR parameters, such as proton-proton and proton-nitrogen -couplings and chemical shifts, we compared NMR spectra of three different isotopomers of AB: the doubly N labeled azobenzene, N,N'-AB, and two partially deuterated AB isotopomers with a single N atom. For the total lineshape analysis of NMR spectra, we used the recently developed ANATOLIA software package.

Complete magnetic field dependence of SABRE-derived polarization.

Signal amplification by reversible exchange (SABRE) is a promising hyperpolarization technique, which makes use of spin-order transfer from parahydrogen (the H molecule in its singlet spin state) to a to-be-polarized substrate in a transient organometallic complex, termed the SABRE complex. In this work, we present an experimental method for measuring the magnetic field dependence of the SABRE effect over an ultrawide field range, namely, from 10 nT to 10 T. This approach gives a way to determine the complete magnetic field dependence of SABRE-derived polarization.

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.

Nuclear Spin Singlet Order Selection by Adiabatically Ramped RF Fields.

We describe an NMR method to generate singlet order in spin pairs from longitudinal spin magnetization and suppress residual background signals. This method can also be used for generating and observing long-lived spin states. A singlet order selection (SOS) filter is proposed, which allows us to find signals of the spin pair of interest buried in a crowded NMR spectrum.

Robust conversion of singlet spin order in coupled spin-1/2 pairs by adiabatically ramped RF-fields.

We propose a robust and highly efficient NMR technique to create singlet spin order from longitudinal spin magnetization in coupled spin-½ pairs and to perform backward conversion (singlet order)→magnetization. In this method we exploit adiabatic ramping of an RF-field in order to drive transitions between the singlet state and the T triplet states of a spin pair under study. We demonstrate that the method works perfectly for both strongly and weakly coupled spin pairs, providing a conversion efficiency between the singlet spin order and magnetization, which is equal to the theoretical maximum.

Magnetic Resonance Characterization of One-Electron Oxidized Cyclic Dipeptides with Thioether Groups.

Photo-oxidation of seven cyclic dipeptides containing methionine, Met, and/or S-methylcysteine, Cys(Me) by electron transfer from the sulfur atom was studied in aqueous solution by time-resolved and field dependent CIDNP (chemically induced dynamic nuclear polarization). Hyperpolarized high resolution NMR spectral patterns of the starting peptides detected immediately after pulsed laser excitation show signals of all protons that are bound to carbons neighboring the sulfur atom, thus proving the involvement of sulfur-centered cation radicals. The magnetic field dependence of CIDNP shows a pronounced maximum that is determined by the g-factors and hyperfine coupling constants of the transient radical species.

Level crossing analysis of chemically induced dynamic nuclear polarization: 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.

A fast field-cycling device for high-resolution NMR: Design and application to spin relaxation and hyperpolarization experiments.

A device for performing fast magnetic field-cycling NMR experiments is described. A key feature of this setup is that it combines fast switching of the external magnetic field and high-resolution NMR detection. The field-cycling method is based on precise mechanical positioning of the NMR probe with the mounted sample in the inhomogeneous fringe field of the spectrometer magnet.

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.

Spin polarization transfer mechanisms of SABRE: A magnetic field dependent study.

We have investigated the magnetic field dependence of Signal Amplification By Reversible Exchange (SABRE) arising from binding of para-hydrogen (p-H2) and a substrate to a suitable transition metal complex. The magnetic field dependence of the amplification of the (1)H Nuclear Magnetic Resonance (NMR) signals of the released substrates and dihydrogen, and the transient transition metal dihydride species shows characteristic patterns, which is explained using the theory presented here. The generation of SABRE is most efficient at low magnetic fields due to coherent spin mixing at nuclear spin Level Anti-Crossings (LACs) in the SABRE complexes.

Long-lived spin states as a source of contrast in magnetic resonance spectroscopy and imaging.

A method is proposed to create Long-Lived spin States (LLSs) from longitudinal spin magnetization, which is based on adiabatic switching of a Radio-Frequency (RF) field with proper frequency. The technique is simple to implement with standard Nuclear Magnetic Resonance (NMR) equipment, providing an excellent conversion of population from the triplet T+ (or T-) state to the singlet state of a pair of spins and back. The method has been tested for the amino acid tyrosine and its partially deuterated isotopomer; for the deuterated compound, we have achieved a LLS lifetime, which exceeds the longitudinal relaxation time by a factor of 21.

Highly Efficient Polarization of Spin-1/2 Insensitive NMR Nuclei by Adiabatic Passage through Level Anticrossings.

A method is proposed to transfer spin order from para-hydrogen, that is, the H2 molecule in its singlet state, to spin-1/2 heteronuclei of a substrate molecule. The method is based on adiabatic passage through nuclear spin level anticrossings (LACs) in the doubly rotating frame of reference; the LAC conditions are fulfilled by applying resonant RF excitation at the NMR frequencies of protons and the heteronuclei. Efficient conversion of the para-hydrogen-induced polarization into net polarization of the heteronuclei is demonstrated; the achieved signal enhancements are about 6400 for (13)C nuclei at natural abundance.

RF-SABRE: A Way to Continuous Spin Hyperpolarization at High Magnetic Fields.

A new technique is developed that allows one to carry out the signal amplification by reversible exchange (SABRE) experiments at high magnetic field. SABRE is a hyperpolarization method, which utilizes transfer of spin order from para-hydrogen to the spins of a substrate in transient iridium complexes. Previously, it has been thought that such a transfer of spin order is only efficient at low magnetic fields, notably, at level anti-crossing (LAC) regions.

Importance of polarization transfer in reaction products for interpreting and analyzing CIDNP at low magnetic fields.

The magnetic field dependence of Chemically Induced Dynamic Nuclear Polarization (CIDNP) was studied for the amino acids N-acetyl histidine, N-acetyl tryptophan and N-acetyl tyrosine. It is demonstrated that at low field CIDNP is strongly affected by polarization redistribution in the diamagnetic molecules. Such a polarization transfer is of coherent nature and is due to spin coherences formed together with non-equilibrium population of the spin states.

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.

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.