RESUMEN
Zero- to ultralow-field nuclear magnetic resonance (ZULF NMR) allows molecular structure elucidation via measurement of electron-mediated spin-spin J-couplings. This study examines zero-field J-spectra from molecules with quadrupolar nuclei, exemplified by solutions of various isotopologues of ammonium cations. The spectra reveal differences between various isotopologues upon extracting precise J-coupling values from pulse-acquire measurements. A primary isotope effect, â³ J = γ 14 N / γ 15 N J 15 N H - J 14 N H ≈ - 58 mHz, is deduced by analysis of the proton-nitrogen J-coupling ratios. This study points toward further experiments with symmetric cations containing quadrupolar nuclei, promising applications in biomedicine, energy storage, and benchmarking quantum chemistry calculations.
RESUMEN
In nuclear magnetic resonance, long-lived coherences constitute a class of zero-quantum (ZQ) coherences that have lifetimes that can be longer than the relaxation lifetimes T2 of transverse magnetization. So far, such coherences have been observed in systems with two coupled spins with spin quantum numbers I = 1/2, where a term S0T0+T0S0 in the density operator corresponds to a coherent superposition between the singlet S0 and the central triplet T0 state. Here, we report on the excitation and detection of collective long-lived coherences in AA'MM'XX' spin systems in molecules containing a chain of at least three methylene (-CH2-) groups. Several variants of excitation by polychromatic spin-lock induced crossing (poly-SLIC) are introduced that can excite a non-uniform distribution of the amplitudes of terms such as S0S0T0S0S0T0, S0T0S0S0T0S0, and T0S0S0T0S0S0. Once the radio frequency fields are switched off, these are not eigenstates, leading to ZQ precession involving all six protons, a process that can be understood as a propagation of spin order along the chain of CH2 groups before the reconversion into observable magnetization by a second poly-SLIC pulse that can be applied to any one or several of the CH2 groups. In the resulting 2D spectra, the ω2 domain shows SQ spectra with the chemical shifts of the CH2 groups irradiated during the reconversion, while the ω1 dimension shows ZQ signals in absorption mode with linewidths on the order of 0.1 Hz that are not affected by the inhomogeneity of the static magnetic field but can be broadened by chemical exchange as occurs in drug screening. The ZQ frequencies are primarily determined by differences ΔJ between vicinal J-couplings.
RESUMEN
Long-lived states (LLSs) have lifetimes TLLS that can be much longer than longitudinal relaxation times T1. In molecules containing several geminal pairs of protons in neighboring CH2 groups, it has been shown that delocalized LLSs can be excited by converting magnetization into imbalances between the populations of singlet and triplet states of each pair. Since the empirical yield of the conversion and reconversion of observable magnetization into LLSs and back is on the order of 10â¯% if one uses spin-lock induced crossing (SLIC), it would be desirable to boost the sensitivity by dissolution dynamic nuclear polarization (d-DNP). To enhance the magnetization of nuclear spins by d-DNP, the analytes must be mixed with radicals such as 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL). After dissolution, these radicals lead to an undesirable paramagnetic relaxation enhancement (PRE) which shortens not only the longitudinal relaxation times T1 but also the lifetimes TLLS of LLSs. It is shown in this work that PRE by TEMPOL is less deleterious for LLSs than for longitudinal magnetization for four different molecules: 2,2-dimethyl-2-silapentane-5-sulfonate (DSS), homotaurine, taurine, and acetylcholine. The relaxivities rLLS (i.e., the slopes of the relaxation rate constants RLLS as a function of the radical concentration) are 3 to 5 times smaller than the relaxivities r1 of longitudinal magnetization. Partial delocalization of the LLSs across neighboring CH2 groups may decrease this advantage, but in practice, this effect was observed to be small, for example, when comparing taurine containing two CH2 groups and homotaurine with three CH2 groups. Regardless of whether the LLSs are delocalized or not, it is shown that PRE should not be a major problem for experiments combining d-DNP and LLSs, provided the concentration of paramagnetic species after dissolution does not exceed 1â¯mM, a condition that is readily fulfilled in typical d-DNP experiments. In bullet d-DNP experiments however, it may be necessary to decrease the concentration of TEMPOL or to add ascorbate for chemical reduction.
RESUMEN
Photochemically induced dynamic nuclear polarization (photo-CIDNP) enables nuclear spin ordering by irradiating samples with light. Polarized spins are conventionally detected via high-field chemical-shift-resolved NMR (above 0.1 T). In this Letter, we demonstrate in situ low-field photo-CIDNP measurements using a magnetically shielded fast-field-cycling NMR setup detecting Larmor precession via atomic magnetometers. For solutions comprising mM concentrations of the photochemically polarized molecules, hyperpolarized 1H magnetization is detected by pulse-acquired NMR spectroscopy. The observed NMR line widths are about 5 times narrower than normally anticipated in high-field NMR and are systematically affected by light irradiation during the acquisition period, reflecting a reduction of the transverse relaxation time constant, T2*, on the order of 10%. Magnetometer-detected photo-CIDNP spectroscopy enables straightforward observation of spin-chemistry processes in the ambient field range from a few nT to tens of mT. Potential applications of this measuring modality are discussed.
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Simulating NMR experiments may appear mysterious and even daunting for those who are new to the field. Yet, broken down into pieces, the process may turn out to be easier than expected. Quite the opposite, it is in fact a powerful and playful means to get insights into the spin dynamics of NMR experiments. In this tutorial paper, we show step by step how some NMR experiments can be simulated, assuming as little prior knowledge from the reader as possible. We focus on the case of NMR at zero and ultralow fields, an emerging modality of NMR in which the spin dynamics are dominated by spin-spin interactions rather than spin-field interactions, as is usually the case with conventional high-field NMR. We first show how to simulate spectra numerically. In a second step, we detail an approach to construct an eigenbasis for systems of spin-1/2 nuclei at zero field. We then use it to interpret the numerical simulations.
RESUMEN
In nuclear magnetic resonance (NMR), the lifetimes of long-lived states (LLSs) are exquisitely sensitive to their environment. However, the number of molecules where such states can be excited has hitherto been rather limited. Here, it is shown that LLSs can be readily excited in many common molecules that contain two or more neighboring CH2 groups. Accessing such LLSs does not require any isotopic enrichment, nor does it require any stereogenic centers to lift the chemical equivalence of CH2 protons. LLSs were excited in a variety of metabolites, neurotransmitters, vitamins, amino acids, and other molecules. One can excite LLSs in several different molecules simultaneously. In combination with magnetic resonance imaging, LLSs can reveal a contrast upon noncovalent binding of ligands to macromolecules. This suggests new perspectives to achieve high-throughput parallel drug screening by NMR.
RESUMEN
Long-lived states (LLS) involving pairs of magnetically inequivalent but chemically equivalent proton spins in aliphatic (CH_{2})_{n} chains can be excited by simultaneous application of weak selective radio frequency fields at n chemical shifts by polychromatic spin-lock induced crossing. The LLS are delocalized throughout the aliphatic chains by mixing of intrapair singlet states and by excitation of LLS comprising products of four and six spin operators. The measured lifetimes T_{LLS} in a model compound are about 5 times longer than T_{1} and are strongly affected by interactions with macromolecules.
RESUMEN
Zero- to ultralow-field nuclear magnetic resonance (ZULF NMR) is a rapidly developing form of spectroscopy that provides rich spectroscopic information in the absence of large magnetic fields. However, signal acquisition still requires a mechanism for generating a bulk magnetic moment for detection, and the currently used methods only apply to a limited pool of chemicals or come at prohibitively high cost. We demonstrate that the parahydrogen-based SABRE (signal amplification by reversible exchange)-Relay method can be used as a more general means of generating hyperpolarized analytes for ZULF NMR by observing zero-field J-spectra of [13C]-methanol, [1-13C]-ethanol, and [2-13C]-ethanol in both 13C-isotopically enriched and natural abundance samples. We explore the magnetic field dependence of the SABRE-Relay efficiency and show the existence of a second maximum at 19.0 ± 0.3 mT. Despite presence of water, SABRE-Relay is used to hyperpolarize ethanol extracted from a store-bought sample of vodka (%PH ~ 0.1%).
RESUMEN
Photochemically induced dynamic nuclear polarization (photo-CIDNP) is a method to hyperpolarize nuclear spins using light. In most cases, CIDNP experiments are performed in high magnetic fields and the sample is irradiated by light inside a nuclear magnetic resonance (NMR) spectrometer. Here we demonstrate photo-CIDNP hyperpolarization generated in the Earth's magnetic field and under zero- to ultralow-field (ZULF) conditions. Irradiating a sample containing tetraphenylporphyrin and para-benzoquinone for several seconds with light-emitting diodes produces strong hyperpolarization of 1H and 13C nuclear spins, enhancing the NMR signals more than 200 times. The hyperpolarized spin states at the Earth's field and in ZULF are different. In the latter case, the state corresponds to the singlet order between scalar-coupled 1H-13C nuclear spins. This state has a longer lifetime than the state hyperpolarized at Earth's field. The method is simple and cost-efficient and should be applicable to many molecular systems known to exhibit photo-CIDNP, including amino acids and nucleotides.
RESUMEN
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 15 N2 -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. Azobenzene exists in two isomers, trans and cis. We show that all nuclear spins in cis-azobenzene can be efficiently hyperpolarized by SABRE at suitable magnetic fields. Enhancement factors (relative to 9.4â T) reach up to 3000 for 15 N spins and up to 30 for the 1 H spins. We compare two approaches to observe either hyperpolarized magnetization of 15 N/1 H spins, or hyperpolarized singlet order of the 15 N spin pair. The results presented here will be useful for further experiments in which hyperpolarized cis-15 N2 -azobenzene is switched by light to trans-15 N2 -azobenzene for storing the produced hyperpolarization in the long-lived spin state of the 15 N pair of trans-15 N2 -azobenzene.
RESUMEN
The field of magnetic resonance imaging with hyperpolarized contrast agents is rapidly expanding, and parahydrogen-induced polarization (PHIP) is emerging as an inexpensive and easy-to-implement method for generating the required hyperpolarized biomolecules. Hydrogenative PHIP delivers hyperpolarized proton spin order to a substrate via chemical addition of H2 in the spin-singlet state, but it is typically necessary to transfer the proton polarization to a heteronucleus (usually 13C) which has a longer spin lifetime. Adiabatic ultralow magnetic field manipulations can be used to induce the polarization transfer, but this is necessarily a slow process, which is undesirable since the spins continually relax back to thermal equilibrium. Here we demonstrate two constant-adiabaticity field sweep methods, one in which the field passes through zero, and one in which the field is swept from zero, for optimal polarization transfer on a model AA'X spin system, [1-13C]fumarate. We introduce a method for calculating the constant-adiabaticity magnetic field sweeps, and demonstrate that they enable approximately one order of magnitude faster spin-order conversion compared to linear sweeps. The present method can thus be utilized to manipulate nonthermal order in heteronuclear spin systems.
RESUMEN
Algorithmic cooling methods manipulate an open quantum system in order to lower its temperature below that of the environment. We achieve significant cooling of an ensemble of nuclear spin-pair systems by exploiting the long-lived nuclear singlet state, which is an antisymmetric quantum superposition of the "up" and "down" Zeeman states. The effect is demonstrated by nuclear magnetic resonance experiments on a molecular system containing a coupled pair of near-equivalent 13C nuclei. The populations of the system are subjected to a repeating sequence of cyclic permutations separated by relaxation intervals. The long-lived nuclear singlet order is pumped well beyond the unitary limit. The pumped singlet order is converted into nuclear magnetization which is enhanced by 21% relative to its thermal equilibrium value.
RESUMEN
An amendment to this paper has been published and can be accessed via a link at the top of the paper.
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The fundamental concept of phase discussed in this tutorial aimed at providing students with an explanation of the delays and processing parameters they may find in nuclear magnetic resonance (NMR) pulse programs. We consider the phase of radio-frequency pulses, receiver, and magnetization and how all these parameters are related to phases and offsets of signals in spectra. The impact of the off-resonance effect on the phase of the magnetization is discussed before presenting an overview of how adjustment of the time reference of the free induction decay avoids first-order correction of the phase of spectra. The main objective of this tutorial is to show how the relative phase of a pulse and the receiver can be used to change the reference frequency along direct and indirect dimensions of NMR experiments. Unusual of phase incrementation with non-90° angles will be illustrated on one- and two-dimensional NMR spectra.
RESUMEN
The signal-to-noise ratio is an important property of NMR spectra. It allows to compare the sensitivity of experiments, the performance of hardware, etc. Its measurement is usually done in a rudimentary manner involving manual operation of selecting separately a region of the spectrum with signal and noise, respectively, applying some operation and returning the signal-to-noise ratio. We introduce here a simple method based on the analysis of the distribution of point intensities in one- and two-dimensional spectra. The signal/artifact/noise plots, (SAN plots) allows one to present in a graphical manner qualitative and quantitative information about spectra. It will be shown that besides measuring signal and noise levels, SAN plots are also quite useful to visualize and compare artifacts within a series of spectra. Some basic properties of the SAN plots are illustrated with simple application.
RESUMEN
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.
RESUMEN
Some nuclear spin systems support long-lived states, which display greatly extended relaxation times relative to the relaxation time of nuclear spin magnetization. In spin-1/2 pairs, such a long-lived state is given by singlet order, representing the difference of the population of the nuclear singlet state and the mean population of the three triplets. In many cases, the experiments with long-lived singlet order are very time-consuming because of the need to wait for singlet order decay before the experiment can be repeated; otherwise, spin order remaining from a previous measurement may lead to experimental artifacts. Here, we propose techniques for fast and efficient singlet order destruction. These methods exploit coherent singlet-triplet conversion; in some cases, multiple conversion steps are introduced. We demonstrate that singlet order destruction enables a dramatic reduction of the waiting time between consecutive experiments and suggest to use this approach in singlet-state Nuclear Magnetic Resonance (NMR) experiments with nearly equivalent spins.
RESUMEN
A method is implemented to perform "fast" adiabatic variation of the spin Hamiltonian by imposing the constant adiabaticity condition. The method is applied to improve the performance of singlet-state Nuclear Magnetic Resonance (NMR) experiments, specifically, for efficient generation and readout of the singlet spin order in coupled spin pairs by applying adiabatically ramped RF-fields. Test experiments have been performed on a specially designed molecule having two strongly coupled 13C spins and on selectively isotopically labelled glycerol having two pairs of coupled protons. Optimized RF-ramps show improved performance in comparison, for example, to linear ramps. We expect that the methods described here are useful not only for singlet-state NMR experiments but also for other experiments in magnetic resonance, which utilize adiabatic variation of the spin Hamiltonian.
RESUMEN
We present approaches for an efficient excitation of singlet-triplet coherences in pairs of nearly-equivalent spins. Standard Nuclear Magnetic Resonance (NMR) pulse sequences do not excite these coherences at all or with very low efficiency. The single quantum singlet-triplet coherences, here termed the outer singlet-triplet coherences, correspond to lines of low intensity in the NMR spectrum of a strongly-coupled spin pair (they are sometimes referred to as "forbidden transitions"), whereas the zero-quantum coherences, here termed the inner singlet-triplet coherences, do not have a direct spectral manifestation. In the present study, we investigated singlet-triplet coherences in a pair of nearly-equivalent carbon spins of the 13C-isotopomer of a specially designed naphthalene derivative with optimized relaxation properties. We propose and compare several techniques to drive the singlet-triplet coherence in strongly coupled spin pairs. First, we study different methods for efficient excitation of the outer singlet-triplet coherences. The achieved conversion efficiency of magnetization to the coherences of interest is close to the theoretically allowed maximum. Second, we propose methods to convert the outer coherences into the inner singlet-triplet coherence. The inner singlet-triplet coherence is insensitive to field inhomogeneity and can be long-lived. By probing this coherence, we perform a very precise measurement of the spin-spin J-couplings. A remarkable property of this coherence is that it can be preserved even in absence of a spin-locking radiofrequency field. Consequently, it is possible to shuttle the sample between different magnetic fields preserving the coherence. This allows one to study the field dependence of the relaxation time, TIST, of the inner singlet-triplet coherence by performing field-cycling experiments. We observed dramatic changes of the ratio TIST/T1 from about 1 (in strong fields) up to 2.4 (in weak fields), which is the evidence of a significant influence of the chemical shift anisotropy on relaxation. We have detected a remarkably long lifetime of the inner singlet-triplet coherence of about 200 s at the magnetic field of 5 mT.
RESUMEN
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.