Two-dimensional (2D) NMR is an important tool for elucidating molecular structure and dynamics 1 . However, the method is limited by the low sensitivity inherent to NMR techniques, resulting in typical acquisition times for 2D NMR spectra ranging from minutes to hours. A number of hyperpolarization techniques have been explored to boost NMR's sensitivity, including an ex situ dynamic nuclear polarization method capable of yielding-for an array of molecules and under conventional observation conditions for liquid samplessignals that exceed those currently afforded by the highestfield spectrometers by several orders of magnitude 2 . Whereas this methodology is able to provide the sensitivity equivalent of ∼10 6 scans, it is constrained to extract its 'super-spectrum' within a single transient, making it a poor starting point for conventional 2D NMR acquisitions. Here, we show that if the ex situ dynamic nuclear polarization approach is suitably merged with spatially encoded ultrafast NMR spectroscopy 3 , 2D NMR spectra of liquid samples at submicromolar concentrations can be acquired within ∼0.1 s.The future of NMR hinges on the efforts devoted to increase the resolution and sensitivity of this branch of spectroscopy. Key among the physical concepts that define NMR's resolving power are multidimensional (nD) acquisitions, which spread and correlate the resonances arising from individual sites onto multiple frequency axes 1,4,5 . Although these experiments are intrinsically lengthier than their conventional one-dimensional (1D) counterparts, nD NMR's success is best portrayed by its nearly ubiquitous contemporary practice in chemistry and biology, as well as by its various NMR imaging (MRI) clinical derivations. Comparable improvements regarding NMR's sensitivity, a particularly pressing issue given the weak signals associated with magnetic resonance observations in bulk, have been slower in coming. Moreover, the moderate dependence that sensitivity exhibits on the magnetic field, B NMR , has led to diminishing returns despite investments on ever-larger NMR magnets. Driven by this reality, recently there has been an increased interest in devising alternatives that prepare nuclei in 'hyperpolarized' states, whose spin population differences depart from the usual ≈10 −5 Boltzmann distributions and approach unity values. Methods proposed and demonstrated for achieving such metastable spin states include chemical synthesis and parahydrogen 6,7 , optical pumping 8,9 and microwave-driven transfers of magnetization from electrons to nearby nuclei via dynamic nuclear polarization 10-13 (DNP). DNP is arguably the most generally applicable of these methods, providing relatively high enhancements reaching up to the γ electron /γ nucleus ratio between the spins' magnetogyric constants, while having the relatively modest requirement that the targeted system be mixed with a free radical to be irradiated at its Larmor frequency.A logical approach to exploit the benefits of DNP within a high-resolution liquid-state NMR setting wou...
Here we demonstrate how para-hydrogen can be used to prepare a two-spin system in an almost pure state which is suitable for implementing nuclear magnetic resonance (NMR) quantum computation. A 12 ns laser pulse is used to initiate a chemical reaction involving pure para-hydrogen (the nuclear spin singlet of H2). The product, formed on the µs timescale, contains a hydrogen-derived two-spin system with an effective spin-state purity of 0.916. To achieve a comparable result by direct cooling would require an unmanageable (in the liquid state) temperature of 6.4 mK or an impractical magnetic field of 0.45 MT at room temperature. The resulting spin state has an entanglement of formation of 0.822 and cannot be described by local hidden variable models.PACS numbers: 03.67. Lx, 03.67.Mn, Introduction. While quantum computing [1] offers the potential of using new quantum algorithms to tackle problems that are intractable for classical processors, its implementation requires the development of quantum devices, which are as yet unavailable. The most complex implementations of quantum algorithms to date have used techniques adapted from nuclear magnetic resonance (NMR) spectroscopy [2,3,4,5], but current liquid state NMR approaches cannot be extended to systems with many quantum bits, as it is not possible to prepare pure initial states by directly cooling the spin system into its ground state [6]. Furthermore, it has been shown that current NMR experiments involve only separable states [7], and thus could in principle be described by local hidden variable models.The conventional approach in NMR quantum computing [4] is to use an ensemble of spins, and to prepare a pseudo-pure ground state [2,4] of the form
We report pump-probe experiments employing laser-synchronized reactions of para-hydrogen (para-H2) with transition metal dihydride complexes in conjunction with nuclear magnetic resonance (NMR) detection. The pump-probe experiment consists of a single nanosecond laser pump pulse followed, after a precisely defined delay, by a single radio frequency (rf) probe pulse. Laser irradiation eliminates H2 from either Ru(PPh3)3(CO)(H)2 1 or cis-Ru(dppe)2(H)2 2 in C6D6 solution. Reaction with para-H2 then regenerates 1 and 2 in a well-defined nuclear spin state. The rf probe pulse produces a high-resolution, single-scan (1)H NMR spectrum that can be recorded after a pump-probe delay of just 10 μs. The evolution of the spectra can be followed as the pump-probe delay is increased by micro- or millisecond increments. Due to the sensitivity of this para-H2 experiment, the resulting NMR spectra can have hydride signal-to-noise ratios exceeding 750:1. The spectra of 1 oscillate in amplitude with frequency 1101 ± 3 Hz, the chemical shift difference between the chemically inequivalent hydrides. The corresponding hydride signals of 2 oscillate with frequency 83 ± 5 Hz, which matches the difference between couplings of the hydrides to the equatorial (31)P nuclei. We use the product operator formalism to show that this oscillatory behavior arises from a magnetic coherence in the plane orthogonal to the magnetic field that is generated by use of the laser pulse without rf initialization. In addition, we demonstrate how chemical shift imaging can differentiate the region of laser irradiation thereby distinguishing between thermal and photochemical reactivity within the NMR tube.
The study of reaction mechanisms by NMR spectroscopy normally suffers from limitations in sensitivity that arise from the physical constraints of the detection method. An overview is presented of how chemical reactions can be studied using parahydrogen assisted NMR spectroscopy where detected signal strengths can exceed those normally seen by factors of over 28,000.
The study of reaction mechanisms by NMR spectroscopy normally suffers from limitations in sensitivity that result in substantial difficulties in the detection and characterisation of reaction intermediates. One technique that can circumvent this problem is the use of para‐enriched hydrogen. Here, an overview of the developments in the study of chemical reactions using parahydrogen‐assisted NMR spectroscopy is presented, with the characterisation of a number of new hydride complexes of iridium, rhodium, ruthenium and tantalum being described. A number of studies involving transition metal clusters and catalytic applications are also considered. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)
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