Matthias Bechmann

Genetic algorithms and solid state NMR pulse sequences

The use of genetic algorithms for the optimisation of magic angle spinning NMR pulse sequences is discussed. The discussion uses as an example the optimisation of the C7(2)(1) dipolar recoupling pulse sequence, aiming to achieve improved efficiency for spin systems characterised by large chemical shielding anisotropies and/or small dipolar coupling interactions. The optimised pulse sequence is found to be robust over a wide range of parameters, requires only minimal a priori knowledge of the spin system for experimental implementations with buildup rates being solely determined by the magnitude of the dipolar coupling interaction, but is found to be less broadbanded than the original C7(2)(1) pulse sequence. The optimised pulse sequence breaks the synchronicity between r.f. pulses and sample spinning.

From binary to continuous gates - and back again

We describe how nuclear magnetic resonance (NMR) spectroscopy can serve as a substrate for the implementation of classical logic gates. The approach exploits the inherently continuous nature of the NMR parameter space. We show how simple continuous NAND gates with sin/sin and sin/sinc characteristics arise from the NMR parameter space. We use these simple continuous NAND gates as starting points to obtain optimised target NAND circuits with robust, error-tolerant properties. We use Cartesian Genetic Programming (CGP) as our optimisation tool. The various evolved circuits display patterns relating to the symmetry properties of the initial simple continuous gates. Other circuits, such as a robust XOR circuit built from simple NAND gates, are obtained using similar strategies. We briefly mention the possibility to include other target objective functions, for example other continuous functions. Simple continuous NAND gates with sin/sin characteristics are a good starting point for the creation of error-tolerant circuits whereas the more complicated sin/sinc gate characteristics offer potential for the implementation of complicated functions by choosing some straightforward, experimentally controllable parameters appropriately.

X-[1H, 19F] triple resonance with a X-[1H] CP MAS probe and characterisation of a 29Si-19F spin pair.

An economic approach for implementing X-[1H,19F] double-decoupling MAS NMR experiments with a conventional X-[1H] dual-channel CP MAS probe is demonstrated. The parameters characterising the isolated 29Si-19F spin pair in an organosilicon compound R(3)SiF (R = 9-anthryl) are determined. In addition, we discuss the optimum choice of experimental parameters for determining all 29Si-19F spin-pair parameters from straightforward 29Si MAS NMR spectra with only 1H decoupling applied during acquisition.

Spin Noise Detection of Nuclear Hyperpolarization at 1.2 K

We report proton spin noise spectra of a hyperpolarized solid sample of commonly used “DNP (dynamic nuclear polarization) juice” containing TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxide) and irradiated by a microwave field at a temperature of 1.2 K in a magnetic field of 6.7 T. The line shapes of the spin noise power spectra are sensitive to the variation of the microwave irradiation frequency and change from dip to bump, when the electron Larmor frequency is crossed, which is shown to be in good accordance with theory by simulations. Small but significant deviations from these predictions are observed, which can be related to spin noise and radiation damping phenomena that have been reported in thermally polarized systems. The non-linear dependence of the spin noise integral on nuclear polarization provides a means to monitor hyperpolarization semi-quantitatively without any perturbation of the spin system by radio frequency irradiation.

Heterotic computing examples with optics, bacteria, and chemicals

Unconventional computers can perform embodied computation that can directly exploit the natural dynamics of the substrate. But such in materio devices are often limited, special purpose machines. To be practically useful, unconventional devices are usually be combined with classical computers or control systems. However, there is currently no established way to do this, or to combine different unconventional devices. In this position paper we describe heterotic unconventional computation, an approach that focusses on combinations of unconventional devices. This will need a sound semantic framework defining how diverse unconventional computational devices can be combined in a way that respects the intrinsic computational power of each, whilst yielding a hybrid device that is capable of more than the sum of its parts. We also describe a suite of diverse physical implementations of heterotic unconventional computers, comprising computation performed by bacteria hosted in chemically built material, sensed and controlled optically and chemically.

Selectivity of Double-Quantum Filtered Rotational-Resonance Experiments on Larger-than-Two-Spin Systems

Characterizing the orientation and molecular conformation of small organic molecules bound to the inner or outer surfaces of proteins represents an important step in drug design and in understanding the mechanisms of biochemical reactions, and similarly, of non-biological catalytic reactions. In a biochemical context, such molecular units or subunits may often contain only three or four carbon atoms, examples being the pyruvate anion, fumaric and maleic acid derivatives, or the phosphenolpyruvate moiety in differing degrees of ionization. Magic-angle spinning (MAS) NMR experiments, capable of delivering reliable information about the conformational properties of these molecular units, have to combine several properties in order to be able to fulfill these tasks in realistic application situations. First, the 13C resonances originating from the (fully or partially) 13C enriched substrate molecules of interest have to be separable from additional natural-abundance 13C resonances; this calls for the application of double-quantum filtration (DQF) techniques. Second, many of these small substrate molecules feature structural subunits that require using the orientation dependence of 13C chemical shielding as the source of information about molecular conformation; this calls for MAS NMR experiments where magnitudes and orientations of chemical shielding tensors are sensitively reflected. Third, for reasons of synthetic feasibility, the chosen MAS NMR techniques must be applicable in a quantifiable manner to larger-than-two-spin systems. The ease and robustness of the experimental and numerical implementations are an additional consideration.

31P and 13C chemical shielding tensors in the phosphoenolpyruvate moiety from rotary resonance recoupling 13C and 31P MAS and single crystal 31P NMR

A 31P and 13C NMR study of powder and single crystal samples of two phosphoenolpyruvate (PEP) compounds, the tris-ammonium salt monohydrate (NH4)3(PEP)·H2O (1), and the mono-ammonium-salt (NH4)(H2PEP) (2) is presented. The P chemical shielding tensors in 1 are measured by 31P single crystal NMR on four minuscule samples and assigned without ambiguity by exploiting the orientation-dependent 31P-31p dipolar splittings of the resonance lines. The orientation of the 31P chemical shielding tensor is discussed in terms of the C2v — and C3-type distortions of the phosphate PO4-coordination sphere. From 13C MAS NMR experiments with 31P rotary resonance recoupling on polycrystalline powder samples the orientations of the 31P chemical shielding tensors in 1 and 2 are obtained, for 1 in very good agreement with the 31P single crystal NMR results. Only some of the orientational parameters of the three 13C chemical shielding tensors in the PEP moiety of 1 could be derived from 13C MAS NMR experiments with 31P rotary resonance recoupling.

On the Tautomerism of N-Substituted Pyrazolones: 1,2-Dihydro-3H-pyrazol-3-ones versus 1H-Pyrazol-3-ols

The tautomerism of 1-phenyl-1,2-dihydro-3H-pyrazol-3-one was investigated. An X-ray crystal structure analysis exhibits dimers of 1-phenyl-1H-pyrazol-3-ol units. Comparison of NMR (nuclear magnetic resonance) spectra in liquid state (1H, 13C, 15N) with those of “fixed” derivatives, as well as with the corresponding solid state NMR spectra reveal this compound to exist predominantly as 1H-pyrazol-3-ol molecule pairs in nonpolar solvents like CDCl3 or C6D6, whereas in DMSO-d6 the corresponding monomers are at hand. Moreover, the NMR data of different related 1H-pyrazol-3-ol derivatives are presented.

Spin-Noise-Detected Two-Dimensional Nuclear Magnetic Resonance at Triple Sensitivity

Abstract A major breakthrough in speed and sensitivity of 2 D spin‐noise‐detected NMR is achieved owing to a new acquisition and processing scheme called “double block usage” (DBU) that utilizes each recorded noise block in two independent cross‐correlations. The mixing, evolution, and acquisition periods are repeated head‐to‐tail without any recovery delays and well‐known building blocks of multidimensional NMR (constant‐time evolution and quadrature detection in the indirect dimension as well as pulsed field gradients) provide further enhancement and artifact suppression. Modified timing of the receiver electronics eliminates spurious random excitation. We achieve a threefold sensitivity increase over the original snHMQC (spin‐noise‐detected heteronuclear multiple quantum correlation) experiment (K. Chandra et al., J. Phys. Chem. Lett. 2013, 4, 3853) and demonstrate the feasibility of spin‐noise‐detected long‐range correlation.