Klaus Woelk

Ortho- and Parahydrogen Induced Nuclear Spin Polarization

Emission and enhanced absorption lines occur in the NMR spectra recorded during homogeneous hydrogénation reactions, if enriched fractions of the spin isomers of dihydrogen are used, namely either parahydrogen or orthohydrogen. As such the spectra resemble those resulting from the Chemically Induced Dynamic Nuclear Polarization (CIDNP) phenomenon, which is caused by free radical intermediates. Here, however, a change of the symmetry due to the hydrogénation causes seemingly similar effects,

Design and application of robust rf pulses for toroid cavity NMR spectroscopy

We present robust radio frequency (rf) pulses that tolerate a factor of six inhomogeneity in the B₁ field, significantly enhancing the potential of toroid cavity resonators for NMR spectroscopic applications. Both point-to-point (PP) and unitary rotation (UR) pulses were optimized for excitation, inversion, and refocusing using the gradient ascent pulse engineering (GRAPE) algorithm based on optimal control theory. In addition, the optimized parameterization (OP) algorithm applied to the adiabatic BIR-4 UR pulse scheme enabled ultra-short (50 μs) pulses with acceptable performance compared to standard implementations. OP also discovered a new class of non-adiabatic pulse shapes with improved performance within the BIR-4 framework. However, none of the OP-BIR4 pulses are competitive with the more generally optimized UR pulses. The advantages of the new pulses are demonstrated in simulations and experiments. In particular, the DQF COSY result presented here represents the first implementation of 2D NMR spectroscopy using a toroid probe.

NMR Studies of the Kinetics of Homogeneously Catalyzed Hydrogénations Using Parahydrogen Induced Polarization at Variable Pressure

Quantitative evaluation of Parahydrogen Induced Polarization (PHIP) experiments provides for the determination of kinetic data of homogeneously catalyzed hydrogénations. Especially developed NMR probes facilitate investigations at reproducible reaction conditions and constant hydrogénation rates. Rate equations, describing the experimentally achieved steady state kinetics, are used to determine the pressure dependent ratio between free and complexed catalyst in the reactive solution during the hydrogénation. A method is presented to determine the rate constants of the catalytic cycle of such hydrogénations, which follow the hydride route.

EXponentially Converging Eradication Pulse Train (EXCEPT) for Solvent-Signal Suppression in Investigations with Variable T 1 Times

Selective presaturation is a common technique for suppressing excessive solvent signals during proton NMR analysis of dilute samples in protic solvents. When the solvent T1 relaxation time constant varies within a series of samples, parameters for the presaturation sequence must often be re-adjusted for each sample. The EXCEPT (EXponentially Converging Eradication Pulse Train) presaturation pulse sequence was developed to eliminate time consuming pulse-parameter re-optimization as long as the variation in the solvents T1 remains within an order of magnitude. EXCEPT consists of frequency-selective inversion pulses with progressively decreasing interpulse delays. The interpulse delays were optimized to encompass T1 relaxation times ranging from 1 to 10s, but they can be easily adjusted by a single factor for other ranges that fall within an order of magnitude with respect to T1. Sequences with different numbers of inversion pulses were tested to maximize suppression while minimizing the number of pulses and thus the total time needed for suppression. The EXCEPT-16 experiment, where 16 denotes the number of inversion pulses, was found satisfactory for many standard applications. Experimental results demonstrate that EXCEPT provides effective T1-insensitive solvent suppression as predicted by the theory. The robustness of EXCEPT with respect to changes in solvent T1 allows NMR investigations to be carried out for a series of samples without the need for pulse-parameter re-optimization for each sample.

Predicting the Effect of Relaxation during Frequency-Selective Adiabatic Pulses

Adiabatic half and full passages are invaluable for achieving uniform, B1-insensitive excitation or inversion of macroscopic magnetization across a well-defined range of NMR frequencies. To accomplish narrow frequency ranges with adiabatic pulses (<100Hz), long pulse durations at low RF power levels are necessary, and relaxation during these pulses may no longer be negligible. A numerical, discrete recursive combination of the Bloch equations for longitudinal and transverse relaxation with the optimized equation for adiabatic angular motion of magnetization is used to calculate the trajectory of magnetization including its relaxation during adiabatic hyperbolic secant pulses. The agreement of computer-calculated data with experimental results demonstrates that, in non-viscous, small-molecule fluids, it is possible to model magnetization and relaxation by considering standard T1 and T2 relaxation in the traditional rotating frame. The proposed model is aimed at performance optimizations of applications in which these pulses are employed. It differs from previous reports which focused on short high-power adiabatic pulses and relaxation that is governed by dipole-dipole interactions, cross polarization, or chemical exchange.