High-resolution compact NMR spectroscopy for structure elucidation, process control and reaction monitoring

Singh, Kawarpal; Blümich, Bernhard (Thesis advisor); Liauw, Marcel (Thesis advisor)

Aachen (2018, 2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2018

Abstract

Nuclear magnetic resonance is known for its methodological diversity and analytical sensitivity. Since the early days of NMR efforts have been made to obtain chemical and physical information of materials with high-resolution and good sensitivity. In this process, NMR instruments with superconducting magnets have replaced the instruments involving permanent and electromagnets. Therefore, the magnetic field strength has been increased drastically from a few MHz to 1 GHz. This increase in magnetic field has expanded the applications of NMR to biopolymers, proteins, metabolomics etc. along with the cost for large and professional instrumentation. In this scenario, current state-of-the-art compact NMR with permanent magnets involving advanced electronics has emerged with sufficient resolution and sensitivity which can help to look into the chemistry of small and medium size molecules. In this work, efforts are made in using compact NMR to elucidate the chemical structure and adulteration of molecules having complex proton NMR spectra by using combinations of different 1D and 2D experiments. The advance in statistical analysis such as chemometrics is combined with compact NMR to resolve the complications associated with peak overlap and data analysis for knowing the concentration of repeat units in SBR by developing the partial least squares regression (PLS-R) models. An acetalization reaction is monitored in real-time using compact NMR to optimize the reaction parameters and results obtained are compared with gas chromatography and high-field NMR. The impact of dense sampling with compact NMR and sparse sampling with gas chromatography on kinetic analysis is discussed. The kinetic isotope effects in chemical reactions are monitored to know the effect of temperature and rate determining steps of the reactions. The isotopic fractionation factors are obtained with the proton inventory method. Lastly, the transesterification reaction for biodiesel synthesis is studied to look into the mechanistic changes occurring with changing catalyst concentrations, molar ratios and temperature. The data analyzed with peak fitting and the PLS-R methods are compared where the PLS-R model provides an automated way to analyze NMR spectra with overlapping peaks.