Studying Fast Reaction Kinetics with Online NMR Spectroscopy

Abstract

A liquid thermostated microreactor Nuclear Magnetic Resonance (NMR) probe head is presented that facilitates the investigation of kinetics of fast reactions. With this setup, reaction kinetics can be studied in a broad temperature (–20 – 100 °C) and pressure range (0 – 60 bar). The setup and NMR probe head are discussed from a reaction engineering standpoint based on experiments and simulations. The microreactor probe head widens significantly the range of applications of online NMR spectroscopy for reaction and process monitoring.

Stopped‐Flow 19F NMR Spectroscopic Analysis of a Protodeboronation Proceeding at the Sub‐Second Time‐Scale

“…Whilst the IR/quench techniques allowed us to determine a wide range of protodeboronation kinetics, n = 0 to 5, Scheme 2, they did not provide the key insights that NMR spectroscopy can. These aspects led us to explore application of the two general techniques to monitor relatively fast irreversible reactions by NMR: rapid-injection, [30][31][32][33][34][35] and stopped-flow, [36][37][38][39][40][41][42][43] Figure 1. The primary difference between the two techniques lies in the way they mix the analytes.…”

“…An early publication from Ernst et al [38] highlighted the potential of stopped-flow NMR in detecting fast irreversible reactions, despite the limitations of instrumentation at the time, and line-shape distortions caused by oscillations induced by the rapid ('hard') stopping motion. More recent developments have included rapid mixing cells using pneumatic drives and solenoid valves, [40] customized probes for greater reagent polarization and temperature control, [42][43][44] and high pressure stopped-flow devices. [39] Landis and Christianson have also reported simulations of the changes in intensity, line-width, and phase, in NMR spectra arising from very rapid reactions.…”

“…One of the advantages of custom-built stopped-flow NMR probes (Figure 1b) is that a single, highlyeffective, fluid-based thermostatting system can be incorporated in the probe head where the reaction is conducted. [42,43] In contrast, a system based on insertion of a stopped-flow cell into a standard NMR probe (Figure 1c) requires two thermostatic systems. An auxiliary system provides thermostatting to the pre-magnetization, mixing, and sample transport stages, [52] with control transitioning to the spectrometer during measurement.…”

Application of a new method for simultaneous phase and baseline correction of NMR signals (SINC)

“…The experimental setup used for the kinetic study of the formation of methyl acetate is described in detail by Brächer et al The measurements were carried out using a microreactor NMR probe, which is well suited for the monitoring of fast reactions (for details, see previous studies). Thus, the time between mixing of the reactants (i.e., the start of the reaction) and analysis in the NMR flow cell is very short (less than 2 s), which allows monitoring of fast reactions .…”

Online reaction monitoring by single-scan 2D NMR under flow conditions

“…Early implementations of flow NMR reaction monitoring relied on dedicated flow probes, initially designed for hyphenation with liquid chromatography. [9][10][11][12] More recently, solutions based on a "flow cell" or "flow tube" that can be inserted into virtually any high-field, high-resolution NMR probes have been described and commercialized. 13,14 These accessories are very useful for the analysis of batch chemical reactions with reaction times of a few minutes or more, in a wide range of reaction conditions.…”