Gas Phase NMR for the Study of Chemical Reactions: Kinetics and Product Identification

Abstract

NMR spectroscopy is a uniquely convenient and information-rich tool for the study of chemical reactions in the gas phase. This report discusses strategies employed to address the special experimental challenges posed by elevated pressure and temperature, and the spectroscopic peculiarities of the gas phase. The extensive literature on the application of gas phase NMR to the study of reaction kinetics (consisting of 11 publications) is surveyed. A study of the gaseous decomposition of di-tert-butyl peroxide, as conducted with modern instrumentation and software, is described in detail. The report concludes with a summary of recent work applying two-dimensional NMR correlation and diffusion experiments to gaseous analytes.

Multi-stage Stern–Gerlach experiment modeled

“…Typical T 1 relaxation times in electron paramagnetic resonance are on the µs scale [35], consistent with the previous estimation of the collapse time scale of ⃗ µ e [5]. In contrast, typical T 1 relaxation times in gas-phase nuclear magnetic resonance are on the ms scale [36], indicating the order-of-magnitude collapse time of ⃗ µ n . In a typical Stern-Gerlach experiment [5,37], the main external field B 0 along z is at least 0.3 T (B 0 > B e ≫ B n , the Paschen-Back regime [16]), the length of the main field is ∼35 mm, and the most likely atomic speed v is ∼800 m s −1 .…”

“…Besides the two distinct collapse branches due to the quantization of ⃗ µ e , no additional branches due to the quantization of ⃗ µ n have been observed by Frisch and Segrè [9] despite the prediction of up to eight branches total [38]. For N c ∼220 (equation ( 11)) estimated from the Frisch-Segrè experimental data shown in figure 4, the collapse time constants (T c , equation (12) and its nuclear counterpart) at the main field strength are computed to be ∼3 × 10 −8 and ∼4 × 10 −4 s for ⃗ µ e and ⃗ µ n , respectively, which are consistent with the above-mentioned corresponding T 1 relaxation times in orders of magnitude [35,36]. This postulate, extended to the weaker-field IR chamber, is consistent with the selection rule for observing an electron-spinresonance transition, stating that the magnetic quantum number of the nuclear spin remains constant (i.e.…”

Solvent-Free Nuclear Magnetic Resonance Spectroscopy of Charged Molecules

“…Gas-phase NMR spectroscopy was popularized in the 1970s. Still, it remains less commonly used than its solution-phase counterpart. , It allows for the structural and kinetics analysis of neutral gaseous species and is the closest technique to SoF-NMR. Consequently, SoF-NMR can take advantage of some of the unique strengths of gas-phase NMR.…”