Determination of Gas Concentrations in Liquids by Nuclear Magnetic Resonance: Hydrogen in Organic Solvents

Abstract

An important parameter for the calculating of rate constants or for process enhancement, both for sciences and engineering, is the knowledge of gas concentrations of gas–liquid systems. We introduce the method SelPULCON for the determination of gas concentrations in liquids for NMR active substances (e.g., hydrogen) by means of NMR spectroscopy. This method is based on the “principle of reciprocity” and correlates the absolute signal intensity of the sample with that of an external reference. As a selective excitation is applied, this method is independent of whether the solvent is deuterated or not. In this way one can measure the gas concentration in almost all solutions and utilize all of the advantages of the NMR spectroscopy (e.g., for in situ or OPERANDO investigations). To demonstrate this method, we determined the hydrogen saturation concentrations in eight organic solvents at 25 °C and 0.101 MPa total pressure, for example, in dichloromethane (c = 1.51 mmol·dm–3, b = 1.15 mmol·kg–1) and in 2,2,2-trifluoroethanol (c = 3.28 mmol·dm–3, b = 2.39 mmol·kg–1).

A tert-butyl-substituted amino-borane bound to an iridium fragment: A latent source of free H2BNtBuH

“…The NMR data for 2 are consistent with its formulation and are similar to those reported for other amino-borane complexes [17,18,26,30,33e35]. In particular the 31 [12,20]. NMR data for complex 3 are as reported previously [17].…”

“…This process occurs with quantitative yield (by NMR spectroscopy). The small amount of H 2 (dissolved) that would be formed was not observed, but as the reaction is slow and its solubility in 1,2-C 6 H 4 F 2 would be expected to be low, [31], this is not unreasonable. We speculate this process operates via a constant oxidation-state sigma-CAM type mechanism [32], as calculated for the dehydrogenation of closely related H 3 B$NMe 2 H to give H 2 B]NMe 2 when bound to the same {Ir} fragment [17].…”

Direct 1H NMR evidence of spin-rotation coupling as a source of para → ortho-H2 conversion in diamagnetic solvents

“…This is not by chance. Indeed, as already discussed in the theoretical section, if the p-H 2 relaxation rates in the liquid and gaseous phases are very close to each other, one actually measures the p-H 2 relaxation rate characteristic of the net gas phase due to is the one of the gas phase (nevertheless quasi-identical to that of the liquid phase and therefore still denoted as R para 1 in the following), much easier to obtain experimentally from the liquid phase (notwithstanding the low H 2 solubility in solvents [35][36][37] ) than from a direct measurement in the gas phase due to the excessive line-broadening 6,13 in the latter case (of about 500 Hz in our experimental conditions, see Figure S3.1 in the supplementary material). According to the considerations developed in the theoretical section, this finding also suggests that the interactions (probably very small) between H 2 molecules and CS 2 molecules in the liquid state are similar to the interactions between H 2 molecules themselves in the gas phase.…”

Pyridyl‐Decorated Self‐Folding Heptaamide Cavitands as Ligands in the Rhodium‐Catalyzed Hydrogenation of Norbornadiene

“…We learned during the course of this study that the reaction conditions used to perform the hydrogenation experiments result in a slow diffusion of hydrogen30 into the reaction mixture. For this reason, and for the particular case of catalyst 3 + , we must conclude that the reactions producing the hydrogenated dimer 8 and non‐hydrogenated 10 become competitive.…”