Assessment of the Association Constant of the CO₂@CB[6] Complex Combining ¹H and ¹³C NMR Spectroscopic Titrations

A methodology for assessing the association constants of a molecular host binding a ¹³C-enriched gaseous guest, that is, ¹³CO₂, using NMR spectroscopic titrations is described. The method relies on injecting the gaseous guest into the headspace of a gas-tight NMR tube filled with the host's solution. The fraction of the gas dissolved in the liquid solution is quantified via ¹³C NMR spectroscopy using the Electronic Reference To Access In Vivo Concentrations technique without requiring assumptions based on Henry's law. By subsequently recording the ¹H NMR spectrum of the solution mixture, the relationship between the chemical shift changes experienced by the proton signals of the host and the precise concentration of the added gaseous guest can be established. Repeating the process over a range of incremental gas concentrations allows for generating the binding isotherm curve. The nonlinear computer fit of the data to a suitable theoretical binding model returns the values of the association constant and the complexation-induced shift for the protons in the complex. The method is applied to the assessment of the association constant values of the complexes formed from the interaction of ¹³CO₂ with two macrocyclic receptors.