A rich literature exists that provides classroom and laboratory NMR instruction and praxis for every level of the undergraduate curriculum. Experiments have been devised to introduce the foundations of the technique, to solve structures using one- and two-dimensional methods, and to determine thermodynamic and kinetic parameters of chemical reactions. Our students have been challenged by the concept that there is one analyte in a sample for structure determination whereas samples for kinetic and thermodynamic measurements typically contain more than one analyte. Herein is presented an experiment designed to foster student understanding as they bridge the conceptual gap between the foundations of structure analysis (1 sample = 1 analyte) and the more complicated analyses required to evaluate spectra related to the determination of kinetic and thermodynamic parameters (one sample ≥ 1 analyte). This experiment was designed such that multiple, non-reacting analytes are present in one NMR sample tube. By selecting compounds with easily interpreted and non-overlapping signals, students may apply their knowledge of electronegativity and spin-spin splitting to unambiguously assign signals to the individual, pure compounds. Students carry those findings forward to the interpretation of spectra taken from samples containing varying amounts of the different analytes. After assigning peaks, students compare the normalized integrals for their assigned peaks to determine the composition of the mixtures. A dry-lab exercise coupled to the instrumental portion asks students to predict NMR spectra for the individual analytes. Thus, all students – those using the instrument and those waiting their turn – are productively occupied during the full laboratory period. In summary, students found this laboratory helpful in providing a conceptual foundation for the determination of equilibrium constants (keto-enol equilibrium in 1,3-dicarbonyl compounds; cis and trans equilibrium in N-acetyl-L-proline) and the evaluation of enzyme activity (the kinetics of hydrolysis of N-trifluoroacetylglycine).

Preparing students to interpret NMR data from kinetics and thermodynamics experiments

251st National Meeting (2016)

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