The use of NMR in the curriculum need not be limited to the organic chemistry laboratory. There are many ways that NMR can also be used in exercises designed for upper level courses since NMR of neat liquids, aqueous solutions (using water) and vapor molecules can be obtained using standard double methods. For example, the chemical shifts for aqueous solutions of acetic acid change as a function of concentration since the amount of associated species in the solutions decrease as they becomes more dilute. The plot of chemical shift verses concentration can be fit using two linear relationships that intersect at approximately 12M. Since vinegar is approximately 5% acetic acid, the linear plot in the lower concentration range could be used for an instrumental analysis laboratory to determine the concentration of acetic acid in vinegar. The 13C chemical shift for oxalic acid is also a function of concentration since it results from a weighted average of the chemical shifts from oxalic acid and the hydrogen oxalate ion. Measuring the chemical shift as a function of concentration was used as a laboratory exercise to determine the first acid dissociation constant, Ka1. IR and NMR coupled with DFT calculations have also be used to determine of the structure of the C2O42- ion in aqueous solution. The 13C NMR neat liquid CHCl3 and CDCl3 was used to answer the question, “Does isotope substitution affect the NMR?” as part of a discovery exercise on the effect of isotope substitution on molecules.

NMR in the instrumental analysis or physical chemistry laboratories

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.