An Organic Chemistry Honors Option Focus on 13C Chemical Shift Studies for Substituted Benzenes and 4-Substitued Pyridines: A Historical and Retrospective View of the Data

The results of a first-year organic chemistry study of 1H and 13C chemical shifts for substituted benzenes and 4-substiuted pyridines is given from a historical perspective. The study examines data from the early days of NMR spectroscopy when tetramethylsilane (TMS) was not the standard by which chemical shifts were measured for all nuclei, and samples from a sequence of compounds were often run in different solvents. In the 1960s some studies involved the NMR spectra for compounds in the same series being run in different solvents, when it was already known that solvent affected chemical shifts. The trend in chemical shifts at the C1 carbon in monosubstituted benzenes has been shown to be in concert with the C4 carbon in a like-substituted set of 4-substituted pyridines. However, further review of the correlations indicates a marked improvement when data is used where all the samples are run in CDCl3 as solvent and referenced using TMS, which is the norm for most NMR spectral acquisitions today. Advances in spectral acquisition also allow for investigating the 15N chemical shift variations with substituents in the substituted pyridines. The revisited correlations have been utilized to predict the 13C and 15N chemical shifts for the less favored tautomer of 4-pyridinone; 4-hydroxypyridine.