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Investigation of Atropisomerism in ortho-Substituted Tetraphenylporphyrins: An Experimental Module Involving Synthesis, Chromatography, and NMR Spectroscopy

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Ruth Freitag Beeston , Shannon E. Stitzel and Mitchell A. Rhea

Davidson College, Department of Chemistry, P.O. Box 1749, Davidson, NC 28036

J. Chem. Educ., 1997, 74 (12), p 1468

DOI: 10.1021/ed074p1468

Publication Date (Web): December 1, 1997

Section:

History, Education, and Documentation

Abstract

An experimental module suitable for advanced undergraduates is described. This module begins with the synthesis of tetra(o-tolyl)porphyrin (T(o-Tol)P), which exists as a statistical distribution of atropisomers due to hindered rotation about the porphine-phenyl bonds. The six types of nonequivalent methyl protons in the four atropisomers (4,0; 3,1; cis 2,2; and trans 2,2) give rise to six methyl peaks in the proton NMR spectrum of T(o-Tol)P. Due to slight differences in the migration rates of the four atropisomers, samples of T(o-Tol)P which are enriched and depleted in certain atropisomers can be obtained chromatographically. NMR spectra of these samples are recorded, and an examination of the changes in relative intensities of the six methyl peaks leads to positive peak assignments. The farthest upfield methyl peak is attributed to the 4,0 isomer (in which all four methyl groups are cofacial), which has the fastest migration rate on silica gel. It is shown that as the number of cofacial methyl neighbors for a particular methyl group decreases, the chemical shift of the methyl protons increases. This experiment leads to a greater understanding of chromatography and NMR spectroscopy in addition to introducing students to porphyrin synthesis and the concepts of atropisomerism and statistical distributions.

Keywords (Audience):

Second-Year Undergraduate

Keywords (Domain):

Laboratory Instruction

Keywords (Pedagogy):

Hands-On Learning / Manipulatives

Keywords (Subject):

Instrumental Methods

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