| Abstract
| - The infrared (IR) and vibrational circular dichroism (VCD) spectra of S-2,2‘-dimethyl-biphenyl-6,6‘-dicarboxylic acid, S-1, in CDCl3 solution are concentration-dependent, showing that oligomerizationoccurs with increasing concentration. DFT calculations support the conclusion that the oligomer formed isthe cyclic tetramer (S-1)4, in which S-1 monomers are linked by hydrogen(H)-bonded (COOH)2 moieties.Due to the existence of two inequivalent tautomeric conformations of each (COOH)2 moiety, six inequivalentconformations of (S-1)4 are possible. B3LYP/6-31G* DFT calculations predict that the conformation “aaab”,possessing three equivalent (COOH)2 conformations, a, and one tautomeric conformation, b, has the lowestfree energy. B3LYP/6-31G* IR and VCD spectra vary substantially with conformation. The B3LYP/6-31G*IR and VCD spectra of the CO stretch modes of “aaab” are in excellent agreement with the experimentalspectra, while those of all other conformations exhibit poor agreement, confirming the prediction that the“aaab” conformation is the predominant conformation. Comparison of the calculated IR and VCD spectraof the six conformations to the experimental spectra in the range 1100−1600 cm-1 further supports thisconclusion. The study is the first to use VCD spectroscopy to determine the structure of a supramolecularspecies.
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