What is syncrystallization? States of the pH indicator methyl red in crystals of phthalic acid

Journal of the American Chemical Society
Jason B BenedictBart Kahr

Abstract

The concept of syncrystallization was reinvestigated by focusing on phthalic acid (PA) grown with methyl red (MR). Crystals are alternately red and yellow in adjacent growth sectors. X-ray structures of MR and its cocrystals, revealing MR in the neutral, zwitterionic, and protonated states, as well as measurements of linear birefringence and linear dichroism of mixed crystals, were used to investigate mechanisms of PA coloring. These experiments were complemented by force field calculations of the lowest energy stable surfaces of expressed facets and energies of MR on and in crystals, as well as molecular orbital calculations of MR. Two MR species were detected in PA having distinct energies, polarizations, and face selectivities. Assignments of structures to these MRs, previously thought to be neutral and protonated, required a nuanced analysis of hydrogen bonds. The essential difference between yellow and red species is whether the MR carboxylic acid proton is inter- or intramolecularly hydrogen bound. Inferences about mixed crystal structure drawn from an examination of cocrystals of PA and MR are inconsistent with polarization spectroscopy signaling caution when using stoichiometric compounds as models of dilute solid solut...Continue Reading

References

Apr 1, 1990·Acta Crystallographica. Section B, Structural Science·M C EtterJ Bernstein
Dec 22, 1999·Angewandte Chemie·J BernsteinJ O Henck
Nov 17, 2001·Chemical Reviews·B Kahr, R W Gurney

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Citations

Mar 28, 2008·Chemical Communications : Chem Comm·Brian J JordanVincent M Rotello
Nov 20, 2016·Nature Communications·David C GreenFiona C Meldrum
Aug 2, 2013·Chemical Communications : Chem Comm·Ian M WaltonJason B Benedict
May 8, 2010·Journal of the American Chemical Society·Yonghong BingBart Kahr
Feb 26, 2010·Accounts of Chemical Research·Bart KahrErica Gunn
Nov 23, 2017·Chemical Reviews·Alexander G ShtukenbergBart Kahr

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