Conformational isomerism in squaraines: saturation transfer NMR studies on hydroxy squaraines

The proton and 13 C NMR of azulene and dialkylaminohydroxyphenyl squaraines have been investigated. The four-membered ring carbon resonances occur in the 180-190 ppm range and are attributed to the cyclobutenediylium character of this structure. Some charge delocalization from the central ring to th...

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Bibliographic Details
Published in:Canadian journal of chemistry 1990-04, Vol.68 (4), p.530-536
Main Authors: Kazmaier, Peter M, Hamer, Gordon K, Burt, Richard A
Format: Article
Language:English
Subjects:
Atomic and molecular physics
Exact sciences and technology
Molecular properties and interactions with photons
Nuclear resonance and relaxation
Physics
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Summary:The proton and 13 C NMR of azulene and dialkylaminohydroxyphenyl squaraines have been investigated. The four-membered ring carbon resonances occur in the 180-190 ppm range and are attributed to the cyclobutenediylium character of this structure. Some charge delocalization from the central ring to the pendent aromatic substituents was also observed, indicative of a substituent effect of the squaraine moiety on aminoaromatic systems equivalent to that of a formyl or acetyl group. Hydroxy squaraines 10 and 11 show the presence of two isomeric squaraine species in the proton NMR in deuterochloroform. Saturation transfer experiments confirm that these species are readily interconverted, and cis/trans isomerism is consistent with these observations. The free energy difference between the two isomers at 318 K is calculated to be 3.0 and 3.2 kJ/mol for hydroxysquaraines 10 and 11 respectively. Keywords: squaraines, saturation transfer, NMR.
ISSN:0008-4042
1480-3291
DOI:10.1139/v90-081