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A study of hydrogen-bond dynamics in carboxylic acids by NMR T1 measurements: isotope effects and hydrogen-bond length dependence

✍ Scribed by T. Agaki; F. Imashiro; T. Terao; N. Hirota; S. Hayashi

Publisher: Elsevier Science
Year: 1987
Tongue: English
Weight: 516 KB
Volume: 139
Category: Article
ISSN: 0009-2614
DOI: 10.1016/0009-2614(87)80567-5

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✦ Synopsis

Proton (deuteron) transfer ofhydrogen bonds in benzoic, glutark and pformylbenzoic acids was studied by proton (deuteron) T, measurements. Deuteration of carboxylic protons was found to increase the barriers to classical proton jumping as well as quantum-mechanical tunneling. The former barriers increase as the hydrogen-bond distance increases. NMR spectroscopy has shown that the two carboxylic protons in some carboxylic acid dimers undergo a simultaneous classical proton jump and quantum-mechanical tunneling along the hydrogen bonds [l-6]. The simultaneous proton transfer interconverts two different configurations A and B. R-C Qo--HO, C-R = R-C ,OH--0% ~O"__O~ C-R *0_+0 A B Whereas the two configurations are energetically degenerate in the free dimers, the degeneracy is generally removed in the solid state by distortion of the double-minimum intermolecular potential due to intermolecular interactions, producing a small enthalpy difference ALI between con~gurations A and B, as shown in fig. 1. The dynamics in this interesting system are studied mainly by proton T, meas-

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