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A study of the inelastic electron tunnelling spectra (IETS) of propynoic acid, propenoic acid and 3-methyl-but-2-enoic acid shows that these are chemisorbed from the vapour on to plasma-grown aluminium oxide as surface-bound carboxylate species. In the case of propynoic acid there is clear evidence for reaction of the triple bond at some of the adsorption sites. Similar considerations apply to the propenoic acid but not to the sterically more crowded 3-methyl-but-2-enoic acid. It is suggested that these surface reactions of the adsorbed carboxylate species may involve a Brønsted acid protonation of the π-electron systems present. Observed band intensities are held to be indicative of an adsorbate orientation to the surface which is close to the vertical.
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Brown et al. (1979) studied this question.
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