Carboxylate ions, Raman frequencies

The vibrational spectrum of 4-pyridine-carboxylic acid on alumina in Fig. 4d is equivalent to an infrared or Raman spectrum and can provide a great deal of information about the structure and bonding characteristics of the molecular layer on the oxide surface. For example, the absence of the characteristic > q mode at 1680 cm 1 and the presence of the symmetric and anti-symmetric O-C-O stretching frequencies at 1380 and 1550 cm indicate that 4-pyridine-carboxylic acid loses a proton and bonds to the aluminum oxide as a carboxylate ion. 

If the reacting molecules are already in contact, the rate is expected to be similar to that found for the recombination of H30+ and OH in ice, i.e. similar to the rate of vibration of a proton in a hydroxyl group. Under certain specialised conditions, the rates of recombination of H30 + and A in contact may be calculated from the width of lines in the Raman spectrum of a mixture of the acid and its conjugate base [15]. The trifluoracetate ion has a fairly narrow Raman peak at 1435 cm 1, attributed to the symmetrical carboxylate stretching frequency, which does not appear in acid solutions of trifluoracetic acid. Trifluoracetate— trifluoracetic acid buffers show a lower, wider peak than that for the trifluoracetate ion, and the width of this peak is inversely proportional to the time a proton spends on the carboxylate group. The variation of the width of the line with acid and anion concentrations gives a measure of the rate of transfer of a proton from an adjacent H30 + to the anion, and a rate coefficient of about 101 2 lmole 1 sec 1 has been calculated. 

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