Infra-red and proton magnetic resonance spectra of solid substances containing very short hydrogen bonds

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According to the type of spectra obtained, the substances investigated are divided into two groups. The infra-red spectra of the first group are characterized by the presence of two OH stretching bands in the region 3000 to 1900 cm −1 separated by 300 to 500 cm −1. The PMR signals are strong and narrow (3–5 G) at room temperature and slightly broader at −180°. To this group belong some metal hydrogen phosphates, arsenates, periodates, phthalates and p-nitrobenzoates. The i.r. spectra of the second group are characterized by the absence of the OH stretching band in the region above 1800 cm −1. The PMR signals are narrow and weak or very weak at room temperature and remain essentially unchanged at low temperature. To this group belong potassium hydrogen bisphenylacetate, bisbenzoate, sodium sesquicarbonate and potassium hydrogen maleate. The latter compound shows a particularly weak signal. These experimental results are interpreted in terms of a potential function for the proton motion having two minima of potential energy separated by a barrier of different size in each of the two groups. In the first one, the barrier is sufficiently small to allow for the tunnelling of protons with subsequent splitting of vibrational levels. With regard to PMR such a function results in a short correlation time which is temperature independent at low temperature when the proton is tunnelling at the lowest vibrational level. In the second group the potential barrier is vanishingly small, the hydrogen bond approaching the symmetrical type. The OH stretching frequency becomes small and the characteristic property of the vibration is lost owing to strong vibrational interactions with other modes. In the limiting case the tunnelling motion ceases. As there is no more large proton motion the relaxation time becomes long: the signal is weak and saturation occurs even at low r.f. power. This extreme case seems to be approached by KH-maleate.