Polarization IR spectra of hydrogen bonded 1-naphthoic acid and 2-naphthoic acid crystals: electronic effects in the spectra

Abstract

This article deals with measurements and a theoretical interpretation of the polarization IR spectra of the hydrogen bond in h 8-1-naphthoic and h 8-2-naphthoic acid crystals and crystals of their deuterium isotopomers: d 7-1-naphthoic acid and d 7-2-naphthoic acid. Polarization spectra were measured at the room temperature and at 77 K. Similarly as for other carboxylic acid crystal cases, the intensity distribution in the bands may be discussed on the basis of the strong-coupling model, when assuming that the isolated (COOH) 2 and (COOD) 2 cycles determine basic spectral properties of hydrogen bonds in the crystals. Such approach appeared to be sufficient for explaining most of the isotopic and the temperature effects in the spectra. Another band shaping mechanism, i.e. a vibronic mechanism, promoting the symmetry forbidden transition in the IR for the totally symmetric proton stretching vibrations, in the centrosymmetric dimers of hydrogen bonds, was also considered. It was shown that for 1-naphthoic acid crystal spectra the promotion mechanism is ca. two times weaker than in the case of 2-naphthoic acid crystal. This relation was found independent of the hydrogen isotopes linked with the aromatic rings. A new kind of the long-range isotopic effects H/D in the spectra was indicated, depending on the influence of the aromatic ring hydrogen atoms in the ν O–H band fine structure patterns. The role of the aromatic ring electronic properties was also discussed, in order to explain reasons of extremely effective promotion of the forbidden transition, as well as of the Fermi resonance impact on the crystalline spectra.