Investigations of Interhydrogen Bond Dynamical Coupling Effects in the Polarized IR Spectra of Acetanilide Crystals

Abstract

This Article presents the investigation results of the polarized IR spectra of the hydrogen bond in acetanilide (ACN) crystals measured in the frequency range of the proton and deuteron stretching vibration bands, nu(N-H) and nu(N-D). The basic spectral properties of the crystals were interpreted quantitatively in terms of the "strong-coupling" theory. The model of the centrosymmetric dimer of hydrogen bonds postulated by us facilitated the explanation of the well-developed, two-branch structure of the nu(N-H) and nu(N-D) bands as well as the isotopic dilution effects in the spectra. On the basis of the linear dichroic and temperature effects in the polarized IR spectra of ACN crystals, the H/D isotopic "self-organization" effects were revealed. A nonrandom distribution of hydrogen isotope atoms (H or D) in the lattice was deduced from the spectra of isotopically diluted ACN crystals. It was also determined that identical hydrogen isotope atoms occupy both hydrogen bonds in the dimeric systems, where each hydrogen bond belongs to a different chain. A more complex fine structure pattern of nu(N-H) and nu(N-D) bands in ACN spectra in comparison with the spectra of other secondary amides (e.g., N-methylacetamide) can be explained in terms of the "relaxation" theory of the IR spectra of hydrogen-bonded systems.

Elucidating the Davydov-coupling mechanism in hydrogen bond dimers: Experimental and theoretical investigation of the polarized IR spectra of 3-thiopheneacetic and 3-thiopheneacrylic acid crystals.

“…On the other hand, the majority of crystals with hydrogenbonded molecular chains in their lattices, surprisingly exhibit the spectral properties similar to the analogous properties of cyclic hydrogen bond dimer spectra from the "a" and "c" group, (e.g., acetic acid [44], N-methylthioacetamide [45] or acetanilide [14] crystals). In the latest case the exciton interactions of the "side-toside" (SS)-type involve the closely-spaced hydrogen bonds where each moiety belongs to a different chain.…”

“…Regardless of the increase in the rates of deuterium substitution in the samples, the "residual" υ O-H band still retained its "dimeric" character. This was due to the fact that the hydrogen bonded dimeric spectrum measured in the "residual" υ O-H band range were still under the influence of the inter-hydrogen bond vibrational exciton interactions occurring intra each individual carboxylic acid dimer [12][13][14]. The unusual properties of the "residual" υ O-H bands have proved that the distribution of protons and deuterons between the hydrogen bonds of the isotopically diluted crystalline samples was non-random and in an individual dimer the co-existence of two identical hydrogen isotope atoms, proton or deuterons, was preferred.…”

“…These spectral effects, i.e. the so-called H/D isotopic "self-organization" effects, are the attribute of the "dynamical co-operative interactions" involving hydrogen bonds in the dimers [12][13][14].…”

“…This property results from the "dynamical co-operative interactions" in the hydrogen bonded systems which lead to the appearance of the so-called H/D isotopic "self-organization" effects in the hydrogen bond IR spectra [12][13][14]. The source of these nonconventional interactions in the hydrogen bond dimers is a vibronic coupling mechanism involving the totally-symmetric proton stretching vibrations and the electronic motions in the systems [12][13][14]. According to the theory of the "dynamical co-operative interactions", the symmetric hydrogen bond dimers of the HH or DD-type, with identical hydrogen isotope atoms, are thermodynamically more stable than the non-symmetric dimers of the HD-type.…”

“…They depend on a non-random distribution of protons and deuterons in the crystal lattices of isotopically diluted hydrogen bond systems. These spectral effects may be considered as the manifestation of a Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jpcs new kind of co-operative interactions, common in nature, i.e., the so-called dynamical co-operative interactions involving hydrogen bonds [12][13][14]. This revealing has emphasized the role of the vibronic coupling between the electronic and the proton vibrational motions taking place in H-bond aggregates, in the generation of the very nature of the H-bond as the natural phenomenon and in the inter-H-bond interaction mechanisms.…”