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H.T. Flakus, A. Michta / Journal of Molecular Structure 707 (2004) 17–31
Theoretical models developed to quantitatively interpret IR
spectra of hydrogen bonds, namely the so-called ‘strong-
coupling’ theory [6–8], the elder, as well as the ‘relaxation’
theory [9–12], the latest, see the source of these effects in
anharmonic couplings, involving the proton stretching
vibrations and the low-energy X· · ·Y bond stretching
vibrations. A unique dynamics of the hydrogen bridge
atoms is considered by the both theoretical models as
responsible for the basic spectral properties, not only for
single hydrogen bonds, but also for simple aggregates like
hydrogen bond dimers. Even for these latest systems,
characterized by a relatively low complexity degree, the
spectral behavior does not seem completely understood,
although their basic properties were intensively studied with
the help of many theoretical approaches [6,7,10–16].
However, the greatest challenge in this area is to deliver
adequate theories, to describe coupling mechanism in more
complex hydrogen bond systems than dimers, molecular
crystals included. From these models fine structure patterns
of the nX–H and nX–D bands in the crystalline spectra could
be calculated. Therefore, the experiment stimulates further
development in the theory and on the other hand, the theory
inspires seeking for new experimental methods, able to
deliver most reliable data for testing newly introduce
theoretical models.
only a spectral, but also of a thermodynamic nature. Some
of these effects were connected with breaking of vibrational
dipole selection rules in the spectra of centrosymmetric
hydrogen bond dimeric systems [17]. To the group of new
spectral effects also belonged diverse, non-traditional H/D
isotopic effects in IR spectra of the hydrogen bond in
crystals [18]. A part of the newly revealed effects were
considered as a manifestation of non-conventional, phe-
nomenological interactions, in the area of the hydrogen
bond thermodynamics. The so-called H/D isotopic ‘self-
organization’ effects, depend on a non-random distribution
of proton and deuterons between the hydrogen bridges, in
cyclic and in chain systems of associated molecules, in
mixture systems of hydrogen and deuterium bonds in
crystals [19–21]. Identification of these effects was possible
only when based on a quantitative analysis of the polarized
IR spectra of hydrogen bonded molecular crystals [20,21].
Although the basic principles of the H/D isotopic ‘self-
organization’ mechanism have been recognized only
recently for cyclic dimeric systems of hydrogen bonds
[22], the nature of these co-operative interactions between
hydrogen bonds needs further intensive studies, especially
for open chain hydrogen bond systems. These latter system
spectra not always exhibit effects, confirming a non-random
distribution of protons and deuterons in their hydrogen bond
lattices [20,21,23–25].
From this point of view it seems, however, that
traditional experimental methods of IR spectroscopy,
applied in the area of the hydrogen bond research, have
exhausted most of their potential abilities. This time, the IR
spectroscopy of hydrogen bonded molecular crystals in
polarized light, seems to be a most promising experimental
method for studying diverse aspects of inter-hydrogen bond
interactions. Measurements of the polarized IR spectra of
spatially oriented hydrogen bond systems, in the lattices of
molecular crystals, can deliver data, allowing estimating the
vibrational transition moment directions to the excited states
of the proton vibrations in the crystals, as well as the
symmetry of the proton vibration exciton states.
In this article, we present the results of our studies on the
hydrogen bond IR polarized spectra of imidazole crystals.
Although during the recent 30 years, this crystalline system
was several times the object of spectroscopic studies, as well
as of theoretical considerations [7,26–31], some basic
problems, concerning the band fine structure generation
mechanisms seemed to be still far from a satisfactory
solving. Imidazole crystal IR hydrogen bond spectrum is
characterized by a wide and a well-developed nN–H band
contour. In the spectra, an extremely strong H/D isotopic
effect can be observed, expressed by reduction of the nN–D
band fine structure pattern, practically to a single narrow
line [26–29]. So far no systematic studies of the polarized
IR spectra of imidazole crystals have been published.
Similar remark also concerns the polarized crystalline
spectra of the deuterium isotopomers of imidazole. There-
fore, at this stage a full understanding of the band structure
generation mechanisms for imidazole crystals seems to be
rather impossible.
The solid state, however, is responsible for a substantial
complication of the coupling mechanisms, involving the
hydrogen bond atom vibrations that implies a considerable
growth in difficulties, when interpreting the hydrogen bond
vibrational spectra of crystals, especially their fine structure
patterns. Therefore, elucidation of the nature of the inter-
and the intra-hydrogen bond interactions, based on the
polarized IR spectra, measured in the nX–H band range,
needs solving of a number of theoretical problems within
one of the quantitative theoretical models, elaborated for
interpretation of the hydrogen bond spectra. Unfortunately,
only extremely few researchers undertook this challenge.
Thus, investigations of polarized IR spectra of hydrogen
bonded molecular crystals, accompanied with an advanced
theoretical interpretation, were so far extremely rare.
Studies on polarized IR spectra of hydrogen bonded
crystals, performed in the recent 5 years, allowed revealing
a number of new and highly non-conventional effects, of not
The highly non-regular spectral effects in the crystalline
spectra of imidazole are still considered to be a serious
challenge for the theory. Solving these problems might be
crucial for deeper understanding of the nature of mutual
interactions in the hydrogen bond systems in the lattices of
molecular crystals. Therefore, it seemed justified to measure
reliable high quality, temperature dependent, polarized
spectra of imidazole crystals and also similar quality spectra
of the imidazole deuterium derivative crystals.
In the lattice of imidazole crystal hydrogen bonded
molecules form infinite open chains and in one chain