Polarization IR spectra of the hydrogen bond in phenylacetic acid crystals: H/D isotopic effects - Temperature and polarization effects

Article abstract of DOI:10.1016/S1386-1425(01)00642-4

This paper deals with experimental studies and with quantitative interpretation of the polarized IR crystalline spectra of phenylacetic acid and of its deuterium isotopomers d₂ and d₇. The spectra were measured in the ν_O-H

Full text of DOI:10.1016/S1386-1425(01)00642-4

Spectrochimica Acta Part A 58 (2002) 1867–1880
www.elsevier.com/locate/saa
Polarization IR spectra of the hydrogen bond in
phenylacetic acid crystals: H/D isotopic effects
Temperature and polarization effects
Henryk T. Flakus *, Michał Chełmecki
Institute of Chemistry, Uni6ersity of Silesia, 9 Szkolna, Pl 40-006 Katowice, Poland
Received 7 August 2001; accepted 17 September 2001
Abstract
This paper deals with experimental studies and with quantitative interpretation of the polarized IR crystalline
spectra of phenylacetic acid and of its deuterium isotopomers d2 and d7. The spectra were measured in the wO–H and
in the wO–D band frequency ranges at temperatures of 298 and 77 K. The intensity distribution in the bands was
quantitatively reproduced on the basis of the strong-coupling model, when assuming that the isolated (COOH)2 and
(COOD)2 cycles were the source of the spectral properties of the crystals. Such approach appeared to be sufficient for
explaining most of the isotopic and the temperature effects in the spectra. A vibronic mechanism, promoting the
symmetry forbidden transition in the IR for the totally symmetric proton stretching vibrations in centrosymmetric
hydrogen bond dimers, was found to be of a considerably minor importance, when compared with analogous
properties of arylcarboxylic acid crystals. The spectra of phenylacetic acid crystals, unlike the spectra of arylacetic
acid crystals do not exhibit the so-called H/D long-range isotopic effects, depending on an influence of the aromatic
ring hydrogen atoms on the wO–H band fine structure patterns. Also no Fermi resonance impact on the wO–H band
shape was identified in the phenylacetic acid crystal spectra. These effects were ascribed to weakening of electronic
couplings between the hydrogen bonds and the phenyl rings, due to the separation of these groups in phenylacetic
acid molecules by methylene groups. © 2002 Published by Elsevier Science B.V.
Keywords: Hydrogen bond; Molecular crystals; Polarization IR spectra; Band shape analysis; Model calculations; Strong-coupling
model; Linear dichroism effects; Temperature effects; Fermi resonance; Long-range H/D isotopic effects
1. Introduction
vibration bands wO–H as a purely vibrational one
[1–12]. However, in the recent years, a number of
experimental facts have stimulated us to verify
such an approach, pointing out its inadequacy in
description of the spectral properties of even so
simple hydrogen bond systems like cyclic, cen-
trosymmetric dimers. Carboxylic acids, with their
associated carboxyl groups used to be treated by
Most of the contemporary theories of infrared
spectra of hydrogen bonded systems have treated
the problem of generation of the proton stretching
* Corresponding author. Fax: +48-32-2599-978.
E-mail address: flakus@tc3.ich.us.edu.pl (H.T. Flakus).
1386-1425/02/$ - see front matter © 2002 Published by Elsevier Science B.V.
PII: S1386-1425(01)00642-4

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H.T. Flakus, M. Chełmecki / Spectrochimica Acta Part A 58 (2002) 1867–1880
Electronic effects were recently identified in the
spectra of the hydrogen bonded aromatic car-
boxylic acid crystals i.e. of the benzoic acid crystal
[19] and of the crystals of naphthoic acid 1 and 2
[20]. In these cases couplings, between the hydro-
gen bonds of associated carboxyl groups
(COOH)2 with aromatic rings were considered as
a hypothetical cause of a number of characteristic
spectral effects, that have no counterpart in spec-
tra of aliphatic carboxylic acid crystals. To these
effects belong for instance: (i) an increase of the
longer-wave branch intensity of the wO–H band,
(ii) Fermi resonance effects deforming the fine
wO–H band structures and (iii) the so-called H/D
long-range isotopic effects in the spectra [21].
Verification of the hypothesis concerning an
influence of aromatic rings on the spectra of the
hydrogen bond may be done when based on
measurements and a quantitative analysis of the
IR spectra of carboxylic acid crystals, belonging
to the series of the arylacetic acid crystals. In the
case of these molecular systems, aromatic rings
are being separated from the carboxyl groups by
methylene groups. This separation should prevent
an effective coupling between the proton vibra-
tions in the hydrogen bonds, with electronic mo-
tions in aromatic rings. Therefore, this fact should
find its reflection in spectral properties of ary-
lacetic acid crystals and in appearance of some
noticeable differences, when compared with spec-
tral properties of arylcarboxylic acid crystals.
This paper has been devoted to spectral investi-
gations of the IR spectra of the hydrogen bond in
crystals of phenylacetic acid and of its deuterium
isotopomers d2 and d7. We expect that these
studies should at least partially explain the role of
aromatic rings in modification of the spectral
properties of the hydrogen bonds from the associ-
ated carboxyl groups, coupled with them.
many researchers as model dimers of hydrogen
bonds. Spectral properties of carboxylic acid
dimers have provided a severe test for the subse-
quently developed theoretical models [2,4–6,10].
Recent experimental studies of polarized spec-
tra of hydrogen bonded crystals indicated a neces-
sity
of
taking
into
the
account
of
vibrational–electronic interactions in hydrogen
bonded dimeric systems, in theoretical models of
their spectral properties in the IR. Such approach
to vibrational spectra is not quite new, as a most
adequate model of the IR spectra generation
mechanism is a vibronic one [13–15]. In the limits
of the vibronic model, vibrational spectra occur
within the ground electronic state, between the
vibronic levels of a hydrogen bond system, per-
turbed by the normal vibrations of a molecule.
Some investigation results of the recent years have
proved that a number of spectral effects concern-
ing hydrogen bonded systems can be explained
only in this particular case, when including vi-
bronic interactions in the theoretical models.
For hydrogen bond systems, the magnitude of
the vibronic effects in the spectra seem to be
extremely large. To this particular kind of effects,
ascribed to vibronic interactions, belongs the vi-
brational dipole selection rule breaking effects in
the IR, for centrosymmetric hydrogen bond
dimers [16]. These effects are connected with acti-
vation of vibrational transitions in the IR for the
totally symmetric proton vibrations of the Ag
–
symmetry, in centrosymmetric dimeric systems of
hydrogen bonds.
It results from the recent investigations, that
electronic properties of the proton-acceptor atoms
in hydrogen bridges considerably influence spec-
tral properties of hydrogen bonded systems. Easy-
polarizable sulfur atoms particularly strongly
influence spectral properties of centrosymmetric
dimers linked by N–H…S bonds, being responsi-
ble for the wN–H band forbidden branch intensity
[16–18]. Some suggestions have also appeared,
that aromatic rings with their easy-polarizable
electrons on the p-orbitals, seem to strongly effect
the wX–H band shapes. Also the vibrational dipole
selection rules governing generation of the spectra
seem to be influenced by aromatic rings [19,20].
2. Experimental
2.1. Synthesis of deuterium isotopomers of
phenylacetic acid
Commercial
phenylacetic
acid
C6H5ꢀCH2ꢀCOOH (Sigma-Aldrich) was the start-

H.T. Flakus, M. Chełmecki / Spectrochimica Acta Part A 58 (2002) 1867–1880
1869
ing substance for synthesis of its deuterium
isotopomers.
were selected from a polycrystalline mosaic and
then spatially oriented with the help of a polariza-
tion microscope. From these fragments small ar-
eas were selected by using a diaphragm with a
hole diameter 1.5 mm and then polarized spectra
of these crystalline fragments were recorded by a
transmission method. Measurements were accom-
plished at room temperature and also at the tem-
perature of liquid nitrogen, for the IR beam of
normal incidence with respect to the crystalline bc
plane. The polarized spectra were measured for
two orientations of the incident beam electric field
vector E: parallel to the b axis and also parallel to
the c* axis (c* denotes a vector in the reciprocal
lattice).
The IR spectra were recorded with the Nicolet
Magna 560 FT-IR spectrometer. Measurements
of the spectra were repeated for a number of
crystals of each isotopomer.
The Raman spectra were measured using the
Raman Accessory with the Magna 560 FT-IR
spectrometer. The excitation laser worked in the
near infrared range (at :9600 cm−1).
The d2 – derivative i.e. C6H5ꢀCD2ꢀCOOH, was
obtained by heating during 3 h of a sealed glass
ampoule, containing common phenylacetic acid,
dissolved in D2O, in 200 °C. After repeating of
the process by three times the deuterium substitu-
tion level for the methylene groups was approxi-
mately equal to 98%. Based on Raman spectra
measured in the wC–H and wC–D band frequency
ranges, we found that in these experimental condi-
tions deuterium atoms do not replace hydrogen
atoms of the phenyl rings.
The d7 – derivative, i.e. C6D5ꢀCD2ꢀCOOH,
was synthesized by heating in 100 °C, of a sealed
glass ampoule, containing common phenylacetic
acid dissolved in D2O, in the presence of the
deuterated Raney alloy [22,23]. In these condi-
tions, hydrogen atoms of the phenyl rings and of
the methylene groups were replaced by deuterium
atoms. After repetition of the process by three
times, the deuterium substitution level was esti-
mated as being close to 97%. These deuterium
substitution rates were estimated based on a
quantitative analysis of Raman spectra of the
samples, measured for the wC–H and wC–D fre-
quency ranges.
3. Crystal structure of phenylacetic acid
The deuterium-bonded isotopomers of pheny-
lacetic acid were obtained by evaporation under
reduced pressure at room temperature, of D2O
solution of each compound.
The crystalline structure of phenylacetic acid
was determined recently by Hodgson and As-
plund [24]; it crystallizes in space group P21/a=
C5 with Z=4 and cell constants a=10.201(5)
2h
,
,
,
A, b=4.9568(14) A, c=14.437(10) A and i=
99.17(5)°. The molecules of phenylacetic acid are
linked by O–H…O hydrogen bonds forming cen-
trosymmetric dimers. The O…O distance is
2.2. Measurement of the phenylacetic acid crystal
spectra
,
,
Phenylacetic acid and its deuterium isoto-
pomers d2 and d7 were used without further
purification. Crystals suitable for further studies
were obtained by melting solid samples between
two CaF2 windows, followed by a very slow cool-
ing of the liquid film, while the windows were
pressed tightly. In these conditions, phenylacetic
acid crystals develop the (1 0 0) face (i.e. the bc
plane). This fact was confirmed by X-ray diffrac-
tion. Very thin crystalline plates, which were ob-
tained, were seen grey between crossed polarizers
of a polarization microscope. Monocrystalline
fragments, having dimensions at least 2×2 mm2,
2.679(3) A, H…O 1.61(3) A and the O–H…O
angle is 174 (2)°. When crystallizing from melt,
the bc crystalline face is usually being developed.
Melting point is equal to 78 °C.
4. Results
The spectra of the phenylacetic acid samples in
the CCl4 solution are shown in Fig. 1.
In Fig. 2 are shown the spectra of the polycrys-
talline samples of phenylacetic acid in KBr pellets,
measured at two different temperatures. One can

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Fig. 1. The wO–H band in the IR spectra of phenylacetic acid in the CCl4 solution.
Fig. 2. The wO–H band in the IR spectra of polycrystalline samples of phenylacetic acid dispersed in KBr pellets: The Raman spectra
indicate the impact of the wC–H bands on the spectrum.

H.T. Flakus, M. Chełmecki / Spectrochimica Acta Part A 58 (2002) 1867–1880
1871
Fig. 3. The polarization spectra of the phenylacetic acid crystals measured at the room temperature for the wO–H band frequency
ranges. The IR beam of the normal incidence with respect to the bc was used. The component spectra were obtained for the two
orientations of the electric field vector E: (I) E parallel to the c axis; (II) E perpendicular to the c axis (parallel to the b axis); and
(III) Spectrum (II) re-normalized to full scale with the spectrum (I). Spectra (I) and (II) were drawn in a common scale.
Fig. 4. The polarization spectra of the phenylacetic acid crystals measured at 77 K at the frequency ranges of the wO–H bands. Other
experimental conditions and the way of presentation of the spectra were identical with those given for Fig. 3.
find a considerably strong temperature induced
evolution of the wO–H and wO–D band contour
shapes. To identify the wC–H bands, and to esti-
mate the influence of these peaks on the spectra of
the crystalline wO–H band, Raman spectra of the
polycrystalline sample of phenylacetic acid were
also measured.
In Fig. 3, the polarization spectra of the pheny-

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H.T. Flakus, M. Chełmecki / Spectrochimica Acta Part A 58 (2002) 1867–1880
lacetic acid crystals are presented, measured at the
room temperature and in Fig. 4 one can see the
spectra measured at 77 K.
In Fig. 5, the polarization spectra of the deu-
terium bonded phenylacetic acid crystals are
shown for the frequency range of the wO–D band
and Fig. 6 presents the corresponding low-temper-
ature spectra.
In Fig. 7, the low-temperature polarization
spectra of phenylacetic acid are shown, measured
Fig. 5. The polarization spectra of the phenylacetic acid crystals at the frequency ranges of the wO–D bands measured at 298 K. Other
experimental conditions and the way of presentation of the spectra were identical with those given for Fig. 3 and Fig. 4.
Fig. 6. The low temperature polarization spectra of the phenylacetic acid crystals, measured at the frequency ranges of the wO–D
bands. Other experimental conditions and the way of presentation of the spectra were identical with those given for Fig. 3 and Fig.
4.

H.T. Flakus, M. Chełmecki / Spectrochimica Acta Part A 58 (2002) 1867–1880
1873
Fig. 7. The residual wO–H bands from the low-temperature polarization spectra measured for phenylacetic acid crystals in the isotopic
dilution. The other experimental conditions and presentation of the spectra were identical with those for Fig. 3 and Fig. 4.
in the frequency range of the residual wO–H bands,
for highly deuterated samples (:90% of the D-
atoms in carboxyl groups). These studies aimed to
find any changes in the wO–H band shapes, accom-
panying to the isotopic dilution of hydrogen
bonds by deuterium ones. Such changes would be
ascribed to vanishing of Da6ydow couplings be-
tween translationally non-equivalent hydrogen
bonds in each unit cell.
5.1. Model calculations of wO–H and wO–D band
shapes
Model calculations of the wO–H and wO–D band
contour shapes were performed in the frames of
the strong-coupling model, assuming that a cen-
trosymmetric hydrogen bond dimer in a (COOH)2
cycle is a basic source of the spectral properties of
the crystal. The method of calculations of spectra,
proposed in recent papers has been utilized
[5,6,17]. The calculation results are shown in Fig.
8 and Fig. 9.
5. Discussion
These results show that when based on a rela-
tively simple, quantitative theoretical model, as-
suming a strong anharmonic coupling between
different-frequency normal vibrations of the
dimer hydrogen bonds, it was possible to satisfac-
torily enough reproduce the analyzed band
shapes. Comparison of the calculation results with
the experiment (Fig. 4 and Fig. 7) confirms not
only the dimeric model adequacy for description
of crystal spectral properties, but also confirms
existence of an H/D isotopic effects i.e. the iso-
tope self-organization effects for centrosymmetric,
cyclic hydrogen bond dimers: In the spectra of
It is easy to notice, that the measured spectra of
phenylacetic acid crystals apparently differ from
spectra of arylcarboxylic acid crystals, e.g. of the
benzoic acid and the crystals of naphthoic acid 1
and 2 [19,20]. A more complete discussion of the
phenylacetic acid crystal have to be performed,
when based on a model calculations, in frames of
one of the quantitative theories of hydrogen bond
system IR spectra [5–9]. The model calculations
of wO–H and wO–D band contours from the crys-
talline spectra can be done, when based on the
so-called strong-coupling theory [5,6,17].

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H.T. Flakus, M. Chełmecki / Spectrochimica Acta Part A 58 (2002) 1867–1880
samples, characterized by even high deuterium
content, spectral properties of the wO–H residual
bands are determined by hydrogen bond dimers,
containing identical hydrogen isotope atoms [25].
From our model calculations some conclusions
can be derived, being crucial for estimation of
differences between spectral properties of pheny-
lacetic acid crystals and the corresponding proper-
ties of other aromatic acid crystals. It has
appeared that the wO–H band, similarly as the
wO–D band, is a superposition of two component
bands of a different origin. Very similar calcula-
tion results were obtained in the case of interpret-
ing the spectra of carboxylic acid crystals:
aliphatic [26,27] as well as aromatic ones [19,20].
The shorter-wave branch of each band can be
reproduced by the so-called plus band, corre-
sponding to the dipole allowed transition to the
excited state of the non-totally stretching proton
vibrations, in a centrosymmetric hydrogen bond
dimer. In turn the longer-wave branch of each
band wO–H and wO–D is reproduced by the minus
band, ascribed to the symmetry-forbidden excita-
tion of the totally symmetric proton vibrations in
the centrosymmetric dimer of hydrogen bonds.
This forbidden transition becomes activated due
to a vibronic promotion mechanism and thus this
transition appears in the spectra [16].
Similarly, as in the case of the spectra of
aliphatic carboxylic acid crystals [26,27], a com-
plex structure of the wO–H and the wO–D bands is
considered to be the source of temperature effects
in the IR hydrogen bond spectra. These effects
basically depend on changes in the relative inten-
sity of the longer-wave, forbidden branch of each
band, with respect to the shorter-wave, allowed
transition branch, namely on an increase of the
forbidden branch intensity in the spectra [26,27].
Fig. 8. Numerical reconstitution within the limits of the strong-coupling model, for the dimer approximation of the low-temperature
wO–H band shapes, from the phenylacetic acid crystalline spectra: (a) The Plus band corresponding to the symmetry-allowed
transition; (b) The activated symmetry-forbidden Minus band; and (c) The superposition of the Plus and the Minus band,
reconstituting the wO–H band shape with E parallel to the c axis. The coupling parameter values: bH=1.5, C0=0.9, and C1= −0.5.
Transition energies are calculated with respect to the gravity center of the hypothetical spectrum of the single O–H…O bond from
the phenylacetic acid dimers. Coupling parameters and transition energies are in the wO…O vibration quantum units. Intensities of
transitions are in arbitrary units. On the right-upper edge of each picture is the respective wO–H experimental band contour.

H.T. Flakus, M. Chełmecki / Spectrochimica Acta Part A 58 (2002) 1867–1880
1875
Fig. 9. Numerical reconstitution within the limits of the strong-coupling model, of the low-temperature wO–D band shapes, from the
phenylacetic acid crystalline spectra: (a) The Plus band corresponding to the symmetry-allowed transition; (b) The activated
symmetry-forbidden Minus band; (c) The superposition of the Plus and the Minus band, reconstituting the wO–D band shape with
E parallel to the c axis. The coupling parameter values: bD=0.4, C0=0.6, and C1= −0.3. Transition energies are calculated with
respect to the center of gravity of the hypothetical spectrum of the single O–D…O bond from the phenylacetic acid dimers.
Coupling parameters and transition energies are in the wO…O vibration quantum units. Intensities of transitions are in arbitrary units.
On the right-upper edge of the picture is the respective wO–D experimental band contour.
Temperature effects found in the spectra of the
phenylacetic acid crystals were identified as rela-
tively small (Figs. 2–4). Most probably, it is
connected with a relatively low integral intensity
of the lower-frequency transitions contributing in
the wO–H and the wO–D bands.
The complexity of the wO–H and the wO–D band
generation nature is also responsible for some
polarization effects, expressed by slight differences
in the vibrational transition moment direction,
characterizing individually the two spectral
branches in the spectra of carboxylic acid crystals
(second kind polarization effects) [26,29]. As the
forbidden transition participates in the two band
forming mechanism with a relatively low statisti-
cal weight, these polarization effects are negligibly
small (Fig. 4 and Fig. 6) and the polarization
effects in the spectra are basically determined by
orientation of O–H bonds in the crystal lattice.
5.2. Nature of the spectral properties of the
phenylacetic acid crystal
It results from our model calculations that the
integral intensity of the lower-frequency, forbid-
den component in the spectra of phenylacetic acid
crystals is relatively low, when compared with the
analogous value, estimated from the spectra of the
benzoic acid crystal [19], or of the 2-naphthoic
acid crystal [20]. With this regard the phenylacetic
acid crystal spectra are fairly similar to the spectra
of aliphatic carboxylic acid crystals [26–29]. For
phenylacetic acid spectra the intensity ratio of the
wO–H component bands Minus:Plus is approxi-
mately equal to 1: 4. A similar relation concerning
the component band intensities of wO–H and wO–D
bands is also valid for spectra of aliphatic acid
crystals. In all these cases the minus band contri-
bution in these spectra is of a statistical weight

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H.T. Flakus, M. Chełmecki / Spectrochimica Acta Part A 58 (2002) 1867–1880
being 2–4 times lower in comparison with the plus
band contribution.
the longer-wave branch of the wO–H band in the
phenylacetic acid crystal spectra. In this case, in
the promotion mechanism, only the hard-core
electrons of the O–H…O bond forming atoms
seem to be active. It is very similar, as in the case
of aliphatic carboxylic acid crystals, which might
explain similarity of the spectra of system com-
pared, namely of the phenylacetic acid crystal
spectra and of aliphatic carboxylic acid crystals
[26–29].
In the case of the benzoic acid [19] and the
2-naphthoic acid crystal spectra [20], the intensity
ratio of the longer- and the shorter-wave intensi-
ties, and simultaneously of the statistical weights
of the minus and plus component bands, reconsti-
tuting these spectra is quite different. For these
arylcarboxylic acid crystal spectra the wO–H band
longer-wave branch is just more intense one when
compared with the shorter-wave branch proper-
ties. It also must be added that by their proper-
ties, the spectra of the 1-naphthoic acid crystal
fairly resembled the spectra of aliphatic carboxylic
acid crystals [26–29]. This fact seems to indicate a
contribution of electronic interactions in the hy-
drogen-bonded molecules in generation mecha-
nisms of the wO-H band fine structures.
The observed differences in spectral properties
of phenylacetic acid crystal, when compared with
benzoic acid crystal [19] and 2-naphthoic acid
crystals [20] finds their explanation in a vibronic
theory of the symmetry forbidden vibrational
transitions in the IR, for the proton totally sym-
metric vibrations in centrosymmetric dimers [16].
Activation of the forbidden transitions, generating
the longer-wave branch of a wO–H band in a
dimeric spectra, is connected with a vibronic cou-
pling mechanism. In this mechanism, a key role is
played by polarizability by vibrating proton of the
electronic cloud in a hydrogen bond and its clos-
est neighborhood. In this effect, polarizability of
aromatic rings in aromatic carboxylic acid
molecules seems to contribute with a magnitude
comparable with the hydrogen bond correspond-
ing contribution. Another interactions co-respon-
sible for the promotion mechanism are the proton
vibration anharmonicity and also a resonance in-
teraction of vibrationally excited hydrogen bonds
in a dimer [16]. These latest two factors in the
promotion mechanism seem to be independent
from couplings between the proton vibrations
with electronic motions of the aromatic rings.
Separation of the carboxyl groups from the
phenyl rings by methylene groups in phenylacetic
acid molecules is responsible for an evident
diminution of the forbidden transition effect mag-
nitude. In such a way, a relatively low intensity of
6. Do the long-range H/D isotopic effects exist in
the phenylacetic acid crystal spectra?
Only recently, in the IR spectra of benzoic acid
crystals some new kind of H/D isotopic effects
have been found, depending on the influence of
the aromatic ring hydrogen atoms on the wO–H
band fine structure pattern [19]. These effects
depended on much dense fine structure pattern for
the band in the case of d5-benzoic acid (i.e.
C6D5COOH) crystals, which corresponds with a
:1.5 times lower magnitude of the wO…O low-en-
ergy hydrogen bond stretching vibration quan-
tum. This effect was not connected with a
substantial change in the general shape of the
wO–H band envelope. Very similar H/D isotopic
effects, in the spectra of the hydrogen bond, have
been also found for the 1- and 2- naphthoic acid
crystals [20]. Such effects still remain obscure,
having no explanation within purely vibrational
models e.g. via the wO…O vibration reduced mass
changes due to deuteration of the aromatic rings.
These unorthodox H/D isotopic effects i.e. the
long-range isotopic effects have been identified
only recently [[19,21,30]]. These differ substan-
tially from the most familiar H/D isotopic effects
concerning IR spectra of the hydrogen bond,
corresponding with the replacement of the pro-
tons by deuterons in hydrogen bonds [1–6].
It seemed to be interesting to investigate if
similar effects appear in the IR spectra of hydro-
gen bonded crystals of d2 and d7 deuterium isoto-
pomers of phenylacetic acid. In Fig. 10 and Fig.
11 spectra are presented of d2 (C6H5ꢀCD2
ꢀCOOH) and d7 (C6D5ꢀCD2ꢀCOOH) pheny-

H.T. Flakus, M. Chełmecki / Spectrochimica Acta Part A 58 (2002) 1867–1880
1877
Fig. 10. The polarization spectra of the d2-phenylacetic acid (C6H5ꢀCD2ꢀCOOH) crystals measured at 77 K at the frequency ranges
of the wO–H bands. Other experimental conditions and the way of presentation of the spectra were identical with those given for Fig.
3.
Fig. 11. The polarization spectra of the d7-phenylacetic acid (C6D5ꢀCD2ꢀCOOH) crystals measured at 77 K at the frequency ranges
of the wO–H bands. Other experimental conditions and the way of presentation of the spectra were identical with those given for Fig.
3.

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H.T. Flakus, M. Chełmecki / Spectrochimica Acta Part A 58 (2002) 1867–1880
7. Does the Fermi resonance influence the §O–H
bands?
lacetic acid isotopomer crystals, measured for
the wO–H band frequency range.
It is easy to notice that these spectra do not
considerably differ from the crystalline spectra
of the common compound (Fig. 4). The deu-
terium substitution of deuterium in the methyl-
ene groups and also in the phenyl rings do not
change the relative intensity of the longer- to the
shorter-wave branch of the wO–H band. The
changes in wO–H band shapes are related with
the disappearance of a number of lines at-
tributed to wC–H normal vibrations of phenyl
rings and methylene groups. Also the wO…O vi-
bration quantum magnitude, derived from the
fine structure patterns of the compared spectra,
remains almost identical. From this comparison
the following conclusions can be derived: Sepa-
ration of the hydrogen bonds from the phenyl
rings is responsible for disappearance of the H/
D long-range isotopic effects in the wO–H band
frequency range. This fact seems to identify a
general nature of the effect discussed. With a
high probability level, it can be assumed that
these mechanisms are being of a vibronic nature.
Most probably, these are some aspects of a cou-
pling mechanism, concerning proton stretching
vibrations and electronic motions, also aromatic
ring electrons included. Methylene groups in as-
sociated arylacetic acid molecules make such
coupling effectively impossible.
With regard of the H/D long-range isotopic
effects in the IR, spectral properties of the
phenylacetic acid crystals resemble spectral prop-
erties of aliphatic carboxylic acid crystals e.g. of
malonic acid [31]. A precise recognition of such
isotopic effects needs, however, next and more
detailed investigations in the future. Recently,
another kind of the H/D long-range isotopic ef-
fects in the IR spectra was discussed in the case
of imidazole crystals [21], where these effects
concerned changes in vibrational dipole selection
rules in the IR hydrogen bond spectra. Also dif-
ferent H/D long-range isotopic effects were ob-
served in the spectra of pyrazole crystals [30].
The effects measured for phenylacetic acid crys-
tals differ from these, measured for the crys-
talline systems mentioned above.
Analysis of the wO–H band shapes from Figs. 4
and 10 and Fig. 11 allows concluding that in
each case the band fine structures are relatively
regular. There is no local lowering of the band
intensity, similar to E6ans hole effects [3], which
was observed in the solid-state spectra of ben-
zoic acid and its d5 – isotopomer and also in
the crystalline spectra of 1- and 2-naphthoic acid
crystals and their d7 – isotopomers [19,20]. In
these latter cases, the band fine structure irregu-
larity effects have been ascribed to the Fermi
resonance mechanisms, concerning the wO–H pro-
ton stretching vibrations in the associated car-
boxyl groups, with the lO–H proton bending-in
plane vibrations in their first overtone state (its
frequency :2700 cm−1).
In a centrosymmetric hydrogen bond dimer,
the Fermi resonance concerns two normal vibra-
tions in the dimer, characterized by identical
symmetry properties: the symmetry of the lO–H
proton bending-in plane vibrations in their first
overtone state belong to the Ag irreducible rep-
resentation of the Ci group. Thus, only the to-
tally symmetric proton stretching vibrations in
the dimer can effectively anharmonically couple
with the proton bending vibrations. Therefore,
the Fermi resonance effects in the carboxylic acid
crystal spectra, as the E6ans hole-type effects
may be only observed in the frequency range of
the longer-wave branch of the wO–H band. Al-
though a direct excitation of the proton totally
symmetric vibration is forbidden by symmetry
rules, the dimer vibrational energy levels and its
vibrational wave functions remain influenced by
the Fermi resonance mechanism. However, this
transition activated by a vibronic promotion
mechanism [16]. The Fermi resonance effects may
be observed only in the case of a proper lower
frequency shift of the wO–H band. Nevertheless,
these spectral effects cannot appear, when the
shorter-wave branch, corresponding to the sym-
metry allowed transition to the Au – state of the
proton stretching vibrations, is shifted just to the
resonance frequency range. In such a case, the
Fermi resonance mechanism is expected to be

H.T. Flakus, M. Chełmecki / Spectrochimica Acta Part A 58 (2002) 1867–1880
1879
non-effective, as the two vibration symmetries
differ from one another.
aromatic rings. These couplings seem to be respon-
sible for a considerably high intensity of the forbid-
den, longer-wave branch of the wO–H band in the
spectra of arylcarboxylic acid crystals. Most prob-
ably these are a primary cause of the H/D long-
range isotopic effects and also of spectral effects,
which were ascribed to the Fermi resonance effects.
In the spectra of arylacetic acid crystals, the
intensity of the forbidden branch of the wO–H band
in the spectra is relatively low. In addition no H/D
long-range isotopic effects and no Fermi resonance
effects can be observed.
In the case of the spectra of the benzoic acid
crystals and of the phenylacetic acid crystals, the
above-discussed circumstances, necessary for an
effective Fermi resonance are fulfilled. Neverthe-
less, these effects were solely identified in the case
of the benzoic acid crystal spectra. Similarly, no
Fermi resonance effects were found in spectra of the
1- and 2-naphthylacetic acid crystal spectra [32],
when such effects were observed for the 1- and
2-naphthoic acid crystals [20].
All these facts presented above suggest that the
lack of the Fermi resonance effects in the spectra of
arylacetic acid crystals should be attributed to the
presence of methylene groups in their molecules. In
this case an extremely effective Fermi resonance
mechanism most probably results from a coupling
of the electrons in (COOH)2 rings formed the
associated carboxyl groups, with the aromatic ring
electrons. This coupling might be a source of an
additional stabilization of arylcarboxylic acid
dimers in crystal lattices. In a consequence of
effective anharmonic couplings of the dimer vibra-
tions leading to the Fermi resonance effects in the
spectra of the benzoic acid and the 1- and 2-naph-
thoic acid crystals [19,20]. Methylene groups in
molecules of arylacetic acids unable an effective
coupling of the hydrogen bonds with the aromatic
rings. This would explain a lack of the Fermi
resonance effects in the IR spectra of arylacetic acid
crystals.
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Our investigations seems to directly point out on
an influence of aromatic rings coupled with cyclic
dimeric systems of hydrogen bonds of associated
carboxyl groups in crystals of aromatic carboxylic
acid crystals, on their IR spectra.
All these above-presented spectral properties
measured in the spectra of the phenylacetic acid
crystals, in confrontation with the spectral proper-
ties of benzoic acid, 1- and 2-naphthoic acid and
1- and 2-naphthylacetic acid crystals, are pointing
out on a role of couplings concerning proton
stretching vibrations and electronic motions in
[19] H.T. Flakus, M. Chetmecki, Spectrochimica Acta, 58
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[20] H.T. Flakus, M. Chełmecki, Polarization IR spectra of
hydrogen bonded 1-naphthoic acid and 2-naphthoic acid

1880
H.T. Flakus, M. Chełmecki / Spectrochimica Acta Part A 58 (2002) 1867–1880
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