Far infrared study of matrix isolated propionic acid dimers
J. Ceponkus, M. Pucetaite, S. Tamosaityte and V. Sablinskas
Department of General Physics and Spectroscopy, Vilnius University,
Universiteto str. 3, Vilnius-01513, Lithuania
Hydrogen bond (H-bond) is a subject of the numerous experimental and theoretical studies
since the beginning of the 20th century. Particularly interesting case of hydrogen bonded systems is
the ones where formation of H-bond is leading to the increase of the system symmetry. Saturated
carboxylic acid dimers are good examples of such kind of systems. Despite numerous experimental
and theoretical studies some aspects of structure and dynamics of the dimers are still under question. In the smallest members of carboxylic acids family: formic and acetic acids a number of cyclic
and open dimers were observed, when for the propionic acid only cyclic dimers has been reported.
In this work we exploit possibilities of MAX-I ring to deliver very bright far infrared (FIR) radiation, in order to study propionic acid absorption in this part of the spectrum. The absorptions in this
spectral region are associated with two kinds of vibrations, large amplitude motions of C-C radical
chain and vibrations associated with the intermolecular H-bonds. Therefore measurements in this
region provide direct information on the types of the dimers formed, thus allowing as with the aid of
theoretical calculations to identify possible dimers of propionic acid. Unfortunately these studies
can not be performed in the normal conditions. In order to decrease the dynamic of the system and
environmental influence samples has to be isolated in the cold inert media so called matrix.
The samples were prepared by codepositing neon gas and propionic acid on the cold OFHC
copper mirror cooled down to 3.6 K temperature in the liquid helium cryostat. Spectra were obtained using Bruker IFS120 spectrometer, MAX-I as FIR source and liquid He cooled bolometer
detector. In order to interpret experimental spectra quantum chemistry calculations were performed.
The structural parameters, barrier heights and energies of the propionic acid monomers and dimers
were calculated using ab intio method at MP2 6-311++G(df,pd) level of approximation. It is well
established that in the most cases vibrational frequencies calculated using DFT method have better
agreement with the experimental values than the ones calculated even at rather high ab initio levels
such as MP2. Therefore we have used DFT B3LYP 6-311++G(3df,3pd) level of theory to calculate
vibrational frequencies of the monomers and dimers. The calculated frequencies were scaled by
0.982 in this work, what resulted an exact fit between frequency of the experimental (C=O) spectral band of propionic acid monomer isolated in the neon matrix with the calculated one.
It has been found (a) ΔE=65 kJ/mol
(b) ΔE=27 kJ/mol
(c) ΔE=28 kJ/mol
that four strong absorption
bands between 650 and
450 cm-1 very well coincide with the calculated
spectral bands of the most
1.16 Å
1.91 Å
2.5 Å
stable monomer. The calculated spectrum of the
1.16 Å
1.98 Å
cyclic dimer shows only
1.81 Å
very weak spectral bands
in this region (Fig. 2 (d)),
therefore the absorption
bands of the cyclic dimer
are not expected to be ob- Fig. 1 The most stable structures of the propanoic acid dimers predicted by ab initio
servable in the experimen- calculations at MP2/6-311++G(df, pd) theory level: (a) cyclic dimer; (b) open CH
dimer with additional C-H...O-H hydrogen bond; open OH dimer with additional
tal spectra. Three possi- H-O...HO hydrogen bond.
bilities should be considered in order to explain the remaining weak bands in the 650-450 cm-1 spectral region: open O-H
Absorbance 
dimer, open C-H dimer and propionic acid-water complex. The calculated far infra red spectra of
the propionic acid-water complex show two strong absorption bands around 400 cm-1 and 300 cm-1.
Such bands are not observed in the investigated spectra. This fact does not completely rules out the
presence of the propionic acid-water complex in the matrix. Intermolecular spectral band involving
high amplitude of water molecule motions are known to be very broad and weak, making very hard
to observe them at low concentrations, while the bands in the medium infrared region at the same
concentrations are observed without essential difficulties1. A comparison of the calculated spectra
of the O-H dimer and experimental spectra of propionic acid in neon suggests that the bands at 614,
582 and 535 cm-1 should be assigned to the O-H dimer. The calculated O-H dimer bands at 511
(b.i.p) and 459 cm-1 (b.o.p) (Fig. 2) are not observed. Those bands involve vibrations of the hydrogen bond which is usually highly anharmonic and resulting spectral bands are very broad1. Most
likely this causes them to be undetectable with relatively low concentrations of propionic acid used
in our experiments. The calculated spectra of the C-H dimer has a strong absorption band at
660 cm-1, which is expected to be approximately 10 cm-1 blue shifted from the monomer band. Such
band is not observed experimentally which supports the hypothesis that the C-H dimer is not observed in the neon matrixes. This result brings us to the conclusion that propionic acid can form
only open dimers of the O-H type. Contrary, only open dimer of the C-H type is experimentally observed for formic acid isolated in noble gas matrixes2. Similar open C-H type dimer was observed
for acetic acid isolated in argon3, and existence of the O-H dimer together with the C-H dimer is
proposed from the jet experiments4.
(a)
(b)
b.o.p.
b.i.p.
(c)
(d)
650
600
550
500
450
400
350
300
250
200
Wavenumber [cm-1]
Fig. 2. Far infrared region of absorption spectrum of propanoic acid: (a) experimental spectrum of
acid isolated in neon matrix (T=3 K), (b) calculated spectrum of trans conformer of monomeric acid,
(c) calculated spectrum of open dimer of acid and (d) calculated spectrum of cyclic dimer of acid.
b.i.p – anharmonic intermolecular in-plane bending vibration; b.o.p. – anharmonic intermolecular
out-of-plane bending vibration. Calculated spectra are scaled with scaling factor equal to 0.982
1
J. Ceponkus, B. Nelander, J. Phys. Chem. A 108 (2004) 6499.
M. Gantenberg, M. Halupka, W. Sander, Chem. Eur. J. 6 (2000) 1865.
3
W. Sander, M. Gantenberg, Spectrochim. Acta A 62 (2005) 902.
4
C. Emmeluth, M. Suhm, Phys. Chem. Chem. Phys. 5 (2003) 3094.
2