A cryospectroscopic study of the oligomers of deuterium chloride in liquid argon, liquid krypton and in liquid nitrogen

Article abstract of DOI:10.1016/S0022-2860(00)00839-5

The infrared spectra of HCl and DCl dissolved in liquid argon (94-148 K) in liquid krypton (117-167 K) and in liquid nitrogen (94-148 K) with mole fractions varying between 0.5×10^-4 and 2.5 × 10^-3 are discussed. In all solutions, bands proving the existence of oligomeric species (DCl)_x were observed. Analysis of the spectra shows that these bands must be assigned to dimeric, trimeric and tetrameric species.

Full text of DOI:10.1016/S0022-2860(00)00839-5

Journal of Molecular Structure 563±564 ,2001) 249±255
www.elsevier.nl/locate/molstruc
A cryospectroscopic study of the oligomers of deuterium chloride
in liquid argon, liquid krypton and in liquid nitrogen
*
W.A. Herrebout, J. Van Gils, B.J. van der Veken
Department of Chemistry, Universitair Centrum Antwerpen, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
Received 20 September 2000; accepted 16 October 2000
Abstract
The infrared spectra of HCl and DCl dissolved in liquid argon ,94±148 K) in liquid krypton ,117±167 K) and in liquid
nitrogen ,94±148 K) with mole fractions varying between 0.5 £ 1024 and 2.5 £ 1023 are discussed. In all solutions, bands
proving the existence of oligomeric species ,DCl)x were observed. Analysis of the spectra shows that these bands must be
assigned to dimeric, trimeric and tetrameric species. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: Vibrational specroscopy; Van der Waals molecules; Cryosolutions; HCl; DCl
1. Introduction
authors were able to separate the contributions due
to monomer and oligomeric species. Concentration
studies in which the spectra of the oligomeric species
were ®tted using Gauss±Lorentz sum functions,
allowed the assignment of the observed complex
bands to a dimer, a trimer and a tetramer, while an
analysis of the temperature dependence of the oligo-
meric band areas yielded the enthalpy of complexa-
tion, the values in LAr being 24.2,3), 213.5,19) and
220.1,32) kJ mol21, respectively [12,13].
In a continuing series of concerted experimental
and ab initio studies of Van der Waals complexes
formed between HCl and a variety of Lewis bases,
we have investigated the formation of weakly bound
complexes in solution, using LAr, LKr and liquid
nitrogen ,LN2) as solvents. The solutions have the
advantage over molecular beams and solid matrices
of being in thermodynamic equilibrium, so that infor-
mation on the stability of the complexes can easily be
obtained. In those cases where the bands of the Lewis
bases strongly overlap with the HCl monomer band,
we have also investigated the spectra of solutions
The behavior of hydrogen chloride as a dilute
species in an environment of inert noble gas atoms
has been the subject of vibrational spectroscopic
research for many years [1±11]. Depending on the
phase of the environment, different aspects of this
behavior were investigated. In 1992, Van der Veken
and De Munck [12] reported a systematic study of
HCl dissolved in liquid argon ,LAr), in liquid krypton
,LKr), and in liquid xenon ,LXe). At suf®ciently low
concentrations, only the spectrum due to monomeric
species was observed in each solvent. At higher
concentrations in LAr and in LKr, however, new
bands proving the existence due to a variety of oligo-
meric species ,HCl)x were observed on the low
frequency side of the monomer band. By comparing
the spectra recorded at several concentrations, the
*Corresponding author. Tel.: 132-3218-0372; fax: 132-3218-
0233.
E-mail address: bvdveken@ruca.ua.ac.be ,B.J. van der Veken).
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-2860,00)00839-5

250
W.A. Herrebout et al. / Journal of Molecular Structure 563±564 22001) 249±255
containing the appropriate Lewis base and DCl
[14,15]. In addition to the bands assigned to the
complexes under study, in some of the spectra weak
features were observed which could not be assigned to
the complexes with the Lewis bases under study. In
agreement with the reported behavior of HCl solu-
tions [12] for the solutions containing only HCl,
these bands were tentatively assigned to DCl dimer
and trimer species. To our knowledge, however, no
detailed description of the behavior of monomer DCl
dissolved in liquid noble gases has yet been reported.
Therefore, we have undertaken a systematic study of
solutions of DCl dissolved in LAr, in LKr, and in LN2.
In addition, for reasons of comparison, some spectra
of HCl in the different solvents were re-investigated.
The results of this study are presented below and it
will be shown that oligomer spectra were obtained
that are very similar to those of the HCl oligomers.
relative intensities, that are due to monomeric and
oligomeric HCl species persisted in the spectra.
Taking into account the lower infrared intensity of
DCl compared to HCl [17], this signals loss of a
small fraction of deuterium on the walls of the ®lling
manifold and the cell during the preparation of the
solutions. In the DCl solutions, therefore, also mixed
complexes ,DCl)x,HCl)y must have been present, and
it is useful to realize that the DCl stretching region
must contain contributions due these mixed species
,Fig. 1).
Previously, the bands due to HCl oligomers were
isolated by subtracting out the monomeric HCl contri-
bution, using spectra recorded at higher dilution [12].
The same procedure was applied in this study, both for
the nHCl region of solutions containing HCl, and for
the nDCl region of solutions containing DCl. As an
example the results obtained at 104 K are given in Fig.
2A for the nHCl region, and in Fig. 2B for the nDCl
region. In all attempts a dip in the baseline of the
oligomer spectrum occurs near the maximum of the
monomer band. Its persistence making it unlikely that
the dip is due to small temperature differences
between the spectrum of the more concentrated and
the more dilute solution, it is believed that it must be
due to an as yet not understood in¯uence of the
concentration of the monomer contour.
The quality of the subtracted spectra in the neigh-
borhood of the oligomer bands is such that a least
squares band ®tting, using Gauss±Lorentz sum
bands, could be used to isolate the different compo-
nents of the multiplets. Such an analysis for the nHCl
region has been made before [12], and was repeated
here. Band ®tting was also applied to the nDCl region.
Results of such ®ttings are shown in Fig. 3 for the
spectra recorded at 104 K. The frequencies of the
component bands are given in Table 1, together with
the corresponding band widths, expressed as Full
Width at Half Height ,FWHH).
Apart from minor differences in band positions and
relative intensities, which are due to the difference in
temperature and concentration, respectively, between
the two studies, the present results for nHCl neatly
reproduce previous ones [12]. In the latter study, a
problem was encountered with the assignments for
the dimer. For this species only two HCl stretches
are expected, while the concentration study [12]
showed that not only the more intense 2828 cm21
2. Experimental
The spectra were recorded on a Bruker IFS 66v
Fourier Transform spectrometer, equipped with a
Globar source, a Ge/KBr beamsplitter and a broad-
band MCT detector. For all spectra, 200 scans
recorded at 0.5 cm21 resolution were averaged,
Happ±Genzel apodized and Fourier transformed
using a zero ®lling factor of 4.
The experimental setup consists of a pressure mani-
fold needed for ®lling and evacuating the cell and for
monitoring the amount of gas used in a particular
experiment, and the actual cell. The cell, with a path
length of 70 mm, is equipped with slightly wedged,
5 mm thick Si windows, using a high pressure window
seal [16].
The samples of HCl and DCl were synthesized in
small amounts by hydrolyzing PCl3 with H2O and
D2O, respectively, and were puri®ed by pumping the
reaction mixture through a 180 K slush, followed by
fractionation. The solvent gases Ar, N2, and Kr, were
supplied by l'Air Liquide and had stated purities of
99.9999, 99.9999, and 99.998%, respectively.
3. Results and discussion
Even after repeated deuteration of the experimental
setup with D2O vapor, bands, with non-negligible

W.A. Herrebout et al. / Journal of Molecular Structure 563±564 22001) 249±255
251
Fig. 1. The nHCl ,A) and nDCl ,B) regions for solutions of HCl ,A, mole fraction ˆ 1.0 £ 1023) and DCl ,B, mole fraction ˆ 1.3 £ 1023) in
liquid argon. From top to bottom, the temperature of the solutions increases, from 101 to 149 K.
band but also the weaker components at 2854 and
2846 cm21 had to be assigned to the dimer. Further
research on HCl complexes has shown that the nHCl
mode tails towards higher frequencies, which was
interpreted to be due to the presence of excited state
transitions [13]. The thermally excited states involved
are the low frequency van der Waals modes and,
therefore, their in¯uence is felt only for the stretching
mode of HCl molecules of which the H atom is part of
the hydrogen bond. Evidently then, the weak, broad
2846 cm21 band represents such thermal structure on
the 2828 cm21 fundamental, while the 2854 cm21
band must be assigned to the free H±Cl bond of the
L-shaped dimer. From Table 1 can be seen that the
largest difference, 4.8 cm21, between present and
previous [12] HCl oligomer band positions is found
for the thermal structure band of the dimer, found here
at 2841.2 cm21. The reason is that the temperature
shifts of the fundamentals is mostly due to the changes
in solvent density with temperature, whereas for the
thermal structure band it is supplementary in¯uenced
by the ,temperature-dependant) relative populations
of the excited states.
Similarly, the 2797 cm21 trimer band and the
2763 cm21 tetramer band should tail towards higher
frequencies. Due to the presence of other fundamen-
tals of these oligomers at 2806 and 2774 cm21
,
respectively, no separate bands representing the
thermal structure of the 2797 and 2763 cm21 bands
were detected in the least squares procedure. It is
believed, however, that the high widths of the 2806
and 2774 cm21 bands re¯ect the presence of some
intensity due to the excited states.
Despite differences in relative intensities and rela-
tive widths, the analogy in the number of bands, and in
their relative positions, of the nDCl and nHCl regions
is striking. This strongly suggests that the bands in the
nDCl region must be assigned in the same way as
those of the nHCl region. These assignments have
been included in Table 1. Inspection of the FWHH
in Table 1 shows that the nDCl bands are systemati-
cally narrower than the corresponding nHCl bands.

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W.A. Herrebout et al. / Journal of Molecular Structure 563±564 22001) 249±255
Fig. 2. The nHCl ,A) and nDCl ,B) regions of solutions of HCl ,A) and DCl ,B) in liquid argon at 104 K. In both panels the top trace was
recorded from a concentrated solution ,mole fraction ˆ 1.3 £ 1023), whereas the middle trace gives the ,rescaled) spectrum recorded from a
dilute solution ,mole fraction ˆ 2.0 £ 1024). The lower trace in A and B was obtained by subtracting the middle from the top trace.
Fig. 3. Results of the least squares band ®tting of the oligomer bands of HCl ,A) and DCl ,B). In each panel the top curves are the superposition
of the experimental with the calculated spectrum, while the component bands are shown below.

W.A. Herrebout et al. / Journal of Molecular Structure 563±564 22001) 249±255
253
Table 1
Vibrational frequencies for the oligomer species ,HCl)x and ,DCl)x observed in liquid argon, at 104 K
,HCl)x
,DCl)x
n ,cm21
)
FWHH ,cm21
)
n ,cm21
)
FWHH ,cm21
)
,H/D±Cl)2
,H/D±Cl)2
,H/D±Cl)2
,H/D±Cl)3
,H/D±Cl)3
,H/D±Cl)4
,H/D±Cl)4
2854.7
2841.2
2829.5
2805.9
2796.8
2774.4
2763.1
10.1
25.5
11.6
18.7
8.5
2068.5
2060.5
2048.5
2030.3
2025.1
2011.7
2002.2
4.8
17.9
9.7
a
14.5
7.0
30.0
11.1
11.6
10.4
a
See text.
Fig. 4. The nHCl ,A) and nDCl ,B) regions for solutions of HCl ,A, mole fraction ˆ 1.3 £ 1023) and DCl ,B, mole fraction ˆ 1.8 £ 1023) in
liquid krypton. From top to bottom, the temperature of the solution increases, from 126 to 194 K.

254
W.A. Herrebout et al. / Journal of Molecular Structure 563±564 22001) 249±255
Fig. 5. The nHCl ,A,C) and nDCl ,B,D) regions for solution of HCl ,A,C) and DCl ,B,D) in liquid nitrogen. The spectra in ,A) and ,B) were
recorded from a dilute solution ,mole fraction ˆ 0.4 £ 1023) at 130 K. No oligomer bands can be observed in these spectra. The spectra in ,C)
and ,D) were recorded from a more concentrated solution ,mole fraction ˆ 1.8 £ 1023), in which measurable concentrations of the dimers are
present. For plots C and D, the temperature for the top, middle and lower traces are 98, 110 and 118 K, respectively.
This characteristic has been observed before for other
complexes involving HCl or DCl [14,15].
ness of the interactions between the monomers in
these complexes, and the relatively high mass of the
interposed chlorine atoms, are considered.
The above analysis of the nDCl region shows that
no indications for bands due to mixed HCl/DCl oligo-
mers have been found. While it is clear from what was
said above that such mixed oligomers must be present,
it follows that their DCl stretches must be very nearly
accidentally degenerate with the corresponding
stretches in the homogenous DCl oligomers. This
must be a consequence of the absence of signi®cant
nDCl/nDCl and nHCl/nHCl couplings in the oligo-
mers. This can be readily rationalized when the weak-
Further analysis of the spectra, including the deter-
mination of the stoichiometry of the oligomers, and
their complexation enthalpy, requires accurate band
areas of the oligomer bands. In view of the presence
of the contributions due to incompletely deuterated
species, it was judged that the band areas derived
from the band ®tting are not suf®ciently representative
for the ,DCl)x oligomers, and, therefore, no further
analysis was attempted.

W.A. Herrebout et al. / Journal of Molecular Structure 563±564 22001) 249±255
255
The study was completed by investigating solutions
of DCl and HCl in LKr and in LN2. In Fig. 4A some
results for HCl in LKr are given, in Fig. 4B the corre-
sponding results for DCl are given. In the spectra
recorded at the lowest temperatures new bands can
be seen to emerge on the low frequency side of the
monomer bands. A weak band at 2824 cm21 has
previously been assigned [12] to ,HCl)2; its DCl-
used in this study. Part of this research was carried
out with ®nancial support from the RAFO-research
grants ,University of Antwerp, RUCA, 1998).
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counterpart can be seen in Fig. 4B at 2043 cm21
.
For the HCl bands a second, weaker oligomer band
is found at 2800 cm21. This band has not been
reported before [12]. Its frequency is very close to
the 2797 cm21 band assigned to ,HCl)3 in LAr [12],
and we assign the 2800 cm21 to the same species.
For HCl in LN2, the nHCl region, at 130 K, is
shown in Fig. 5A. The more intense band at
2866 cm21 is due to N2´HCl, the weaker feature at
2987 cm21 is assigned to HCl monomers [18]. Upon
cooling the solution, a weak, new band becomes
visible on the low frequency side of the N2´HCl
band, at 2827 cm21. Its frequency is very close to
that of the more intense ,HCl)2 band in LAr, and we
have assigned the 2827 cm21 band accordingly [13].
Analogous behavior is observed for DCl, as can be
seen in Fig. 5B and D: the dimer band is found at
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second band to be observed for ,HCl)2 as well as for
,DCl)2. In LN2, also no bands due to trimers or tetra-
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Acknowledgements
W.A.H. is indebted to the Fund for Scienti®c
Research ,FWO, Belgium) for an appointment as
Postdoctoral Fellow. The FWO is also thanked for
®nancial help toward the spectroscopic equipment
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Clark, R.E. Hester ,Eds.), Molecular Cryospectroscopy,
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