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Neutralization of solvated protons and formation of noble-gas hydride molecules: matrix-isolation indications of tunneling mechanisms?

08 Aug 2005-Journal of Chemical Physics (American Institute of Physics)-Vol. 123, Iss: 6, pp 064507-064507

TL;DR: It is proposed that this phenomenon could be explained by the neutralization of the solvated protons by electrons, and the proposed electron-tunneling mechanism should be considered as a possible alternative to the literature models based on tunneling-assisted or radiation-induced diffusion of protons in noble-gas solids.
Abstract: The (NgHNg)+ cations (Ng=Ar and Kr) produced via the photolysis of HF∕Ar, HF∕Kr, and HBr∕Kr solid mixtures are studied, with emphasis on their decay mechanisms. The present experiments provide a large variety of parameters connected to this decay phenomenon, which allows us to reconsider various models for the decay of the (NgHNg)+ cations in noble-gas matrices. As a result, we propose that this phenomenon could be explained by the neutralization of the solvated protons by electrons. The mechanism of this neutralization reaction probably involves tunneling of an electron from an electronegative fragment or another trap to the (NgHNg)+ cation. The proposed electron-tunneling mechanism should be considered as a possible alternative to the literature models based on tunneling-assisted or radiation-induced diffusion of protons in noble-gas solids. As a novel experimental observation of this work, the efficient formation of HArF molecules occurs at 8K in a photolyzed HF∕Ar matrix. It is probable that the low-t...
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Neutralization of solvated protons and formation of noble-gas
hydride molecules : matrix-isolation indications of tunneling mechanisms?
Khriachtchev, Leonid
American Institute of Physics
2005-08-08
J. Chem. Phys. 123, 064507 (2005) (6 pages)
http://link.aip.org/link/?jcp/123/064507
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Neutralization of solvated protons and formation of noble-gas hydride
molecules: Matrix-isolation indications of tunneling mechanisms?
Leonid Khriachtchev,
a
Antti Lignell, and Markku Räsänen
Laboratory of Physical Chemistry, P.O. Box 55, University of Helsinki, Helsinki FIN-00014, Finland
Received 23 February 2005; accepted 23 May 2005; published online 16 August 2005
The NgHNg
+
cations Ng=Ar and Kr produced via the photolysis of HF/Ar, HF/Kr, and
HBr/Kr solid mixtures are studied, with emphasis on their decay mechanisms. The present
experiments provide a large variety of parameters connected to this decay phenomenon, which
allows us to reconsider various models for the decay of the NgHNg
+
cations in noble-gas matrices.
As a result, we propose that this phenomenon could be explained by the neutralization of the
solvated protons by electrons. The mechanism of this neutralization reaction probably involves
tunneling of an electron from an electronegative fragment or another trap to the NgHNg
+
cation.
The proposed electron-tunneling mechanism should be considered as a possible alternative to the
literature models based on tunneling-assisted or radiation-induced diffusion of protons in noble-gas
solids. As a novel experimental observation of this work, the efficient formation of HArF molecules
occurs at 8 K in a photolyzed HF/Ar matrix. It is probable that the low-temperature formation of
HArF involves local tunneling of the H atom to the Ar–F center, which in turn supports the locality
of HF photolysis in solid Ar. In this model, the decay of ArHAr
+
ions and the formation of HArF
molecules observed at low temperatures are generally unconnected processes; however, the
decaying ArHAr
+
ions may contribute to some extent to the formation of HArF molecules.
© 2005 American Institute of Physics. DOI: 10.1063/1.1953467
I. INTRODUCTION
The matrix-isolation technique has been used for several
decades to study the physics and chemistry of trapped
species.
1
One important direction of research concentrates on
photodissociation and photoionization of molecules, and
various ionic and neutral species can be identified in these
photolysis experiments.
2
In particular, the UV photolysis of
hydrogen-containing molecules can lead to the formation of
NgHNg
+
ions Ng=Ar, Kr, and Xe, which is probably the
simplest case of a solvated proton.
2–4
These cations can also
be trapped in noble-gas matrices upon direct deposition of
hydrogen through discharge, as it was done in the pioneering
works on these species.
5–7
An intriguing question concerns
the experimentally observed decay of the cations at low tem-
peratures, which is not fully understood to date. For ex-
ample, it was suggested that room-temperature background
radiation could accelerate the diffusion of protons and deu-
terons in the matrix and explain the faster decay of protons.
8
More recently, it has been proposed that protons could dif-
fuse via a tunneling mechanism.
9
In this model, proton dif-
fusion is several orders of magnitude faster than that of deu-
terons, and the jump rate strongly increases from Xe to Kr
and Ar, which is in qualitative agreement with the available
experimental data on these species.
As a related subject, experiments with the photolysis
of hydrogen-containing species have constructed the basis
for the preparation and identification of noble-gas hydride
molecules with general formula HNgY Y denotes an elec-
tronegative fragment, such as F, OH, CCH, etc..
10
As an
example, the first neutral ground-state chemical compound
containing argon, HArF, was prepared using the vacuum-UV
photolysis of HF in solid Ar at 7 K and by annealing at
20 K.
11
The formation of HNgY molecules via the H
+Ng+Y reaction of the neutral fragments was directly dem-
onstrated in the case of HXeI.
12
It was not an evident fact
because of the charge-transfer nature of these molecules
showing the strong HNg
+
Y
character and the presence of
the NgHNg
+
ions in most of those experiments. Indeed,
one could connect the annealing-induced decay of NgHNg
+
ions and the formation of H NgY molecules, but this does not
seem to be the general case. Moreover, two examples were
later found when photolysis did not produce NgHNg
+
ions
in detectable amounts, but annealing efficiently generated the
HNgY molecules HKrCN and HKrC
4
H.
13,14
The
annealing-induced formation of HNgY molecules 25 and
37 K in Kr and Xe matrices, respectively was used to study
the diffusion of hydrogen in solid Kr and Xe.
15,16
The atomic
hydrogen diffusion in solid Ar has not been studied in detail
so far. Some indications exist that the hydrogen mobility in
solid Ar is rather a local process than a global one,
17
meaning
that the formation of HArF might be somewhat different
from the formation of other HNgY molecules.
Another link between the NgHNg
+
ions and the HNgY
molecules is remarkable. It was shown that in some if not in
all cases the photodissociation of small hydride molecules
in noble-gas solids is a local process. This concept means
that the dissociating hydrogen atom is localized at a short
distance comparable with the lattice parameter from the
parent cage. First, this locality allows direct light-induced
a
Electronic-mail: leonid.khriachtchev@helsinki.fi
THE JOURNAL OF CHEMICAL PHYSICS 123, 064507 2005
0021-9606/2005/1236/064507/6/$22.50 © 2005 American Institute of Physics123, 064507-1
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formation of the HNgY molecules. Next light pulses
can decompose the formed molecules but, in some cases
HXeNCO, HKrCl, and HArF, their formation is detected
directly.
11,18,19
Based on the data obtained for HXeI, this di-
rect formation takes place also for other HNgY species but
their steady-state concentration is minor because of the very
fast photodecomposition.
20
Second, the formation of the
NgHNg
+
ions can influence the final spatial distribution of
H atoms. In this image, the electronegative fragment en-
hances the electron transfer from the surrounding host, and
the dissociated hydrogen atom captures the created hole. The
repeated neutralization and formation of NgHNg
+
ions can
move the hydrogen atom to a longer distance from the parent
cage, as discussed elsewhere.
20
In the present work, we analyze the experimental data on
the decay of NgHNg
+
ions and the formation of HNgY
molecules in low-temperature matrices. We concentrate
mainly on experiments in solid Ar, where HArF is formed,
and present some comparison with experiments in Kr matri-
ces. We discuss various mechanisms that can explain the
experimental data. For the decay of the NgHNg
+
ions, we
propose a possibility of their neutralization via an electron-
tunneling mechanism. For the low-temperature formation of
HArF molecules, the hydrogen atom tunneling is a probable
mechanism.
II. EXPERIMENT
A. Experimental details
The HF/Ar, HF/Kr, and HBr/Kr solid mixtures were
studied in a closed-cycle helium cryostat APD, DE 202A at
temperatures down to 8 K. The HF/Ar and HF/Kr matrices
共⬃100
m thick were deposited onto a cold CsI substrate
by passing Ar and Kr gas AGA over a HF-pyridine poly-
mer Fluka at room temperature. In order to prepare deuter-
ated samples, the HF/Ng mixture was passed through a line
with deuterated sulfuric acid, and the deuteration degree was
up to 90%.
11
The photolysis of HF was performed with a Kr
continuum lamp Opthos emitting in the 127160 nm spec-
tral interval, the decomposition proportion being typically
20% after 3060 min of irradiation, limited probably by
photogenerated light absorbers.
21
The HBr/Kr gas mixture
1/500 was prepared by mixing HBr and Kr in a bulb. The
193-nm radiation of an excimer laser MSX-250, MPB,
pulse energy density 10 mJ cm
−2
was used to photolyze
HBr molecules, and typically 90% of HBr was decom-
posed after 1000 pulses. The IR absorption spectra in the
4000 to 400 cm
−1
region were recorded with a Nicolet 60
SX FTIR spectrometer using a resolution of 1 cm
−1
. Most of
the HF/Ng matrices were quite monomeric with respect to
HF, and the estimated HF/Ng ratio was 1/2000. In par-
ticular, the doublet band of HF monomer at 3962.5 and
3953.8 cm
−1
dominated in the IR absorption spectra in solid
Ar, in agreement with the literature data.
22
B. Experimental results
The IR absorption spectra in Fig. 1 demonstrate various
steps of an experiment with HF in solid Ar. Some amount of
HArF is seen already after the vacuum ultraviolet VUV
photolysis and, simultaneously, ArHAr
+
ions are formed,
evidenced by the IR absorption band
3
at 903 cm
−1
. Anneal-
ing at 20 K further increases the concentration of HArF with
triplet absorption at 1965.7, 1969.4, and 1972.3 cm
−1
and a
broad feature around 1992 cm
−1
and promotes the doublet
absorption at 2016.3 and 2020.8 cm
−1
that belongs to HArF
as well.
23,24
The triplet absorption and the broad feature de-
crease and disappear upon annealing above 28 K. The dou-
blet absorption has a larger thermal stability, and it was as-
signed to HArF in a thermally relaxed solid-state
configuration matrix site and was called “stable” HArF in
order to distinguish it from the “unstable” HArF absorbing at
1970 cm
−1
.
23
The stable and unstable configurations corre-
spond to molecules in certain local matrix morphologies. The
1992-cm
−1
band probably originates from the hindered rota-
tion of HArF in an Ar matrix, in analogy with some other
HNgY molecules.
25
Upon annealing at 20 and 33 K the
ArHAr
+
concentration decreases; however, even after the
33-K annealing the cations are very visible in the spectra.
Figure 2 presents the ArHAr
+
concentration versus the
annealing temperature for four experiments with similar an-
nealing periods. No reliable correlation between the deposi-
tion temperature and the decomposition rate of ArHAr
+
was found. It seems that the decay depends in a complicated
way on the HF/Ar concentration and the efficiency of pho-
tolysis. Unfortunately, these two parameters are difficult to
reproduce precisely in such experiments. We can conclude
here qualitatively that the ArHAr
+
concentration is quite
stable in these experiments, especially taking into account
that one annealing-measurement cycle is typically 20 min
long.
Figure 3 compares the stability of NgHNg
+
and
NgDNg
+
cations in Ar and Kr matrices. The annealing
above 20 K for 3 min accelerates the decay process con-
FIG. 1. ArHAr
+
and HArF in solid Ar. Shown are from top to bottom
fragments of IR absorption spectrum after the VUV photolysis of the HF/Ar
matrix, the spectrum after annealing of the photolyzed matrix at 20 K, and
the spectrum after annealing of the same sample at 33 K. The annealing
time was 3 min. The spectra were measured at 8 K. The deposition tem-
perature was 8 K. The multiplying factor of 3.5 is applied to the higher-
frequency part of the plot.
064507-2 Khriachtchev, Lignell, and Räsänen J. Chem. Phys. 123, 064507 2005
Downloaded 15 Aug 2007 to 128.214.3.188. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp

siderably as compared with the lowest working temperature
see below for experiments at 8 K. The NgDNg
+
cations
are more stable than NgHNg
+
, and the ratio of the corre-
sponding rate constants kNgHNg
+
/kNgDNg
+
at the el-
evated temperatures is estimated to be 1.4 for Ar 30 K and
3.5 for Kr 24 K. Furthermore, the stability of ArHAr
+
and KrHKr
+
is also comparable at the elevated tempera-
tures.
The ArHAr
+
decay kinetics was measured at 8 K,
which is the lowest temperature of our apparatus see Fig. 4.
The ArHAr
+
cations were found to decompose at 8 K on a
time scale of hours. It should be mentioned that the start of
measurement is delayed with respect to the formation of the
cations mainly due to the photolysis time of 3060 min. The
decay upon globar irradiation solid symbols in Fig. 4a兲兴
was compared with the decay in dark open symbols. The
present data show that the vibrational excitation of ArHAr
+
by globar radiation does not accelerate the decay process.
The ArDAr
+
decay at 8 K is slower by a factor of 200
compared with the ArHAr
+
decay, which is much larger
than the corresponding difference estimated at elevated tem-
peratures. Annealing at 20 K accelerates the ArDAr
+
decay
by a factor of 10
3
as compared with 8 K, which is a signifi-
cantly larger enhancement than that for ArHAr
+
, explain-
ing the closer decay rates at the elevated temperatures.
A remarkable observation of this work concerns the low-
temperature formation of HArF, and this similarly takes
place under globar irradiation and in dark as seen in Fig.
4a. It should be stressed that the delayed formation of the
HNgY molecules has been usually promoted by annealing
activating diffusion of hydrogen atoms in the matrix, with an
important exclusion of IR-decomposed HXeI.
10,12
In the
present work, it is found that HArF preferably the unstable
configuration can be slowly but efficiently produced in a
photolyzed HF/Ar matrix at 8 K. The formation of DArF at
8 K is much slower, by a factor of 50, than the formation
of HArF.
FIG. 2. Annealing-induced decay of ArHAr
+
cations. The annealing time
was 3 min. The annealing-measurement cycle was typically 20 min long.
The ArHAr
+
concentrations are obtained by integrating the 903-cm
−1
band
3
and are normalized by the initial value. The deposition temperatures are
shown in the plot. The spectra were measured at 8 K.
FIG. 3. Protons and deuterons solvated in a Ar and b Kr matrices. The IR
absorption spectra correspond to the situations after the photolysis of
HFDF/Ar and HFDF/Kr matrices and after annealing of the photolyzed
matrices. The annealing time was 3 min. The deposition temperatures
were 8 and 20 K for Ar and Kr matrices, respectively. The spectra were
measured at 8 K.
FIG. 4. a Decay of ArHAr
+
and formation of HArF unstable configu-
ration in an Ar matrix at 8 K and b the same processes for the deuterated
species. Open and solid symbols in panel a correspond to experiments in
dark and under globar irradiations, respectively. The HArF and DArF con-
centrations are normalized by the values after additional annealing at 20 K.
Photolysis-produced HArF and DArF concentration is excluded from the
kinetic data. The concentrations are obtained by integrating the correspond-
ing IR absorption bands.
064507-3 Neutralization of solvated protons J. Chem. Phys. 123, 064507 2005
Downloaded 15 Aug 2007 to 128.214.3.188. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp

The formation of HArF at 8 K follows different kinetics
from the decay of the ArHAr
+
cations. We fitted the data
points using a stretched exponent expkt
and obtained k
=0.16±0.02 h
−1
and
=0.84±0.12 for the HArF forma-
tion kinetics and k =0.51±0.06 h
−1
and
=0.67±0.07 for
the ArHAr
+
decay kinetics. This distinction between the
fitting parameters suggests that the two processes are prob-
ably independent.
In some experiments with photolyzed HF/Ar matrices,
annealing above 20 K produced a band at 1377 cm
−1
. This
band was assigned previously to the FHF
anion.
26
The
formation of negative ions along the decay of positive ions
was somewhat surprising because one would rather expect
the neutralization of the negative ions if the cations were
globally mobile, as suggested in Ref. 9. In order to test this
observation further, we performed experiments on HBr/Kr
solid mixtures, and the result is presented in Fig. 5. Irradia-
tion at 193 nm decomposes HBr efficiently and a strong
band of KrHKr
+
is built up. A minor amount of BrHBr
is
also present after photolysis.
27
After 135 min at 8 K, about
half of KrHKr
+
decomposes, and the decomposition pro-
portion is similar after several min at 34 K. After 135 min at
8 K, a small increase of BrHBr
takes place, and the for-
mation of the anions is strongly enhanced at 34 K. As ex-
pected, our present annealing experiment did not show a for-
mation of HKrBr species. The present data indicate similar
decay rates for KrHKr
+
generated in HF/Kr and HBr/Kr
matrices, at least at 34 K. The stability of KrHKr
+
in
HBr/Kr matrices at 8 K is quite comparable with the sta-
bility of ArHAr
+
in solid Ar. It was pointed earlier that the
same qualitative conclusion is valid for elevated tempera-
tures. Comparing the data at the lowest and elevated tem-
peratures, we see that the annealing at 30 K accelerates the
decay by about two orders of magnitude, and this is similar
for ArHAr
+
and KrHKr
+
cations.
III. DISCUSSION
A. Decay of NgHNg
+
ions
The tunneling and light-induced mechanisms for the glo-
bal mobility of protons solvated in noble-gas solids were
proposed by Beyer et al.
8,9
These models could explain some
previous experimental observations on the decay phenom-
enon of the cations, in particular, the isotope effect. Based on
the present experimental data, the proposed models and ad-
ditional possibilities can be reexamined. It seems that the
effect is strongly dependent on sample preparation. For ex-
ample, the short lifetime of ArHAr
+
共⬃15 min as reported
previously
6
is not reproduced in our experiments, which can
be due to the different methods of sample preparation. The
stability of ArHAr
+
and KrHKr
+
species is quite compa-
rable in our experiments while the previous experiments
showed a four-fold larger stability for KrHKr
+
,
6
and the
proton-tunneling model suggests the decay difference by
several orders of magnitude. The decay of deuterons in Ar
and Kr matrices is quite visible in our experiments, which
does not fit accurately the proton-tunneling model predicting
a slower decay of six to seven orders of magnitude for deu-
terons. A serious problem for models based on proton diffu-
sion is introduced by the observed annealing-induced in-
crease of the YHY
concentration. It looks natural to
expect the bleaching of the negative centers upon global mo-
bility of the positive charges, but the opposite effect has been
documented see Fig. 5. No reaction of the solvated protons
with YHY
seems to occur, featuring, rather, the immobil-
ity of both species. The globar radiation is inefficient in pro-
moting the proton decay so that the blackbody radiation is
improbable to do this either.
An alternative explanation of the NgHNg
+
decay is
suggested here. The decomposition of the cations can be
caused by their reactions with electrons stored in the matrix
as a result of photolysis, and the transfer of the electrons to
the positive center occurs via tunneling. The tunneling of
electrons through very long distances is known in frozen
matrices.
28–30
Brus and Bondybey reported long-range
1–2 nm electron tunneling from C
2
to a nearby cation in a
Ne matrix.
31
It is plausible to assume that the positive and negative
charges generated in the matrix bulk upon photolysis are in
equilibrium. The most probable electron trap is the electrone-
gative fragment Y. It follows from the properties of solid-
state photolysis that the negative and positive centers are
formed in the vicinity of each other, and the distance is com-
parable with the lattice parameter 共⬃0.5 nm.
19
The energet-
ics of this system allows the electron-tunneling process be-
cause the NgHNg
+
+e
neutralization reaction is strongly
exothermic due to the large ionization energy of hydrogen
13.5 eV. The total solvation energy of a proton with two
Ng atoms is computationally 4.4, 4.9, and 5.3 eV for Ar, Kr,
and Xe,
32
respectively, and the electron affinity of Cl is
3.6 eV the largest related value. The solvent reorganization
energy, which results from the response of the medium to the
change of the charge distribution between the initial and final
states, and the electrostatic energy should also be considered;
however, these contributions are not very large. In order to
FIG. 5. KrHKr
+
and BrHBr
in solid Kr. Shown are from top to bottom
the fragment of an IR absorption spectrum after the 193-nm photolysis of
the HBr/Kr 1/500 matrix, the spectrum after 135 min at 8 K, and the
spectrum after annealing of the same sample at 34 K for 3 min. The band
at 844.5 cm
−1
belongs to BrHBr
. The matrix was deposited at 27 K. The
spectra were measured at 8 K.
064507-4 Khriachtchev, Lignell, and Räsänen J. Chem. Phys. 123, 064507 2005
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Journal ArticleDOI
TL;DR: Quantum chemical calculations at the MP2/aug-cc-pVTZ and CCSD(T) levels have been carried out for the title compounds and suggest a significant covalent character for the hydrogen bonds to the noble gas atoms in [NgHNg](+) and to the halogen atoms in[XHX](-) .
Abstract: Quantum chemical calculations at the MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ levels have been carried out for the title compounds. The electronic structures were analyzed with a variety of charge and energy partitioning methods. All molecules possess linear equilibrium structures with D∞h symmetry. The total bond dissociation energies (BDEs) of the strongly bonded halogen anions [XHX](-) and [XAuX](-) decrease from [FHF](-) to [IHI](-) and from [FAuF](-) to [IAuI](-) . The BDEs of the noble gas compounds [NgHNg](+) and [NgAuNg](+) become larger for the heavier atoms. The central hydrogen and gold atoms carry partial positive charges in the cations and even in the anions, except for [IAuI](-) , in which case the gold atom has a small negative charge of -0.03 e. The molecular electrostatic potentials reveal that the regions of the most positive or negative charges may not agree with the partial charges of the atoms, because the spatial distribution of the electronic charge needs to be considered. The bonding analysis with the QTAIM method suggests a significant covalent character for the hydrogen bonds to the noble gas atoms in [NgHNg](+) and to the halogen atoms in [XHX](-) . The covalent character of the bonding in the gold systems [NgAuNg](+) and [XAuX](-) is smaller than in the hydrogen compound. The energy decomposition analysis suggests that the lighter hydrogen systems possess dative bonds X(-) →H(+) ←X(-) or Ng→H(+) ←Ng while the heavier homologues exhibit electron sharing through two-electron, three-center bonds. Dative bonds X(-) →Au(+) ←X(-) and Ng→Au(+) ←Ng are also diagnosed for the lighter gold systems, but the heavier compounds possess electron-shared bonds.

34 citations


References
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Journal ArticleDOI
Abstract: A general quantum mechanical description of exothermic electron transfer reactions is formulated by treating such reactions as the nonradiative decay of a ’’supermolecule’’ consisting of the electron donor, the electron acceptor, and the polar solvent. In particular, the role of the high‐frequency intramolecular degrees of feedom on the free energy relationship for series of closely related reactions was investigated for various model systems involving displacement of potential energy surfaces, frequency shift, and anharmonicity effects. The free energy plots are generally found to pass through a maximum and to be asymmetric with a slower decrease in the transition probability with increasing energy of reaction. For high‐frequency intramolecular modes this provides a rationalization of the experimental observation of ’’activationless’’ regions. Isotope effects are discussed as also are the oscillatory free energy relationships, predicted for low temperatures and high frequencies, and which are analogous t...

626 citations


Journal ArticleDOI
Leonid Khriachtchev1, Mika Pettersson1, Nino Runeberg1, Jan Lundell1  +1 moreInstitutions (1)
24 Aug 2000-Nature
TL;DR: It is reported that the photolysis of hydrogen fluoride in a solid argon matrix leads to the formation of argon fluorohydride (HArF), which is identified by probing the shift in the position of vibrational bands on isotopic substitution using infrared spectroscopy and indicates that HArF is intrinsically stable, owing to significant ionic and covalent contributions to its bonding.
Abstract: The noble gases have a particularly stable electronic configuration, comprising fully filled s and p valence orbitals. This makes these elements relatively non-reactive, and they exist at room temperature as monatomic gases. Pauling predicted1 in 1933 that the heavier noble gases, whose valence electrons are screened by core electrons and thus less strongly bound, could form stable molecules. This prediction was verified in 1962 by the preparation of xenon hexafluoroplatinate, XePtF6, the first compound to contain a noble-gas atom2,3. Since then, a range of different compounds containing radon, xenon and krypton have been theoretically anticipated and prepared4,5,6,7,8. Although the lighter noble gases neon, helium and argon are also expected to be reactive under suitable conditions9,10, they remain the last three long-lived elements of the periodic table for which no stable compound is known. Here we report that the photolysis of hydrogen fluoride in a solid argon matrix leads to the formation of argon fluorohydride (HArF), which we have identified by probing the shift in the position of vibrational bands on isotopic substitution using infrared spectroscopy. Extensive ab initio calculations indicate that HArF is intrinsically stable, owing to significant ionic and covalent contributions to its bonding, thus confirming computational predictions11,12,13 that argon should form a stable hydride species with properties similar to those of the analogous xenon and krypton compounds reported before14,15,16,17,18.

498 citations


Journal ArticleDOI
V. A. Apkarian1, Nikolaus Schwentner1Institutions (1)
09 Jun 1999-Chemical Reviews
TL;DR: Photon-Induced Harpooning, Channeled Migration, and Forced and Delayed Cage Exit 1503 A. Cl2 in XesDissociation without cage Exit 1502 IX.
Abstract: III. Sample Preparation and Morphology 1492 IV. Photon-Induced Dissociation 1493 V. Perfect Caging 1494 A. Direct Observation of the Caging Process 1494 B. Stabilization of Fragments and Isomerization 1495 1. OClO 1495 2. ICN 1496 VI. Predissociation 1497 A. Electronic Caging by Solvent Symmetry 1497 B. Differential Solvation 1497 VII. Dissociation through Sudden Cage Exit 1499 A. F(2P) 1500 B. O(1D) 1500 C. S(1D) 1500 VIII. Delayed versus Sudden Exit 1500 A. HI, HCl 1500 B. H2O, D2O 1502 C. H2S, D2S 1502 D. Cl2 in XesDissociation without Cage Exit 1502 IX. Forced and Delayed Cage Exit 1503 A. Cl2 in Ar 1503 B. Cl2 in Kr and Xe 1504 X. Channeled Migration 1504 A. F Atoms 1504 B. O Atoms 1505 C. H Atoms 1506 XI. Photon-Induced Harpooning 1508 XII. Acknowledgments 1510 XIII. References 1510

254 citations


Book
01 Jan 1989-
TL;DR: The chapters in this book describe many of the contributions of matrix-isolation spectroscopy to chemistry and physics in the last decade, and predict the continued evolution of this technique over the next decade.
Abstract: Matrix-isolation spectroscopy, as a technique for studying unstable species, has had tremendous lasting powers. Since 1954, matrix-isolation has been able to assimilate new technology rather than being replaced by it. The matrix-isolation technique for producing and trapping new chemical species has been applied to an ever increasing range of chemical and physical problems since its inception. The last 12 years have seen a substantial number of new developments and applications of the matrix technique. The chapters in this book describe many of the contributions of matrix-isolation spectroscopy to chemistry and physics in the last decade, and as such predict the continued evolution of this technique over the next decade. Experimental techniques are generally closely related to the development of new apparatus, and each chapter has a section describing innovations in instrumentation such as the closed-cycle cryogenic cooler that has revolutionized the technique. In addition, each chapter describes many of the specialized methods used to prepare, trap and study particular new subject species.

217 citations


Journal ArticleDOI
TL;DR: An additional solid-state configuration of HArF with higher thermal stability is reported, which proves that the doublet at ∼2020 cm-1 originates from the * Address correspondence to this author: (e-mail) Leonid.Fi
Abstract: During the past decade a number of HRgY molecules (H ) hydrogen; Rg ) Ar, Kr, Xe; Y ) an electronegative fragment) have been characterized experimentally in rare-gas solids and computationally by using ab initio methods.1,2 These species are formed from neutral fragments,3 and experiments support their intrinsic stability.4 These molecules constitute an important intermediate during UV photolysis of HY molecules in rare-gas hosts demonstrating the locality of the primary photolysis.5 One of the HRgY molecules, HXeI, has recently been observed in Xe clusters.6 A stable Ar-containing molecule, HArF, was identified in an Ar matrix.2 The high-level ab initio calculations on HArF confirmed its intrinsic stability.7,8 An unclear experimental fact on HArF is its decrease upon annealing above 27 K,2 which contradicts the calculated decomposition barrier of 0.33 eV.7 In this communication, we report an additional solid-state configuration of HArF with higher thermal stability. The HF/Ar solid mixtures were studied in a closed-cycle helium cryostat (APD, DE 202A) at temperatures down to 7.5 K. The samples were deposited onto a cold CsI substrate by passing Ar gas (40Ar from AGA and 36Ar from ICON Services) over an HFpyridine polymer (Fluka) at room temperature. Photolysis of HF was performed with a Kr continuum lamp (Opthos) emitting in the 127-160 nm spectral interval. Our HF/Ar matrixes are quite monomeric as indicated by the IR absorption bands at 3962.5 and 3953.8 cm-1.9 HArF molecules are prepared in the following. First, HF is photodissociated, which stabilizes H and F atoms in solid Ar and leads to some formation of HArF. Next, the photolyzed sample is annealed, which mobilizes the atoms and leads to an increase in the HArF concentration below 20 K.2 The formation of HArF molecules is demonstrated by strong absorption bands at 1965.7, 1969.4, 1972.3 (νH-Ar), 686.9 (δH-Ar-F), and 435.7 cm-1 (νAr-F). The H-Ar stretching region is presented in Figure 1 (see the upper spectrum). The observed bands agree well with the calculated values and show proper shifts upon H and Ar isotopic substitutions. Surprisingly, annealing above 27 K destroys all bands listed above, which was tentatively explained by secondary reactions of mobile matrix species with HArF molecules.2 It was not noticed in the original paper on HArF2 that the decrease of “unstable” HArF molecules is accompanied with an increase of other bands in the H-Ar stretching (2016.3 and 2020.8 cm-1) and bending (693.5 and 697.0 cm-1) regions. This central observation of the present work is illustrated by the lower spectrum in Figure 1. A sign of the novel bands is visible already after annealing at 20 K. We suggest that this novel set of bands belong to HArF in a different solid-state configuration. This additional configuration is thermally rather “stable” and the corresponding bands decrease only with evaporation of the sample. The H-Ar stretching doublet shifts to 1494.0 and 1496.9 cm-1 upon deuteration, and to 2018.5 and 2023.1 cm-1 upon 36Ar/40Ar substitution [see Figure 2a], i.e., in accord with the calculations and experiment on “unstable” HArF.2 These observations prove that the doublet at ∼2020 cm-1 originates from the * Address correspondence to this author: (e-mail) Leonid.Khriachtchev@ Helsinki.Fi. (1) Lundell, J.; Khriachtchev, L.; Pettersson, M.; Rasanen, M. Low Temp. Phys. 2000, 26, 680. (2) Khriachtchev, L.; Pettersson, M.; Runeberg, N.; Lundell, J.; Rasanen, M. Nature (London) 2000, 406, 874. (3) Pettersson, M.; Nieminen, J.; Khriachtchev, L.; Rasanen, M. J. Chem. Phys. 1997, 107, 8423. (4) Lorenz, M.; Rasanen, M.; Bondybey, V. E. J. Phys. Chem. A 2000, 104, 3770. (5) Khriachtchev, L.; Pettersson, M.; Lundell, J.; Rasanen, M. J. Chem. Phys. 2001, 114, 7727. (6) Baumfalk, R.; Nahler, N. H.; Buck, U. J. Chem. Phys. 2001, 114, 4755 (7) Runeberg, N.; Pettersson, M.; Khriachtchev, L.; Lundell, J.; Rasanen, M. J. Chem. Phys. 2001, 114, 836. (8) Lundell, L.; Chaban, G. M.; Gerber, R. B. Chem. Phys. Lett. 2000, 331, 308. (9) Anderson, D. T.; Winn, J. S. Chem. Phys. 1994, 189, 171. Figure 1. Formation of stable HArF in solid Ar. The HF/Ar matrix is deposited and photolyzed at 7.5 K and annealed at 20 and 35 K. Annealing above 27 K destroys unstable HArF (on the right) and forms stable HArF (on the left). The spectra are measured at 7.5 K.

153 citations


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