Infrared spectrum of matrix isolated ClSiO and ab initio calculations

Article abstract of DOI:10.1063/1.478267

Cl atoms and SiO molecules are trapped in a solid Ar matrix at 16 K. The formation of the as yet unknown ClSiO molecule has been followed via its IR spectrum. The stretching frequencies of the isotopic isomer ³⁵Cl²⁸Si¹⁶O are observed at 1160.9 and 509.4 cm^-1. Experiments with the ¹⁸O isotopic isomers were performed in order to confirm the assignment of the absorptions and to characterize the force field. With the help of quantum chemical calculations (DFT) the optimized ClSiO bond angle is obtained at 125.2°. The computed bond lengths are determined to be 153.6 pm for d(Si-O) and 207.8 pm for d(Si-Cl). The SiCl bond is weak in comparison with that of Cl₂SiO (203.4 pm) which is in line with a decrease in the corresponding Si-Cl force constant. The calculation of its thermodynamic data, Δ_fH⁰(298) = -167.2 kJ/mol; Δ_fS⁰(298) = +279.1 J/(mol·K), is of high importance for high temperature gas phase reactions of industrial processes, e.g., the combustion of SiCl₄ by O₂.

Full text of DOI:10.1063/1.478267

Infrared spectrum of matrix isolated ClSiO and ab initio calculations
Markus Junker and Hansgeorg Schnöckel
Citation: The Journal of Chemical Physics 110, 3769 (1999); doi: 10.1063/1.478267
View online: http://dx.doi.org/10.1063/1.478267
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 110, NUMBER 8
22 FEBRUARY 1999
Infrared spectrum of matrix isolated ClSiO and ab initio calculations
¨
Markus Junker and Hansgeorg Schnockel
¨
¨
Institut fur Anorganische Chemie, Universitat Karlsruhe (TH), D-76128 Karlsruhe, Germany
͑Received 14 October 1998; accepted 16 November 1998͒
Cl atoms and SiO molecules are trapped in a solid Ar matrix at 16 K. The formation of the as yet
unknown ClSiO molecule has been followed via its IR spectrum. The stretching frequencies of the
isotopic isomer 35Cl28Si16O are observed at 1160.9 and 509.4 cmϪ1. Experiments with the 18O
isotopic isomers were performed in order to confirm the assignment of the absorptions and to
characterize the force field. With the help of quantum chemical calculations ͑DFT͒ the optimized
ClSiO bond angle is obtained at 125.2°. The computed bond lengths are determined to be 153.6 pm
for d(Si–O) and 207.8 pm for d(Si–Cl). The SiCl bond is weak in comparison with that of Cl2SiO
͑203.4 pm͒ which is in line with a decrease in the corresponding Si–Cl force constant. The
calculation of its thermodynamic data, ⌬fH0͑298͒ϭϪ167.2 kJ/mol; ⌬fS0͑298͒ϭϩ279.1 J/͑mol•K͒,
is of high importance for high temperature gas phase reactions of industrial processes, e.g., the
combustion of SiCl4 by O2. © 1999 American Institute of Physics.
͓S0021-9606͑99͒00108-7͔
I. INTRODUCTION
FT-IR spectrometer. An MCT detector and a Ge/KBr beam-
splitter have been applied in order to obtain the spectra pre-
sented. The DFT calculations are carried out on IBM RS
6000/390 workstations using the B3-LYP functional and a
Some time ago we studied the reactive species Cl2SiO,1
ClCO,2 and ClPO3,4 under matrix conditions. For ClCO and
for ClPO the bonding to chlorine is significantly weaker than
that in Cl2CO2 and Cl3PO.5 For comparison with similar
Si–Cl bonding we were interested in the as yet unknown
radical ClSiO. Furthermore, this reactive species is of inter-
est, since we are involved in studies on the mechanism of the
high temperature reaction between SiCl4 and O2.6 Therefore,
in addition to principal interest in bonding of reactive spe-
cies, the results presented here may contribute to elucidate
one of the first steps of this SiCl4 combustion.
9
TZVPP basis. The program packages GAUSSIAN94 and
10
TURBOMOLE have been used. Contour line diagrams have
been realized using the program MOLDEN.11
III. EXPERIMENTAL RESULTS
Figures 1͑a͒ and 1͑b͒ show the two dominant features of
the infrared spectrum of ClSi16O and ClSi18O in solid argon.
The Si–O stretching vibrations are observed with strong in-
tensity at 1160.9 cmϪ1 (ClSi16O) and 1120.9 cmϪ1
(ClSi18O). The corresponding frequencies of the 29Si isoto-
pic isomer can be detected unambiguously ͑Table I͒. They
confirm the assignment to a Si–O motion. the Si–Cl stretch-
ing modes are observed at 509.4 cmϪ1 (ClSi16O) and 507.6
cmϪ1 (ClSi18O). The correctness of this assignment is un-
derlined by a 35Cl/37Cl splitting. The deformation mode has
not been observed so far which seems plausible, since, as a
result of DFT calculations, its intensity should be only about
10% of that of the Si–Cl stretching mode. Even without the
observation of the deformation mode these experimental data
clearly show that only a single species is responsible for all
these absorptions and that there is present only a single
Si–Cl as well as a single Si–O bond in the molecule. No
formation of Cl2SiO was observed in these experiments.
Even after irradiation of the matrix with the 254 nm wave-
length of a low pressure Hg lamp, no absorptions of Cl2SiO
could be detected. In contrast, former experiments exhibit
strong absorptions of Cl2SiO after Cl2 /SiO codeposition fol-
lowed by the same irradiation.1
II. EXPERIMENT
Chlorine atoms are produced by thermal dissociation of
chlorine molecules. Gaseous chlorine ͑99.8%, Messer
Griesheim GmbH, Krefeld͒ was passed through a resistor
heated oven made of alumina at an input flow rate of about
0.1 mmol/h. The temperature in the center of this furnace
was about 1200 °C and the pressure about 10Ϫ4 atm. Using
thermochemical data7 the fraction of chlorine atoms gener-
ated was determined to be more than 99%.
The chlorine atoms were co-condensed with SiO and an
excess of argon on a cooled copper block8 at about 16 K. The
high temperature molecule SiO was generated by passing an
O2 stream ͑0.02 mmol/h͒ over silicon heated to 1200 °C in
an alumina furnace. The advantage of this technique is a
continuous stream of SiO which can be regulated reproduc-
ibly. Furthermore, Si18O can be prepared easily by applica-
tion of an 18O2 stream. Thus, the relative amount of Ar:Si:Cl
in a typical experiment is 100:1:2. After a condensing period
of 30 min, IR absorptions were recorded by means of a re-
flection unit.
IV. DFT CALCULATIONS
Photochemical activation has been performed with the
¨
help of a low pressure Hg lamp ͑Grantzel, Karlsruhe, Ger-
DFT calculations have been performed in order to get:
many͒. All spectra were recorded with a BRUKER IFS113v
͑a͒ the equilibrium geometry for the normal coordinate
0021-9606/99/110(8)/3769/4/$15.00
3769
© 1999 American Institute of Physics
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¨
3770
J. Chem. Phys., Vol. 110, No. 8, 22 February 1999
M. Junker and H. Schnockel
TABLE II. Experimental and calculated thermodynamic data for Cl2CO at
298 K.
Experimental data for Cl2CO ͑Ref. 16͒
DFT calculated values
⌬fH0ϭϪ220.08Ϯ3.3 kJ/mol
⌬fS0ϭϩ283.796 J/͑mol•K͒
Ϫ227.2 kJ/mol
ϩ283.84 J/͑mol•K͒
mol/Ϫ44.058 J/͑mol•K͒ with the experimental data for SiO
and Cl2, the following ⌬fH0 ͑298͒ and ⌬fS0 ͑298͒ values in
Table III are obtained.
The calculated structural parameters for the molecules
SiO, ClSiO, Cl2SiO, SiCl2, and SiCl4 are listed in Table IV.
The Si–Cl distance decreases going from SiCl2 /OSiCl to
SiCl4 /OSiCl2. This is in line with the increasing oxidation
state of the Si atoms and the atomic charges calculated by the
Mulliken population analysis. However, as expected this
bond seems to be stronger in OSiCl than in SiCl2. A different
situation is found for the Si–O-bonds: This bond is signifi-
cantly longer for OSiCl compared to SiO and OSiCl2. For
ClSiO the three calculated vibrational frequencies and their
IR-intensities ͑in km/mol͒ are listed in Table V.
The general valence force field is described by the fol-
lowing force constants f ͑mdyn/Å, normalized to unit
length͒:
fSiO
8.03,
fSiCl
2.30,
fClSiO
fSiO/SiCl
fSiO/ClSiO
0.194,
0.146,
0.405,
FIG. 1. ͑a͒ Part of the infrared spectrum of 16OSiCl in solid argon. Isotopic
isomers containing 37Cl, 29Si, and 30Si are marked ͑↑͒. ͑b͒ Part of the infra-
red spectrum of 18OSiCl in solid argon. Isotopic isomers containing 37Cl,
29Si, and 30Si are marked ͑↑͒.
fSiCl/ClSiO Ϫ0.007.
The electronic structures of the SOMO (2A ) of OSiCl and
Ј
for comparison of the HOMO (1A ) of OPCl with an addi-
analysis, and ͑b͒ the energetic relation to solid SiO2 and
SiCl4 that means a thermodynamic data ⌬fH0 and ⌬fS0 of
OSiCl and OSiCl2. In order to check the method applied, we
first calculated the corresponding data for OCCl2 and ob-
tained an excellent agreement in comparison with the experi-
mental values, which is shown by Table II.
Ј
tional electron are visualized in Fig. 2. It was calculated on
the basis of a Mulliken population analysis. There are
p-contributions of all three atoms. In order to get SENs, a
Roby–Davidson population analysis has been performed us-
ing 9 MAOs for Si, P, and Cl and only 5 MAOs for O. In
Fig. 3 the results of the population analysis with the charges
q and the SEN values ͑DFT/B3-LYP; TZVPP basis͒ are
summarized.
The same method has been applied in order to obtain the
thermodynamic data for the following reactions:
Cl g͒ϩSiO g͒ꢀCl SiO g͒,
͑1͒
͑2͒
͑
2
͑
͑
2
1Cl g͒ϩSiO g͒ꢀClSiO g͒.
͑
2
͑
͑
2
V. NORMAL COORDINATE ANALYSIS
Combining the calculated reaction energies and entropies for
͑1͒ Ϫ335.6 kJ/mol/Ϫ132.3 J/͑mol•K͒ , and ͑2͒ Ϫ62.789 kJ/
On the basis of the geometrical data obtained from the
DFT calculations and of the experimental data from matrix
experiments, a normal coordinate analysis has been per-
formed. Due to the lack of experimental information about
the frequency of the bending mode it was assumed to be
equal to the theoretical value. With the help of the measured
TABLE I. Observed vibrational modes of different isotopic isomers of
ClSiO in solid argon. All values are given in cmϪ1
.
␯͑SiO͒
␯͑SiCl͒
35Cl28Si16O
37Cl28Si16O
35Cl29Si16O
35Cl28Si18O
37Cl28Si18O
35Cl29Si18O
1160.9
•••
1153.7
1121.0
•••
509.4
503.1
•••
507.7
501.5
•••
TABLE III. Calculated thermodynamic data for ClSiO and Cl2SiO.
Cl2SiO
ClSiO
⌬fH0 ͑298͒ ͓kJ/mol͔
Ϫ440.0
302.4
Ϫ167.2
279.1
1112.9
⌬fS0 ͑298͒ ͓J/͑mol•K͔͒
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¨
J. Chem. Phys., Vol. 110, No. 8, 22 February 1999
M. Junker and H. Schnockel
3771
TABLE IV. Calculated geometrical data for SiO, ClSiO, Cl2SiO, SiCl2, and
SiCl4.
SiO
ClSiO
Cl2SiO
SiCl2
SiCl4
d͑SiO͒/pm
d͑SiCl͒/pm
Є͑ClSiO͒/°
Є͑ClSiCl͒/°
152.1
•••
•••
153.6
207.8
125.2
•••
151.9
202.8
125.7
108.6
•••
210.5
•••
•••
204.7
•••
•••
101.9
109
FIG. 2. Contour diagram ͑Ref. 11͒ of the HOMO of OPCl (1A ) and of the
Ј
SOMO (2A ) of OSiCl, as discussed in the text. The contour lines are
plotted in the molecular plane with the values ͑in a.u.͒ 0.0, Ϯ0.025, Ϯ0.075,
Ϯ0.100, Ϯ0.125, Ϯ0.150, Ϯ0.175.
Ј
isotopic shifts (16O/18O, 35Cl/37Cl and 28Si/29Si) a suitable
force field which fits in with all experimental data was ob-
tained. The measured isotopic shifts were corrected for miss-
ing differences in anharmonicity ͑e.g., between that of 16O
and of 18O species͒ by application of the following
simplification:12
with the decrease of the bond length 207.7 ͑ClSiO͒—203.4
(Cl2SiO) and a drastically lower dissociation energy for
ClSiO than for Cl2SiO ͑calculated values͒:
⌬␯corrϭ⌬␯expϩ2xe⌬␯
.
͑3͒
ex
ClSiOꢀClϩSiO 184 kJ/mol,
͑4͒
͑5͒
With the help of the anharmonicity constants xe for SiO and
SiCl (xSiOϭ0.00481 and xSiClϭ0.00403,13͒ the following
12Cl2SiOꢀClϩ12SiO
291 kJ/mol,
⌬␯
values of Table VI are obtained. Only the corrected
corr
14SiCl4ꢀClϩ14Si
isotopic shifts, which represent nearly ‘‘harmonic’’ shifts
should restrict the many possible force field data from a nor-
mal coordinate analysis. With the following general valence
force field ͑in mdyn/Å, normalized to unit length͒ the correct
frequency shifts were calculated:
365 kJ/mol.
͑6͒
The Si–Cl bond energy of ClSiO is only 63% of that of
Cl2SiO; for the analogous carbon compounds the situation is
even more drastic. For ClCO the Cl–C bond energy is only
18% of that in Cl2CO ͑32 kJ/177 kJ͒.2 SiCl4 possesses the
strongest Si–Cl bonds due to the high ionic contributions,
although bond distances are longer compared to those of
Cl2SiO.
This discussion clearly shows that the radical character
of ClSiO with the single electron in the HOMO weakens the
Si–Cl as well as the Si–O bond in comparison with bonds of
the normal valent species SiO, SiO2, and Cl2SiO. This be-
havior is also observed when going from the 16 valence elec-
tron molecule CO2 to COϪ2 ͑17 valence electrons͒ for which
longer C–O distances and a bond angle of ϳ130 to 140° is
found.15
In comparison with the analogous carbon compounds, it
is evident that in principle the trends are similar for OSiClx
species. However, especially the strength of the SiO bond
exhibits a much smaller variation strength of the CO bonds
where multiple bonding up to a triple bond is usual. In con-
trast, for bonding in the SiO system a maximum bond order
is already reached for Cl2SiO since for the SiO molecule
itself no stronger bond is observed. A further comparison
seems to be fruitful: By adding an electron to ClSiO the high
temperature molecule ClPO is formed ͑cf. Fig. 2͒. A very
weak P–Cl bond f(PCl)ϭ2.2 and a strong P–O bond with
f(PO)ϭ9.8 is observed for this species which is similar to
the situation in ClNO for which the description ClϪNOϩ
visualizes the presence of nearly an NO triple bond. Further-
fSiO
8.00,
2.52,
0.226,
0.12,
0.03,
fSiCl
fClSiO
fSiO/SiCl
fSiO/ClSiO
fSiCl/ClSiO Ϫ0.05.
These force constants represent the bonding of ClSiO in
solid argon. These constants are slightly different from those
of the molecules in the gas phase and also from those of the
calculated DFT frequencies. However, these ‘‘argon matrix
constants’’ can be compared with those of other matrix iso-
lated species like SiO14 and Cl2SiO.1
VI. DISCUSSION
The results obtained are discussed on the basis of the
experimentally determined force constants. This force field is
confirmed by that obtained from DFT calculations. The Si–O
force constant ͑in mdyn/Å͒ fSiO in ClSiO ͑8.00͒ is signifi-
cantly lower than in the normal valent species SiO ͑9.01͒ and
Cl2SiO ͑8.95͒. This trend is also demonstrated by the calcu-
lated Si–O distances ͑in pm͒ ͑SiO 152.4, ClSiO 153.6,
Cl2SiO 152.2͒. Likewise the Si–Cl bond in ClSiO is signifi-
cantly weaker than in Cl2SiO: f(SiCl): 2.55 ͑ClSiO͒/3.31
(Cl2SiO). This weakness of the Si–Cl bond is also in line
TABLE V. Calculated frequencies and intensities ͑in km/mol͒ of OSiCl.
␯͑SiO͒ϭ1149.3 cmϪ1
␯͑SiCl͒ϭ486.6 cmϪ1
␦͑ClSiO͒ϭ229.4 cmϪ1
͑49͒
͑98.5͒
͑12.8͒
FIG. 3. Geometry, charges, and SENs of OSiCl.
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¨
M. Junker and H. Schnockel
3772
J. Chem. Phys., Vol. 110, No. 8, 22 February 1999
5
6
TABLE VI. Experimental and corrected isotopic shifts of OSiCl.
¨
M. Binnewies and H. Schnockel, Chem. Rev. 90, 321 ͑1990͒.
¨
M. Junker, A. Wilkening, M. Binnewies, and H. Schnockel ͑to be pub-
Experimental
Corrected
lished͒.
7 I. Barin, Thermochemical Data of Pure Substances, Part I & II ͑Verlag
16O/18O␯͑SiO͒
28Si/29Si␯͑SiO͒
35Cl/37Cl␯͑SiCl͒
39.9
7.2
6.1
40.3
7.3
Chemie, Weinheim, 1989͒.
8
¨
Leybold RGD 510 closed-cycle cryostat, Leybold AG, Koln.
6.2
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͑109°͒ in comparison with 125.2° in ClSiO plausible.
Note added in proof. While correcting the proof we got
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