Romano et al.
either X or Y ) Cl is the familiar radical ClCO•,13 which
subsequently takes up a second halogen atom to form the
appropriate carbonyl dihalide XC(O)Cl (X ) Cl, Br, or I).
Scheme 1. Members of the Carbonyl Dihalide Family. The Formulas
in Red Correspond to Previously Unknown Species
By contrast, the photoinduced reaction with Br
2
gives rise,
•
initially, to the radical BrCO , which has hitherto eluded
detection; unlike its chloro counterpart, but in keeping with
the results of earlier sophisticated quantum chemical calcula-
1
4
tions, this is most aptly formulated as a weakly bound van
•
der Waals complex, engaging CO to a Br atom.
Experimental Section
Commercial samples of ICl and IBr (Aldrich) were purified by
repeated trap-to-trap condensation in vacuo, mainly to remove the
I
2
impurity, the presence of which hampered photochemical studies
by its inherently high absorption cross section throughout much of
the visible and UV regions. Cl and Br (Aldrich) were used without
further purification. BrCl was produced by mixing equimolar
amounts of Cl and Br , leading to an equilibrium mixture of BrCl,
Cl , and Br . CO and Ar gases (BOC, research grade) were used
2
2
9
ClC(O)SBr, has served to point the way to a means of
2
5
2
1
completing the carbonyl dihalide family. Broad-band UV-
visible irradiation was then shown to lead not only to the
partial transformation of the more stable syn form into the
anti form of the molecule but also to the formation of its
isomer BrC(O)SCl, as well as the photoevolution fragments
2
2
without further purification.
Gas mixtures of the dihalogen or interhalogen molecules (XY),
CO, and Ar, typically with the composition XY/CO/Ar ) 1:1:200,
were prepared by standard manometric methods. Each such mixture
was deposited on a CsI window cooled to ca. 15 K by a Displex
closed-cycle refrigerator (Air Products, model CS202), using the
pulsed deposition technique.1 Alternatively, a modified assembly
was used in some experiments with IBr to overcome its low vapor
pressure at ambient temperatures. In this case, the CO/Ar mixture
was passed through a U trap containing IBr cooled to -45 °C before
9
BrCl, BrSCl, CO, and OCS. The mechanisms proposed for
the various processes imply active rather than passive roles
for the stable products CO and OCS. Hence, it has already
been shown, for example, that hitherto unknown compounds
such as BrC(O)SBr can be formed under matrix conditions
by the photoactivation of a dihalogen in the presence of
OCS.10
6,17
15
deposition on the CsI window, continuous deposition being carried
out at the rate of ca. 1-2 mmol h
-1
.
Here, we report similar reactions involving a dihalogen
and CO isolated together in an Ar matrix at about 15 K and
show that broad-band UV-visible photolysis results in the
formation of one or more carbonyl dihalides, XC(O)Y, where
X and Y ) Cl, Br, or I and may be the same or different
atoms. Hence, carbonyl iodide chloride, IC(O)Cl, and car-
bonyl iodide bromide, IC(O)Br, two of the missing members
of this well-known family of molecules, have finally been
produced (by the reactions of CO with ICl and IBr,
respectively). These and the known carbonyl dihalides
The IR spectrum of each matrix sample was recorded at a
-1
resolution of 0.5 cm , with 256 scans and a wavenumber accuracy
-
1
of (0.1 cm , using a Nicolet Magna-IR 560 FTIR instrument
equipped with either an MCTB or a DTGS detector (for the ranges
-
1
-1
4
000-400 cm and 600-250 cm , respectively). Following
deposition and IR analysis of the resulting matrix, the sample was
exposed to broad-band UV-visible radiation (200 e λ e 800 nm)
issuing from a Spectral Energy Hg-Xe arc lamp operating at
8
00 W. The output from the lamp was limited by a water filter to
absorb infrared radiation and to minimize any heating effects. The
IR spectrum of the matrix was then recorded at different times of
irradiation to closely scrutinize any decay of the absorptions due
to the reactants and the growth of absorptions due to the respective
products.
2 2
OCCl , OCBr , and BrC(O)Cl, formed in similar reactions
involving the relevant halogen atoms, have been character-
ized experimentally by their infrared (IR) spectra and
theoretically by the results of ab initio [Hartree-Fock (HF)
and Moeller-Plesset second-order (MP2)] and density
functional theory (DFT) calculations. As reported re-
All of the quantum chemical calculations were performed using
the Gaussian 98 program system18 under the Linda parallel
execution environment using two coupled PCs. Different ab initio
and DFT methods were tried, in combination with a 6-31+G* basis
1
1,12
cently,
trapping XY and CO together in an Ar matrix
1
9
set for C, O, and Cl atoms and a LANL2DZ basis set including
an effective core potential (ECP) for Br and I atoms. The ECP
chosen is that proposed by Hay and Wadt,20 which incorporates
results, prior to photolysis, only in the formation of a weakly
bound adduct, XY‚‚‚CO. The first product of photolysis when
(
8) (WMO) World Meteorogical Organization. Scientific Assessment of
Ozone Depletion: 2002; Global Ozone Research and Monitoring
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Switzerland, 2003.
(13) Schn o¨ ckel, H.; Eberlein, R. A.; Plitt, H. S. J. Chem. Phys. 1992, 97,
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(14) Dixon, D. A.; Peterson, K. A.; Francisco, J. S. J. Phys. Chem. A 2000,
104, 6227-6232.
(
9) Romano, R. M.; Della V e´ dova, C. O.; Downs, A. J.; Greene, T. M. J.
Am. Chem. Soc. 2001, 123, 5794-5801.
(15) Mattraw, H. C.; Pachucki, C. F.; Hawkins, N. J. J. Chem. Phys. 1954,
22, 1117-1119.
(16) (a) Almond, M. J.; Downs, A. J. AdV. Spectrosc. 1989, 17, 1-511.
(b) Dunkin, I. R. Matrix-Isolation Techniques: A Practical Approach;
Oxford University Press: New York, 1998.
(17) Perutz, R. N.; Turner, J. J. J. Chem. Soc., Faraday Trans. 2 1973, 69,
452-461.
(
(
(
10) Romano, R. M.; Della V e´ dova, C. O.; Downs, A. J. Chem. Commun.
2001, 2638-2639.
11) Schriver, A.; Schriver-Mazzuoli, L.; Chaquin, P.; Bahou, M. J. Phys.
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3242 Inorganic Chemistry, Vol. 44, No. 9, 2005