Chertihin, Bare, and Andrews: Reactions of chromium with dioxygen
2803
The weak 844.8 cmϪ1 band shifts to 800.1 cmϪ1 with
18O2, gives only these bands with the mechanical mixture
and a quartet of nearly equal components with the statistical
mixture. The 16/18 ratio ͑1.0559͒ characterizes an O–O
stretching mode. As one chromium atom appears to be inca-
pable of transferring two electrons to form the peroxide, ex-
pected in this region, perhaps Cr2 can do so. The 844.8 cmϪ1
band is tentatively assigned to such a Cr2–OO complex with
the Cr2 arrangement undetermined but with an end-on ar-
rangement of OO, as indicated by the quartet splitting pattern
observed in experiments with scrambled O2.
the absorbance at 965.4 cmϪ1. This band is 10.1 cmϪ1 less
than the sum of and for OCrO. It produced an almost
1
3
symmetrical triplet with scrambled oxygen, and its 16/18 iso-
topic ratio ͑1.0447͒ is the average of the and isotopic
1
3
ratios. Taking into account anharmonicity, the weak
1869.7 cmϪ1 band is assigned to the 1ϩ combination
3
band. Note that the 52Cr16O2–53Cr16O2 isotopic splitting in
the 1ϩ band, 4.1 cmϪ1, is the sum of these splittings, 3.0
3
and 1.1 cmϪ1, in the and bands; similar agreement is
3
1
found for Cr18O2. Observation of this combination band con-
firms that the present and assignments are correct.
1
3
The dioxide OCrO is known from high-temperature
mass spectrometric studies, and its formation from O2 is
highly exothermic.27 However, the 965.4 and 914.4 cmϪ1
bands decreased upon annealing, which means that the inser-
tion reaction requires significant activation energy. The diox-
ide OCrO can be formed by reaction ͑3͒ on the matrix sur-
face during deposition or through photolysis with energetic
Cr atoms. Cold reaction ͑4͒ produces the CrOO molecule, as
proposed for the room temperature process.26
Similar 1ϩ combination bands have been observed for
3
the OCoO, OPbO and OVO molecules.17,24,25
Experiments with N2O produced the 973.7, 970.5, and
965.4 cmϪ1 bands on deposition. The first band increased
markedly on annealing; the second and third bands increased
on initial then decreased on further annealing, Experiments
with O2 /N2 mixture and its oxygen isotopic modifications
revealed strong 976.3 and 973.3 cmϪ1 bands after deposition.
First annealing decreased the 976.3 and increased the
973.3 cmϪ1 bands. Experiments with mechanical mixtures
revealed doublets for each band and 16/18 isotopic ratios
near 1.0386. The latter bands produced triplets with
scrambled oxygen, indicating two equivalent oxygen atoms.
These bands are assigned to (N2)xCrO2 complexes.
27
*
Cr ϩO →OCrO ⌬EϭϪ113Ϯ11 kcal/mol͒,
͑3͒
͑4͒
͑
2
CrϩO2→CrOO
CrO3
CrOO
The sharp 968.4 cmϪ1 band appeared in the spectra after
annealing and was stronger with 1% than with 0.5% O2.
Isotopic substitution with both mixtures revealed four bands
at 968.4, 960.2, 950.7, and 932.1 cmϪ1 that increased to-
gether on annealing to 40 K with an intensity distribution of
approximately 2:1:1:2, and the ͑16/18͒ isotopic ratio 1.0391
͑slashes in Fig. 4͑b͒ separate absorbance increase at these
frequencies on 40 K annealing from absorbance present be-
fore annealing͒. This isotopic pattern is characteristic of the
3(e) fundamental for C3v or D3h XY3 molecules, where
the nondegenerate antisymmetric stretching vibrations of
mixed isotopic molecules of lower symmetry also make con-
tributions to the absorptions due to the degenerate vibration
of the pure isotopic molecules,28 and assignment to CrO3 is
suggested. The harmonic 16/18 isotopic ratio for the D3h
structure ͑1.404͒ is slightly higher than the observed value,
which may be accounted for by anharmonicity. Although the
1(a1) vibrations of the mixed isotopic molecules of lower
symmetry are allowed, they are too weak to detect here.
The yield of CrO3 increased on annealing: two reactions
͑5͒ and ͑6͒ are expected to be highly exothermic27 and to
require little activation energy. The 968.4 cmϪ1 band was
observed after deposition and markedly increased on anneal-
ing in the CrϩN2O/Ar system. This suggests that reaction ͑7͒
is exothermic27 and leads to CrO3 formation in this system
Spectra in the 1200–1000 cmϪ1 region are shown in Fig.
2. Three bands at 1153.9, 1134.2, and 1108.7 cmϪ1 appeared
in the spectra after annealing to 25 K. Further annealing
increased the first two bands and decreased the last band.
The second band has been assigned to the OOCrOO
molecule.8 The 1108.7 cmϪ1 band revealed 1125 and
1056 cmϪ1 components in the 16O2ϩ18O2 experiments,
which is analogous to the pattern for the 1134.2 cmϪ1 band.
However, the 1153.9 cmϪ1 band which shifted to
1090.6 cmϪ1 with 18O2, showed no intermediate with
16O2ϩ18O2, and a single intermediate at 1121.1 cmϪ1 with
scrambled oxygen. The 1153.9 cmϪ1 band is here assigned to
the CrϩO2 complex, but the structure ͓CrOO vs Cr͑O2)͔
cannot be determined. Such a complex has been proposed for
reaction of ground state Cr atoms with O2 at room
temperature.26 DFT calculations show that the most stable
Cr͑O2) state, the quintet should have an O–O vibration be-
low 1000 cmϪ1, which makes it similar to the analogous
cyclic peroxo complexes in the Fe, Co, and NiϩO2
systems.14–17 On the other hand, the next Cr͑O2) state, the
triplet, has a calculated frequency of 1189 cmϪ1. It is known
that the 3d5 configuration of Crϩ is energetically favorable,
which suggests that chromium is unlikely to transfer two
electrons to O2 to form a peroxo complex. DFT calculations
suggest that quintet is the most stable state for the end-
bonded superoxo isomer, and a strong 1165 cmϪ1 band is
predicted, which correlates with the present assignment. The
question about the relative stability of CrOO and Cr͑O2) re-
mains open, and a higher level of theory must be applied to
answer it.
CrOϩO2→CrO3,
͑5͒
͑6͒
͑7͒
OCrOϩO→CrO3,
OCrOϩN2O→CrO3ϩN2.
J. Chem. Phys., Vol. 107, No. 8, 22 August 1997
Chertihin, George V.
