Matrix isolation infrared spectroscopic and density functional study of the mechanism of the oxidation of CH3OH by CrCl2O2

Article abstract of DOI:10.1021/ja9808752

The matrix isolation technique has been employed to isolate several intermediates, in sequence, in the oxidation of CH₃OH by CrCl₂O₂. Consistent with previous theoretical calculations, a hydrogen-bonded complex formed initially after twin jet deposition and was enhanced by matrix annealing to 33 K. This complex was photodestroyed by Hg are irradiation with λ < 500 nm, and led to the production of HCl and ClCrO₂OCH₃. These species were also produced by room temperature reaction of the two precursors in a flow system followed by rapid matrix trapping. Heating the flow reaction zone above 150°C led to destruction of ClCrO₂OCH₃, and production of CH₂O and HCl. Above 250°C bands due to CH₂O were destroyed and bands due to CO and CO₂ grew in. High-level density functional calculations (B3LYP/6-311G*) were carried out to identify potential intermediates in this system and to provide theoretical vibrational spectra for comparison to experiment.

Full text of DOI:10.1021/ja9808752

J. Am. Chem. Soc. 1998, 120, 6105-6112
6105
Matrix Isolation Infrared Spectroscopic and Density Functional Study
of the Mechanism of the Oxidation of CH OH by CrCl O
3
2
2
Bruce S. Ault
Contribution from the Department of Chemistry, UniVersity of Cincinnati, Cincinnati, Ohio 45221-0172
ReceiVed March 16, 1998
Abstract: The matrix isolation technique has been employed to isolate several intermediates, in sequence, in
the oxidation of CH3OH by CrCl2O2. Consistent with previous theoretical calculations, a hydrogen-bonded
complex formed initially after twin jet deposition and was enhanced by matrix annealing to 33 K. This complex
was photodestroyed by Hg arc irradiation with λ < 500 nm, and led to the production of HCl and
ClCrO2OCH3. These species were also produced by room temperature reaction of the two precursors in a
flow system followed by rapid matrix trapping. Heating the flow reaction zone above 150 °C led to destruction
of ClCrO2OCH3, and production of CH2O and HCl. Above 250 °C bands due to CH2O were destroyed and
bands due to CO and CO2 grew in. High-level density functional calculations (B3LYP/6-311G*) were carried
out to identify potential intermediates in this system and to provide theoretical vibrational spectra for comparison
to experiment.
Introduction
characterization of these intermediates would contribute sub-
stantially to our understanding of these important chemical
High valent transition metal oxyhalide compounds, including
chromyl chloride CrCl2O2, are well-known to serve as potent
oxidizing agents for a variety of organic substrates.1-4 Selective
oxidation is important in a range of chemical industries, and
systems.
The matrix isolation technique1
9-21
was developed in the
1
950s to facilitate the isolation and characterization of short-
lived intermediates. This approach has been applied to the study
of diverse species, including radicals, ions, and weakly bound
molecular complexes. Recently, a simple flowing gas reactor
was coupled to matrix isolation to permit the study of gas-phase
numerous studies have sought to determine the mechanism by
which these compounds serve as oxidizing agents.5
-10
None-
theless, many of the mechanistic details have not been well
established. These compounds have also been invoked as
models for the active sites of a number of metalloenzymes,
2
2
bimolecular reactions, with subsequent trapping of the inter-
mediates species in cryogenic matrixes. Very recently, one
group reported a study of the photochemical reactions of
CrCl2O2 with olefins in argon matrixes and ethylene oxide in
1
1
including cytochrome P-450. In addition, chromyl chloride and
a number of CrVI compounds are suspected carcinogens, very
likely due to their great oxidizing power. Concurrently, the
development of more powerful and sophisticated computers has
led to several theoretical studies of the mechanism of reaction
of CrCl2O2 and related compounds with small organic
2
3,24
low-temperature solution.
Also, while the majority of
studies of the reactivity of chromyl chloride have been in
solution, CrCl2O2 has substantial vapor pressure and some gas-
9
,10
phase reaction chemistry has been explored.
Since the
1
2-15
molecules.
Several of these studies have focused on the
flowing gas reactor limits the time available for reaction before
matrix deposition, this approach should be appropriate for the
study of the intermediates in the reaction of chromyl chloride
with small organic substrates. In this study, intermediates and
reaction products of the system CrCl2O2 + CH3OH are reported.
reaction of CrCl2O2 with CH3OH, calculating pathways and
intermediates in the reaction sequence.1
6-18
Observation and
(
(
1) Hartford, W. H.; Darrin, M. Chem. ReV. 1958, 58, 1.
2) Sharpless, K. B.; Teranishi, A. Y.; Backvall, J.-E. J. Am. Chem. Soc.
1
1
1
977, 99, 3120.
3) Freeman, F.; McCart, P. D.; Yamachika, N. J. J. Am. Chem. Soc.
970, 92, 4621.
4) Scott, S. L.; Bakac, A.; Espenson, J. H. J. Am. Chem. Soc. 1992,
14, 4205.
(
Experimental Section
(
All of the experiments in this study were conducted on conventional
25
matrix isolation apparatus that has been described. In the flowing
(
(
(
5) Westheimer, F. H.; Chang, Y. W. J. Phys. Chem. 1959, 63, 438.
6) Wiberg, K. B.; Eisenthal, R. Tetrahedron 1964, 20, 1151.
7) Necsoiu, I.; Balaban, A. T.; Pascaru, I.; Sliam, E.; Elian, M.;
22
gas reactor, referred to as merged jet deposition, the two gaseous
(17) Deng, L.; Ziegler, T. Organometallics 1996, 15, 3011.
(18) Deng, L.; Ziegler, T. Organometallics 1997, 16, 716.
(19) Hallam, H. E. Vibrational Spectroscopy of Trapped Species; John
Wiley and Sons: New York, 1973.
(20) Craddock, S.; Hinchliffe, A. Matrix Isolation; Cambridge University
Press: Cambridge, 1975.
(21) Chemistry and Physics of Matrix Isolated Species; Andrews, L.,
Moskovitz, M., Eds.; Elsevier Science Publishers: Amsterdam, 1989.
(22) Limberg, C.; Koppe, R.; Schnockel, H. Angew. Chem., Int. Ed. Engl.
In press.
(23) Limberg, C.; Cunskis, S.; Frick, A. J. Chem. Soc., Chem. Commun.
1998, 225.
Nenitzescu, C. D. Tetrahedron 1963, 19, 1133.
(
(
(
(
(
8) Freeman, F.; Arledge, K. W. J. Org. Chem. 1972, 37, 2656.
9) Cook, G. K.; Mayer, J. M. J. Am. Chem. Soc. 1994, 116, 1855.
10) Cook, G. K.; Mayer, J. M. J. Am. Chem. Soc. 1995, 117, 7139.
11) Sharpless, K. B.; Flood, T. C. J. Am. Chem. Soc. 1971, 93, 2316.
12) Rappe, A. K.; Goddard, W. A., III J. Am. Chem. Soc. 1980, 102,
5
4
3
114.
(
48.
(
13) Rappe, A. K.; Goddard, W. A., III J. Am. Chem. Soc. 1982, 104,
14) Rappe, A. K.; Goddard, W. A., III J. Am. Chem. Soc. 1982, 104,
287.
(
(
15) Rappe, A. K.; Goddard, W. A., III Nature 1980, 285, 311.
16) Ziegler, T.; Li, J. Organometallics 1995, 14, 214.
(24) Carpenter, J. D.; Ault, B. S. J. Phys. Chem. 1991, 95, 3502.
(25) Ault, B. S. J. Am. Chem. Soc. 1978, 100, 2426.
S0002-7863(98)00875-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/06/1998

6106 J. Am. Chem. Soc., Vol. 120, No. 24, 1998
Ault
reactants, each diluted in argon, were prepared in separate gas-handling
manifolds. The two deposition lines were then joined with an Ultratorr
tee at a distance from the cryogenic surface, and the flowing gas samples
were permitted to mix and react during passage through the merged
region. The length of this region was variable, from 10 to 300 cm. In
addition, the merged region could be heated to as high as 400 °C, to
induce further reaction. The flowing gas mixture exited the tip of the
deposition line and was sprayed onto the 14 K cold surface for 20-24
h before final infrared spectra were recorded either on a Bruker 113v
or a Perkin-Elmer Spectrum 2000 Fourier transform infrared spectrom-
-
1
eter at 1 cm resolution. Additional experiments were conducted in
the twin jet mode, where the two reactants, each diluted in argon, were
co-deposited on the cold surface from separate nozzles. A number of
these matrixes were subsequently warmed to 33-35 K to permit
annealing and limited diffusion, and then recooled to 14 K and
additional spectra recorded. Also, many of these matrices were
irradiated with the filtered output of a medium-pressure Hg arc after
which additional spectra were recorded.
Chromyl chloride (Aldrich) was introduced into the vacuum system
as the vapor above the room-temperature liquid, after purification in a
glass finger by freeze-pump-thaw cycles. CH
3
OH (Aldrich), CD
OH, CD OD, CH OH, and CH 18OH (all Cambridge Isotope
3
-
1
3
3 3 3
Laboratories, 99% isotopic enrichment) were also introduced as the
vapor above the room-temperature liquid, after repeated freeze-pump-
2
thaw cycles. In several experiments, CH O was employed as a reactant;
this species was produced by sublimation of solid paraformaldehyde
CH O) at approximately 50 °C from a metal finger attached to the
Figure 1. Infrared spectra over selected spectral regions of matrixes
(
2
x
prepared by twin jet deposition of samples of Ar/CrCl
2 2
O ) 100 and
deposition line. In a few experiments, HCl (Matheson) was employed
as a reactant after introduction into the vacuum system from a lecture
bottle and purification by freeze-pump-thaw cycles at 77 K. Argon
was used as the matrix gas in all experiments, and was used without
further purification.
Ar/CH OH ) 250. The upper trace was taken before annealing while
3
the lower trace was taken after annealing to 33 K and recooling to 14
K. Bands denoted by an asterisk are attributable to species A.
addition, blank experiments were conducted in the merged jet
mode with the merged region heated to as high as 380 °C, for
comparison to the merged jet pyrolysis experiments. The
spectra of Ar/CrCl2O2 ) 100 samples after pyrolysis to 380 °C
were unchanged relative to deposition through a room-temper-
ature line. In contrast, there was some evidence of decomposi-
Ab initio calculations were conducted on the likely intermediate
species in this study, using the Gaussian 94 suite of programs.26 Both
restricted Hartree-Fock and density functional calculations employing
the Becke functional B3LYP were conducted to locate stable minima,
determine structures, and calculate vibrational spectra. Final calcula-
tions with full geometry optimization employed the 6-311G* triple-ú
basis set, after initial calculations with smaller basis sets were run to
approximately locate energy minima. Calculations were carried out
on a Silicon Graphics Indigo 2 workstation.
tion of Ar/CH OH samples above 300 °C, leading to the
3
production of CH4, CO, and CO2 in the matrix. There was no
evidence of decomposition of Ar/CH2O samples during high-
temperature pyrolysis.
CrCl2O2 + CH3OH. (a) Twin Jet Experiments. The
reaction chemistry of these two compounds was initially
explored in a twin jet experiment in which a sample of Ar/
CrCl2O2 ) 100 was co-deposited with a sample of Ar/CH3OH
Results
Blank experiments were carried out in the twin jet mode for
each of the reagents alone in argon prior to any co-deposition
experiments. The spectra obtained were in good agreement with
)
250. The resultant spectrum showed intense bands due to
literature spectra.2
7-33
the two parent species, along with a set of very weak new
absorptions at 429, 443, 944, 978, 1010, 1313, 3586, and 3596
Several of these blank experiments were
subsequently annealed to around 34 K, recooled, and scanned.
Some aggregation was noted in spectra of CH3OH samples,
while no distinct changes were noted in spectra of CrCl2O2
samples. A blank experiment of each reagent was also irradiated
with the Pyrex/H2O-filtered output of a medium-pressure Hg
arc lamp. No changes were seen as a result of irradiation. In
-
1
-1
cm , with the 944, cm band appearing as a triplet with
-
1
maxima at 938, 944 and 951 cm . This matrix was then
annealed to 33 K and recooled to 14 K and an additional
spectrum recorded. All of the new, weak bands listed above
(hereafter referred to as set A) grew by an factor of nearly 10,
as shown in Figure 1. In addition, all of these bands grew by
the same amount (i.e. they maintained a constant intensity ratio
with respect to one another). A very slight reduction in parent
monomer intensities was noted, and some growth in bands due
to the parent CH3OH dimer (also seen in annealed blank
experiments). This annealed matrix was then irradiated for 1
h with the output of a medium-pressure Hg arc, using a Pyrex/
H2O filter, and an additional spectrum recorded. The bands of
set A were greatly decreased as a result of irradiation, and new
infrared absorptions appear at 676, 1129, 1150, 1425, and 1445
(
26) Gaussian 94, Revision E.1; Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.;
Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.;
Cioslowski, J.; Stefanov, B. B.; Nanayakkara, M.; Challacombe, M.; Peng,
C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Binkley, J. S.;
Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.;
Pople, J. A.; Gaussian, Inc.: Pittsburgh, PA, 1995.
(
(
27) Varetti, E. L.; Muller, A. Spectrochim. Acta 1978, 34A, 895.
28) Beattie, I. R.; Marsden, C. J.; Ogden, J. S. J. Chem. Soc., Dalton
Trans. 1980, 535.
(
(
29) Barnes, A. J.; Hallam, H. E. Trans. Faraday Soc. 1970, 66, 1920.
30) Barnes, A. J.; Hallam, H. E.; Scrimshaw, G. F. Trans. Faraday Soc.
-1
cm (hereafter referred to as set B) as well as a group of bands
-
1
1
969, 65, 3150.
31) Maillard, D.; Schriver, A.; Perchard, J. P. J. Chem. Phys. 1979, 71,
05.
at 2704, 2731, 2752, 2764, 2784, and 2800 cm (set C), as
shown in Figure 2. Further irradiation resulted in complete
destruction of bands in set A, and some additional growth of
bands in sets B and C.
(
5
(
(
32) Nelander, B. J. Mol. Struct. 1978, 50, 223.
33) Bach, S. B. H.; Ault, B. S. J. Phys. Chem. 1984, 88, 3600.

Mechanism of the Oxidation of CH3OH by CrCl2O2
J. Am. Chem. Soc., Vol. 120, No. 24, 1998 6107
Figure 3. Irradiation time dependence of the absorptions of species
A, B, and C, as a fraction of the total absorbance change due to
irradiation. For species A, the values are the average of the four major
product bands, while for species B and C, the values are the average
of the three major product bands.
light of the Hg arc, using a quartz/H2O filter that transmitted λ
>
200 nm. Complete destruction of the bands in set A was
noted, along with a significant growth of the bands of sets B
and C. No other changes were noted in the spectrum. In all of
these irradiation experiments, no changes were noted in bands
due to the parent species (consistent with irradiated blank
experiments).
In another experiment, an annealed matrix containing CH3-
OH and CrCl2O2 was irradiated for a number of short intervals
with use of the Hg arc with Pyrex/H2O filter. In the initial 15
min irradiation period, bands in set A were reduced by
approximately 50% of their original intensity, while bands of
sets B and C grew to approximately 50% of their final
absorbance (see below). Another 15 min irradiation period
reduced set A to 25% of the original intensity, while sets B
and C grew to 75% of their final intensity. Additional irradiation
intervals were employed, followed by a long (120 min) interval.
By the end of the irradiation intervals, set A was completely
destroyed and sets B and C had reached a maximum and did
not grow further. From the intensities as a function of irradiation
time, a destruction curve could be constructed for set A and
growth curves for sets B and C; these are shown in Figure 3.
(b) Merged Jet Experiments. These two reactants were co-
deposited in the merged jet mode in a large number of
experiments, employing a wide range of experimental condi-
tions. In an initial experiment with the merged region held at
room temperature and with Ar/CrCl2O2 ) 100 and Ar/CH3OH
) 500, eight new product bands of medium intensity were
observed in the resulting spectrum, at 676, 1129, 1150, 1425,
1445, 2815, 2863, and 2888 cm , as shown in Figure 4. The
first five bands match the position and relative intensities of
the set B, above, while the latter three form set D. Quite intense
parent bands were also observed. The overall yield of the set
B bands was somewhat higher in the merged jet experiments
than in the twin jet irradiation experiments. Also, the bands
comprising sets A and C (above) were observed very weakly
in this and all merged jet experiments. In several subsequent
experiments employing the same experimental conditions, these
bands were reproducible.
Figure 2. Infrared spectra over selected spectral regions of matrixes
prepared by twin jet deposition of samples of Ar/CrCl
Ar/CH OH ) 250. The lower trace was recorded after annealing and
before irradiation, while the upper trace was recorded after 90 min of
irradiation with a medium-pressure Hg arc, with a Pyrex/H O filter.
2 2
O ) 100 and
3
2
Bands denoted by an asterisk are attributable to species B and C. A
decrease in bands due to species A can be seen as well.
A number of twin jet experiments were conducted after this
initial run. In one set of experiments, the concentrations of the
two reactant samples were varied from as concentrated as 50/1
to as dilute as 500/1. In all cases, the weak product bands of
set A remained, and they again grew significantly upon
annealing of the matrix to between 33 and 36 K. It is
noteworthy that these bands maintained a constant intensity ratio
with respect one another throughout all of the experiments within
uncertainty limits on the absorbances of the very weak bands.
A number of matrices were irradiated under different condi-
tions to further explore the loss of bands in set A and the growth
of bands in sets B and C. In one experiment, the wavelength
dependence of photolysis was investigated. When an annealed
matrix was irradiated with a medium-pressure Hg arc with use
of a H2O filter and a glass filter that only transmitted λ > 600
nm, no changes were observed in the spectrum (set A did not
decrease and sets B and C did not grow in). When a glass
filter was employed that transmitted λ > 540 nm, a slight
decrease in sets A and a slight growth in sets B and C were
noted after 130 min of irradiation. When a filter was used that
transmitted λ > 400 nm, a large reduction in sets A was seen
with a significant growth in sets B and C. Finally, in a separate
experiment the reagents were co-deposited in the twin jet mode,
and the matrix was annealed and then irradiate with the full
-
1
After these initial experiments, a number were conducted in
which the concentrations of the two reactants were each varied
over the range 100/1 to 500/1, again using a room-temperature
merged deposition line. In these experiments, the medium
intensity bands of sets B and D and the very weak bands of

6108 J. Am. Chem. Soc., Vol. 120, No. 24, 1998
Ault
Figure 4. Infrared spectrum over selected spectral regions of a matrix
prepared by twin jet deposition of samples of Ar/CrCl ) 100 and
Ar/CH OH ) 250 (lower trace) compared to the infrared spectrum of
a matrix prepared by merged jet deposition of samples of Ar/CrCl
100 and Ar/CH OH ) 250 (upper trace). The reaction zone was
2
O
2
Figure 5. Infrared spectrum over selected spectral regions of a matrix
prepared by merged jet deposition of samples of Ar/CrCl
2 2
O ) 100
3
2
O
2
and Ar/CH OH ) 250, with the reaction zone held at 200 °C.
3
)
3
1170, 1499, 1732, 1744, 2620, 2799, and 2855 cm- (set E),
while the bands in set D increased in intensity. As the
temperature of the reaction zone was increased in a subsequent
experiment to 200 °C, these bands grew in intensity. In the
highest yield experiments, these bands were very intense,
1
held at room temperature. Bands denoted by an asterisk are attributable
to species B and D. Note also the absence of bands due to species C.
sets A and C were seen reproducibly, although the absolute yield
of product varied with the sample concentrations. Optimal
product yield was obtained when the reactants were at a 1:1
stoichiometric ratio. Additional experiments were then con-
ducted by using reaction zones (merged regions) of different
lengths, from as short as 15 cm to as long as 300 cm. In these
experiments, the same product bands (medium intensity sets B
and D and very weak sets A and C) were observed with the
same relative intensities. The absolute yield of the product
bands was somewhat decreased but still observable with the
short reaction zone while the yield of product with the 300 cm
reaction zone was no greater than with the initial 125 cm
reaction region. Further, no additional product bands were seen
in these experiments with different lengths of the reaction zone.
A series of experiments was then conducted by using the
original length (125 cm), and heating the reaction zone to
increasingly higher temperatures. When the temperature of the
reaction zone was held at or below 100 °C, the same product
bands were observed, with approximately the same intensities.
However, heating above 100 °C led to a reduction in intensity
of all of the bands of sets A, B, and C; above 150 °C, these
bands were completely absent from the spectrum. At the same
time (above 100 °C), new bands appeared in the spectrum, near
-
1
particularly the 1744 cm absorption, as shown in Figure 5.
-1
In addition, bands at 663, 2140, and 2344 cm became apparent
in this experiment. As the temperature of the reaction zone
was increased to 250, 300, 350, and then 380 °C in an additional
sequence of experiments, the bands in set E decreased in
intensity and then ultimately were completely absent in the very
high temperature experiments. On the other hand, the bands
-
1
near 663, 2140, and 2344 cm became very intense, with an
-1
additional band noted at 2275 cm . Also, the set D bands grew
in intensity as the temperature was increased.
(c) Isotopic Labeling Studies. To more completely identify
and characterize the species responsible for new absorptions in
the above experiments, CrCl2O2 was deposited with different
1
3
18
isotopomers of CH3OH, including CH3OH, CH3 OH, CD3-
OH, and CD3OD. While isotopic purity of the commercially
purchased enriched samples was 99% in the labeled element,
12,13
16,18
50% enriched samples of
CH3OH and CH3
OH were also
prepared by mixing equal volumes of the natural abundance
and enriched compounds. Several experiments were run with
each level of enrichment of each isotopomer, both in the twin
jet mode and in the merged jet mode with the 125 cm length

Mechanism of the Oxidation of CH3OH by CrCl2O2
J. Am. Chem. Soc., Vol. 120, No. 24, 1998 6109
a
Table 1. Infrared Spectra and Band Assignments for the 1:1 Hydrogen Bonded Complex of CrCl
O
2 2
3
with CH OH
12CH
16OH
13CH
16OH
12CH
18OH
12CD
16OH
12CD
16OD
assignment
3
3
3
3
3
4
29
43
44
78
429
443
944
978
429
443
944
978
984
429
443
944
978
972
429
443
944
978
972
784
1105
2655
Cr-Cl str
4
9
9
010
313
Cr-Cl str
Cr-O str (H-bonded O)
Cr-O str
1
1
C-O str
1306
3591
1306
1253
C-O-H bend
1
105
3
CH def
3
591
3580
3591
O-H str
a
Band positions in cm-1
.
a
Table 2. Infrared Spectrum and Band Assignments for ClCrO
2
OCH
3
12CH
16OH
76
129
13CH
16OH
12CH
18OH
12CD
16OH
12CD
16OD
assignment
3
3
3
3
3
6
671
1122
1143
1421
1440
662
1123
1144
1423
1442
652
652
Cr-O str
CH3 rock
CH3 rock
CH3 def
CH3 def
CH3 def
1
1
1
1
151
425
443
1054
1081
1054
1081
2176
2
176
a
Band positions in cm-1
.
reaction zone. The twin jet experiments paralleled closely those
conducted with normal isotopic CH3OH, above. For each
isotopomer, a set of weak absorptions was noted upon initial
deposition, analogous to set A, and are listed in Table 1. These
were observed to grow markedly upon sample annealing to 34
K and were destroyed by Hg arc irradiation. New bands
counterpart to set B were observed to grow in upon irradiation,
with shifts in band position as a result of isotopic labeling. The
isotopic set B bands are listed in Table 2. The set C bands
formed upon irradiation did not show any shifts when
CrCl2O2 + CH2O. Formaldehyde is a likely intermediate
oxidation product of CH3OH, and may itself be oxidized further.
To explore this possibility, CH2O was sublimed from solid
paraformaldehyde and passed through the reaction zone with
samples of Ar/CrCl2O2 ) 100. When CH2O and CrCl2O2,
diluted in argon, flowed together through the reaction zone held
at 160 °C, no new bands were observed in the spectrum while
very intense bands due to CH2O and CrCl2O2 were noted. When
similar experiments were conducted with the reaction zone at
330 °C, all of the bands of parent CrCl2O2 and CH2O were
absent, and strong new bands were observed at 663, 2140, and
13
18
CH3OH, CH3 OH or CD3OH was employed. However, when
CD3OD was employed, counterpart bands were observed at
-
1
2344 cm , as well as a multiplet of bands between 2800 and
-
1
-1
1
961, 1980, 1997, 2003, 2019, and 2027 cm .
2900 cm . In a final experiment, CH2O and CrCl2O2 were
reacted at the intermediate temperature of 240 °C. In this
experiment, some reduction of parent band intensities was noted,
along with some growth of the bands at 663, 2140, and 2344
In the merged jet experiments with the reaction region held
at room temperature, counterpart bands were observed to those
observed in the natural abundance experiments. The set B bands
-
1
-
1
cm , although not nearly to the same intensity as in the 330
at 676, 1129, 1150, 1425, and 1445 cm showed distinct
isotopic shifts as are listed in Table 2, again identical to the
bands that grew in as a result of irradiation in the twin jet
°
C experiments.
CrCl2O2 + HCl. In one experiment, a sample of Ar/HCl )
experiments. When 50% mixtures of 1 C and
2,13
16,18
O isoto-
pomers were employed in different experiments, the bands at
250 was co-deposited with a sample of Ar/CrCl O )100 in
2
2
the merged jet mode to look for possible interactions between
-1
-1
these two species. Only a single new absorption, at 2837 cm ,
was noted in this experiment after comparison to a comparable
blank experiment of Ar/HCl ) 250.
6
76, 1129, 1150, and 1425 cm each appeared as clear, distinct
doublets, with peaks at positions matching the pure C, C,
1
2
13
1
6
18
O, and O isotopic experiments. The bands in set D showed
no shift with 13C, O, or -CD3 substitution, but did shift with
18
-
1
Discussion
-
OD substitution, to near 2100 cm .
Experiments employing these different isotopomers were also
Numerous product bands and several product species were
isolated in the reaction of CrCl2O2 with CH3OH under a range
of reaction conditions. As noted above, many of the product
bands can be grouped into sets, based on the conditions under
which they appeared, and the fact that bands within a given set
maintained a constant intensity ratio with respect to other bands
in that set. It is also apparent that the different product species
are formed in sequence, as reaction conditions are altered and
additional energy deposited into the system. The identity of
the species responsible for each set of product bands will be
discussed, followed by the results of ab initio calculations and
an overview of the mechanism of reaction.
Product Identification. The bands in set A were formed
under the conditions of shortest reaction time and lowest reaction
temperature, namely in the twin jet deposition experiments
where mixing of the two reactants occurs on the surface of the
condensing matrix. This indicates the species A is the initial
conducted while heating the reaction zone to 150, 200, 250,
and 350 °C. A pattern similar to the natural abundance isotopic
experiments was observed: the bands of sets A, B and C were
destroyed by heating above 150 °C, and new product bands
grew in as isotopic counterparts to the set E bands. In the 50%
1
2,13
16,18
C and
O experiments, each of these bands appeared as
a distinct isotopic doublet. These are listed in Table 2. On the
-
1
other hand, the band at 2620 cm and the set D bands did not
1
3
18
shift with C and O isotopic labeling. However, when
CD3OH was employed, the set D bands shifted to near 2100
-
1
-1
-1
cm and the 2620 cm band shifted to 1916 cm . Above
50 °C, the set E bands were themselves destroyed, and bands
2
-
1
counterpart to the 663, 2140, and 2344 cm high-temperature
bands described above grew in. The 663 and 2344 cm bands
-1
12,13
appeared as doublets in the mixed
C experiments and triplets
in the mixed 1 O experiments.
6,18

6110 J. Am. Chem. Soc., Vol. 120, No. 24, 1998
Ault
intermediate in the reaction between CrCl2O2 and CH3OH.
Further, while the yield of species A was quite low during
deposition, species A was produced in significant amounts by
annealing the matrix to 33 K. Thus, the barrier to formation of
A from the isolated reactants must be very low. This is
indicative of the formation of a molecular complex between
Table 3. Optimized Structures
Cl
2
CrO(OH)OCH
3
2 3
ClCrO OCH
parameter
B3LYP/6-311g* a
parameter
B3LYP/6-311g* a
R(Cr-Cl)
2.205
2.305
1.529
1.787
0.966
1.729
1.405
1.098
1.097
1.090
85.9
110.6
96.7
135.4
82.4
113.5
103.0
117.1
90.8
159.9
127.4
109.3
108.2
108.3
R(Cr-Cl)
2.147
1.554
1.551
1.720
1.424
1.094
1.093
1.091
110.4
109.0
110.3
109.4
109.4
109.2
127.0
110.1
110.0
107.5
R(Cr-Cl)
R(CrdO)
R(CrdO)
R(CrdO)
34
the two subunits. Spectroscopically, this would be manifested
by a shifting of certain vibrational modes of the two subunits,
particularly modes involving atoms at the site of complexation.
In addition, since CH3OH is known to serve as a proton donor
in hydrogen bonding interactions and CrCl2O2 has four very
electronegative ligands, hydrogen bond formation is reasonable.
Spectroscopically, the three vibrational modes of the CH3OH
subunit most perturbed by the formation of the complex were
R(Cr-O)
R(Cr-O)
R(O-H)
R(C-O)
R(Cr-O)
R(C-H)
R(O-C)
R(C-H)
R(C-H)
R(C-H)
R(C-H)
a(OdCrdO)
a(OdCr-Cl)
a(OdCr-Cl)
a(Cl-Cr-O)
a(OdCr-O)
a(OdCr-O)
a(Cr-O-C)
a(O-C-H)
a(O-C-H)
a(O-C-H)
R(C-H)
a(Cl-Cr-Cl)
a(Cl-CrdO)
a(Cl-CrdO)
a(Cl-Cr-O)
a(Cl-Cr-O)
a(OdCr-O)
a(OdCr-O)
a(Cr-O-H)
a(Cl-Cr-O)
a(C1-Cr-O)
a(Cr-O-C)
a(O-C-H)
a(O-C-H)
a(O-C-H)
-
1
the O-H stretch (shifted from 3667 cm in the parent to 3591
-
1
cm in the complex), the C-O-H bend, and the C-O stretch
as shown in Table 1. These observations, particularly the
distinctly red-shifted hydrogen stretching mode, are consistent
with hydrogen bond formation.35 Also shifted to lower energy
-
1
is one of the CdO stretching modes of CrCl2O2 (from 990 cm
in the parent to 944 cm-1 in the complex) as well as the two
Cr-Cl stretching modes. Finally, detailed ab initio calculations
by Ziegler et al. concluded that the initial product in this system
is a hydrogen bonded complex.18 All of these observations point
to the identification of species A as a hydrogen bonded complex
between CH3OH and CrCl2O2, the initial intermediate for this
pair of reactants. Band assignments for this hydrogen bonded
complex are given Table 1.
a
This work.
reaction. DCl was produced only when CD3OD was employed.
This demonstrates that the -OH group leads to HCl production
(or -OD to DCl formation). The second fragment arising from
The initial complex was destroyed by Hg arc irradiation while
trapped in the cryogenic matrix, giving rise to the bands in sets
B and C. Set B was also formed in the room-temperature
merged jet reaction of CrCl2O2 with CH3OH, accompanied by
the production of bands in set D. The bands in set D can be
readily assigned to matrix isolated HCl by comparison to
authentic matrices containing HCl, as well as to the literature.
HCl elimination would be ClCrO2OCH3, a species analogous
to parent chromyl chloride but with a methoxy group replacing
one chloride ligand. Chemically, one might anticipate that this
species should be stable and the calculations of Ziegler et al.
1
6
did find an energy minimum for this species, as did earlier
1
4
calculations by Goddard and co-worker. ClCrO2OCH3 has
four quite electronegative ligands, three of which are different,
providing multiple sites for hydrogen bonding to the HCl that
is produced photochemically in the same cage (species C). Also,
the observation of distinct doublets when 50-50 mixtures of
This is consistent with a lack of 13C and 18O shift and a strong
deuterium shift for these bands. The bands in set C showed
similar isotopic behavior to HCl, with a lack of 1 C and
3
18
O
shift, and a strong deuterium shift. Specifically, the H/D
frequency ratio for each band of species C is 1.38, exactly the
1
2
13
16
18
CH3OH/ CH3OH and CH3 OH/CH3 OH were used dem-
36
onstrates that the product species contains 1 carbon atom and 1
oxygen atom (from the methanol reactant), consistent with
addition of a methoxy ligand to the chromium center. In
addition, ab initio calculations on ClCrO2OCH3 described below
predict very well the infrared spectrum observed for species B,
particularly the isotopic shifts of the Cr-O stretch of the
methoxy ligand. Thus, species B is identified as the HCl
elimination product of the initial hydrogen bonded complex,
ClCrO2OCH3. As will be discussed below, this conclusion is
not fully consistent with the calculations of Ziegler which
suggest that the hydrogen shift product, Cl2CrO(OH)OCH3, is
the second intermediate in the reaction pathway, but is consistent
with the calculations of Goddard et al. Band positions and
assignments for ClCrO2OCH3 are collected in Table 2.
While the photochemical reaction producing species B and
C did not lead to additional products, heating the reaction zone
in the merged jet experiments did lead to further reaction.
Above approximately 150 °C, bands due to species B were
destroyed and intense bands due to species D and E grew in.
Species D, as noted above, is identified as HCl, while the bands
of species E are readily assigned to CH2O and the complex of
ratio observed for HCl in the gas phase and in argon
3
0,31
matrixes.
1
However, the bands in set C were approximately
-
1
00 cm to lower energy of matrix-isolated HCl, suggesting
that they are due to hydrogen bonded HCl. From the growth
curves as a function of irradiation time (Figure 3), set C is
produced at the same time and at the same rate as set B, and
they both arise from the photodestruction of the initial hydrogen
bonded complex (species A). Thus, species B and species C
are very likely trapped within the same matrix cage. If HCl is
formed in the photoreaction, then it would be expected to form
a hydrogen bond with species B. In the thermal, merged jet
reaction the HCl that is produced is able to separate from species
B in the gas phase prior to matrix deposition, and is trapped
primarily as free HCl. Thus, species D is identified as free,
matrix-isolated HCl while species C is identified as HCl weakly
hydrogen bonded to species B.
Species B must arise from the photodestruction of species A
and the concomitant production of HCl. It is noteworthy that
when CD3OH was employed, DCl was not a product in either
the thermal, merged jet reaction or the photochemical twin jet
(
34) Ault, B. S. ReV. Chem. Intermed. 1988, 9, 233.
CH2O with HCl, by comparison with authentic samples and with
(35) Pimentel, G. C.; McClelland, A. L. The Hydrogen Bond; W. H.
32,33
the literature.
Further, the isotopic shifts of the bands in
Freeman: San Francisco, 1960.
36) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
set E matched exactly the known bands of CH2O. It is also
important to note that when CD3OH was employed, a high yield
(
th
Coordination Compounds, 5 ed.; Wiley-Interscience: New York, 1997.

Mechanism of the Oxidation of CH3OH by CrCl2O2
J. Am. Chem. Soc., Vol. 120, No. 24, 1998 6111
a
b
Table 4. Comparison of Calculated and Experimental Band Positions for ClCrO
2
OCH
3
and Cl
2
CrO(OH)OCH
3
band position
13C shift
18O shift
CD
exptl
-24
3
shift
calcd
-22
molecule
ClCrO OCH
mode
exptl
calcd
exptl
calcd
exptl
calcd
2
3
Cr-O stretch
676
1129
1151
1425
1443
676
1129
1151
1425
1443
684
1127
1143
1435
1447
620
1091
1147
1393
1432
-5
-7
-8
-4
-3
-5
-7
-8
-4
-3
-5
-8
-8
-5
-1
-3
-8
-9
-4
-4
-14
-6
-7
-2
-1
-14
-6
-7
-2
-1
-15
-3
-5
-2
0
CH
CH
CH
CH
3
3
3
3
rock
rock
not observed
not observed
deformation
deformation
-344
-389
-24
not observed
not observed
-371
-362
-342
-402
-22
Cl
2
CrO(OH)OCH
3
Cr-O stretch
-8
-4
-7
-2
-3
CH
CH
CH
CH
3
3
3
3
rock
rock
deformation
deformation
-395
-407
a
Calculated using density functional theory, with theB3LYP functional and the 6-311 g* basis set. b Band positions in cm-1
.
of DCl was formed, and the complex of DCl with CD2O.33 This
result is in agreement with earlier experimental work and with
ab initio calculations. When the reaction zone was heated above
additional bands are also calculated for ClCrO2OCH3, they are
fewer in number (due to the smaller number of atoms in the
molecule); those that are calculated to have substantial intensity
fall very close to intense parent bands of CH3OH and CrCl2O2
and may well be obscured.
2
50 °C, bands due to CH2O decreased in intensity and strong
-
1
bands grew in at 663, 2150, and 2344 cm . These are readily
assigned to CO and CO2, as well as the appropriate isotopomers
in the isotopic labeling experiments. Finally, when CH2O was
sublimed directly into the reaction zone of the merged jet system
and reacted with CrCl2O2 above 250 °C, bands due to CH2O
diminished in intensity and the bands due to CO and CO2
became very intense.
Calculations were also carried out at the same level (B3LYP/
6
-311G*) for CrO2, HCl, and CH2O, to evaluate the energetics
of the different species and the possible decomposition pathways
of ClCrO2OCH3 and Cl2CrO(OH)OCH3. These calculations
parallel those done by Ziegler and by Goddard, but with a
different set of functionals. The present calculations indicate
that ClCrO OCH is energetically more stable than Cl CrO-
Ab Initio Calculations. Hartree-Fock and density functional
DFT) calculations were carried out in support of the experi-
2
3
2
(
(OH)OCH by approximately 19 kcal/mol, in qualitative agree-
3
mental results, using the 3-21G* and 6-311G* basis sets; the
ment with findings of Ziegler and Goddard. The calculations
2
5
B3LYP functional was used for the DFT calculations. On
the basis of previous work, ClCrO2OCH3 and Cl2CrO(OH)OCH3
were the two species most likely to be identified as species B.
Both of these species optimized to stable minima; the structure
found for Cl2CrO(OH)OCH3 was quite similar to that calculated
by Ziegler et al. using density functional theory. The optimized
structure for ClCrO2OCH3 was relatively similar, as shown in
Table 3. Harmonic vibrational frequencies were then calculated
at both the RHF and DFT levels of theory and compared to the
observed spectrum of species B, including all isotopomers.
While only five bands were observed for species B, the fit was
overall better for ClCrO2OCH3. Both species are predicted to
further indicate that there is a large barrier, slightly greater than
100 kcal/mol, to the decomposition of ClCrO OCH to CrO ,
2
3
2
HCl, and CH O. This is in reasonable agreement with the
2
1
8
barrier calculated by Ziegler of +92 kcal/mol.
Mechanistic Inferences. A series of intermediates have been
suggested over the years to explain the role of CrCl2O2 in the
oxidation of CH3OH and other organic substrates. The earlier
calculations of Goddard et al. proposed specific intermediates
and the recent calculations of Ziegler et al. extended this work.
Ziegler in particular identified the initial intermediate as a
18
hydrogen bonded complex, experimentally observed for the
first time in the present study as species A. Both Ziegler and
Goddard identified Cl2CrO(OH)OCH3 and ClCrO2OCH3 as
potential second intermediates; both concluded that O-H bond
addition was energetically favorable to C-H bond addition.
Goddard calculated the latter species to be slightly lower in
energy than the former, as did Ziegler. These calculations are
consistent with the identification here of species B as ClCrO2-
OCH3. While lower in energy, Ziegler et al. concluded that
ClCrO2OCH3 was not an active intermediate in the oxidation
of CH3OH due to the substantial endothermicity they calcu-
-
1
have a Cr-O-C stretching mode in the 600-700 cm range
in the DFT/6-311G* calculation. The position and all of the
-
1
isotopic shifts clearly identify the band at 676 cm in the
merged jet experiments as this mode. The unscaled DFT/6-
-
1
3
11G* calculation puts this mode at 705 cm while for Cl2-
-
1
CrO(OH)OCH3 this mode is calculated at 639 cm , below the
experimental value. While no widely accepted scaling factor
has been adopted for B3LYP/6-311G*, a value around 0.97 is
a reasonable estimate given accepted scaling factors for B3LYP
with other basis sets (e.g. 0.961 for the 6-31G* basis set ). A
value slightly less than 1.00 is anticipated, based on the
harmonic approximation built into the calculation. Thus, as
3
7
16
lated for the dissociation of this species into CH2O, HCl, and
CrO2. As noted above, the present calculations reproduce this
substantial endothermicity. In the present study, species B
clearly decomposed or was consumed at temperatures above
-1
shown in Table 4, the scaled value for ClCrO2OCH3 is 684 cm
-
1
while for Cl2CrO(OH)OCH3 the scaled value is 620 cm ,
substantially lower than the experimental value. The fit to the
other observed bands of species B was also distinctly better for
ClCrO2OCH3 than for Cl2CrO(OH)OCH3. In addition, ad-
ditional intense bands are calculated for Cl2CrO(OH)OCH3 in
spectral regions that are not congested (e.g. the O-H stretch
of the hydroxyl group); these bands were not observed. While
150 °C; the loss of B was clearly accompanied by the formation
of both CH2O and HCl. It is also important to note that when
CD3OH was used as the starting material, formation of species
B was accompanied by formation of DCl. This demonstrates
that the HCl formed in this step arises from the methyl group
of the parent methanol. The mechanism of decomposition of
ClCrO2OCH3 to HCl, CH2O, and (presumably) CrO2 is not clear
in view of the large endothermicity of the gas-phase reaction.
One reasonable explanation is that the decomposition of species
B at elevated temperatures is surface-mediated by the walls of
(37) Foresman, J. B.; Frisch, A. E. Exploring Chemistry with Electronic
Structure Methods; Gaussian, Inc.: Pittburgh, PA, 1996; and references
therein.

6112 J. Am. Chem. Soc., Vol. 120, No. 24, 1998
Ault
the deposition system. Under these conditions, the calculations
and experiments cannot be compared, since the calculations are
for isolated, gas-phase species and reactions. In any event, the
experimental work and present ab initio calculations argue for
the identification of species B in these experiments as ClCrO2-
OCH3, and this species is destroyed at elevated temperatures
as CH2O and HCl are produced.
temperature to eliminate HCl and produce ClCrO2OCH3. This
reaction could also be induced by visible irradiation of the
hydrogen bonded complex trapped in an argon matrix. Pyrolysis
above 150 °C of ClCrO2OCH3 in a flowing gas reactor followed
by matrix trapping led to destruction of this species, and the
production of CH2O in high yield. This species, in turn, could
be oxidized further by CrCl2O2 to CO and CO2 at temperatures
above 250 °C.
Conclusions
Acknowledgment. The National Science Foundation is
gratefully acknowledged for partial support of this research
through Grant No. CHE 93-22622.
Twin jet and merged jet matrix isolation experiments led to
the formation of a series of intermediate products in the
oxidation of CH3OH by CrCl2O2. Initially formed was a 1:1
hydrogen bonded complex, a species that further reacted at room
JA9808752