Matrix isolation investigation of the photochemical reaction of activator-substituted benzenes with CrCl2O2

Article abstract of DOI:10.1016/j.molstruc.2007.12.023

The matrix isolation technique, combined with infrared spectroscopy and theoretical calculations, has been used to characterize the products of the photochemical reactions of aniline, anisole, and N,N-dimethylaniline with CrCl₂O₂. While initial twin jet deposition of the reagents did not lead to product formation, new bands were noted following irradiation with light of λ > 300 nm. In the reactions of CrCl₂O₂ and aniline, irradiation led to HCl formation, very likely accompanied by CrClO₂(H)NC₆H₅ formation, and a mixture of isomers of CrCl₂O-aminocyclohexa-2,4-dienone via oxygen atom transfer. CrCl₂O₂ and aniline were also codeposited as cryogenic thin films, producing new bands. Irradiation of these films led to further growth of these bands. The same products were identified in the thin film studies as were observed in the argon matrix experiments. The irradiation of the matrices containing CrCl₂O₂ and anisole and N,N-dimethylaniline were shown to lead to oxygen atom transfer, forming complexes between the corresponding cyclic ketone derivatives and CrCl₂O. The identification of the complexes were supported by density functional calculations at the B3LYP/6-311G++(d,2p) level and isotopic labeling (in the case of aniline).

Full text of DOI:10.1016/j.molstruc.2007.12.023

Available online at www.sciencedirect.com
Journal of Molecular Structure 888 (2008) 277–290
www.elsevier.com/locate/molstruc
Matrix isolation investigation of the photochemical reaction
of activator-substituted benzenes with CrCl2O2
Michael D. Hoops, Bruce S. Ault *
Department of Chemistry, University of Cincinnati, 404 Crosley Tower, P.O. Box 210172, Cincinnati, OH 45221-0172, USA
Received 29 October 2007; received in revised form 17 December 2007; accepted 17 December 2007
Available online 31 December 2007
Abstract
The matrix isolation technique, combined with infrared spectroscopy and theoretical calculations, has been used to characterize the
products of the photochemical reactions of aniline, anisole, and N,N-dimethylaniline with CrCl2O2. While initial twin jet deposition of
the reagents did not lead to product formation, new bands were noted following irradiation with light of k > 300 nm. In the reactions of
CrCl2O2 and aniline, irradiation led to HCl formation, very likely accompanied by CrClO2(H)NC6H5 formation, and a mixture of iso-
mers of CrCl2O-aminocyclohexa-2,4-dienone via oxygen atom transfer. CrCl2O2 and aniline were also codeposited as cryogenic thin
films, producing new bands. Irradiation of these films led to further growth of these bands. The same products were identified in the
thin film studies as were observed in the argon matrix experiments. The irradiation of the matrices containing CrCl2O2 and anisole
and N,N-dimethylaniline were shown to lead to oxygen atom transfer, forming complexes between the corresponding cyclic ketone deriv-
atives and CrCl2O. The identification of the complexes were supported by density functional calculations at the B3LYP/6-311G++(d,2p)
level and isotopic labeling (in the case of aniline).
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Matrix isolation; Infrared spectroscopy; Theoretical calculations; Reaction intermediates
1. Introduction
Aniline is considered to be a probable human carcinogen
and a concentration as low as 600 lg/L in water sources
is referred as having high probability of developing cancer
upon water ingestion [10]. Catalytic wet air oxidation
(CWAO) is one of the available treatment technologies,
which consists in the total oxidation of the organic species
present in the effluent to CO2, N2, and H2O at moderate
temperature and pressure. Transition metals have been
packed in modified molecular sieves to be used as catalysts
in the oxidation of various organic compounds [11–13].
Recently, chromium, vanadium, and copper have been
tested using CWOA to break down aniline [11].
The matrix isolation technique was developed to facili-
tate the isolation and spectroscopic characterization of
reactive intermediates [14–16] and may provide access to
the study of initial intermediates in the above reactions.
Past studies in this laboratory have examined the matrix-
isolated products of the thermal and photochemical
reactions of CrCl2O2, OVCl3, and OVF3 with organic
The oxidation of organic substrates with metal oxo com-
pounds, including CrCl2O2, is of continuing interest to a
range of chemists. Of particular interest is the selectivity
of the oxidation of organic compounds and the specificity
of the product(s) formed. CrCl2O2 has been shown to be
a versatile oxidant, but the mechanism is complex and
not well understood, so its mechanism has been the object
of many studies throughout the years [1–7]. While theses
studies represent a significant advance in the in the overall
understanding of these reactions, the work was limited to
product studies and did not identify the initial intermedi-
ates in these reactions.
Aniline is a toxic chemical that is often present in dyes,
rubber production, and agrochemical wastewater [8,9].
*
Corresponding author. Tel.: +1 513 556 9238; fax: +1 513 556 9239.
E-mail address: bruce.ault@uc.edu (B.S. Ault).
0022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.12.023

278
M.D. Hoops, B.S. Ault / Journal of Molecular Structure 888 (2008) 277–290
and inorganic substrates [17–31]. Recent studies in this lab-
oratory [32–34] have examined the reactions of CrCl2O2
with benzene, methyl-substituted benzenes, and benzenes
substituted with electron withdrawing groups. In the first
system, light of k > 590 nm led to reaction and oxygen
atom transfer, forming a complex between 2,4-cyclohexadi-
enone and CrCl2O. In the second series of reactions, irradi-
ation led to reaction and oxygen atom transfer, forming
complexes between methylcyclohexadienone derivatives
and CrCl2O. With the xylene isomers and mesitylene, di-
and trimethylphenols, complexed to CrCl2O, were also
observed, respectively. This was the first observation of
CAH bond activation and oxygen atom insertion into a
CAH bond from CrCl2O2. In the third series of reactions,
irradiation led to reaction and oxygen atom transfer, form-
ing complexes between the corresponding cyclic ketone
derivatives and CrCl2O. With benzonitrile, cyanophenol
complexed to CrCl2O was also observed. The present study
examines the photochemical reactions of CrCl2O2 with ani-
line, anisole, and N,N-dimethylaniline to further explore
the nature of the oxidation reaction and the effects of elec-
tron-donating, i.e., activator, substituents on the product
distribution.
region was varied, ranging from 32 to 100 cm in length and
heated to temperatures as high as 100 °C. In both twin and
merged jet, matrices were deposited at the rate of 2 mmol/h
from each sample manifold onto the cold window.
Cryogenic thin films of CrCl2O2 + aniline and
CrCl2O2 + aniline-d7 were prepared by spraying the pure
reagent gases onto the cold window without adding argon
for approximately 6.5 h. Variation of the gas flow rates per-
mitted differing mole ratios of reagents in the thin film. The
thin films were warmed slowly to 160 K or higher with
spectra recorded at several temperatures during the
warm-up process, or irradiated for 0.5–5.0 h with the
H2O/Pyrex-filtered output of a 200 W medium-pressure
Hg arc lamp, after which additional spectra were recorded.
Final spectra were recorded on a Perkin-Elmer Spectrum
One Fourier transform infrared spectrometer at 1 cmÀ1
resolution.
Theoretical calculations were carried out on likely inter-
mediates in this study, using Gaussian 03W suite of pro-
grams [37]. Density functional calculations using the
hybrid B3LYP functional were used to locate energy min-
ima, determine structures, and calculate vibrational spec-
tra. Final calculations with full geometry optimization
employed the 6-311G++(d,2p) basis set, after initial calcu-
lations with smaller basis sets were run to approximately
locate energy minima. Thermodynamic functions for the
reactants and potential intermediates were also calculated
at this level of theory.
2. Experimental
All of the experiments in this study were carried out on a
conventional matrix isolation apparatus that has been
described [35]. Chromyl chloride, CrCl2O2, (Acros), was
introduced into the vacuum system as the vapor above
the room temperature liquid, after purification by freeze-
pump-thaw cycles at 77 K. Anisole (Acros) was introduced
in a similar manner into a separate vacuum manifold and
was purified by repeated freeze-pump-thaw cycles at
77 K. Aniline (Acros), aniline-d7 (99% D, Acros), and
N,N-dimethylaniline (Acros) were introduced into the vac-
uum system by placing 1 mL of the neat liquid in a small
stainless steel sidearm connected to the deposition line
through a needle valve to an UltraTorr tee. The needle
valve was opened slightly, allowing a fraction of the vapor
pressure of the samples at room temperature to be
entrained in flowing argon, carried to the cold window,
and deposited. Argon (Wright Brothers) was used as the
matrix gas without further purification.
3. Experimental results
Prior to any codeposition experiments, blank experi-
ments were run on each of the reagents used in this study.
Blank experiments were also run on CrCl2O2 and aniline
independently as thin films at 14 K. In each case, the
blanks were in good agreement with literature spectra
[38–41], and with blanks run previously in this laboratory.
Each blank experiment was then irradiated by the H2O/
Pyrex-filtered output of a 200 W Hg arc lamp for 1.0 h
and no changes were noted, other than in the Ar/CrCl2O2
blank, where the growth of very weak bands due to photo-
chemical reaction with impurity H2O was seen [20]. Weak
bands due to HCl impurity were noted at 2883 and
2863 cmÀ1 in all of the experiments involving CrCl2O2.
Matrix samples were deposited in both the twin jet and
merged jet modes. In the former, the two gas samples were
deposited from separate nozzles onto the 14 K window,
allowing for only a brief mixing time prior to matrix depo-
sition. In addition, all of these matrices were subsequently
irradiated for 1.0 or more hours with the H2O/Pyrex-fil-
tered output of a 200 W medium-pressure Hg arc lamp,
after which additional spectra were recorded.
A few experiments were conducted in the merged jet
mode [36], in which the two deposition lines were joined
with an UltraTorr tee at a distance from the cryogenic sur-
face, and the flowing gas samples were permitted to mix
and react during passage through the merged region. This
3.1. CrCl2O2 + aniline
In an initial twin jet experiment, a sample of Ar/
CrCl2O2 = 500 was codeposited with a sample of Ar/ani-
line. After 23.0 h of deposition, no new bands were
detected. The matrix was irradiated for 6 h with the filtered
(k > 300 nm) output of a medium-pressure Hg arc lamp,
which led to the observation of many new, product bands,
as listed in Tables 1 and 2. The more intense product bands
were seen at 2704, 2658, 1580, 1528, 1213, 1164, 1014, 673,
and 443 cmÀ1. Most of the observed product bands were
split into doublets or multiplets. Fig. 1 shows a representa-

M.D. Hoops, B.S. Ault / Journal of Molecular Structure 888 (2008) 277–290
279
Table 1
Table 2
Calculated (B3LYP/6-311G++(d,2p)) and observed frequenciesb and
isotropic shifts for CrClO2(H)NC6H5 (structure 8e) via twin jet deposition
Product band positions arising from the irradiation of matrices containing
CrCl2O2 and aniline and the calculateda bands of a representative CrCl2O-
aminocyclohexa-2,4-dienone complex
Normal isotope
2H
Assignment
CrCl2O2 + aniline CrCl2O-
Calcd.
Assignment
Calcd.
freq.a
Exp.
freq.
Calcd.
shift
Exp.
shift
(exp. freq.b)
Aminocyclohexa- intensity
2,4-dienone
(calcd. freq.)
(km/mol)
442
492
542
605
644
668
701
754
443
À5
À76
À2
CrACl st.
443
407
443
489
517
524
560
191.3
2.8
6.0
49.9
0.1
22.9
CrACl st.
543
À19
Ring st.
À25
639
673
À103
À161
À52
NAH wag
CAH wag
543
639
673
Ring st.
768
779
816
858
CAH wag
809
861
894
956
976
CAH wag
Ring st.
583
739
767
3.1
14.7
60.9
À106
À259
À179
À166
À40
768
779
816
858
CAH wag
923
975
991
À153
À38
CAH wag
CAH wag
Ring st.
831
877
0.3
3.4
5.6
3.3
0.1
6.3
78.8
113.5
CAH wag
Ring st.
981
1015
1036
1053
1077
1149
1166
1206
À183
À2
895
923
975
908
986
Ring st.
CAH wag
NH2 rock; ring st.
CrAO st.
CAH rock; NH2
rock; ring st.
CAH rock
1014
1096
1
0
CrAO st.
CAH rock; NAH rock
À260
À316
À307
À238
991
996
1014
1096
1071
1080
1164
1214
CAH rock; ring st.
CAH rock; NAH rock;
ring st.
À238
À56
1164
1153
1181
111.6
0.6
1224
1257
1258
À60
CAH rock; NAH rock;
ring st.
1214
1224
1317
1353
1458
1470
1563
1578
2658
2704
3065
3078
3088
3099
3110
3423
a
À25
À291
À122
À115
À42
1250
1257
1337
1373
1393
1409
1490
1548
6.9
261.1
65.4
27.8
28.1
94.5
1125.6
78.9
1257
Ring st.
0
À135
2
À22
1489
1528
1580
CAH rock; ring st.
NAH rock; ring st.
Ring st.
HCl
HCl
À38
1489
1528
1575
1580
1585
1595
C@C st.
À771
À803
À806
À804
À804
À806
À920
1598
1627
119.8
306.4
NH2 scissor; ring st.
NH2 scissor; ring st.
Frequencies scaled by a factor of 0.97.
2658
2704
HCl
HCl
Frequencies in cmÀ1
.
b
2935
2962
3067
3084
3118
3496
3610
3.0
0.2
10.5
10.4
4.0
tive region of this spectrum in the difference mode (after–
before irradiation).
The above experiment was repeated numerous times
using different absolute and relative concentrations of
CrCl2O2 and aniline in argon. The same product bands
were observed after irradiation throughout, with relative
intensities that appeared to be consistent. Band intensities
were also consistent with the concentrations employed over
all of the experiments.
159.9
46.3
a
Calculated values have been scaled by a factor of 0.97, B3LYP/6-
311G++(d,2p).
Frequencies in cmÀ1
.
b
The reaction of this pair of reactants was studied in a
number of merged jet experiments, using a 32 and 100 cm
merged or reaction region, held at both room temperature
and 100 °C. In all of these experiments, the parent bands
were either reduced in intensity or absent (compared to
the twin jet experiments run at the same sample concentra-
tions), indicating that reaction was occurring. However, no
new bands were observed in the spectrum, other than an
increase of the intensity of the HCl bands at 2883 and
2863 cmÀ1
.

280
M.D. Hoops, B.S. Ault / Journal of Molecular Structure 888 (2008) 277–290
0.250
0.12
0.11
0.10
0.16
0.15
a
b
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
-0.02
-0.04
0.14
0.13
A
A
0.09
0.08
0.12
0.11
0.10
0.07
0.09
A
0.060
0.080
2800.0
2600
2050.0
1900
-0.070
1600.0
1550
1500
1450
1400
1350
1300
1250
1200
1150
1100
1050
1000
950.0
cm-1
Fig. 1. Infrared difference spectrum before and after 6 h irradiation of a matrix formed by the twin jet deposition of a sample of Ar/CrCl2O2 = 500 with a
sample of Ar/aniline. The upper regions are of the (a) HACl stretch and the (b) DACl stretch. The negative bands are due to loss of intensity in the parent
absorptions after irradiation.
Cryogenic thin films of each pair of reagents were
formed by the deposition of the pure vapor of each reagent
onto the 14 K cold surface. Product bands were observed
after deposition of the pure reagent gases onto the 14 K
window as seen in Table 3. After 30 min of irradiation with
the Hg arc lamp, these bands grew in intensity along with
the appearance of three weak bands at 1546, 1188, and
1167 cmÀ1. These product bands were similar to, but
broader than, the product bands for the system in solid
argon. Fig. 2 shows blank spectra of the two reagents each
alone as a thin film, and Fig. 3 shows a representative
region of the thin film of CrCl2O2 and aniline.
detected. The matrix was irradiated for 24.5 h with the fil-
tered (k > 300 nm) output of a medium-pressure Hg arc
lamp, which led to the observation of many weak new
product bands, as listed in Table 4. The more intense bands
were seen at 1568, 1548, 1431, 1279, 1241, 1204, 1017, 820,
708, and 440 cmÀ1. Most of the observed product bands
were split into doublets or multiplets. Fig. 4 shows a repre-
sentative region of this spectrum.
Numerous additional twin jet experiments were
conducted, at sample concentrations ranging from Ar/
CrCl2O2 = 250 to Ar/CrCl2O2 = 500 and Ar/anisole = 300
to Ar/anisole = 400. The same product bands were
observed after irradiation throughout, with relative intensi-
ties (i.e., relative to all other bands in the set) that appeared
to be consistent. Band intensities were also consistent with
the concentrations employed over all of the experiments.
A series of merged jet experiments was conducted with
samples of Ar/CrCl2O2 = 500 and Ar/anisole = 300. With
the merged region held at room temperature, the resulting
spectrum was identical to that observed in the twin jet
experiments with no new product bands. In a subsequent
experiment, with the merged region heated to 100 °C, no
new product bands were observed.
3.2. CrCl2O2 + aniline-d7
Multiple twin jet experiments were conducted in which
samples of Ar/CrCl2O2 were codeposited with samples of
Ar/aniline-d7. After 21.0 h of deposition, the matrix was
irradiated for 6.0 h. This irradiation produced many weak
new bands (see Table 1), with the more intense bands at
1933, 1558, 1544, 1529, 1393, 1201, 1009, 922, and
441 cmÀ1. Most of the observed product bands were split
into doublets or multiplets. Fig. 1 shows the DACl stretch-
ing region for this spectrum.
3.4. CrCl2O2 + N,N-dimethylaniline
3.3. CrCl2O2 + anisole
In an initial twin jet experiment, a sample of Ar/
CrCl2O2 = 500 was codeposited with a sample of Ar/
N,N-dimethylaniline. After 23.0 h of deposition, no new
bands were detected. The matrix was irradiated for 25.0 h
In an initial twin jet experiment, a sample of Ar/
CrCl2O2 = 500 was codeposited with a sample of Ar/ani-
sole = 400. After 23.0 h of deposition, no new bands were

M.D. Hoops, B.S. Ault / Journal of Molecular Structure 888 (2008) 277–290
281
Table 3
Product band positions arising from the irradiation of thin films of CrCl2O2 and aniline and the calculateda bands of CrClO2(H)NC6H5 and a
representative CrCl2O-aminocyclohexa-2,4-dienone complex
CrCl2O2 + aniline
(exp. freq.b)
CrClO2(H)NC6H5
(calcd. freq.)
Calcd. intensity
(km/mol)
CrCl2O-Aminocyclohexa-
2,4-dienone (calcd. freq.)
Calcd. intensity
(km/mol)
422
637
442
492
542
605
644
668
701
754
809
136.0
17.2
7.5
417
483
499
506
518
581
626
717
754
174.9
32.2
4.1
11.3
9.3
38.0
17.6
2.7
0.1
22.8
29.9
19.7
59.1
1.8
788
855
9.1
39.9
2.5
861
894
10.3
2.0
813
877
956
0.4
895
0.9
976
0.3
922
4.5
981
2.0
971
5.6
1015
1036
1053
1077
1149
1166
1206
1258
1317
1353
1458
1470
1563
1578
2.5
974
1.4
4.4
929
954
144.5
171.3
13.1
0.1
6.8
14.8
2.6
1045
1072
1160
1172
1247
1293
1332
1355
1385
1440
1513
1529
1619
183.5
58.2
6.6
157.9
67.8
2.0
54.1
9.9
70.8
1144.9
42.7
93.0
1167
1188
1.5
1378
1431
1469
1493
1546
1572
1673
2610
2658
2704
13.1
11.8
4.2
3.7
14.8
1649
361.5
HCl
16.3
2.2
4.2
5.5
3.7
168.7
53.9
3065
3078
3088
3099
3110
3423
3.2
0.2
10.2
10.8
2.5
2909
2937
3067
3087
3089
3497
3612
55.1
a
Calculated values have been scaled by a factor of 0.97, B3LYP/6-311G++(d,2p).
b
Frequencies in cmÀ1
.
with the filtered (k > 300 nm) output of a medium-pressure
Hg arc lamp, which led to the observation of many weak
new product bands, as listed in Table 5. The more intense
bands were seen at 1655, 1569, 1500, 1462, 1050, 1006, 974,
936, 775, and 450 cmÀ1. Most of the observed product
bands were split into doublets or multiplets. Fig. 5 shows
a representative region of this spectrum.
line. With the merged region held at room temperature,
the resulting spectrum was identical to that observed in
the twin jet experiments with no new product bands. In a
subsequent experiment, with the merged region heated to
100 °C, again no new product bands were observed.
4. Results of calculations
The above experiment was repeated numerous times
using different absolute and relative concentrations of
CrCl2O2 and N,N-dimethylaniline in argon. The same
product bands were observed after irradiation throughout,
with relative intensities that appeared to be consistent.
Band intensities were also consistent with the concentra-
tions employed over all of the experiments.
The structures, energetics, and vibrational spectra of a
number of possible products of the multiple systems were
calculated using the B3LYP hybrid functional and basis
sets as high as 6-311G++(d,2p). Since experimental evi-
dence for oxygen atom transfer was observed (see below),
calculations focused on possible products and isomers of
oxygen atom transfer to form cyclic diene ketones, as deter-
mined from previous studies, complexed to the remaining
A series of merged jet experiments was conducted with
samples of Ar/CrCl2O2 = 500 and Ar/N,N-dimethylani-

282
M.D. Hoops, B.S. Ault / Journal of Molecular Structure 888 (2008) 277–290
2.00
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
*
*
A
*
*
0.0
-0.13
1650.0
1500
1400
1300
1200
1100
1000
900
800
700
600
500
400.0
cm-1
Fig. 2. Infrared spectra of thin films of each blank of CrCl2O2 and aniline-d7. CrCl2O2 blank bands are marked with (Ã).
2.20
2.0
1.8
1.6
1.4
1.2
A
1.0
0.8
0.6
0.4
0.2
0.10
1750.0
1600
1500
1400
1300
1200
1100
cm-1
1000
900
800
700
600
500
400.0
Fig. 3. Infrared spectra of a thin film by the twin jet deposition of CrCl2O2 with aniline. The lower trace is before irradiation, while the upper trace is after
30.0 min of irradiation with light of k > 300 nm.
metal species, CrCl2O which is trapped within the same
matrix cage. In addition, phenols formed through the inser-
tion of an O atom into a CAH bond were considered, each
complexed to CrCl2O, plus the product of HCl elimination
from the initial cage pair of CrCl2O2 + aniline. The possi-
bility of ketene formation through ring opening was con-
sidered briefly. Similar to systems studied previous, this
alternative is calculated to be much less favorably energet-
ically, and the calculated spectra of the possible product
ketenes do not correspond at all to the experimental spec-
tra. Consequently, the structures and energetics of all of the
possible ketene isomers were not calculated. Fig. 6 shows

M.D. Hoops, B.S. Ault / Journal of Molecular Structure 888 (2008) 277–290
283
Table 4
of CrCl2O2 with aniline (a cyclohexadienone, a phenol, two
ketenes, and HCl elimination). The Supporting Informa-
tion shows graphic representations of all of these calculated
structures (structures S1a-i and S2a-j) as well as tables list-
ing the calculated energies relative to CrCl2O2 + the activa-
tor-substituted benzenes. All of the structures, except the
HCl elimination product, correspond to a single oxygen
transfer from the chromyl chloride to an activator-substi-
tuted benzene. All of the proposed complexes optimized
to energy minima on their respective potential energy sur-
faces, with all positive vibrational frequencies.
Product band positions arising from the irradiation of matrices containing
CrCl2O2 and anisole and the calculateda bands of a representative CrCl2O-
methoxycyclohexa-2,4-dienone complex
CrCl2O2 + anisole CrCl2O-
(exp. freq.)b
Calcd.
Assignment
Methoxycyclohexa- intensity
2,4-dienone
(calcd. freq.)
(km/mol)
440
454
415
434
475
487
532
590
611
741
761
780
814
894
930
981
1011
1028
181.8
3.1
12.3
2.7
2.8
3.4
CrACl st.
Ring st.
5. Discussion
5.9
708
750
845
820
901
37.8
14.6
17.7
38.7
1.4
4.5
0.1
59.4
8.4
Ring st.
CAH wag
Ring st.
CAH wag
CAH wag
No distinct product bands were observed upon initial
codeposition of any of the activator-substituted benzenes
with CrCl2O2 into argon matrices. However, in each case
subsequent irradiation of these matrices led to the forma-
tion of clear product bands. These results are quite similar
to the previous systems studied in this laboratory [22,31–
34]. For a photochemical reaction to occur, the reactants
must be trapped in the same matrix cage. It is likely that
there are cage pairs formed from the statistical distribution
of components of the system during the matrix condensa-
tion process. It is very probable that the stoichiometry of
the cage pairs is 1:1, i.e., one molecule of CrCl2O2 and
one molecule of an activator-substituted benzene. This
conclusion is reasonable because of the low concentrations
employed in these studies, on the order of 1:1:1000, which
makes dimer or aggregate formation unlikely, and the
approximate 1:1 stoichiometry of the reactants. In addi-
tion, the relative intensities of the product bands remained
constant as the concentration of the initial reagents was
varied. This suggests that either a single reaction product
arises from the initial 1:1 cage pair, or that more than
one product is formed but the ratio of products is constant.
There are two probable modes of photochemical reac-
tion between CrCl2O2 and an organic substrate: (1) HCl
elimination from the initial cage pair and (2) oxygen atom
transfer from CrCl2O2 to the organic substrate, yielding an
oxidized product and CrCl2O. In HCl elimination, the HCl
that is produced is trapped within the matrix cage with the
second photoproduct. The spectrum will be that of HCl
molecule weakly hydrogen bonded to the second photo-
product, with bands appearing between 2650 and
2800 cmÀ1, shifted from isolated monomeric HCl which
absorbs at 2863 and 2888 cmÀ1 in solid argon [42,43]. Such
absorptions were noted for the aniline system studied here,
but not for the anisole and dimethylaniline systems. For
the second possible mode of reaction, CrCl2O has been
observed by several research groups [24,44,45], and has
characteristic absorptions near 1014 and 450 cmÀ1. The
exact position of these absorptions varied slightly from sys-
tem to system, due to complexation of CrCl2O to the oxi-
dized product. In the present study, product bands were
observed near 1014 and 440 cmÀ1 for all three systems
studied here, and are very reasonably assigned to CrCl2O.
1032
1017
1148
CAH rock; ring
st.
CrAO st.
1074
1136
1138
1167
1168
1216
1245
155.2
5.5
1.0
31.7
0.5
188.6
29.5
CH3 twist
1204
1241
CAO st.; ring st.
CAH rock; ring
st.
CAH rock; CH2
wag; ring st.
1279
1303
33.8
1328
1355
1410
9.4
23.1
58.7
1354
1396
CH2 scissor
CAH rock; ring
st.; CH3 wag
1417
1431
1440
2.6
CAH rock; ring
st.; CH3 wag
CH3 twist
CH3 scissor
C@C st.; C@O
st.
1453
1467
1548
1452
1463
1537
11.3
19.8
373.0
1568
1560
4.4
C@C st.; C@O
st.
1587
1641
1639
2909
2916
2921
2978
3050
3085
3108
3109
56.2
4.1
0.1
32.8
27.8
16.1
1.9
C@C st.
1
4.9
a
Calculated values have been scaled by a factor of 0.97, B3LYP/6-
311G++(d,2p).
b
Frequencies in cmÀ1
.
the energies of the calculated likely structures (products)
relative to CrCl2O2 + aniline; the energies are also listed
in Table 6. Fig. 7 shows the calculated structures of five
possible representative complexes arising from the reaction

284
M.D. Hoops, B.S. Ault / Journal of Molecular Structure 888 (2008) 277–290
0.500
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
A
-0.050
1700.0 1650
1600
1550
1500
1450
1400
1350
1300
1250
1200
1150
1100
1050
1000
950.0
cm-1
Fig. 4. Infrared difference spectrum before and after 25 h irradiation of a matrix formed by the twin jet deposition of a sample of Ar/CrCl2O2 = 250 with a
sample of Ar/anisole = 400. The negative bands are due to loss of intensity in the parent absorptions after irradiation.
5.1. Aniline + CrCl2O2
potential matches. Thus, based on these potential matches
and the observation of hydrogen bonded HCl as a photo-
product, it is very likely that one of the products formed
in the photochemical reactions of CrCl2O2 and aniline in
argon matrices is the HCl elimination product,
CrClO2(H)NC6H5.
After irradiation of matrices containing these two reac-
tants, two new bands appeared at 2704 and 2658 cmÀ1
,
indicative of HCl formation with hydrogen bonding to a
second photoproduct. These new bands shifted to a broad
band at 1933 cmÀ1 in experiments with aniline-d7. This is
precisely the isotopic dependence anticipated for HCl; the
mH/mD ratio of 2704/1933 = 1.39 is almost identical to that
observed for both gas phase [46] and matrix isolated [42,43]
HCl, namely, 1.38. Given the isotopic behavior of these
bands and the analogy to a number of well-established sys-
tems, the bands at 2704 and 2658 cmÀ1 are assigned to
photoproduced HCl. Further, the HCl that is produced is
trapped within the same matrix cage, and weakly hydrogen
bonded to the species responsible for at least some of the
photochemical bands. Based on previous studies
[17,20,29], HCl would be formed from an amino hydrogen
and a chlorine from CrCl2O2. Following these analogies,
the HCl elimination product would be CrClO2(H)NC6H5.
This molecule should have a large number of vibrational
modes and may well be responsible for a number of the
product bands observed here. However, a second product
(discussed in the following paragraph) is the CrCl2O-
aminocyclohexa-2,4-dienone complex (and isomers
thereof). Structurally, these molecules are quite similar
and will have many comparable vibrational modes, making
assignment to one product versus the other nearly impossi-
ble. Table 1 compares the experimental bands for the
CrCl2O2 + aniline system to those calculated for
CrClO2(H)NC6H5. As is apparent, there are numerous
As noted above, the second mode of photochemical reac-
tion of CrCl2O2 with an organic substrate is oxygen atom
transfer from CrCl2O2 to the organic substrate, yielding
an oxidized product and CrCl2O. Since CrCl2O was clearly
observed, this reaction pathway is also being followed.
Based on calculated energetics and previous studies, the
two classes of potential oxidation products are phenols
(via insertion into a CAH bond) and cyclic dienones (via
addition at one of the carbons). Each of these two classes
has distinctive infrared signatures. Phenols are character-
ized by a distinct OAH stretch above 3400 cmÀ1, and the
cyclic dienones have strong absorptions from 1700 to
1500 cmÀ1 due to the C@O and C@C stretches (with the
C@O group complexed to CrCl2O and shifted to lower
energies relative to the isolated molecule). On this basis,
phenols can be ruled out, as a band corresponding to the
OAH stretch is completely absent. Rather, the spectral data
strongly support formation of a cyclic dienone, consistent
with the results from the previous studies of CrCl2O2 + ben-
zene and benzene derivative reactions. There are five possi-
ble isomers for the cyclic dienone product, depending on the
positions of the C@O and amino groups with respect to one
another. Computationally, oxygen transfer reactions to
form the separated isomer dienones and CrCl2O species
were all calculated to be endothermic at the B3LYP/6-

M.D. Hoops, B.S. Ault / Journal of Molecular Structure 888 (2008) 277–290
285
Table 5
311G++(d,2p) level of theory (see Fig. 6). However, when
complexation energy of the dienone to the CrCl2O species
was included, the DE0 ’s to form the products from
Product band positions arising from the irradiation of matrices
containing CrCl2O2 and N,N-dimethylaniline and the calculateda
bands of a representative CrCl2O-(dimethylamino)cyclohexa-2,4-dienone
complex
o
CrCl2O2 and aniline were calculated to be exothermic,
except for the CrCl2O-6-aminocyclohexa-2,4-dienone com-
CrCl2O2 + N,N- CrCl2O-
dimethylaniline (Dimethylamino)
(exp. freq.)b
Calcd.
intensity
(km/mol)
Assignment
o
plex, where DE0 was calculated to be +9.1 kcal/mol. As
discussed above, definitive assignment of product bands
(other than the C@O stretch of the cyclic carbonyl) versus
the HCl elimination product is very difficult. Table 2 com-
pares the experimental bands for the CrCl2O2 + aniline sys-
tem to those calculated for one isomer of CrCl2O-
aminocyclohexa-2,4-dienone. Again, there are numerous
potential matches, but no definitive assignments can be
made since there are potential matches to the HCl elimina-
tion product as well. Thus, the products formed in the pho-
tochemical reaction of CrCl2O2 with aniline in solid argon
matrices are identified as a mixture of isomers of CrCl2O-
aminocyclohexa-2,4-dienone complexes and very likely the
HCl elimination product, CrClO2(H)NC6H5.
cyclohexa-2,4-
dienone(calcd. freq.)
450
477
415
423
498
501
515
599
620
693
779
788
179.8
19.2
2.4
3.8
9.3
11.0
14.4
19.5
18.0
1.4
CrACl st.
Ring st.
696
775
816
Ring st.
Ring st.
CAH wag; CH2
rock
CAH wag; CH2
twist
834
828
21.4
906
928
932
26.9
0.2
1.9
Cryogenic thin film experiments were conducted to fur-
ther study the reaction between CrCl2O2 + aniline. A band
at 2610 cmÀ1 was observed, which increased in intensity
after irradiation, and assigned to HCl hydrogen bonded
to the species responsible for some of the photochemical
bands. This is assignment is further supported by the shift
of the DCl band to 1969 cmÀ1 upon using a deuterated
sample of aniline. Thus, it is likely that CrClO2(H)NC6H5
is formed in these thin film experiments. Additionally, there
is evidence of carbonyl formation with a strong band at
1673 cmÀ1 indicating that one or more isomers of
CrCl2O-aminocyclohexa-2,4-dienone are formed. Given
the increased breadth of the product bands in the thin film
experiments, individual band assignment cannot be made
to either product, except for the identification of HCl as
a product, and the presence of a C@O group from
CrCl2O-aminocyclohexa-2,4-dienone. Table 3 compares
the observed product bands to those calculated for both
the HCl elimination product and the dienone product.
Finally, it should be noted that the Cr@O and CrACl
stretches shifted more in the cryogenic thin films than in
the argon matrices, consistent with the more strongly inter-
acting environment in the thin film.
The thin films were gradually heated and spectra were
recorded in 10 K intervals up to 160 K. The first noticeable
change in the spectrum occurred at 40 K, and the same
results were observed during the warming of the films as
after the irradiation of the films. Slight growth of the bands
continued until about 90 K, after which the films started to
degrade. From the cryogenic thin film studies, the key con-
clusion is the products formed between CrCl2O2 and aniline
are identified as a mixture of isomers of CrCl2O-aminocy-
clohexa-2,4-dienone and very likely CrClO2(H)NC6H5.
936
974
Ring st.
CAH wag
985
0.9
1007
1026
1073
1083
1094
1142
1147
1164
1184
1240
1275
1291
18.7
14.7
153.7
5.1
42.6
1.5
32.5
1.7
19.2
9.8
1050
1006
CH3 wag
CrAO st.
1094
1154
Ring st.; CH3 wag
CH2 twist
13.4
2.6
1293
1371
CAH rock; ring st.;
CH2 wag
1316
1342
16.9
30.5
1404
1410
1428
1444
1456
1458
1474
1520
1550
1645
0.1
18.2
0.3
6.9
28.3
6.2
1422
1436
CH3 scissor
CH3 scissor
1462
CH3 scissor
7.0
1500
1569
1655
1710
189.0
140.8
11.9
C@C st.; C@O st.
C@C st.; C@O st.
C@C st.
2843
2848
2910
2918
2969
2971
3013
3015
3071
3090
3101
35.9
115.6
6.9
0.1
14.3
47.3
11.0
28.4
2.1
1.4
0.3
5.2. Anisole + CrCl2O2
a
Calculated values have been scaled by a factor of 0.97, B3LYP/6-
A significant number of product bands were observed
in the photochemical reaction of CrCl2O2 with anisole,
311G++(d,2p).
b
Frequencies in cmÀ1
.

286
M.D. Hoops, B.S. Ault / Journal of Molecular Structure 888 (2008) 277–290
0.600
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
-0.10
A
-0.150
1700.0 1650
1600
1550
1500
1450
1400
1350 1300
cm-1
1250
1200
1150
1100
1050
1000
950.0
Fig. 5. Infrared difference spectrum before and after 25 h irradiation of a matrix formed by the twin jet deposition of a sample of Ar/CrCl2O2 = 250 with a
sample of Ar/N,N-dimethylaniline. The negative bands are due to loss of intensity in the parent absorptions after irradiation.
CrCl2O + 2-aminohexa-1,3,5-trien-1-one
55.2 kcal/mol
Energy
CrCl2O + 4-aminohexa-1,3,5-trien-1-one
CrCl2O + 5-aminohexa-1,3,5-trien-1-one
CrCl2O + 3-aminohexa-1,3,5-trien-1-one
46.5 kcal/mol
46.1 kcal/mol
45.9 kcal/mol
CrCl2O + 6-aminohexa-1,3,5-trien-1-one
44.8 kcal/mol
44.7 kcal/mol
44.0 kcal/mol
43.5 kcal/mol
42.4 kcal/mol
41.3 kcal/mol
CrCl2O-2-aminohexa-1,3,5-trien-1-one,η2(C=O)
CrCl2O-5-aminohexa-1,3,5-trien-1-one,η2(C=O)
CrCl2O-3-aminohexa-1,3,5-trien-1-one,η2(C=O)
CrCl2O-2-aminohexa-1,3,5-trien-1-one,η1(end on)
CrCl2O-4-aminohexa-1,3,5-trien-1-one,η2(C=O)
CrCl2O + 6-aminocyclohexa-2,4-dienone
38.2 kcal/mol
27.0 kcal/mol
39.0 kcal/mol
CrCl2O-6-aminohexa-1,3,5-trien-1-one,η2(C=O)
35.9 kcal/mol
35.8 kcal/mol
33.8 kcal/mol
31.2 kcal/mol
CrCl2O-5-aminohexa-1,3,5-trien-1-one,η1(end on)
CrCl2O-3-aminohexa-1,3,5-trien-1-one,η1(end on)
CrCl2O-4-aminohexa-1,3,5-trien-1-one,η1(end on)
CrCl2O-6-aminohexa-1,3,5-trien-1-one,η1(end on)
CrCl2O + 4-aminocyclohexa-2,4-dienone
CrCl2O + 2-aminocyclohexa-2,4-dienone
22.2 kcal/mol
20.8 kcal/mol
19.7 kcal/mol
CrCl2O + 5-aminocyclohexa-2,4-dienone
CrCl2O + 3-aminocyclohexa-2,4-dienone
10.1 kcal/mol
CrCl2O + 4-aminophenol
9.1 kcal/mol
CrCl2O-6-aminocyclohexa-2,4-dienone
CrCl2O + 2-aminophenol
CrCl2O + 3-aminophenol
8.4 kcal/mol
8.1 kcal/mol
CrCl2O2 + Aniline
-2.7 kcal/mol
-2.8 kcal/mol
CrCl2O-4-aminocyclohexa-2,4-dienone
CrClO2(H)NC6H5 + HCl
-7.0 kcal/mol
-11.4 kcal/mol
CrCl2O-2-aminocyclohexa-2,4-dienone
CrCl2O-5-aminocyclohexa-2,4-dienone
-12.2 kcal/mol
-12.7 kcal/mol
-12.8 kcal/mol
CrCl2O-4-aminophenol
CrCl2O-2-aminophenol
CrCl2O-3-aminophenol
-15.1 kcal/mol
CrCl2O-3-aminocyclohexa-2,4-dienone
uncomplexed species
complexed species
Fig. 6. Relative energies of the uncomplexed and complexed products as compared to CrCl2O2 and aniline.

M.D. Hoops, B.S. Ault / Journal of Molecular Structure 888 (2008) 277–290
287
Table 6
Calculated (B3LYP/6-311G++(d,2p)) relative energiesa of possible products (uncomplexed and complexed) from the CrCl2O2 and aniline reaction
Uncomplexed products
Rel. energies
Complexed products
Rel. energies
CrCl2O + 2-Aminohexa-1,3,5-trien-1-one
CrCl2O + 4-Aminohexa-1,3,5-trien-1-one
CrCl2O + 5-Aminohexa-1,3,5-trien-1-one
CrCl2O + 3-Aminohexa-1,3,5-trien-1-one
CrCl2O + 6-Aminohexa-1,3,5-trien-1-one
55.2 kcal/mol
46.5
46.1
45.9
44.8
CrCl2O-2-Aminohexa-1,3,5-trien-1-one, g2 (C@O)
CrCl2O-5-Aminohexa-1,3,5-trien-1-one, g (C@O)
44.7 kcal/mol
44.0
43.5
42.4
41.3
39.0
35.9
35.8
33.8
2
CrCl2O-3-Aminohexa-1,3,5-trien-1-one, g2 (C@O)
CrCl2O-2-Aminohexa-1,3,5-trien-1-one, g1 (end on)
CrCl2O-4-Aminohexa-1,3,5-trien-1-one, g2 (C@O)
CrCl2O-6-Aminohexa-1,3,5-trien-1-one, g2 (C@O)
CrCl2O-5-Aminohexa-1,3,5-trien-1-one, g1 (end on)
CrCl2O-3-Aminohexa-1,3,5-trien-1-one, g1 (end on)
CrCl2O-4-Aminohexa-1,3,5-trien-1-one, g1 (end on)
CrCl2O-6-Aminohexa-1,3,5-trien-1-one, g1 (end on)
31.2
CrCl2O + 6-Aminocyclohexa-2,4-dienone
CrCl2O + 4-Aminocyclohexa-2,4-dienone
CrCl2O + 2-Aminocyclohexa-2,4-dienone
CrCl2O + 5-Aminocyclohexa-2,4-dienone
CrCl2O + 3-Aminocyclohexa-2,4-dienone
38.2
27.0
22.2
20.8
19.7
CrCl2O-6-Aminocyclohexa-2,4-dienone
CrCl2O-4-Aminocyclohexa-2,4-dienone
CrCl2O-2-Aminocyclohexa-2,4-dienone
CrCl2O-5-Aminocyclohexa-2,4-dienone
CrCl2O-3-Aminocyclohexa-2,4-dienone
CrClO2(H)NC6H5 + HCl
9.1
À2.7
À7.0
À11.4
À15.1
À2.8
CrCl2O + 4-Aminophenol
CrCl2O + 2-Aminophenol
CrCl2O + 3-Aminophenol
a
10.1
8.4
8.1
CrCl2O-4-Aminophenol
CrCl2O-2-Aminophenol
CrCl2O-3-Aminophenol
À12.2
À12.7
À12.8
Energies relative to the reactants, CrCl2O2 + aniline.
as listed in Table 3. CrCl2O was again observed, with
absorptions at 1017 and 440 cmÀ1, so oxidized products
including methoxycyclohexadienone and methoxyphenol
must be considered. The methoxyphenol complexes can
be ruled out, as the characteristic OAH stretch was
absent from the spectra. The spectral data strongly sup-
ports the formation of an isomeric mixture of methoxy-
cyclohexadienones formed in the argon matrices. There
are five possible isomers of this species, and it is not pos-
sible to identify a single isomer as the product. Rather it
appears that a mixture of two or more isomers are
formed, each complexed to CrCl2O. The reactions to
form 3 of the 5 complexes from CrCl2O2 + anisole were
for all of the five isomers and vibrational frequencies that
agreed well with the observed frequencies.
Merged jet experiments were also carried out for the
reactions between CrCl2O2 and anisole and CrCl2O2 and
N,N-dimethylaniline, to provide the reagents with more
time to react before deposition onto the cold window. In
these experiments, no reaction products were detected, with
heating of the reaction zone to as high as 100 °C. Past stud-
ies in this lab have shown that ample collisions occur in the
merged jet region for reaction to occur. Reactions studied
in this manner include HCl elimination [17,20,29] and oxy-
gen atom transfer [23] from the transition metal com-
pound. Of all of the possible products from the two
systems, the calculations demonstrate that the potential
reactions are endothermic and are not likely to occur ther-
mally in the temperature range that was used.
It is noteworthy that HCl elimination was observed only
in the aniline experiments. Previous studies [17,20,29] have
observed the HCl elimination reaction only from reaction
pairs when the substrate has an acidic hydrogen, such as
an amine. Aniline has a pKa of 4.63, which makes it a weak
acid. Dimethylaniline and anisole have only hydrogens
bound to carbon which are not sufficiently acidic to
undergo HCl elimination.
o
found computationally to have DE0 values <0 so that
multiple isomers are possible. These 3 are calculated to
have quite similar vibrational spectra. As such, unique
assignment of each product band to a specific isomer
of CrCl2O-methoxycyclohexa-2,4-dienone is not possible.
The key conclusion is that the product formed in the
photochemical reaction of CrCl2O2 with anisole in solid
argon matrices is identified as a mixture of isomers of
CrCl2O-methoxycyclohexa-2,4-dienone.
5.3. N,N-Dimethylaniline + CrCl2O2
The product bands for the photochemical reaction of
CrCl2O2 with N,N-dimethylaniline followed closely to the
results described above for anisole. Moderately strong
bands were observed in the 1700–1500 cmÀ1 region, and
numerous weaker bands below 1500 cmÀ1. The product
bands are assigned to a mixture of isomers of CrCl2O-dim-
ethylaminocyclohexadienones, while no dimethylamin-
ophenols were observed. Theoretical calculations are
consistent with these conclusions, finding stable minima
6. Further considerations
As seen in the previous studies of CrCl2O2 + benzenes
with electron withdrawing groups, a decrease in the yield
of the photochemical products was observed, compared
to the benzene and methyl-substituted benzenes studies,
as measured by band intensities (computed band intensities
for the complexes in this and previous studies are similar,
suggesting that it is yield rather than absorption coefficient

288
M.D. Hoops, B.S. Ault / Journal of Molecular Structure 888 (2008) 277–290
Fig. 7. Representative structures of the complexes of CrCl2O with (a) an aminocyclohexadienone, (b) an aminophenol, (c) an aminohexa-1,3,5-trien-1-
one, g1 (end on), and (d) an aminohexa-1,3,5-trien-1-one, g2 (C@O). Also, the HCl elimination product, (e) CrClO2(H)NC6H5.
that leads to the low band intensities). In the present study,
the greatest yield of photochemical product was seen in the
reaction of CrCl2O2 with anisole while the yield was much
lower with aniline and N,N-dimethylaniline. This may be a
consequence of either kinetic or thermodynamic factors.
Kinetics may be involved either through the number of ini-
tial cage pairs that form, or through the rate of conversion
of cage pairs to product. While the low vapor pressure and
method of deposition of N,N-dimethylaniline does not per-
mit a direct measurement of sample concentration, a com-
parison of band intensities for analogous modes of known
systems (e.g. benzene) indicates that the concentrations of
N,N-dimethylaniline are similar to others employed in
these studies. Thus, the low intensities are not a result of
low reactant concentration and number of cage pairs.
The product bands for anisole and N,N-dimethylaniline
continued to grow in with the longer irradiation times dem-
onstrating that the yields in these photochemical reactions
are likely kinetically controlled. However, the product
bands for aniline reached maximum intensity after 6 h of
irradiation.
From a thermodynamic perspective, it is possible to get
an approximation of the extent of the reaction by estimat-
ing the equilibrium constant at 14 K. K can be calculated
from the free energy differences calculated by Gaussian.
However, the uncertainties in the calculated values of
DG° at this level of theory suggest that only a qualitative
argument can be made. All of the CrCl2O-aminocyclo-
hexa-2,4-dienone complexes isomers, except CrCl2O-6-
aminocyclohexa-2,4-dienone, had favorable equilibrium
constants (negative DG). Thus, qualitatively it appears
that the product yield for the reaction between CrCl2O2
and aniline is thermodynamically controlled. However,
this does not suggest that the reaction is in dynamic equi-

M.D. Hoops, B.S. Ault / Journal of Molecular Structure 888 (2008) 277–290
289
librium at 14 K. Rather, the initially photochemically
Appendix A. Supplementary data
excited cage pair can either lose excess energy and reform
the initial cage pair, or can proceed to form the oxidized
product complex. This apportioning between cage pair
and product during the energy loss process appears to
qualitatively follow the calculated thermodynamics for
each system.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.
2007.12.023.
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is significantly less than the Cr@O bond strength in
CrCl2O2.
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Acknowledgments
The National Science Foundation is gratefully acknowl-
edged for support of this research through Grant CHE 02-
43731. The Ohio Supercomputer Center is also apprecia-
tively acknowledged for computer time.

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