4108
J. Phys. Chem. A 2000, 104, 4108-4114
Photochemical Reactions of Oxygen Atoms with Toluene, m-Xylene, p-Xylene, and
Mesitylene: An Infrared Matrix Isolation Investigation
James K. Parker and Steven R. Davis*
Department of Chemistry, UniVersity of Mississippi, UniVersity, Mississippi 38677
ReceiVed: August 10, 1999
The photolysis of toluene/ozone, m-xylene/ozone, p-xylene/ozone, and mesitylene/ozone mixtures in argon
matrices at 12 K with UV light of λ g 280 nm leads to products with ketene functionalities. The identification
of the molecular structures of these products is based on deuterium isotopic shifts and agreement with density
functional calculations. Other products from the toluene/O atom reaction include methylphenol and benzyl
alcohol; phenols are also formed from the reaction of xylenes with O atoms and from the reaction of mesitylene
with O atoms.
Introduction
hydrocarbon. The shifts in the infrared spectra, due to isotopic
substitution, allow the assignment of molecular structure and
assessment of directing effects that the methyl group has on
the oxygen atom reaction with methylbenzenes in the formation
of the ketene products. We have performed density functional
calculations on ketene products at the B3LYP/6-311G(d,p) level
of theory in order to aid in the assignment of the infrared bands.
The focus of this paper will be on the ketene products; products
containing the O-H functional group will also be discussed.
The gas-phase reactions of methyl-substituted benzenes with
oxygen atoms have been the subject of several studies.1-7 As a
result, many data are available for these reactions including
specific rate constants, activation energies, product identifica-
tions and distributions, as well as mechanistic information.
Cvetanovic1 studied the reactions of toluene with oxygen atoms,
prepared from mercury-photosensitized decomposition of nitrous
oxide, at room temperature and found o-cresol, p-cresol, carbon
monoxide, water, and a polymeric material (major product) of
unknown composition as products. Sloane3 studied the reactions
of toluene and mesitylene with oxygen atoms under single
collision conditions in crossed molecular beams where the
average collision energy was 2.6 kJ/mol; benzaldehyde and 3,5-
dimethylbenzaldehyde were found as products. Mulder and
coworkers7 studied the reactions of toluene with oxygen atoms,
using microwave discharge of nitrous oxide as the source of
oxygen atoms, in a flow reactor at ambient temperature in a
helium atmosphere. They found hydroxytoluenes, phenol, and
benzaldehyde as products. There has been one report of the
photochemical reactions of benzene and methylbenzenes in solid
oxygen matrices.8 Products of these reactions include 2,4-
hexadiendials and methylated 2,4-hexadiendials. In view of the
contrasting gas-phase results, we have undertaken a matrix
isolation study of the reaction of oxygen atoms with methyl-
benzenes in order to better understand the mechanism of
electrophilic oxygen atom additions with aromatic systems and
the directing effects which methyl groups have on these
reactions.
We have recently reported a novel photochemical reaction9
between benzene and O(3P) atoms in a solid argon matrix at 12
K. It was shown that oxygen atom addition to benzene leads to
the formation of 2,4-cyclohexadienone which subsequently
photolyzes at λ g 360 or 280 nm to yield butadienylketene
(1,3,5-hexatrien-1-one). In the present study, we report the
results of photochemical reactions of oxygen atoms with the
following methylated benzenes: toluene, m-xylene, p-xylene,
and mesitylene. In each case, ketenes are products of reaction.
Each reaction was studied using 16O and 18O isotopes of oxygen,
as well as standard and deuterated isotopomers of each
Experimental
Normal toluene (99.5+%, Aldrich) was distilled over sodium
prior to use. Toluene, toluene-R,R,R-d3 (99.8 atom % D, C/D/N
isotopes), toluene-2,3,4,5,6-d5 (99.7 atom % D, C/D/N isotopes),
toluene-d8 (99.7 atom % D, C/D/N isotopes), m-xylene-H10
(99+%, Aldrich), m-xylene-d10 (98.5 atom % D, C/D/N
isotopes), p-xylene-H10 (99+%, Aldrich), p-xylene-d10 (99+
atom % D, Aldrich), mesitylene-H12 (98%, Aldrich), and
mesitylene-d12 (98 atom % D, Aldrich) were freed of relatively
volatile impurities by three consecutive freeze-pump-thaw
cycles at liquid nitrogen temperature. The substrate vapors were
warmed to room temperature and expanded into an evacuated
sample chamber and diluted with argon (99.995%, Air Products)
to an Ar:substrate mole ratio of 100:1. Standard ozone 16O-
16O-16O, and the 18O-18O-18O isotopomer of ozone, were
made by passing an electric discharge, from a Tesla coil, through
a sample of oxygen gas (99.996%, Air Products; 18O isotope
from Euriso-top, 98.01% enrichment) contained in a glass U
tube which was submerged in liquid nitrogen. In this way the
ozone condensed to the solid phase on the walls of the glass
tube as it formed. The solid ozone was freed of residual oxygen
molecules by pumping at 77 K. The purified ozone was then
warmed to room temperature, expanded into a separate evacu-
ated sample chamber, and diluted with argon to an Ar:O3 mole
ratio of 100:1.
The IR spectrometer and accompanying vacuum chamber
have been described in detail elsewhere.10 The samples were
codeposited on a gold-mirrored matrix support at a combined
rate of 8.4 mmol/h for a total of 24.2 mmol. The matrix support
was held at 12 K with a closed-cycle, single-stage helium
refrigerator (APD Cryogenics, model DE-202). The temperature
was measured at the cold plate with a gold/chromel thermo-
* Corresponding author.
10.1021/jp992832t CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/06/2000