mixture after the end of photolysis. Under these conditions,
the recombination of peroxy radicals with NO2 (reactions (5)
and (7)) cannot compete with their reaction with NO,
RC(H)O, O and NO , using nitrogen as a bu†er gas. The
RCO radicals formed in reaction (4) undergo reactions (1) and
2
2
(2); the peroxy radicals are scavenged by NO (reaction (5)).
2
In Fig. 3, IR spectra of a mixture of 0.025 mbar 2,2-
RC(O)O ] NO ] R ] CO ] NO
(8)
(9)
dimethylpropionaldehyde (\pivalaldehyde), 0.1 mbar Br ,
2
2
2
2
0
.006 mbar NO , 5.0 mbar O , and 995 mbar N before and
RO ] NO ] RO ] NO ,
2
2
2
2
2
after 4 min of photolysis at j P 420 nm are shown. The loss of
2,2-dimethylpropionaldehyde is accompanied by the forma-
tion of CO and 2,2-dimethylpropionyl peroxynitrate, in
accord with reactions (1), (2) and (5). The photolysis product
spectra of other RC(O)HÈBr ÈO ÈNO ÈN mixtures were
and the e†ective lifetime of the peroxynitrate is identical to its
thermal lifetime. In these experiments, a fast change of the
typical peroxynitrate bands in the photolysis product spectra
was not observed after the addition of NO, indicating the
absence of short-lived alkyl peroxynitrates and thus setting
an upper limit of about 10% for the initial abstraction of
non-aldehydic H atoms by Br. Consequently, the observed
carbonyl bands at 1840 cm~1 were assigned to the desired
acyl peroxynitrate product.
2
2
2
2
equivalent. IR spectra of the corresponding peroxynitrates are
shown in Fig. 4. The positions of the predominant absorption
bands are the following (in cm~1): 2982, 1835, 1738, 1301,
1037, 796 (n-butyryl peroxynitrate); 2991, 1827, 1738, 1299,
1037, 797 (2-methylpropionyl peroxynitrate); 2975, 1832, 1736,
1301, 1049, 796 (n-pentanoyl peroxynitrate); 2975, 1831, 1736,
1302, 1048, 796 (3-methylbutyryl peroxynitrate); 2985, 1824,
1736, 1299, 1042, 1006, 796 (2-methylbutyryl peroxynitrate);
2989, 1820, 1737, 1301, 1058, 1011, 796 (2,2-dimethylpropionyl
peroxynitrate). The IR spectrum of n-butyryl peroxynitrate is
in good agreement with a spectrum published by Niki et al.29
Pure samples of peroxynitrates are difficult to synthesize
due to their thermal instability. For the calibration experi-
ments, most of the acyl peroxynitrates were thus prepared in
situ by photolysis of Br in the presence of the corresponding
2
aldehyde, NO and an excess of O (P200 mbar). Under these
2
2
conditions, thermal decomposition of RCO is negligible. A
halogen lamp and cut-o† Ðlter combination (j P 420 nm) was
The ratios k /k were derived using eqn. (I). Since the
yields of CO and RC(O)O NO for the same carbonyl rad-
icals strongly depend on the partial pressure of oxygen,
dis O2
used in these experiments in order to avoid photolysis of NO
and the complications induced by its photolysis products NO
2
2
2
and O . IR absorption coefficients of the peroxynitrates were
then determined assuming a stoichiometric consumption of
experiments at di†erent [O ] can be used to test the method
and, eventually, to detect complications in the reaction
3
2
the aldehyde according to
mechanism. According to eqn. (I), the expression [O ]
2
]
*[CO]/*[RC(O)O NO ] should have an O independent
2
2
2
RC(O)H ] Br ] O ] NO ] RC(O)O NO ] HBr.
value which is equal to k /k . In Figs. 5 and 6, this quantity
dis O2
2
2
2
2
is plotted as a function of oxygen partial pressure at 317 K for
The validity of this assumption was supported by the absence
of any unidentiÐed product IR bands under these conditions.
Intensity ratios of strong and weak IR absorption bands were
independent of total absorbance thus suggesting that BeerÏs
law is valid. n-Butyryl peroxynitrate was synthesized both in
situ and by the wet chemical method described by Ga†ney et
al.27 which is based on the reaction of n-butyric acid anhy-
dride with H O and HNO . Di†erent from ref. 27, n-butyryl
3-methylbutyryl, 2-methylpropionyl, and 2-methylbutyryl, and
at 317, 307 and 298 K for 2,2-dimethylpropionyl, demonstrat-
ing that [O ] ] *[CO]/*[RC(O)O NO ] is in fact indepen-
2
2
2
dent of the O concentration. For 3-methylbutyryl, the CO
2
yield was slightly above the detection limit, and for n-butyryl
and n-pentanoyl only upper limits could be derived for the
CO yields. In these cases, experiments were performed only at
2
2
3
peroxynitrate was recondensed from its solution in n-tridecane
at liquid nitrogen temperature before transferring it to the
reaction cell. Attempts to prepare branched peroxynitrates by
this method were not successful. The IR absorption coeffi-
cients of n-butyryl peroxynitrate from samples prepared by
the wet chemical method were smaller by 15È20% as com-
pared to the in-situ mixtures, possibly due to saturation of the
IR bands since the mixing ratios were larger by a factor of
5
00 28 in this case. For the evaluation of the kinetic data,
absorption coefficients based on the stoichiometric conversion
of RC(O)H to RC(O)O NO were used.
2
2
Reaction temperatures were measured in the gas phase with
two platinum resistance gauges. Silicon oil was used as a
heating liquid; the temperature distribution of the reactor
walls including the end Ñanges was around the nominal value
within a range of ^0.5 K. At high temperatures, the accessible
temperature region of the kinetic experiments was limited by
the thermal instability of the acyl peroxynitrates.
The following chemicals were used as received from the
manufacturer: Br (Merck, 99.8%), CO (Messer Griesheim,
2
9
9.997%), NO (Messer Griesheim, 99.5%), NO (Messer
2
Griesheim,
98%),
n-butyraldehyde
(Aldrich,
99%),
i-butyraldehyde (Aldrich, 98%), 2-methylbutyraldehyde
Aldrich, 95%), 3-methylbutyraldehyde (Merck, 98%),
,2-dimethylpropionaldehyde (Aldrich, 97%), n-pentanal
Merck, 98%).
(
2
(
Fig. 3 IR spectra of
a mixture of 0.025 mbar 2,2-dimethyl-
propionaldehyde, 0.1 mbar Br , 0.006 mbar NO , 5.0 mbar O , and
2
2
2
9
95 mbar N before (top) and after (middle) 4 min of photolysis at
2
Results and discussion
Carbonyl radicals RCO were generated by stationary photoly-
j P 420 nm; the product spectrum consists of absorptions from CO,
2
,2-dimethylpropionyl peroxynitrate, t-butyl peroxynitrate, and
residual 2,2-dimethylpropionaldehyde; bottom: reference spectrum of
the product 2,2-dimethylpropionyl peroxynitrate.
sis of molecular bromine in the presence of the aldehyde
Phys. Chem. Chem. Phys., 2000, 2, 1175È1181
1177