tion is not available. The rate constants for the OH reactions of
the homologous aldehydes, n-butanal,26,27 n-pentanal,24 and n-
hexanal were estimated to be 2.35 × 10Ϫ11, 2.7 × 10Ϫ11 and 1.9 ×
10Ϫ11 cm3 moleculeϪ1 sϪ1, respectively. Although no data on the
rate constant of the n-heptanal OH reaction are available, it is
expected to be in line with these measurements, resulting in
a reactive lifetime of 5–7 hours under atmospheric conditions
(average noon-time OH concentration of 2 × 106 molecules
cmϪ3 was used).
Maximum daytime photolysis rates for n-butanal and n-
pentanal were estimated during in-situ measurements in the
photochemical outdoor reactor,24 as kph,b = (1.0 0.2) × 10Ϫ5 sϪ1
for n-butanal and kph,p = (1.6 0.15) × 10Ϫ5 sϪ1 for n-pentanal,
corresponding to a photolytic lifetime of 28 hours and 17
hours, respectively. Our previous work on n-pentanal4 and this
study reveal similar behavior of n-heptanal, n-hexanal and n-
pentanal photolysis parameters under laboratory conditions,
including absolute quantum yield values, sensitivity to total
pressure, and Norrish type I/II ratio. This would indicate that
the dominant removal process in the lower troposphere for n-
heptanal, like for other homologous aldehydes, is the reaction
with OH radicals, but that photolytic processes still take some
part in the degradation.
detection and monitoring of all the IR-active products and the
starting material.
Photolysis was achieved with six radially mounted lamps,
TL/12-sunlamps (Philips 40 W TL/12 lamps λ = 275–380 nm).
Spectra were taken every 5 min with a total irradiation time of
60 min. The reaction was followed until the aldehyde concentra-
tion had decreased by 30%.
The use of a continuous broad band light source allows
only the determination of an integral, effective quantum yield
Φint for the photoactive spectral region. Quantum yields were
calculated according to the following equation (for carbonyl
compound, C, and actinometer, Act):
For all TL/12-experiments, n-butanal was used as the
actinometer (with φ = 0.48 0.02, 0.44 0.01, 0.35 0.05, and
0.32 0.01 at 100, 300, 500, and 700 Torr, respectively). The
quantum yield is the only unknown parameter in the equation.
The photolysis rate for the compound Kphot(C) could be directly
measured, and the terms ΣOV(C) and ΣOV(Act) represent the
calculated overlap of lamp emission and absorption spectrum
of substrate. Fig. 4 displays the emission spectra of the lamps
and the spectra of homologous aliphatic aldehydes.
The absolute radical yield under atmospheric conditions can
be estimated for n-heptanal as 10.08 × 0.31 = 3.1%. If quantum
yield values for aldehydic compounds are not known (e.g. for
use in atmospheric modeling), it is common to assume that
they are unity, which is seldom the case as shown in this, and
previous studies.3–6,24 This often leads to an overestimation of
the calculated radicals produced by photolysis processes.
The knowledge of the UV absorption spectra of n-heptanal
was a basic prerequisite for these experiments. The absorption
spectrum has been recently measured by Zabel,25 and is
practically identical to the spectra of homologous aldehydes,
n-butanal and n-pentanal.4,24,30 The spectrum displays a broad
absorption band between 250 and 350 nm, with a maximum
absorption observed at 295 nm with a cross section of 6.0 ×
Conclusions
In this work we achieved several goals.
We identified the primary products of n-heptanal photolysis
following Norrish type I and II decomposition channels,
detectable in the IR region under the experimental conditions.
We quantified the products and partially deduced the pri-
mary photodecomposition pattern.
We determined the absolute quantum yields at different pres-
sures, providing values necessary for atmospheric modeling,
at the same time examining the influence of the total pressure
on the absolute quantum yield and evaluating the importance
of collisional deactivation, and indicating other relaxation
channels.
There is evidence that for up to C4 aldehydes the decom-
position upon absorption of light mainly follows the free
radical channel (Norrish type I), forming a formyl radical and
alkyl radicals.4,6,24 Higher than C4 aldehydes, starting with
n-pentanal, mainly decompose by internal rearrangement of
the molecule (Norrish type II), forming vinyl alcohol, and the
corresponding alk-1-ene. Our results on n-heptanal photolysis
and previous work support this pattern. However, the decrease
of the absolute yields of Norrish type I and II processes indi-
cates that other, so far unidentified, processes become more and
more important in the photolysis of longer chain aldehydes.
10Ϫ20 cm2 moleculeϪ1
.
Experiments were carried out at room temperature (298 K),
at pressures between 100 and 700 Torr (1 Torr = {101325/
760}Pa), with an initial aldehyde concentration of approx.
100 ppm. Qualitative and quantitative data evaluation was
carried out by comparing the product spectra with reference
spectra obtained in the same cell and using calibration curves at
corresponding pressures and resolution.
Carbonyl compounds were obtained from Sigma-Aldrich
Company with purity higher than 95%. Before use, all samples
were degassed by several freeze–pump–thaw cycles. The purity
of the compounds was checked by FTIR spectral measure-
ments and no impurities were found. The conversion of vinyl-
alcohol and ethanal is 1 : 1 (keto–enol tautomerism) according
to reaction (10).
CH ᎐CH–OH
CH –CH᎐O
(10)
᎐
᎐
2
3
References
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Experimental
The apparatus employed in this work has been described else-
where28,29 and so will only be briefly discussed here. The central
part of the apparatus is a 44.2 liter (1.40 m length and 20 cm
diameter) quartz cell equipped with two independent sets of
White-optic mirror arrangements. Sapphire-coated aluminium
mirrors were used in the infrared region (l = 33.6 m) for the
measurements of the educts and products. Infrared spectra at
0.5 cmϪ1 resolution (450–4000 cmϪ1) were measured with a
Bomem DA8-FTIR spectrometer. For the UV measurements
the same diode array detector as previously described was
used.28 This method provides the possibility of simultaneous
J. Chem. Soc., Perkin Trans. 2, 2002, 135–140
139