Photooxidation of n-octanal in air: Experimental and theoretical study

Article abstract of DOI:10.1021/jo102133m

Dilute mixtures of n-octanal in synthetic air (up to 100 ppm) were photolyzed with fluorescent UV lamps (275-380 nm) at 298 K. The main photooxidation products were 1-hexene, CO, vinyl alcohol, and acetaldehyde. The photolysis rates and the absolute quantum yields were found to be slightly dependent on the total pressure. At 100 Torr,φ100 ± 0.41(0.06, whereas at 700 Torr the total quantum yield was φ700 ± 0.32 ( 0.02. Two decomposition channels were identified: the radical channel C ₇H₁₅CHO → C₇H₁₅ + HCO and the molecular channel C₇H₁₅CHO → C₆H ₁₂ + CH₂dCHOH, having absolute quantum yields of 0.022 and 0.108 at 700 Torr. The product CH2dCHOH tautomerizes to acetaldehyde. Carbon balance data lower than unities suggest the existence of unidentified decomposition channel(s) which substantially contributes to the photolysis. On the basis of experimental and theoretical evidence, n-octanal photolysis predominantly proceeds to form Norrish type II products as the major ones.

Full text of DOI:10.1021/jo102133m

ARTICLE
pubs.acs.org/joc
Photooxidation of n-Octanal in Air: Experimental and
Theoretical Study
Jovan M. Tadiꢀc,†,* Lai Xu,‡ K. N. Houk,‡ and Geert K. Moortgat§
†NASA Ames Research Center, Moffett Field, California 94035, United States
‡Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
§Max-Planck-Institut f€ur Chemie, Atmospheric Chemistry Department, Postfach 3060, 55020 Mainz, Germany
S Supporting Information
b
ABSTRACT:
Dilute mixtures of n-octanal in synthetic air (up to 100 ppm) were photolyzed with fluorescent UV lamps (275-380 nm) at 298 K.
The main photooxidation products were 1-hexene, CO, vinyl alcohol, and acetaldehyde. The photolysis rates and the absolute
quantum yields were found to be slightly dependent on the total pressure. At 100 Torr, Φ100 = 0.41 ( 0.06, whereas at 700 Torr the
total quantum yield was Φ700 = 0.32 ( 0.02. Two decomposition channels were identified: the radical channel C7H15CHO f
C7H15 þ HCO and the molecular channel C7H15CHO f C6H12 þ CH2dCHOH, having absolute quantum yields of 0.022 and
0.108 at 700 Torr. The product CH2dCHOH tautomerizes to acetaldehyde. Carbon balance data lower than unities suggest the
existence of unidentified decomposition channel(s) which substantially contributes to the photolysis. On the basis of experimental
and theoretical evidence, n-octanal photolysis predominantly proceeds to form Norrish type II products as the major ones.
’ INTRODUCTION
such as C3H7CHO, C4H9CHO,3,21 isopentanal and tert-
pentanal,5 n-hexanal,6,7 and n-heptanal.7-9
Photodissociation of aldehydes represents an important
source of free radicals in the lower atmosphere and thus may
significantly influence the atmospheric oxidation capacity.1 n-
Hexanal and higher aldehydes have been observed in ambient air
and in emissions of various plants, especially grasses, with
comparable emissions rates as the monoterpenes.2 The examina-
tion of longer chain aldehydes has recently come into the
research focus, providing quantum yield and decomposition
patterns data necessary for atmospheric modeling.3-9 This is
the first reported study on the photolysis of n-octanal.
n-Hexanal and n-heptanal photodissociation patterns6-9 and
reaction schemes for n-pentanal3 suggest that processes 1 and 2
play an important role in the photolysis of longer chain alde-
hydes:
n-octanal þ hv f n-C7H15 þ HCO
ð1Þ
ðNorrish type IÞ
f C6H12 þ ½CH2 ¼ CHOHꢀ
ð2Þ
ðNorrish type IIÞ
Aliphatic aldehydes exhibit a weak absorption band in the
wavelength range 240-360 nm as a result of a symmetry
forbidden n-π* transition.10,11 There have been a number of
studies devoted to the photodissociation of the simplest alkyl
aldehydes, such as HCHO, CH3CHO, and C2H5CHO,12-20 and
a recently growing number devoted to longer chain aldehydes,
Process 1 represents the fragmentation into free radicals, with
an enthalpy of around 350 kJ/mol, corresponding to a photo-
chemical threshold of around 340 nm.3 Process 2, which is
Received: November 9, 2010
Published: February 14, 2011
r
2011 American Chemical Society
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Figure 2. Photolysis of n-octanal-time profile: variation of the partial
pressures of products. Curve of CO (C) shows that CO is not only the
primary product but also a product of the reactions following a primary
step. Ethenol partial pressure reaches maximum and then decreases by
conversion to acetaldehyde.
Figure 1. FTIR spectrum of 100 mTorr n-octanal, before and after
photolysis (6 TL/12 lamps, 100 Torr synthetic air). Major products
1-hexene, ethenol, and CO2, which is formed as a byproduct, are marked
(exact positions of the peaks are given in the text).
common to molecules with a γ-hydrogen atom, is an intramole-
cular rearrangement with enthalpy around 80 kJ/mol (λ e 1454
nm).3 The enthalpy change for process 2 was calculated assuming
that the keto form of acetaldehyde is formed in the primary step.
Thisassumption isnot correct, and therefore, the enthalpy change
for this reaction should be adjusted for the difference between
the heats of formation of enol and keto forms of acetaldehyde
the quantification, is fairly weak, resulting in a large uncertainty in
the 1-hexene concentration at the beginning of the photolysis,
and the first two points have been omitted from the plot.
1-Hexene is formed exclusively as a primary product in the
reaction. Acetaldehyde is secondary product arising from ethenol
conversion, and it undergoes further photolysis, which was
described in our previous work on n-pentanal.4 This time, only
the identification of acetaldehyde was done.
(the equilibrium constant for keto-enol equilibrium is 5 ꢁ 10-6
,
and the corresponding ΔG value is ∼70 kJ/mol).22 The total
contribution of the Norrish I and II processes in the cases of
n-butanal, n-pentanal, n-hexanal, and n-heptanal photolysis is,
however, less than 100%, indicating a further reaction pathway
(the calculation is made on carbon balance data).4,6,9
Norrish type I reaction gives two radicals, which immediately
react with oxygen. One of the products, formyl radical HCO, is
quantitatively converted to CO and HO2,12,23,24 according to
HCO þ O2 f HO2 þ CO
ð3Þ
In this paper, the results obtained from the photolysis of small
quantities of n-octanal in air, using wide band emission lamps, are
reported together with theoretical investigation of the reactions
of triplet aldehyde that results from photoexcitation using density
functional theory. We have investigated the products and abso-
lute quantum yields in the pressure range 100-700 Torr.
This process is followed through the rate of production of CO
(2094-2096 cm-1). The n-heptyl radicals are oxidized forming
n-heptylperoxy radicals
n-C7H15 þ O2 þ M f n-C7H15O2 þ M
ð4Þ
The n-C7H15O2 radicals subsequently react with HO2, re-
combine, or disproportionate to produce a variety of products
(reactions 5-8).25,26
’ RESULTS AND DISCUSSION
The photooxidation experiments were carried out in a long-
path quartz cell with detection of precursors and products by
FTIR spectroscopy. After identification and quantification of the
products, a mechanistic description of the photooxidation/
photolysis is proposed. From the measured decay rate of the
starting material, and from the knowledge of the absorption
spectrum, overall quantum yields for the photolysis were calcu-
lated for various pressures.
Figure 1 shows the FTIR spectra of a mixture of 100 ppm
n-octanal in synthetic air, before and after the photolysis. Major
products observed are CO (νmax = 2037-2235 cm-1), 1-hexene
(880-940, 970-1012, 1420-1500, 1620-1660, 2810-3030,
3087.5 cm-1), ethenol (vinyl alcohol, which is the enol form of
acetaldehyde) (947.6, 1078, 1118.3, 1259.8 cm-1), and acetal-
dehyde (1348.5-1355.5 cm-1).
n-C7H15O2 þ HO2 f n-C7H15OOH þ O2
2n-C7H15O2 f 2n-C7H15O þ O2
ð5Þ
ð6Þ
f n-C7H15OH þ n-C6H13CHO þ O2 ð7Þ
n-C7H15O þ O2 f n-C6H13CHO þ HO2 ð8Þ
However, because of the estimated low reaction rate constant
for the recombination and disproportionation, reactions 6 and
7,26 the major expected product is n-heptyl hydroperoxide
(n-C7H15OOH). This peroxide was not observed in the FTIR
spectrum. Either its concentrations were under the detection
limit (∼1 mTorr) or the signals interfered with those of other
compounds. No evidence of n-heptanal produced via channels
7 and 8 was observed.
Figure 2 shows the concentration-time profiles for two
products used for the estimation of Norrish type I/II absolute
yields and their ratios. The only peak of 1-hexene that does not
interfere with other present compounds, and which was used for
1-Hexene is a stablemolecular species and was usedto calculate
the rate of the Norrish type II process. The coproduct of the
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Table 1. Relative Probabilities for n-Octanal Molecule To
Undergo Different Decomposition Channels (Errors Are
Represented by Experimental Scatter)
total pressure (torr)
Norrish type I
Norrish type II
100
100
100
300
300
300
500
500
500
700
700
700
average
15.46
16.93
16.92
16.96
17.33
16.93
15.81
18.18
20.03
15.04
17.22
18.28
17.1 ( 3.0
84.54
83.07
83.08
83.04
82.67
83.07
84.19
81.82
79.97
84.96
82.78
81.72
82.9 ( 3.0
Figure 3. Photolysis of n-octanal products formed versus loss in n-
octanal. See comments in the Figure 2 caption.
reaction is ethenol CH2dCHOH, which tautomerizes slowly to
acetaldehyde. This is clearly seen in Figure 2, where the peak
integral-time profile of the enol product displays the behavior of
a primary product involved in a secondary reaction. Ethenol
concentration reaches a maximum after ∼40 min photolysis, and
its subsequent decay is caused by its conversion to acetaldehyde.
Figure 3 shows profiles of product concentrations (partial
pressures) of CO and 1-hexene versus loss of n-octanal.
Scheme 1. Pathway to Cis/Trans Isomers of
2-Butylcyclobutanol
According to the suggested mechanism, the yield of 1-hexene
should be identical to the sum of the yields of ethenol and
acetaldehyde, since all compounds are products of the same
decomposition channel. CO originating from the reaction of
HCO radical with oxygen can be treated as a primary product,
but an increase in the yield at long conversion times is observed,
which can be attributed to the photolysis of secondary products
such as acetaldehyde, n-heptanal, etc. Theoretically, another
possible source of CO could be the reaction
the natural logarithm of concentration versus time (first order
decay) and performing a least-squares fit. From these results,
overall quantum yields were calculated according to eq 10 using
the overlap integrals of n-octanal and n-butanal spectra (integral
equals unity) within the range of the TL/12 emission (275-
380 nm).
KphotðCÞ
ΦintðCÞ ¼
ð10Þ
ΣOVðCÞ
n-C8H16O þ hv f n-C7H16 þ CO
ð9Þ
KphotðOVÞ
ΣOVðActÞ ꢁ ΦintðActÞ
n-octanal
n-heptane
In all cases, the absolute quantum yield dependency on the
total pressure was observed. Measurements of all n-alkanal cross
sections from C2-C9 have been performed recently by Zabel
et al.28 This study reveals that the UV spectra of the aldehydes
from n-butanal upward are practically identical.
The absolute quantum yields data are summarized in Table 3
for all experiments performed at total pressures of 100, 300, 500,
and 700 Torr and are also shown in form of the Stern-Volmer
plot in Figure 4.
Since the intercept at zero pressure is not equal to 1 (the
intercept is 2.4 ( 0.1), which should be the case if collisional
deactivation was the only relaxation process besides the photo-
decomposition, it seems very probably that there are other
energy-dissipating processes (the triplet state of n-octanal could
deactivate by phosphorescence, etc.).6 The interaction of the
photoexcited molecules with the walls, resulting in the relaxation
to the ground state, is of minor importance because of the huge
volume-to-surface ratio of the reaction cell. Theslope of the fitting
line, described by (1.061 ꢁ 10-3)P, corresponds to the sensitivity
of absolute quantum yield on the total pressure P (in Torr).
The total quantum yield Φtot can be calculated from the
following eq 1/Φtot = 2.368 þ (1.061 ꢁ 10-3)P. Using the
The stable product of this reaction, n-heptane, was not
identified in our experiments. Decarbonylation was not observed
in the recent similar investigation of n-heptanal photolysis.8 The
relative probability for n-octanal molecules to undergo two
detected decomposition channels is shown in Table 1. The
probability is deduced from the ratio of primary formed CO
(Norrish type I) and 1-hexene (Norrish type II).
Assuming that the formation of CO2 is an artifact, the sum of
both processes Norrish type I and II is 41 ( 7%, indicating the
presence of other unidentified product channels.
n-Octanal molecules could undergo photocyclization, forming
cis/trans isomers of 2-butylcyclobutanol (Scheme 1). This me-
chanism is observed in the photolysis of n-pentanal27 and some
ketones7 and suggested as the important one in the photolysis of
n-heptanal accounting for ∼30% of the total product yields.8
These compounds were not detected in our experiments.
Absolute Quantum Yields. One of the objectives of the
study was to determine the dependence of the absolute quantum
yield on the total pressure. n-Butanal was used as the actinometer,
with absolute quantum yields recently reported.4 From the decay
of n-octanal concentration (partial pressure), the photolytic rate
constants were deduced for the different pressures by plotting
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Table 2. Absolute yields of Norrish Type I and II Decom-
position Products in n-Octanal Photolysis (Errors Represent
the Experimental Scatter)
total pressure
(Torr)
Norrish type I
Norrish type II
(Δprimary CO/-Δoctanal)
(Δ1-hexene/-Δoctanal)
100
100
6.34
34.66
6.94
34.06
100
6.95
34.06
300
6.95
34.05
300
7.11
33.89
300
6.94
34.06
500
6.48
33.55
500
7.45
32.79
500
8.21
34.83
700
6.17
34.83
700
7.06
33.94
Figure 4. Pressure dependency of 1/(absolute quantum yields) in n-
octanal photolysis at different total pressures of synthetic air (Stern-
Volmer plot).
700
7.49
33.51
Average
7.01 ( 0.84
34.02 ( 0.81
of the homologous aldehydes, n-butanal,29,30 n-pentanal,27 and
n-hexanal6 were estimated to be 2.35 ꢁ 10-11, 2.7 ꢁ 10-11, and
1.9 ꢁ 10-11 cm3 molecule-1 s-1, respectively. Although no data
on the rate constant of n-octanal OH reaction is available, it is
expected to be in line with these measurements, resulting in the
reactive lifetime of 5-7 h under atmospheric conditions (average
noontime OH concentration of 2 ꢁ 106 molecules cm-3 was used).
Maximum daytime photolysis rates for n-butanal and n-
pentanal were estimated during in situ measurements in the
photochemical outdoor reactor,27as kph,b= (1.0 ( 0.2) ꢁ 10-5 s-1
for n-butanal and kph,p = (1.6 ( 0.15) ꢁ 10-5 s-1 for n-pentanal,
corresponding to a photolytic lifetime of 28 and 17 h, respec-
tively. Our previous work on other homologous aldehydes and
this study reveals similar behavior of n-octanal, n-heptanal,9 n-
hexanal,6 and n-pentanal4 photolysis parameters under labora-
tory conditions, including absolute quantum yield values, sensi-
tivity on total pressure, and Norrish type I/II ratio. This would
indicate that the dominant removal process in the lower tropo-
sphere for n-octanal, as for other homologous aldehydes, is the
reaction with OH radicals but that photolytic processes still take
some part in the degradation.
The absolute radical yield at atmospheric conditions can be
estimated for n-octanal as 7.01 ꢁ 0.32 = 2.2%. If quantum yields
for aldehydic compounds values are not known (e.g., for the 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.3-6,27 This often leads to an overestimation of the
calculated radicals produced by photolysis.
Theoretical Studies. The optimized structures (triplet n-
octanal, transition structures, and selected biradicals) of impor-
tance for hydrogen abstraction and R-cleavage are shown in
Scheme 2, along with the zero-point energy corrected barrier
heights (in kcal/mol). In comparing the feasibility of the
carbonyl oxygen to extract a β-, γ-, δ-, or ε-hydrogen, it is clear
that the highest barrier heights correspond to the five- and eight-
membered ring transition structures (β- and ε-H). The five-
membered TS involves significant angle strain to achieve the
hydrogen-transfer geometry, while some unfavorable torsional
interactions destabilize the eight-membered TS. Furthermore,
R-cleavage is also a high-energy process, relative to γ- and
δ-hydrogen abstraction, involving a 11.2 kcal/mol barrier height.
estimated absolute yield from Table 2 and quantum yield value
from Table 3, it is possible to calculate the absolute contribution
at atmospheric conditions (700 Torr): for Norrish type I (radical)
process, j(I) = 0.022 ( 0.002 and Norrish type II (molecular)
j(II) = 0.108 ( 0.002 (total quantum yield being 0.32, and a
contribution of both decomposition channels being 41 ( 7%).
Comparison with other aldehydes (Table 3) shows that n-
pentanal, n-hexanal, n-heptanal, and n-octanal have similar
dependencies on the total pressure of synthetic air.4,6,9 The
sensitivity of the quantum yield on the total pressure (Torr) is
described by the slope of the curve in Stern-Volmer plot and
was 1.061 ꢁ 10-3, 1.169 ꢁ 10-3, 4.758 ꢁ 10-4, 7.771 ꢁ 10-4
,
and 1.931 ꢁ 10-3 for n-octanal, n-heptanal, n-hexanal, n-penta-
nal, and n-butanal, respectively.4,6,9 n-Octanal is more or less in
the line of n-heptanal, n-hexanal, and n-pentanal; n-butanal
sensitivity is somewhat higher, which also accompanied with
the substantially different Norrish type I/II ratio. This may point
to the different spin-state distributions upon the photoexcitation.
Future work on the photolysis of the longer chain aldehydes
should concentrate on the determination of the spin states of
photoexcited molecules as precursors of Norrish type I and II
processes and on the analysis of other possible products, since
∼60% of the initial aldehyde cannot be accounted for by Norrish
type I and II reaction mechanisms. There is no conclusive evi-
dence for the formation of cyclobutanol derivatives, and further
examination of the subject should be performed. Possible
examinations of the absolute quantum yield dependency on
the low pressures of pure oxygen may reveal new information.
The triplet ground electronic state of oxygen means that reaction
pathways proceeding from the lowest triplet of the aldehyde (T1)
would be probably quenched more efficiently than the ones
proceeding from the vibrationally excited ground singlet (S0*) or
eventually from the first excited singlet (S1). Thus, a stronger
dependence of the decomposition parameters on oxygen pres-
sure may be observed.
The main degradation processes of carbonyl compounds are
controlled by photolysis and by the reaction with OH radicals.
The atmospheric lifetime of n-octanal can be estimated from the
knowledge of the OH reaction rate constant and the photo-
dissociation rate. Unfortunately, the rate constant for OH
reaction is not available. The rate constants for the OH reactions
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Table 3. Absolute Quantum Yield Values in n-Octanal, n-Heptanal,9 n-Hexanal,6 n-Pentanal4 and n-Butanal4 Photolysis at
Different Pressures of Synthetic Air (Errors Are Represented by Experimental Scatter)
absolute quantum yield
total pressure (Torr)
n-octanal
n-heptanal9
n-hexanal6
n-pentanal4
n-butanal4
100
0.39
100
0.35
100
0.40
0.33
100
0.47
0.41
100
0.37
0.45
0.47
100
0.43
0.37
0.43
0.40
0.50
100
0.38
0.42
0.41
0.44
0.47
100
0.39
0.40
0.44
0.36
0.47
average
300
0.41 ( 0.06
0.44
0.39 ( 0.06
0.38
0.43 ( 0.02
0.40 ( 0.04
0.48 ( 0.02
300
0.35
0.36
300
0.35
0.36
300
0.39
0.41
0.41
0.37
0.44
300
0.38
0.41
0.42
0.39
0.44
300
0.36
0.36
0.43
0.36
0.43
average
500
0.38 ( 0.06
0.38 ( 0.03
0.32
0.42 ( 0.01
0.38 ( 0.02
0.44 ( 0.01
500
0.37
0.32
500
0.33
0.32
500
0.35
0.37
0.38
0.36
0.30
500
0.32
0.33
0.43
0.36
0.37
500
0.33
0.37
0.41
0.33
0.37
average
700
0.34 ( 0.03
0.34
0.32 ( 0.05
0.31
0.40 ( 0.03
0.35 ( 0.02
0.35 ( 0.05
700
0.34
0.30
700
0.33
0.31
700
0.31
0.31
0.40
0.35
0.32
700
0.32
0.29
0.37
0.34
0.33
700
0.30
0.31
0.37
0.33
0.32
average
0.32 ( 0.02
0.31 ( 0.02
0.38 ( 0.02
0.34 ( 0.01
0.32 ( 0.01
Scheme 2. Different Reactive Channels from the Triplet State of n-Octanal (Box) Showing the Transition Structures and Relative
Energies (in kcal/mol)a
a Triplet biradicals generated by γ-H and δ-H abstraction are also shown.
Although γ-hydrogen abstraction has the lowest barrier height
(6.9 kcal/mol), the transition structure for δ-H abstraction lies
only 0.7 kcal/mol above the γ-H channel. Both of these
TSs involve transition states with almost perfect chairlike
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ARTICLE
arrangements of the heavy atoms in the six- and seven-membered
transition structures.
higher than 98%. Before use, all samples were degassed by several
freeze-pump-thaw cycles. The purity of the compounds was checked
by FTIR spectral measurements, and no impurities were found. The
calibration of vinyl alcohol was done indirectly, from the amount of
formed acetaldehyde and the converted vinyl alcohol (peak decrease),
since the conversion of these compounds is 1:1 (keto-enol tauto-
merism) according to
We also calculated the relative energies of the cyclized
biradicals 3BR-γ and 3BR-δ and ground-state products formed
by cyclization of these biradicals. The biradicals are slightly
higher in energy than the triplet aldehyde, but rapid inter-
system crossing to the singlet and cyclization is expected to
occur. The relative energies of the cis- and trans-cyclopentanols
(from cyclization of 3BR-δ) are -65.7 and -65.9 kcal/mol,
respectively. The cleavage of the 3BR-γ yields 1-pentene and
hydroxyethylene at a relative energy of -35.5 kcal/mol.
Furthermore, the cyclization of 3BR-γ yields the cis- and
trans-cyclobutanols at relative energies of -48.7 and -51.4
kcal/mol, respectively.
The theoretical prediction of a high barrier for the Norrish
type I reaction does not agree with the significant amount of this
reaction observed experimentally. The R-cleavage may possibly
also occur from the singlet state.
The results of the theoretical investigation are in good agree-
ment with experimental findings in terms of importance of
Norrish type II channel. Another channel very likely to occur,
δ-H abstraction pathway, is not detected in our experiments. The
lack of standards and the limitations of our experimental method
prevented the detection of possible products of this channel—
derivatives of cyclopentanol. Such products were postulated in
other similar experiments with n-heptanal.8
CH2 ¼ CH-OH T CH3-CHdO
Quantum Mechanical Methods. Quantum mechanical calcula-
tions were carried out using the Gaussian 09 suite of programs.33 Density
functional theory at the UB3LYP/6-31G* level of theory was used to
obtain structure and energetic information along the triplet-state reac-
tion coordinate.34-36 All relative energies reported are zero-point energy
(ZPE) corrected. Vibrational frequency calculations were also used to
confirm that minimum energy structures have no imaginary frequencies,
and transition structures (TS) have the appropriate imaginary frequency
for R-cleavage (type I) or H-abstraction (type II).
’ ASSOCIATED CONTENT
S
Supporting Information. Cartesian coordinates and re-
b
ference. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
’ CONCLUSIONS
Corresponding Author
*E-mail: jotadic@lycos.com.
Whereas simple aldehydes with up to four carbons undergo
photochemical decomposition by Norrish type I reactions to
form formyl and alkyl radicals,4,6,27 higher aldehydes mainly
decompose by the Norrish type II reaction, forming vinyl alcohol
and the corresponding 1-alkene. Our study of n-octanal photo-
lysis shows that the Norrish type II reaction is favored over the
type I reaction. Theoretical calculations also show that the
Norrish type II channel is favored. However, the decrease of
the absolute yields of Norrish type I and II processes indicates
that other, so far unidentified, processes become more and more
important in the photolysis of longer chain aldehydes.
’ ACKNOWLEDGMENT
J.M.T. is supported by the NASA Senior Postdoc Program,
Oak Ridge Associated Universities.
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’ EXPERIMENTAL SECTION
The apparatus employed in this work has been described else-
where31,32 and will be briefly discussed here. The central part of the
apparatus is a 44.2 L (1.40 m length and 20 cm diameter) quartz cell
equipped with two independent sets of White-optic mirror arrange-
ments. Sapphire-coated aluminum mirrors were used in the infrared
region (l = 33.6 m) for the measurements of the educts and products.
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Experiments were carried out at room temperature (298 K) at
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an initial concentrations of approximately 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 a commercial supplier with purity
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