Photolysis of heptanal

Article abstract of DOI:10.1021/jo060596u

Photolysis of heptanal is investigated from an experimental and theoretical point of view. Photoexcited heptanal is believed to undergo rapid intersystem crossing to the triplet manifold and from there undergoes internal H-abstraction to form biradical intermediates. The favored γ-H abstraction pathway can cyclize or cleave to 1-pentene and hydroxyethene, which tautomerizes to acetaldehyde. Yields of 1-pentene and acetaldehyde were measured at 62 ± 7% and 63 ± 7%, respectively, relative to photolyzed heptanal. Additionally, small quantities of hexanal and hexanol were observed. On the basis of combined experimental and theoretical evidence, the remaining heptanal photolysis proceeds to form an estimated 10% HCO + hexyl radical and 30% cyclic alcohols, particularly 2-propyl cyclobutanol and 2-ethyl cyclopentanol.

Full text of DOI:10.1021/jo060596u

Photolysis of Heptanal
Suzanne E. Paulson,* De-Ling Liu,† and Grazyna E. Orzechowska†
Department of Atmospheric and Oceanic Sciences, UniVersity of California,
Los Angeles, California 90095-1565
Luis M. Campos and K. N. Houk
Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90095-1565
paulson@atmos.ucla.edu
ReceiVed March 18, 2006
Photolysis of heptanal is investigated from an experimental and theoretical point of view. Photoexcited
heptanal is believed to undergo rapid intersystem crossing to the triplet manifold and from there undergoes
internal H-abstraction to form biradical intermediates. The favored γ-H abstraction pathway can cyclize
or cleave to 1-pentene and hydroxyethene, which tautomerizes to acetaldehyde. Yields of 1-pentene and
acetaldehyde were measured at 62 ( 7% and 63 ( 7%, respectively, relative to photolyzed heptanal.
Additionally, small quantities of hexanal and hexanol were observed. On the basis of combined
experimental and theoretical evidence, the remaining heptanal photolysis proceeds to form an estimated
10% HCO + hexyl radical and 30% cyclic alcohols, particularly 2-propyl cyclobutanol and 2-ethyl
cyclopentanol.
Introduction
photochemically via ozone reactions with long-chain alkenes
such as 1-octene.6
Straight-chain aldehydes, including heptanal, are ubiquitous
in the Earth’s boundary layer. At remote sites, long-chain
aldehydes are frequently among the most prominent organics
observed. Concentrations typically fall in the mid parts per
trillion to several parts per billion range for clean continental,
coastal, and urban air.1,2 Heptanal has many natural sources,
including trees and grasses, and emission rates can be compa-
rable to terpene emission rates.3,4 Heptanal is also produced by
most if not all combustion sources, including motor vehicles,
wood burning, and cooking.5 Finally, heptanal is generated
Aldehydes are an important source of chain-initiating radicals
throughout the Earth’s troposphere,7 via their Norrish type I
photodecomposition channel (R-cleavage, a, Scheme 1). Pho-
tolysis of larger aldehydes appears to produce fewer radicals in
favor of cyclization (b or c) or cleavage to an alkene and alcohol
(b, the Norrish type II cleavage). Aldehydes also appear to be
key components of secondary organic aerosol formation,
contributing to polymerization reactions,8,9 although this is
debated.10
(5) Schauer, J. J.; Kleeman, M. J.; Cass, G. R.; Simoneit, B. R. T.
EnViron. Sci. Technol. 2002, 36, 567-575.
† Now at the Jet Propulsion Laboratory, Pasadena, CA 91109.
(1) Zielinska, B.; Fujita, E. M. Atmos. EnViron. 2003, 37, S171-S180.
(2) Grossmann, D.; Moortgat, G.; Kibler, M.; Schlomski, S.; Bachmann,
K.; Alicke, B.; Geyer, A.; Platt, U.; Hammer, M.; Vogel, B.; Mihelcic, D.;
Hofzumahaus, A.; Holland, F.; Volz-Thomas, A. J. Geophys. Res. 2003,
108 (D4): Art. No. 8250.
(3) Owen, S.; Boissard, C.; Street, R. A.; Duckham, S. C.; Csiky, O.;
Hewitt, C. N. Atmos. EnViron. 1997, 31, 101-117.
(4) Kirstine, W.; Galbally, I.; Ye, Y. R.; Hooper, M. J. Geophys. Res.-
Atmos. 1998, 103, 10605-10619.
(6) Paulson, S. E.; Chung, M. Y.; Hasson, A. S. J. Phys. Chem. A 1999,
103, 8125-8138.
(7) Paulson, S. E.; Orlando, J. J. Geophys. Res. Lett. 1996, 23, 3727-
3730.
(8) Ziemann, P. J. Faraday Discuss. 2005, 130, 469-490.
(9) Tolocka, M. P.; Jang, M.; Ginter, J. M.; Cox, F. J.; Kamens, R. M.;
Johnston, M. V. EnViron. Sci. Technol. 2004, 38, 1428-1434.
(10) Kroll, J. H.; Ng, N. L.; Murphy, S. M.; Varutbangkul, V.; Flagan,
R. C.; Seinfeld, J. H. J. Geophys. Res.-Atmos. 2005, 110, Art. No. D23207.
10.1021/jo060596u CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/18/2006
J. Org. Chem. 2006, 71, 6403-6408
6403

Paulson et al.
SCHEME 1. Possible Heptanal Photolysis Pathways
The smallest members of the aldehyde family, formaldehyde,
acetaldehyde, and propanal, have been studied extensively.
Aldehydes with four or more carbons have additional reaction
pathways available to them but have been investigated only by
a handful of studies,11-14 and product yields vary widely among
studies.
Here, we provide results of a combined experimental and
theoretical investigation of heptanal photolysis. Products of
photolysis initiated with ultraviolet (UV) lamps have been
measured using gas chromatography with flame ionization and
mass spectral detectors (GC-FID and GC-MS), using on-column
injection, solid-phase microextraction (SPME), and derivatiza-
tion to introduce samples. Theoretical investigations of the
reactions of the triplet aldehyde that results from photoexcitation
were performed using density functional theory.
has only a slight dependence on wavelength,14 consistent with
the notion that pathways a-c are enthalpically accessible
throughout the range of absorption wavelengths.
We focused on the triplet state path, which according to the
fast rate of intersystem crossing previously mentioned is most
likely to be a heavily populated state. Long-chain ketones also
exhibit photochemistry similar to long-chain aldehydes. De
Feyter et al.16 have studied the femtosecond dynamics of ketones
with γ-hydrogens. It is interesting to note that their studies
focused on the higher-energy singlet state reaction and found
that intersystem crossing competed with reactivity in the singlet
manifold. Upon generation of the singlet biradical, cyclization
and cleavage took place without noticeable energetic barriers.
In their studies, they also reported calculations along the singlet
ground-state manifold for the Norrish cleavage and Yang
cyclization of the biradical generated after γ-Η abstraction. In
essence, such a biradical is postulated to react similarly to the
singlet 1,4-biradical generated after intersystem crossing (ISC)
of the triplet 1,4-biradical from heptanal (this study). Wagner
has noted that cyclization and cleavage reactions may occur
from both the singlet and triplet states. That is, because the
geometric changes are similar for product formation and ISC,
the triplet biradical can react to form products without reverting
to the singlet state before product formation.17,18
Mechanism
Heptanal, like other alkyl aldehydes, exhibits a weak,
symmetry forbidden n-π* transition in the 240-360 nm
region,13,14 overlapping with near UV solar wavelengths that
penetrate into the upper troposphere and, to a lesser degree,
into the Earth’s surface. Although H-abstraction can take place
from the singlet state, the very fast rate of intersystem crossing
of aldehydes (ca. 108 s-1 15
) to the triplet state is more likely to
be the predominant path, followed by H-abstraction from the
triplet state.
Results
Heptanal photolysis is thought to proceed via several path-
ways, illustrated in Scheme 1. These include (a) Norrish type I
cleavage leading to n-hexyl and HCO radicals and several
cyclization pathways involving a â-, γ-, δ-, or ꢀ-hydrogen
abstraction; (b) the γ-H abstraction leading to a biradical that
can undergo Norrish II cleavage to 1-pentene and hydroxyethene
(which tautomerizes to acetaldehyde) or can undergo a Yang
cyclization to 2-propylcyclobutanol; (c) the δ-H abstraction
leading to a biradical that can undergo cyclization to 2-ethyl-
cyclopentanol; and (d) decarbonylation to CO and hexane which
have been suggested as products of heptanal photolysis,
analogous to other aldehydes (e.g., Tang and Zhu14). We were
not able to determine a plausible mechanism leading to
decarbonylation (Scheme 1, d, see below). Results of Tang and
Zhu’s studies performed at multiple wavelengths in the 280-
330 range indicate that the Norrish I channel producing HCO
Measurements. UV photolysis of heptanal resulted in several
products, dominated by 1-pentene, acetaldehyde, and hexanal,
together with trace quantities of hexanol and several unidentified
products, a few with yields of up to about 7%. Positive identities
of the first four products were established by comparing
retention times and ionization fingerprints in GC-MS to
authentic standards and the NIST mass spectral library. The
measured photolysis yields (relative to reacted heptanal) of
acetaldehyde, 1-pentene, and n-hexanal were 63 ( 6%, 62 (
7%, and 7 ( 3%, respectively (Figure 1 and Table 1). A small
correction was made to the acetaldehyde data to account for its
photolysis. 1-Hexanol was also observed in both the SPME/
GC-MS and GC-FID samples, but with a yield of less than 0.5%.
No peak was observed at the retention time of n-hexane, which
was established by authentic standards and its Kovats index.
Two products (unknowns indicated in Table 1) with yields of
around 7% each elute after n-heptanal and in approximately
(11) Chen, Y. Q.; Zhu, L.; Francisco, J. S. J. Phys. Chem. A 2002, 106,
7755-7763.
(12) Tadic´, J.; Juranic, I.; Moortgat, G. K. Molecules 2001, 6, 287-
299.
(13) Tadic´, J. M.; Juranic, I. O.; Moortgat, G. K. J. Chem. Soc., Perkin
Trans. 2 2002, 135-140.
(14) Tang, Y.; Zhu, L. J. Phys. Chem. A 2004, 108, 8307-8316.
(15) Hansen, D. A.; Lee, E. K. C. J. Chem. Phys. 1975, 63, 3272-3277.
(16) De Feyter, S.; Diau, E. W.-G.; Zewail, A. H. Angew. Chem., Int.
Ed. 2000, 39, 260-263.
(17) Wagner, P. J. In CRC Handbook of Organic Photochemistry and
Photobiology, 2nd ed.; Horspool, W. M., Lenci, F., Eds.; CRC Press: New
York, 2004; Chapter 58.
(18) Michl, J. J. Am. Chem. Soc. 1996, 118, 3568-3579.
6404 J. Org. Chem., Vol. 71, No. 17, 2006

Photolysis of Heptanal
kcal/mol, respectively. Such values are relatively similar to the
values of the singlet biradicals obtained for the γ-H abstraction
of 2-pentanone.16
Discussion
Table 2 summarizes the theoretical and experimental results
of this work, together with two earlier investigations of heptanal
photolysis.
Norrish I Channel. Theory suggests that the Norrish I
cleavage, which dominates the photolysis products from small
aldehydes, should be essentially shut down for heptanal in the
triplet state. This is due to the competing type II channels that
have barrier heights that are lower by at least 3.5 kcal/mol,
leading to hydrogen abstraction products. It may be possible,
however, that the Norrish I channel may be accessed through
the singlet state, before intersystem crossing to the competing
triplet state, as it has been previously observed for ketones
similar to the case of heptanal.16,19 Alternatively, the reversibility
of the biradicals generated may allow for the triplet state to
access this channel leading to the hexyl and HCO radicals at a
relative energy of 7.4 kcal/mol.
FIGURE 1. Products from UV photolysis of heptanal in air at
atmospheric pressure, experiment 4. The initial heptanal concentration
was 2.6 ppm.
In our experiments, we observed formation of hexanol and
hexanal, which could arise from the hexyl radical produced by
the Norrish I channel. Pathways leading to these products are
shown in Scheme 3: disproportionation of C6H13OO (pathway
a) produces both hexanol and hexanal, and the self-reaction of
C6H13OO producing two primary hexyloxyl radicals (b), fol-
lowed by the reaction of the hexyloxyl radical with O2 (e),
generates hexanal. Little hexanal and hexanol are expected
however, as the self-reaction of C6H13OO, radical formation (b),
is expected to dominate disproportionation (a) by perhaps a
factor of 5.20,21 Further, a substantial body of literature indicates
that the hexyloxyl radical isomerization (d) should be favored
over reaction with O2 (e),20,21 such that only about 4% of
hexyloxyl radicals should eventually generate hexanal in 1 atm
of air (calculated with FACSIMILE). Given that products other
than the hexyl radical account for most of the heptanal
photolyzed, it is difficult to rationalize such a high yield of
hexanal. The observed trace quantities of hexanol, on the other
hand, are consistent with a moderate yield of hexyl radicals, in
the neighborhood of 10% or less.
Norrish I cleavage was monitored with CO and HCO by
Tadic´ et al.13 and Tang and Zhu,14 respectively. Tang and Zhu14
measured the HCO yield at discrete wavelengths and found that
HCO formation varied from 0.10 to 0.15 over the wavelength
range 305-320 nm. Tadic´ et al.13 estimated the R-cleavage
channel at 10% after correcting the CO they observed for
secondary reactions that also generate CO, which accounted for
nearly half of the CO by the end of their experiments.
Interpretation of the HCO observations from high-concentration
(1700-13 000 ppm) cavity ring down experiments14 requires a
kinetics model, increasing the uncertainty in this method.
Taken together, the experimental evidence indicates that the
Norrish I cleavage accounts for ∼10% of heptanal photolysis
products. Although the theoretical explanation with respect to
the triplet state is not entirely clear, R-cleavage may be possibly
accessed from the singlet state.
the correct location for C7 cyclic alcohols that are expected from
the γ- and δ-H abstraction pathways. The identity of these
products could not be confirmed by SPME/GC-MS, or from
their Kovats indices. Products may not have been successfully
recovered from the SPME fiber, either because they decomposed
or were retained by the SPME fiber. Further, authentic standards
and index data are not available for these compounds.
Theoretical. The optimized structures (triplet heptanal,
transition structures, and selected biradicals) of importance for
hydrogen transfer and R-cleavage are shown in Scheme 2, along
with the zero-point energy inclusive barrier heights (in kcal/
mol). In comparing the feasibility of the carbonyl oxygen to
extract a â-, γ-, δ-, or ꢀ-hydrogen, one can clearly see that the
highest barrier heights correspond to the formation of a five-
and an eight-membered ring (â- and ꢀ-H). Furthermore,
R-cleavage is also a high-energy process, relative to γ- and
δ-hydrogen abstraction, requiring 11 kcal/mol to overcome the
barrier height. Although γ-hydrogen abstraction has the lowest
barrier height (6.9 kcal/mol), the transition structure for δ-H
abstraction lies only 0.8 kcal/mol above the γ-H channel.
Kinetically, transition state theory predicts that the biradicals
arising from γ-H and δ-H abstraction will form in ∼80% and
∼20% yields, respectively, whereas trace amounts of the
R-cleavage may be observed. The generation of both biradical
intermediates 3BR-γ and 3BR-δ is slightly endothermic, allow-
ing for a feasibly reversible path that can potentially explore
other channels such as the Norrish I. Following the formation
3
3
of the triplet biradicals, BR-γ and BR-δ, cyclization and/or
cleavage may be accessed.16-18 Note that the percentages
correspond to formation of the biradicals and not the products.
3
The pathways of BR-γ include cleavage to 1-pentene and
hydroxyethylene and cyclobutanol formation (see Scheme 1).
We also calculated the relative energies of the ground-state
3
3
products following biradicals BR-γ and BR-δ (see Scheme
1). The relative energies of the cis- and trans-cyclopentanols
3
(from cyclization of BR-δ) are -65.7 and -65.9 kcal/mol,
3
(19) Diau, E. W.-G.; Ko¨tting, C.; Zewail, A. H. ChemPhysChem 2001,
2, 273-293.
(20) Atkinson, R.; Arey, J. Chem. ReV. 2003, 103, 4605-4638.
(21) Orlando, J. J.; Tyndall, G. S.; Apel, E. C.; Riemer, D. D.; Paulson,
S. E. Int. J. Chem. Kinet. 2003, 35, 334-353.
respectively, and cleavage of the BR-γ yields 1-pentene and
hydroxyethylene at a relative energy of -35.5 kcal/mol.
Furthermore, the cyclization of 3BR-γ would yield the cis- and
trans-cyclobutanols at relative energies of -48.8 and -51.5
J. Org. Chem, Vol. 71, No. 17, 2006 6405

Paulson et al.
TABLE 1. Product Yields for Heptanal Photolysis Experiments
product yields (%)
hexanal
initial heptanal
experiment
(ppm)
1-pentene
acetaldehyde
Unk. 1a
Unk. 2a
1
2
3
4
5
2.1
2.9
2.5
2.6
2.0
58
70
64
59
66
55
71
64
59
66
5.4
5.3
4.3
9.2
8.7
5.1
5.4
6.7
5.6
6.6
6.6
8.5
8.1
7
7
average
63 ( 6
62 ( 7
6.6 ( 2
6.5 ( 3a
6.8 ( 3a
a Unknowns 1 and 2 (see text). Quantification is based on an FID response for compounds with the formula C7HxO, corresponding to 2-propylcyclobutanol
and 2-ethylcyclopentanol.
SCHEME 2. Different Channels of Reactivity from the Triplet State of n-Heptanal (box) Showing the Transition Structures and
Relative Energies (in kcal/mol), Including Those of the Triplet Biradicals Generated after γ-H and δ-H Abstraction
TABLE 2. Summary of Heptanal Photolysis Studiesa
product yields per molecule of heptanal photolyzed (%)
total pressure
(Torr);
[initial aldehyde]
(ppm)
Norrish II.
C7H14O cyclic
alcohols
wavelength range,
max (nm)
Norrish I.
HCO + C6H13
Norrish II.
pentene + C2H4O
study
λ
CO + n-hexane
this work,
exptl
275-380, 312b
755; 2.0-2.9
1- hexanol: trace,
<0.5, hexanal: 7 ( 2
<1
63 ( 7
e80e
possible IDf;
14 ( 7%
g20g
0j
this work,
theor
_
_
0
Tadic´ et al.13a
Tang and Zhu14
275-380, 313b
700; 100
300; 1700-13000
CO: 10 ( 8
38 ( 3
27 ( 3
not observedh
0
308, 308c
HCO: 15 ( 7d
40 ( 4i
23 ( 9
a Total quantum yield for all channels was reported as 0.31 ( 0.01. b TL-12 UV fluorescent bulb light source. λmax does not include the narrow spikes
at 320 nm and other wavelengths. c Excimer laser light source. Tang and Zhu measured HCO over the range 280-330 nm and other products at 308 nm
3
only. d At 310 nm. By 315 nm, the yield had dropped to 10%. e Corresponds to the formation of the biradical BR-γ, which can partition to cyclobutanol
and 1-pentene + C2H4O and undergo disproportionation reactions, assuming irreversibility. f Two peaks, each with yields of about 7%, were observed that
may be cyclic alcohols (see text). g Corresponds to the formation of the biradical 3BR-δ, which can cyclize to form cyclopentanol, assuming irreversibility.
h Using Fourier transform infrared spectroscopy (FTIR). Tang and Zhu14 suggest they were not observed because parent aldehyde absorption peaks overlap
those of cyclic alcohols. i Using MS only (no GC). Identification should be considered tentative (see text). j No hexane was observed in the GC-FID or
SPME/GC-MS analyses (also see text).
Type II Channels. The hydrogen abstraction channels (b and
c, Scheme 1 and Scheme 2) can result in either C7 cyclic
alcohols or decomposition products. All experimental studies
observe substantial amounts of pentene and acetaldehyde,
although reported yields range from 27 to 63%.
Our result, 63 ( 7%, is based on a measurement of both
1-pentene and acetaldehyde, both of which are easily and
reliably quantified via GC-FID and GC-MS. Our yields of
1-pentene and acetaldehyde are in excellent agreement with one
another (Figure 1). The only source of a positive artifact for
this measurement is decomposition of an unknown compound
to produce acetaldehyde and 1-pentene in the column. Although
this cannot be completely ruled out, such a labile source
compound is not known.
Both Tadic´ et al.13 and Tang and Zhu14 quantified 1-pentene
and hydroxyethene and/or acetaldehyde using FTIR but noted
that quantification of each of these products with FTIR is
complicated by the overlap of most of their absorptions with
heptanal. Tadic´ et al.13 assigned a 38 ( 3% yield to the Norrish
II decomposition channel on the basis of 1-pentene, for which
they used a weak absorption peak that did not overlap with
heptanal, and Tang and Zhu14 assigned a yield of 27 ( 3% but
6406 J. Org. Chem., Vol. 71, No. 17, 2006

Photolysis of Heptanal
SCHEME 3. Expected Reaction Pathways Available to the
Hexyl Radical under NOx-Free Atmospheric Conditions
to the Norrish II decomposition channel. They could not detect
hexane with FTIR because of overlap with heptanal and did
not observe evidence for it with MS. As it seems unlikely that
decarbonylation is an accessible pathway for photoexcited
heptanal, it is possible that the other secondary sources of CO
become very significant at the high concentrations used by Tang
and Zhu14 in their experiments.
Experimental Section
Chamber Experiments. Very similar experiments have been
described in detail elsewhere24,25 and are only briefly outlined here.
Investigations of heptanal photolysis were performed in a 240
(experiments 1-3) or 1200 L (experiments 4 and 5) collapsible
Teflon reaction chamber (200 mil) at 296 ( 2 K and atmospheric
pressure. The large chamber was equipped with a Teflon mixing
fan (used while the chamber was filled), and both chambers were
surrounded by eight fluorescent UVB lamps (275-380 nm,
λmax ) 312 nm). In our chamber configuration, these lamps result
did not specify which products were used to derive this value.
A possible source of negative artifacts for these measurements
is reactions with OH or other radicals that may be more
important at the higher reactant concentrations used in these
studies.
Our theoretical results indicate that there is potential for
1-pentene and hydroxyethene to be the predominant products,
as the γ-H abstraction pathway is favored; however, the resulting
biradical can either decompose or cyclize. It must also be noted
that the theoretical percentages in Table 1 correspond to
generation of the biradicals, which can undergo further reactions
and undergo a reversible process22 in the gas phase.
Formation of Cyclic Alcohols. From a theoretical point of
view, a yield of cyclic alcohols of 20% is expected from the
δ-H abstraction pathway, and additional cyclic alcohols are
expected from the γ-H abstraction pathway, assuming an
irreversible pathway following biradical generation.22 Tadic´ et
al.13 postulated that cyclic alcohols were likely products of
heptanal photolysis but could not observe them with FTIR. Tang
and Zhu et al.14 estimated a yield for cyclic alcohols of 40%
using a residual gas analyzer to identify fragment ions that could
arise from cyclic alcohols. Given the complexity of the organics
in their experiments and the lack of authentic standards,
however, quantification is difficult with this method. Our
experimental evidence, together with the other studies, indicates
that yields of cyclic alcohols fall in the 15-30% range,
bracketed by the observation of likely peaks in our chromato-
grams at the low end and the yields of other products, which
account for most of the carbon, at the upper end.
Decarbonylation. We observe no evidence for a channel
producing CO and hexane (d, Scheme 1) in either of our
theoretical or experimental results. No traces of hexane were
observed in the GC-FID or SPME/GC-MS samples. Our
detection limit for n-hexane corresponds to a yield of about
0.1%. From an energetic point of view, it is known that the
experimentally observed activation energy for H-CO dissocia-
tion to CO and atomic hydrogen (path c in Scheme 1) is
relatively high, 15.8 kcal/mol,23 thus making such a process
rather unfavorable. Similarly, radical pair recombination, al-
though thermodynamically more advantageous, cannot compete
with the radical reactions with molecular oxygen.
in a heptanal photolysis rate of (1.65 ( 0.03) × 10-5 s-1
.
A small quantity of heptanal (purity 95%, used as received) was
slowly introduced into the chamber by evaporating it into the
purified air used to fill the chamber. Gas samples were automatically
collected from the chamber every 23 min and introduced, via a 2
mL sampling loop, into a GC-FID, equipped with a capillary column
(DB-1, 0.32 mm ID, 3 µm film, 30 m), programmed to 2 min at
-50 °C, then 14 °C/min to 170 °C. The GC-FID was calibrated
with a cyclohexane standard. Effective carbon numbers26 normalized
to the cyclohexane calibration were measured in our laboratory and
were used to calculate concentrations of hydrocarbons and carbonyl
compounds, as follows: hexanol 5.6, acetaldehyde 1.02, 1-pentene
5.0, heptanal 6.15, and hexanal 5.15.27 Concentrations and yields
derived with this method have overall uncertainties of (10%
(aliphatics) and (15% (oxygenates). Relative errors for a set of
reactants and products normalized to the same calibration standard
are about (5% and (10%, respectively.
The chamber was also sampled with SPME/GC-MS25 before and
after each experiment. Used this way, SPME provides qualitative
mass spectral information. Experiments were also monitored for
the production of organic acids using in-inlet derivatization/GC-
MS,24 but no acid formation was observed.
Quantum Mechanical and Chemical Kinetics Methods.
Quantum mechanical calculations were carried out using the
Gaussian 03 suite of programs.28 Density functional theory at the
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Total nonmethane organic carbon: DeVelopment and Validation of a new
instrument, and measurements of total nonmethane organic carbon and
C2-C7 hydrocarbons in the south coast air basin; California Air Resources
Board Report No. 95-335, 1999.
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M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
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M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
Tadic´ et al.13 also found no evidence for decarbonylation.
Tang and Zhu,14 on the other hand, assigned a large yield based
on FTIR measurements of CO, from which they subtracted HCO
measurements obtained with cavity ring down, assigning this
(22) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Sausalito, 1991.
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J. Org. Chem, Vol. 71, No. 17, 2006 6407

Paulson et al.
UB3LYP/6-31G* level of theory was used to obtain structure and
energetic information along the triplet state reaction coordinate.29-31
All relative energies reported are inclusive of the scaled zero-point
energy (ZPE).32 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).
Marka and Prof. Miguel A. Garcia-Garibay for valuable
discussions. This work was supported by the National Science
Foundation under Grants ATM-0100823 and CHE-0240203.
L.M.C. thanks the National Science Foundation, the Paul and
Daisy Soros Fellowship, and the NSF IGERT: Materials
Creation Training Program (MCTP) (Grant number: DGE-
0114443) for graduate support.
Chemical kinetics calculations were performed by solving the
set of ordinary differential equations describing heptanal photolysis
and subsequent reactions of alkyl, alkyl peroxy, and alkoxy radicals
together with other products, relevant inorganic reactions, and
physical loss processes, using the software package FACSIMILE
(AEA Tech., U.K.). Reaction rate constants were obtained from
the evaluations by Sander et al.33 and Lightfoot et al.34
Supporting Information Available: Cartesian coordinates of
the stationary points. This material is available free of charge via
the Internet at http://pubs.acs.org.
JO060596U
(32) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502-16513.
(33) Sander, S. P.; Friedl, R. R.; Golden, D. M.; Kurylo, M. J.; Huie, R.
E.; Orkin, V. L.; Moortgat, G. K.; Ravishankara, A. R.; Kolb, C. E.; Molina,
M. J.; Finlayson-Pitts, B. J. Kinetics and Photochemical Data for use in
Atmospheric Studies, EValuation #14; Jet Propulsion Laboratory Publication
02-25, 2002.
(34) Lightfoot, P. D.; Cox, R. A.; Crowley, J. N.; Destriau, M.; Hayman,
G. D.; Jenkin, M. E.; Moortgat, G. K.; Zabel, F. Atmos. EnViron. 1992,
26A, 1805-1961.
Acknowledgment. We thank Prof. Jochen Stutz for measur-
ing the emission spectrum of the UV lamps and Dr. Zsuzsanna
(29) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(30) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys.
1980, 72, 650-654.
(31) Head-Gordon, M.; Pople, J. A. Chem. Phys. Lett. 1988, 153, 503-
506.
6408 J. Org. Chem., Vol. 71, No. 17, 2006