Relative reaction rates of HCHO, HCDO, DCDO, H13CHO, and HCH18O with OH, Cl, Br, and NO3 radicals

Article abstract of DOI:10.1021/jp048329k

Formaldehyde (HCHO) is a principal intermediate in the photochemical oxidation of hydrocarbons in the troposphere. Isotopic analysis is an important tool for tracing the atmospheric path of gaseous species, and for this purpose, characterization of the isotope effects in the loss processes for formaldehyde is needed. The main loss pathways for formaldehyde in the atmosphere are photolysis and reactions with the radical species of OH, Cl, Br, and NO ₃. In this study, the kinetic isotope effects in the reactions of five different isotopomers of formaldehyde (HCHO) with OH, Cl, Br, and NO ₃ radicals are studied in a relative-rate experiment at 298 ± 2 K and 1013 ± 10 mbar. The reaction rates of DCDO, HCDO, H ¹³CHO, and HCH¹⁸O with the four radicals are measured relative to H₂CO in a smog chamber using long-path FTIR detection. The experimental data are analyzed with a nonlinear least-squares spectral-fitting method using measured high-resolution infrared spectra and cross sections from the HITRAN database. The reaction rates of HCDO and HCH ¹³O with OH and Cl are determined relative to HCHO as k _OH+HCHO/k_OH+HCDO = 1.28 ± 0.01, k _OH+HCHO/k_OH+HCH18O = 0.967 ± 0.006, k _Cl+HCHO/k_Cl+HCDO = 1.201 ± 0.002, and k _Cl+HCHO/k_Cl+HCH18O = 1.08 ± 0.01. The reaction rates of HCDO and HCH18O with Br and NO₃ are determined relative to HCHO as k_Br+HCHO/k_Br+HCDO = 3.27 ± 0.03, k _Br+HCHO/k_Br+HCH18O = 1.275 ± 0.008, k _NO3+HCHO/k_NO3+HCDO = 1.78 ± 0.01, and k _NO3+HCHO/k_NO3+HCH18O = 0.98 ± 0.01. The errors represent 2σ from the statistical analyses, and do not include possible systematic errors.

Full text of DOI:10.1021/jp048329k

J. Phys. Chem. A 2004, 108, 7393-7398
7393
Relative Reaction Rates of HCHO, HCDO, DCDO, H13CHO, and HCH18O with OH, Cl, Br,
and NO3 Radicals
Karen L. Feilberg,*,† Matthew S. Johnson,† and Claus J. Nielsen‡
Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5, DK-2100 Copenhagen OE,
Denmark, and Department of Chemistry, UniVersity of Oslo, Pb. 1033-Blindern, 0315 Oslo, Norway
ReceiVed: April 16, 2004; In Final Form: June 22, 2004
Formaldehyde (HCHO) is a principal intermediate in the photochemical oxidation of hydrocarbons in the
troposphere. Isotopic analysis is an important tool for tracing the atmospheric path of gaseous species, and
for this purpose, characterization of the isotope effects in the loss processes for formaldehyde is needed. The
main loss pathways for formaldehyde in the atmosphere are photolysis and reactions with the radical species
of OH, Cl, Br, and NO3. In this study, the kinetic isotope effects in the reactions of five different isotopomers
of formaldehyde (HCHO) with OH, Cl, Br, and NO3 radicals are studied in a relative-rate experiment at 298
( 2 K and 1013 ( 10 mbar. The reaction rates of DCDO, HCDO, H13CHO, and HCH18O with the four
radicals are measured relative to H2CO in a smog chamber using long-path FTIR detection. The experimental
data are analyzed with a nonlinear least-squares spectral-fitting method using measured high-resolution infrared
spectra and cross sections from the HITRAN database. The reaction rates of HCDO and HCH18O with OH
18
and Cl are determined relative to HCHO as kOH+HCHO/kOH+HCDO ) 1.28 ( 0.01, kOH+HCHO/kOH+HCH O ) 0.967
18O
( 0.006, kCl+HCHO/kCl+HCDO ) 1.201 ( 0.002, and kCl+HCHO/kCl+HCH ) 1.08 ( 0.01. The reaction rates of
HCDO and HCH18O with Br and NO3 are determined relative to HCHO as kBr+HCHO/kBr+HCDO ) 3.27 ( 0.03,
18
18
k
Br+HCHO/kBr+HCH O ) 1.275 ( 0.008, kNO +HCHO/kNO +HCDO ) 1.78 ( 0.01, and kNO +HCHO/kNO +HCH O ) 0.98
3
3
3
3
( 0.01. The errors represent 2σ from the statistical analyses, and do not include possible systematic errors.
Introduction
The main loss pathways for formaldehyde in the troposphere
are photolysis, reactions with radicals, and deposition.2,7 Pho-
Formaldehyde (HCHO) is one of the most important carbonyl
compounds in the lower atmosphere. It is present in both
polluted and remote areas. It is produced from the combustion
of fossil fuels, the photochemical oxidation of methane and other
hydrocarbons, biomass burning, and microbiological processes.
The decomposition of HCHO plays a significant role as a source
of HOx radicals throughout the atmosphere. About 40% of the
CO source in the atmosphere stems from the oxidation of
methane by OH via formaldehyde.1 While the largest global
photochemical source of HCHO is CH4 oxidation, direct
emission is typically the dominating source in and near urban
areas. In certain regions with high emissions of non-methane
hydrocarbons, such as isoprene (C5H8), the photochemical
oxidation of these compounds can dominate HCHO formation.2
Average mixing ratios of HCHO in the remote atmosphere are
on the order of 10-100 ppt; in polluted areas, they may even
reach 1-20 ppb.3 The sources and sinks of formaldehyde in
the atmosphere and their relative importance are not well-
characterized. In particular, the roles of transport, heterogeneous
chemistry, and halogen reactions are uncertain.4 Recent airborne
measurements of HCHO and other atmospheric species in the
marine boundary layer and the upper troposphere have shown
that HCHO concentrations in these areas are significantly higher
than current models predict.5,6 This suggests that the HOx source
from HCHO in these areas is greater than previously thought,
and that unknown sources of HCHO are present in the upper
troposphere.
tolysis and the reaction with OH (the lifetime of HCHO with
respect to OH is about 24 h)8,9 compete during the day; the
reaction with Cl (the lifetime of HCHO with respect to Cl is
about 2 months)9,10 is important in the marine boundary layer,
and the reaction with the nitrate radical is the dominant loss
process for HCHO at night (the lifetime of HCHO with respect
to NO3 is about 9 years). The reaction with chlorine can
dominate HCHO loss, and thereby the isotopic signature, in the
marine boundary layer where concentrations are sometimes very
high. Aldehyde reactions (HCHO and CH3CHO) have been
identified as the most important sink for Br atoms during
ground-level ozone depletion events in the Arctic marine
boundary layer.11 The reaction of HCHO with Br can also be a
significant sink for ozone-destroying Br atoms in the strato-
sphere.11,12 Characterization of the isotope effects in the loss
processes for formaldehyde is an important step in tracing the
carbon, hydrogen, and oxygen isotopic signatures in the
processes throughout the methane oxidation mechanism.13
Recently, the deuterium isotope effect in the reactions and
photolysis of formaldehyde have generated a great deal of
interest because the photolysis of HCDO is the main source of
atmospheric HD.14 Molecular hydrogen is the basis of new fuel-
cell technologies that may replace fossil fuels; however, the
global budget of H2 and the effect of an increase in its mixing
ratio are not well-understood.15-17 Measurements of D/H ratios
can help to constrain sources and sinks of atmospheric H2;
however, the isotopic signature of the largest source of H2 (the
oxidation of methane via HCHO) has not been fully character-
ized.14 The photolysis of HCHO is the only route transferring
the hydrogen isotope signature in methane and non-methane
* Corresponding author. E-mail: klf@kiku.dk.
† University of Copenhagen.
‡ University of Oslo.
10.1021/jp048329k CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/12/2004

7394 J. Phys. Chem. A, Vol. 108, No. 36, 2004
Feilberg et al.
hydrocarbons to H2.14 The deuterium enrichment in formalde-
hyde is determined by the kinetic isotope effects (KIEs) in the
reactions that produce it, the enrichment in the precursor
hydrocarbons, and the KIE of the chemical reactions of
formaldehyde. The relative reaction rates of HCHO and HCDO
with atmospheric radical species, particularly OH and Cl, are
thus important factors in assessing the budget of H2. Although
the societal relevance of the present study is linked to
environmental issues, the reactions of HCHO with OH, Cl, Br,
and NO3 present some kinetic features that are of theoretical
interest.18,19 In this study, we present the reaction rates of four
isotopomers of HCHO (DCDO, HCDO, H13CHO, and HCH18O)
with OH, Br, Cl, and NO3 radicals relative to the most abundant
isotopomer. We have previously shown that these reactions
exhibit large hydrogen kinetic isotope effects (KIEs) for DCDO
ranging from 30% for Cl to 750% for Br, while the 13C KIEs
are on the order of 3-10%.18-20 The kOH+HCHO/kOH+HCDO KIE
has previously been studied by mass spectrometry,21 and the
constant for the duration of the experiments. Before the
experiments were carried out, the chamber was cleaned by
photolyzing O3 in the presence of water vapor to eliminate
impurities in the system.
HCHO. The HCHO isotopomers used were in the form of
paraformaldehyde, (CH2O)n. DCDO (99 at. % D) was obtained
as paraformaldehyde from Isotech. H13CHO (99.8 at. % 13C)
was obtained from Aldrich as a formalin solution, which was
freeze-dried to isolate the polymer. HCHO was obtained from
Merck, and this chemical was also used to synthesize the
HCH18O by exchange with H218O (95 at. % 18O, from Campro
Scientific). The monodeuterated formaldehyde was prepared
from deuterated bromoform.24 The paraformaldehyde samples
were heated to produce gaseous formaldehyde monomers and
flushed into the reaction chamber with synthetic air via a Pyrex
gas-handling system. The pressures of the reactants were
measured in a standard volume on the gas line using a 10 mbar
range capacitance manometer. After the introduction of all of
the reagents, the chamber was filled to 1013 mbar with synthetic
air.
13CHO
KIEs have been
k
Cl+HCHO/kCl+DCDO and kOH+HCHO/kOH+H
studied by FTIR spectroscopy.22,23 The present study extends
the previous studies of isotope effects in the radical reactions
of atmospheric aldehydes to include measurements for HCDO
and HCH18O.
OH. OH radicals were generated by photolysis of isopropyl
nitrite (IPN-H6) or isopropyl nitrite-1,1,1,3,3,3-d6 (IPN-D6),
which were synthesized from isopropyl alcohol and isopropyl
alcohol-1,1,1,3,3,3-d6, respectively. The deuterated compound
was used to minimize overlap in the spectral region where most
of the formaldehyde isotopomers absorb (2670-2880 cm-1).
However, some of the experiments were carried out using IPN-
H6, as well, to determine if the decomposition of IPN produces
HCHO or DCDO. No differences in the KIEs between the two
OH precursors were seen (see also D’Anna et al. for details).18
These compounds were photolyzed using Philips TLD-08
fluorescence lamps (“black lamp”, λmax ∼ 370 nm).
Experimental Section
The kinetic study was carried out by the relative-rate method
in a static gas mixture, in which the decays of the concentrations
of the reacting species are measured simultaneously as a function
of reaction time. Consider two simultaneous bimolecular reac-
tions with the rate coefficients kA and kB:
kΑ
A + X
B + X
9
kΒ
98 products
8 products
(1)
(2)
Cl and Br. The chlorine and bromine atoms were generated
by photolysis of Cl2 and Br2 using the Philips TLD-08
fluorescence lamps. In this wavelength region, the Cl and Br
atoms produced were in the ground state. The Cl2 and Br2 were
standard laboratory-grade compounds and were purified by two
freeze-pump-thaw cycles prior to use.
Assuming that there are no loss processes other than these
reactions and that there are no other processes producing the
reactants, the following relation is valid:
NO3. The NO3 radicals were generated inside the chamber
by the thermal decomposition of N2O5. N2O5 was prepared by
mixing a gas stream of NO2 with excess O3 and trapping the
product at -78 °C. The N2O5 was subsequently purified by
vacuum distillation.
Each of the four experiments (OH, Cl, Br, and NO3) was
carried out twice for three different mixtures of formaldehyde
isotopomers: HCHO + DCDO, HCHO + HCDO, and HCHO
+ H13CHO + HCH18O. The initial mixing ratios of the radical
precursors were 2.0 ppm for Cl2 and Br2 and 20 ppm for N2O5
and IPN/IPN-D6. The initial mixing ratio of each formaldehyde
isotopomer was 0.5 ppm, giving a maximum ratio of 1.5 ppm
formaldehyde in some of the experiments. The mixing ratio of
formaldehyde was kept as low as possible to minimize interfer-
ing reactions (see below).
The experimental spectra have been analyzed using a global
FTIR nonlinear least-squares spectral-fitting procedure (NLM)
developed by D. W. T. Griffith.25 This method simulates the
spectrum of the mixture of the absorbing species from the initial
concentrations of the absorbing species and, then, iterates to
minimize the residual between the measured and simulated
spectra. In the spectrum calculation, true absorption coefficients
from the HITRAN26 database are used if available; otherwise,
a high-resolution measured spectrum can be used as a good
approximation. A least-squares minimum residual between the
measured and simulated spectra is typically achieved after 5-10
[A]0
[A]t
k
[B]0
[B]t
ln
)
Aln
(3)
{
}
{
}
kB
where [A]0, [A]t, [B]0, and [B]t denote the concentrations of
compounds A and B at times zero and t. A plot of ln([A]0/[A]t)
versus ln([B]0/[B]t) will thus give the relative reaction-rate
coefficient R ) kA/kB as the slope, or in terms of the
fractionation constant, ꢀ ) R - 1.
The experiments were carried out in a 250 L electropolished
stainless steel smog chamber equipped with a White-type
multiple reflection mirror system with a 120 m optical path
length for rotovibrationally resolved infrared spectroscopy. The
infrared spectra were recorded with a Bruker IFS 88 FTIR
instrument equipped with a liquid-nitrogen-cooled InSb detector
and an 1800-4000 cm-1 band-pass filter. At a nominal
resolution of 0.125 cm-1, 128 scans were co-added using boxcar
apodization. Periods of data collection (ca. 10 min) were
alternated with UV photolysis (ca. 1 min) until about 60% of
the HCHO was consumed. For a given gas mixture, the entire
experiment typically lasted 1.5 h. The reaction chamber was
equipped with UV photolysis lamps mounted in a quartz tube
inside the chamber, and all of the experiments were carried out
in synthetic air (99.9990% purity AGA; CO and NOx < 100
ppb) at 298 ( 2 K and 1013 ( 10 mbar. The temperature was
monitored on the outside of the chamber, and it remained

Relative Reaction Rates of HCHO
J. Phys. Chem. A, Vol. 108, No. 36, 2004 7395
TABLE 1: Wavenumber Regions and Compounds Included
in Fitting the Experimental Spectraa
HCHO isotopomer
mixture
spectral regions
compounds included
in spectral fitting
analyzed (cm-1
)
HCHO, DCDO
2670-2855,
2010-2190
HCHO, NO2, HCl,
HBr, DCDO, CO,
CO2, H2O
HCHO, HCDO
2670-2855
2670-2855
HCHO, HCDO,
NO2, HBr, HCl
HCHO, H13CHO,
HCH18O, NO2, HBr,
HCl
HCHO, H13CHO,
HCH18O
a NO2 was formed as a byproduct of N2O5 decomposition in the NO3
reaction experiments.
Figure 2. FACSIMILE model showing the maximum contributions
from OH and HO2 radical reactions in a Cl/HCHO/DCDO experiment.
The halogen and NO3 experiments are potentially susceptible to error
because of the OH and HO2 generated photochemically in the reaction
chamber. This scenario shows that, with the initial concentrations used
in this study, the OH reaction is significant during the first 70 s of
photolysis and becomes negligible thereafter. The worst-case scenario
is a 2% contribution to the relative rates from OH and HO2 radicals.
point offset, with the point (0,0) being the calibration or anchor
point.
Results and Discussion
The reactions of OH, Cl, Br, and NO3 radicals with
formaldehyde can proceed in three possible ways:
Figure 1. Example of a fitted spectrum of HCHO, H13CHO, and
HCH18O in the 2670-2855 cm-1 region. Lines corresponding to each
isotopomer are indicated in the spectrum. The residual of the fit is shown
above the spectrum. The correlation between the concentrations, derived
by the least-squares fit of the spectra, is in all cases less than -0.1.
X + HCHO f H + XCHO
f HX + HCO
(1a)
(1b)
(1c)
f HX + H + CO
iterations, and the maximum correlation between the derived
concentrations is -0.1.
The substitution reaction (eq 1a) is negligible.18,19 In the
presence of O2, reactions 1b and 1c rapidly lead to CO and
HO2 production. HO2 is a potential source of error as it reacts
with formaldehyde to form an adduct, HOO-CH2-O.28 This
adduct further reacts to form HCOOH and other products (see
refs 18 and 19). In the case of the NO3 experiments, HO2 can
also react with NO3 to produce OH radicals: HO2 + NO3 f
NO2 + O2 + OH (k ) 3.5 × 10-16 cm3 s-1).29 Since the reaction
of NO3 with formaldehyde is orders of magnitude slower than
the OH reaction (5.6 × 10-16 cm3 s-1 compared to 9.4 × 10-12
cm3 s-1),9 this is a potential source of error. In addition, the
presence of NOx in the synthetic air (<100 ppb) means that O3
and OH will be formed during the photochemical halogen
reactions. The reaction system was therefore examined in a
FACSIMILE30 kinetic model employing 89 reactions (published
as supporting material in Beukes et al.19) in order to elucidate
the extent of competing chemical reactions. The model showed
that the amount of formaldehyde that reacts with HO2 is strongly
dependent on the initial concentration of formaldehyde. Figure
2 shows the worst-case scenario for a chlorine experiment with
1.5 ppm formaldehyde and 100 ppb NOx in the cell. The OH
reaction is significant during the first 70 s of the reaction, and
beyond that point, the chlorine reaction dominates completely.
On the basis of this model, a maximum of 2% formaldehyde
reacts with something other than chlorine. A similar scenario
was observed for the Br reaction. Following the model results,
we did not include data from the first 70 s of photolysis in the
Cl and Br experiments. The model of the NO3 experiment shows
a 1% contribution to the HCHO degradation from OH. The
The spectral features used in the analysis were the C-H
stretching bands in the 2670-2855 cm-1 region and the C-D
stretching bands in the 2010-2190 cm-1 region. The com-
pounds considered in each spectral region are given in Table 1.
The spectral data needed for the fitting procedure were taken
from the HITRAN database (HCHO, CO, CO2, HCl, HBr, and
H2O); for DCDO, HCDO, H13CHO, and HCH18O, experimental
high-resolution IR spectra were used. These spectra were
recorded with a Bruker IFS 120 FTIR instrument at a 0.01 cm-1
resolution in a 10 cm Pyrex gas cell equipped with CaF2
windows. The partial pressures of formaldehyde isotopomers
were in the range of 6-10 mbar, and the cell was filled to 1013
mbar with synthetic air (Air Liquide, dry technical air). The
gas cell was passivated with ammonia before use to minimize
the acid-catalyzed polymerization of the compound on the walls.
A Ge on KBr beam splitter and an 1800-4000 cm-1 band-
pass filter were used in the interferometer, and a glow bar (SiC2)
was used as the MIR light source. The detector was a liquid-
N2-cooled InSb semiconductor, and 128 scans were co-added
to achieve an acceptable signal/noise ratio in the resultant
spectra. An example of the fitted spectra of HCHO, H13CHO,
and HCH18O and the residuals of the fit are shown in Figure 1.
The FTIR spectra showed accurate values for the relative change
in concentrations. The data were analyzed according to eq 3
using a weighted least-squares procedure, which includes
uncertainties in both reactant concentrations.27 That is, the least-
squares regression was carried out without allowing a zero-

7396 J. Phys. Chem. A, Vol. 108, No. 36, 2004
Feilberg et al.
Figure 3. Plots of ln([HCHO]0/[HCHO]t) vs ln([HCDO]0/[HCDO]t) and ln([HCH18O]0/[HCH18O]t) during the reactions with OH and Cl radicals.
Each plot contains data from two independent experiments with a total of at least 15 data points. The uncertainty assigned to each data point is 2σ
statistical error from the spectral-fitting procedure and does not include possible systematic errors. The results are summarized in Table 2.
model also indicates that about 18 ppb formic acid will form
during the reactions, which is below the detection limit of the
experiment (a spectral window of 1800-4000 cm-1). Indeed,
it was not observed in the spectra during any of the experiments.
The relative-rate plots, ln([A]0/[A]t) versus ln([B]0/[B]t), for
the HCDO and HCH18O experiments are shown in Figures 3
and 4. The plots include two independent experiments with a
total of at least 15 data points for each experiment. The results
of all of the experiments are summarized in Table 2. The relative
reaction rates of HCHO, DCDO, and H13CHO with the four
radicals have been studied previously by this group, and the
results obtained in the present study are in excellent agreement
and NO3. It is straightforward to argue that deuteration should
affect the reaction rate the most since it corresponds to the
largest relative change in mass and involves the active site in
the reaction. The reactions of formaldehyde with OH, Cl, Br,
and NO3 are all known to proceed via hydrogen abstraction in
atmospheric conditions, and the C-O bond is passive in this
context.18 Substituting with deuterium also reduces the tunneling
of the hydrogen. This is likely to have some effect on the
reaction rate of the Br reaction, where there is a significant
barrier for hydrogen to leave the system. For the OH, Cl, and
NO3 reactions, the barriers are small and tunneling is unlikely
to play a major role.
with the earlier work, with the exception of the kBr+HCHO
k
a value of 1.35 ( 0.15 for kCl+HCHO/kCl+DCDO that is in agreement
with our results. In a separate study, the same group found no
KIE outside experimental error for the OH reaction with H13-
CHO.22,23 There is a difference outside experimental error
/
One of the origins of the kinetic isotope effect is the change
in zero-point energy (ZPE) brought about by isotopic substitu-
tion; the large KIEs in the halogen reactions are mainly due to
zero-point vibration effects. The KIEs of the deuterated
compounds are larger than can be explained by changes in ZPE;
it is likely that quantum mechanical tunneling effects need to
be taken into account. The changes in ZPE may however
account for the KIEs in the reactions of HCH18O and H13CHO
with the halogens. The KIEs for the reactions of HCH18O and
H13CHO with OH and NO3 radicals are inverse KIEs; for
example, the heavy isotopomer reacts faster than the light
isotopomer. This is an interesting effect, the origin of which
might lie in the known formation of prereaction adducts in which
the carbonyl oxygen is oriented toward the radical species.
Quantum chemical calculations suggest that the OH-HCHO
adduct is more stable and longer lasting than the Cl/Br-HCHO
adducts, which means that the energetics of the adduct play an
overall greater part in the KIEs of the OH reaction.18,19 The
preadduct of the reaction involves a bond between the OH
hydrogen and the carbonyl oxygen in HCHO, which means that
Br+DCDO relative rate.18,19 Niki and co-workers22 have reported
between the previous and present results for the kBr+HCHO
/
k
Br+DCDO ratio; this is likely due to inaccuracies in the standard
spectral subtraction procedure used in the previous study and
caused by insufficient spectral resolution.19 There is one previous
study of the kOH+HCHO/kOH+HCDO ratio by Morris and Niki that
uses mass spectrometry; however, they were not able to detect
an isotope effect.21 Modified RRKM calculations predict a
k
OH+HCHO/kOH+HCDO KIE value of 1.34, which agrees very well
with the KIE value of 1.284 ( 0.004 found in this study.18 There
are no previous studies to compare with our results for HCH18O.
There are two clear trends in the KIEs in Table 2. One
obvious trend is that deuteration generally leads to larger isotope
effects than substitution by a heavy isotope of C or O; the other
is that the halogens lead to significantly larger KIEs than OH

Relative Reaction Rates of HCHO
J. Phys. Chem. A, Vol. 108, No. 36, 2004 7397
Figure 4. Plots of ln([HCHO]0/[HCHO]t) vs ln([HCDO]0/[HCDO]t) and ln([HCH18O]0/[HCH18O]t) during the reactions with Br and NO3 radicals.
Each plot consists of two independent experiments with a total of at least 15 data points. The uncertainty assigned to each data point is 2σ statistical
error from the spectral-fitting procedure and does not include possible systematic errors. The results are summarized in Table 2.
TABLE 2: Relative Reaction Rates of Formaldehyde Isotopomers Given as r ) k(X + H2CO)/k(X + isotopomer)a
R(OH)
R(Cl)
R(Br)
R(NO3)
HCHO/isotopomer
HCHO/DCDO
9.4 × 10-12 b
7.3 × 10-11 b
1.2 × 10-13 b
5.6 × 10-16 b
1.64 (0.01)
1.67 (0.01)
1.66 (0.01)
1.32 (0.01)
1.30 (0.01)
1.31 (0.01)
6.6 (0.1)
6.8 (0.1)
6.7 (0.1)
3.01 (0.02)
3.04 (0.02)
3.02 (0.01)
ref values
1.62 (0.05)c
1.302 (0.014)d
7.5 (0.2)d
2.97 (0.09)c
1.35 (0.15)e
HCHO/HCDO
1.28 (0.01)
1.29 (0.01)
1.28 (0.01)
1.203 (0.002)
1.191 (0.006)
1.201 (0.002)
3.33 (0.05)
3.25 (0.04)
3.27 (0.03)
1.77 (0.02)
1.79 (0.02)
1.78 (0.01)
ref values
HCHO/H13CHO
∼1f
0.961 (0.008)
0.95 (0.01)
0.952 (0.005)
1.06 (0.01)
1.051 (0.009)
1.058 (0.007)
1.12 (0.01)
1.14 (0.01)
1.13 (0.01)
0.95 (0.01)
0.96 (0.01)
0.96 (0.01)
ref values
0.97 (0.03)c
1.07 (0.03)d,g
1.103 (0.03)d,h
0.97 (0.03)d
1.1 (0.4)i,j
HCHO/HCH18O
0.972 (0.008)
0.960 (0.009)
0.967 (0.006)
1.07 (0.01)
1.08 (0.01)
1.08 (0.01)
1.26 (0.01)
1.278 (0.008)
1.275 (0.008)
0.98 (0.01)
0.99 (0.01)
0.98 (0.01)
aThe weighted mean is given in bold. Errors represent 2σ from the statistical analyses and are given in parentheses. b Absolute rate constant at
298 K (ref 9). HCHO + X (cm3 s-1). c From ref 18. d From ref 19. e From ref 22. f From ref 21. g Calculated from the relative rates kCl+HCHO
/
h Calculated from the relative rates kBr+HCHO/kBr+DCDO and kBr+H CHO/kBr+DCDO
.
i From ref 23. j Calculated from the
13
13
k
Cl+DCDO and kCl+H CHO/kCl+DCDO
.
13
relative rate kCl+H CHO/kCl+C H
.
2
4
the stability of the complex is directly affected by isotopic
substitution in the carbonyl group. The NO3-HCHO adduct
energy has not been calculated correctly; the electronic structure
of the NO3 radical presents a challenge to theoreticians due to
symmetry breaking, and at present, there is no standard size
extensive method applicable to larger systems.31 Inverse isotope
effects are also seen in the reaction of CO with OH, which is
a complex-forming reaction.32,33
For stratospheric applications, it is necessary to consider a
possible temperature dependence of the KIEs in these reactions.
A significant temperature dependence of the total reaction rate
has been reported for the Br reaction; the OH reactions have
little or no temperature dependence, which may be attributed
to the lack of a reaction barrier for these reactions.9 The
temperature dependence of the NO3 reaction has not been
quantified, but since the Arrhenius activation energy is low, it

7398 J. Phys. Chem. A, Vol. 108, No. 36, 2004
Feilberg et al.
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Conclusion
We report new data on the isotopic signatures of the reactions
of five isotopomers of HCHO with OH, Cl, Br, and NO3. These
reactions exhibit distinct, measurable isotope effects containing
information on the reaction potentials, which are useful for
constructing atmospheric isotope budgets. The deuterium isotope
effects are the largest, which might be expected since the
reactions in question involve hydrogen abstraction and because
deuteration represents the largest relative change in mass. In
all of the cases, the active bonds in the reactions are the C-H
and C-D bonds. Bromine shows the largest isotope effects,
particularly for HCDO and DCDO, which may be attributed to
zero-point vibration effects. It is interesting to note that the
isotope effects for H13CHO and HCH18O and their reactions
with OH and NO3 are inverse kinetic isotope effects; for
example, they react faster than HCHO with these radicals. We
attribute this to prereaction adduct formation involving the
carbonyl group. The most important KIEs with respect to the
atmosphere are the those of HCDO, H13CHO, and HCH18O in
the OH and Cl reactions. The carbon and oxygen isotopic
signature in formaldehyde is propagated to CO, which is known
to be enriched in heavy isotopes of oxygen and carbon. These
relative rates can therefore be important pieces of information
when tracing the isotopic enrichment of CO.13,34 Both the
kOH+HCHO/kOH+HCDO KIE, which is most important in the
troposphere, and the kCl+HCHO/kCl+HCDO KIE, which is significant
in the stratosphere, lead to a large enrichment of deuterium in
the remaining formaldehyde and, in turn, a deuterium enrichment
in atmospheric molecular hydrogen.
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Acknowledgment. The experiments were made possible by
a researcher mobility grant from the Nordic Network for
Chemical Kinetics supported by the Nordic Academy for
Advanced Study (NorFA) and by the financial support of the
Danish Natural Science Research Council. The authors thank
Else Philipp for her valuable assistance with the synthesis of
formaldehyde isotopomers, David W. T. Griffith for providing
us with the spectral fitting method, and Flemming M. Nicolaisen
for his help with recording the high-resolution spectra.
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