The reactions of N-methylformamide and N,N-dimethylformamide with OH and their photo-oxidation under atmospheric conditions: Experimental and theoretical studies

Article abstract of DOI:10.1039/c4cp05805d

The reactions of OH radicals with CH₃NHCHO (N-methylformamide, MF) and (CH₃)₂NCHO (N,N-dimethylformamide, DMF) have been studied by experimental and computational methods. Rate coefficients were determined as a function of temperature (T = 260-295 K) and pressure (P = 30-600 mbar) by the flash photolysis/laser-induced fluorescence technique. OH radicals were produced by laser flash photolysis of 2,4-pentanedione or tert-butyl hydroperoxide under pseudo-first order conditions in an excess of the corresponding amide. The rate coefficients obtained show negative temperature dependences that can be parameterized as follows: k_OH+MF = (1.3 ± 0.4) × 10^-12 exp(3.7 kJ mol^-1/(RT)) cm³ s^-1 and k_OH+DMF = (5.5 ± 1.7) × 10^-13 exp(6.6 kJ mol^-1/(RT)) cm³ s^-1. The rate coefficient k_OH+MF shows very weak positive pressure dependence whereas k_OH+DMF was found to be independent of pressure. The Arrhenius equations given, within their uncertainty, are valid for the entire pressure range of our experiments. Furthermore, MF and DMF smog-chamber photo-oxidation experiments were monitored by proton-transfer-reaction time-of-flight mass spectrometry. Atmospheric MF photo-oxidation results in 65% CH₃NCO (methylisocyanate), 16% (CHO)₂NH, and NO_x-dependent amounts of CH₂NH and CH₃NHNO₂ as primary products, while DMF photo-oxidation results in around 35% CH₃N(CHO)₂ as primary product and 65% meta-stable (CH₃)₂NC(O)OONO₂ degrading to NO_x-dependent amounts of CH₃NCH₂ (N-methylmethanimine), (CH₃)₂NNO (N-nitroso dimethylamine) and (CH₃)₂NNO₂ (N-nitro dimethylamine). The potential for nitramine formation in MF photo-oxidation is comparable to that of methylamine whereas the potential to form nitrosamine and nitramine in DMF photo-oxidation is larger than for dimethylamine. Quantum chemistry supported atmospheric degradation mechanisms for MF and DMF are presented. Rate coefficients and initial branching ratios calculated with statistical rate theory based on molecular data from quantum chemical calculations at the CCSD(T?)-F12a/aug-cc-pVTZ//MP2/aug-cc-pVTZ level of theory show satisfactory agreement with the experimental results. It turned out that adjustment of calculated threshold energies by 0.2 to 8.8 kJ mol^-1 lead to agreement between experimental and predicted results.

Full text of DOI:10.1039/c4cp05805d

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The reactions of N-methylformamide and
N,N-dimethylformamide with OH and their
photo-oxidation under atmospheric conditions:
experimental and theoretical studies†
Cite this: Phys.Chem.Chem.Phys.,
2
015, 17, 7046
ab
b
c
c
Arne Joakim C. Bunkan, Jens Hetzler, Tomas Mikoviny, Armin Wisthaler,
´ˇ
a
b
Claus J. Nielsen and Matthias Olzmann*
3 3 2
The reactions of OH radicals with CH NHCHO (N-methylformamide, MF) and (CH ) NCHO (N,N-dimethyl-
formamide, DMF) have been studied by experimental and computational methods. Rate coefficients were
determined as a function of temperature (T = 260–295 K) and pressure (P = 30–600 mbar) by the flash
photolysis/laser-induced fluorescence technique. OH radicals were produced by laser flash photolysis of
2,4-pentanedione or tert-butyl hydroperoxide under pseudo-first order conditions in an excess of the
corresponding amide. The rate coefficients obtained show negative temperature dependences that can
ꢂ12
ꢂ1
3
ꢂ1
be parameterized as follows: kOH+MF = (1.3 ꢀ 0.4) ꢁ 10
exp(3.7 kJ mol /(RT)) cm s and kOH+DMF
ꢂ1
s . The rate coefficient kOH+MF shows very weak positive
=
ꢂ13
ꢂ1
3
(5.5 ꢀ 1.7) ꢁ 10
exp(6.6 kJ mol /(RT)) cm
pressure dependence whereas kOH+DMF was found to be independent of pressure. The Arrhenius
equations given, within their uncertainty, are valid for the entire pressure range of our experiments.
Furthermore, MF and DMF smog-chamber photo-oxidation experiments were monitored by proton-
transfer-reaction time-of-flight mass spectrometry. Atmospheric MF photo-oxidation results in 65% CH
methylisocyanate), 16% (CHO) NH, and NO -dependent amounts of CH QNH and CH NHNO as primary
products, while DMF photo-oxidation results in around 35% CH N(CHO) as primary product and 65% meta-
stable (CH NC(O)OONO degrading to NO -dependent amounts of CH NQCH (N-methylmethanimine),
CH NNO (N-nitroso dimethylamine) and (CH NNO (N-nitro dimethylamine). The potential for nitramine
3
NCO
(
2
x
2
3
2
3
2
3
)
2
2
x
3
2
(
3
)
2
3
)
2
2
formation in MF photo-oxidation is comparable to that of methylamine whereas the potential to form
nitrosamine and nitramine in DMF photo-oxidation is larger than for dimethylamine. Quantum chemistry
supported atmospheric degradation mechanisms for MF and DMF are presented. Rate coefficients and initial
branching ratios calculated with statistical rate theory based on molecular data from quantum chemical
calculations at the CCSD(T*)-F12a/aug-cc-pVTZ//MP2/aug-cc-pVTZ level of theory show satisfactory
agreement with the experimental results. It turned out that adjustment of calculated threshold energies by
Received 11th December 2014,
Accepted 6th February 2015
DOI: 10.1039/c4cp05805d
ꢂ1
www.rsc.org/pccp
0.2 to 8.8 kJ mol lead to agreement between experimental and predicted results.
important products in the atmospheric degradation of
dimethylamine and trimethylamine, respectively. The toxicol-
1
. Introduction
2
N-methylformamide (MF) and N,N-dimethylformamide (DMF) ogy of dimethyl and monomethyl derivatives of acetamide and
3
are High Production Volume (HPV) chemicals on the US HPV formamide was recently reviewed; DMF is handled under the
4
Challenge Program Chemical List along with around 2800 OECD HPV Screening Information Data Set (SIDS) Program
1
1
other compounds. MF and DMF have also been reported as while the test plan for MF has not yet been implemented.
5
Barnes et al. reviewed the scarce information on amides,
presented new kinetic and product information, and outlined
the photo-oxidation mechanisms for formamides and acetamides.
a
b
c
Center for Theoretical and Computational Chemistry, Department of Chemistry,
University of Oslo, P. O. Box 1033 – Blindern, 0315 Oslo, Norway
Institut f u¨ r Physikalische Chemie, Karlsruher Institut f u¨ r Technologie (KIT),
Kaiserstr. 12, 76131 Karlsruhe, Germany. E-mail: matthias.olzmann@kit.edu
Institute for Ion Physics and Applied Physics, University of Innsbruck,
Technikerstrasse 25, A-6020 Innsbruck, Austria
6
Recently, Borduas et al. reported photo-oxidation products and
kinetic data for OH reactions with five amides and presented
additional mechanistic insight from quantum chemistry calcula-
tions. In short: the atmospheric gas phase chemistry of amides is
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cp05805d
not yet fully understood. The amide + OH and amide + NO
3
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ꢂ11
reactions are relatively fast (rate coefficients in the order of 10
and 10
Test measurements with a second bottle in a two-stage arrangement
ꢂ14
3
ꢂ1
5
cm s , respectively), and the average global lifetimes gave identical results. The amide concentrations were calculated
of amides with respect to reactions with OH and NO radicals with the ideal gas law from the measured vapor pressures by
3
will be less than a few days; reactions with Cl atoms and O do assuming complete saturation.
3
not constitute important sinks for amides. The gas phase UV
To assess the measured values, the vapor pressure of DMF was
11
spectra of amides are structureless and show low absorption calculated from Antoine equation, log(P/bar) = A ꢂ [B/(T/K + C)],
ꢂ20
2
7
cross-sections (o10
tropospheric photolysis of amides is not an important loss ꢂ82.648 (determined in the temperature range 303–363 K).
process. Due to their solubility in water, uptake into droplets For T = 294 K, one obtains a vapor pressure of P = 3.99 mbar.
and subsequent deposition may be an important sink. For MF the Antoine constants in ref. 12, A = 4.99796, B =
The present communication reports on experimental kinetic 2134.031, and C = ꢂ45.071, were obtained in the temperature
cm ) above 270 nm. Consequently, with the constants from ref. 12: A = 3.93068, B = 1337.716, and C =
1
2,13
1
2,14
studies of MF and DMF reactions with OH radicals at atmo- range 370–472 K.
Because this is much higher than the
spherically relevant temperatures and from product studies in temperature in our saturator, we determined the vapor pressure
3
photo-oxidation experiments of MF and DMF. The results are of MF at 294 K in separate experiments. We used a 200 cm
corroborated by high-level quantum chemical calculations and stainless steel vacuum line connected to a turbo pump and a
statistical rate theory.
capacitance manometer. The leak rate of this vacuum line was
determined to be 0.002 mbar min . The MF was purified by
ꢂ1
first purging it with helium in a gas washing bottle for 3 hours
and then thoroughly degassing it in repeated freeze–pump–
thaw cycles. The gas washing bottle was connected to the
vacuum line, and the pressure rise was measured until the
2
. Experimental methods
2.1. Laser photolysis experiments
The kinetic experiments were performed by using the flash change in pressure reached the leak rate determined before. In
photolysis/laser-induced fluorescence technique in a coolable this way, we obtained a vapor pressure of P = 0.33 mbar at
slow-flow reactor with helium as bath gas. The experimental T = 294 K. This value is somewhat higher than the value of
8
setup was described in detail elsewhere, so only a brief description 0.266 mbar obtained with the above Antoine parameters.
will be given here.
However, we note that extrapolations with the Antoine equation
3
The reaction cell has an inner volume of ca. 640 cm and is outside the temperature range, where the parameters were
enclosed in a steel housing that can be evacuated to prevent ice determined, have to be considered with great caution.
formation on the windows and to improve thermal insulation.
The mixtures of 2,4-pentanedione and t-BuOOH in helium
The gas flows were controlled by calibrated mass flow control- were manometrically prepared in gas cylinders and allowed to
lers, and accumulation of reaction products is avoided by homogenize for at least 16 hours before use.
choosing appropriate flow rates. OH radicals were produced
2,4-Pentanedione (99%, Sigma Aldrich), t-BuOOH (80% in
O, Sigma Aldrich), MF (99% Sigma Aldrich), and DMF (99%
9
by pulsed photolysis of 2,4-pentanedione (for OH + DMF) or
H
2
1
0
tert-butyl hydroperoxide (t-BuOOH, for OH + MF) with a KrF Alfa Aesar) were purified by repeated freeze–pump–thaw cycles,
excimer laser at 248 nm. The OH concentration-time profiles and the gas washing bottles containing the amides were purged
were monitored by recording the laser-induced fluorescence before each experiment by a helium flow for at least 30 minutes.
(
2
LIF) in a wavelength range of (308 ꢀ 7.5) nm after excitation at The purity of the helium was 499.999% (Air Liquide).
81.9 nm. For excitation a frequency-doubled dye laser was In our kinetic experiments pseudo-first order conditions
used that was operated with rhodamine 6G and pumped with a were ensured by keeping the amide concentrations in large
Nd:YAG laser. The fluorescence signal was detected with a excess of the initial OH concentration. Typical amide concen-
1
4
16
ꢂ3
photomultiplier oriented perpendicularly to the antiparallel trations were in the range 10 –10 cm whereas OH con-
excimer and dye laser beams. A variation of the pulse energy centrations can be estimated from absorption cross-sections of
1
1
ꢂ3
of the photolysis laser between 60 and 300 mJ corresponding to our precursors as being typically in the order of 10 cm . The
ꢂ2
fluences of 20–100 mJ cm had no significant effect on the rate decay of the OH concentration, of which an example is shown
coefficient, which indicates that unwanted radical–radical reac- in Fig. 1, is then given by a first-order rate law, d[OH]/dt =
tions are unimportant under our experimental conditions.
A bath gas flow with a well-known concentration of the the bimolecular rate coefficient of the OH + amide reaction and
corresponding amide was produced by flowing helium through characterizing other first-order loss processes of the OH
a saturator that consisted of a conventional gas washing bottle radicals (diffusional loss, reaction with precursor etc.). If k is
d
ꢂkpseudo[OH], where kpseudo = kbim[amide] + k , with kbim being
k
d
d
with a fritted disc. The washing bottle containing the pure assumed to be constant for a given temperature and pressure,
amide was kept in a water bath at 294 K, and the possibility of the bimolecular rate coefficient kbim is given as the slope of
evaporative cooling was checked by monitoring the tempera- kpseudo plotted vs. amide concentration. An example plot is
ture at the highest flow rates for 90 minutes. No change in given in Fig. 2. We note that in some experiments we observed
temperature was observed. In general, one bottle (height of the biexponential decays. However, because the scattering of the
liquid column: ca. 10 cm, helium flow: 5–50 sccm) was found to low LIF intensities at longer times was considerable, we
be sufficient for saturation under our experimental conditions. refrained from performing biexponential analyses in these
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302 K and 306 K. OH radicals were produced by photolysis of
isopropyl nitrate. The initial reaction conditions were 250 ppbV
of the reagent amide, 100 ppb of NO and 250 ppbV of isopropyl
nitrite. The reagent mixture was irradiated for 15 minutes.
During photo-oxidation, NO was continuously added to the
ꢂ1
chamber at a rate of 6.8 ppb min . Synthetic air was con-
tinuously added to the chamber at a flow rate of 3.26 slpm to
replenish instrumental sampling flows. A large amount of NO
was injected into the chamber 10 minutes after the lamps had
been turned off for identification of rapidly decaying thermally
unstable acylperoxynitrate compounds. Duplicate experiments
were carried out for each reagent.
The chemicals and gases used in the experiments and their
stated purities were as follows: N-methylformamide (99%,
Sigma-Aldrich), N,N-dimethylformamide (99.8%, Sigma-Aldrich),
synthetic air (5.0, Messer Austria), nitrogen oxide in nitrogen
ꢂ3
(
401 mg m NO; Linde). Isopropyl nitrite was synthesized from
Fig. 1 LIF intensity–time profile and least-squares fit for MF + OH
16
ꢂ3
experiment at T = 295 K, P = 600 mbar, and [MF]
0
= 1.4 ꢁ 10 cm
.
sodium nitrite in water, isopropanol and sulfuric acid.
Analytical instrumentation coupled to the chamber included a
high-resolution proton-transfer-reaction time-of-flight mass spectro-
meter (PTR-TOF 8000, Ionicon Analytik GmbH) for amide and
amide photo-oxidation product measurements, a chemilumines-
cence NO-detection instrument (CLD770 AL ppt, ECO PHYSICS)
combined with a photolytic converter (PLC 760 MH, ECO PHYSICS)
16
2
for NO detection, a UV photometric ozone analyzer (49i, Thermo
Scientific) and a temperature/relative humidity sensor (UFT75-AT,
MELTEC). The PTR-ToF-MS was interfaced to the chamber via a
PEEK capillary tube (temperature: 50 1C; flow: 0.15 slpm). The
PTR-ToF-MS was operated at a reduced electric field of 52 Td
and 85 Td, respectively, during the duplicate experiments.
+
+
Chemical ionization reagent ions were H
respectively, for the two PTR-ToF-MS operating modes.
For PTR-ToF-MS data analysis, we used the ‘‘PTR-TOF Data
3 3 2
O and H O (H O),
17
Analyzer’’ software. Instrumental response factors for amides and
amide photo-oxidation products were derived from ion-molecule
collision theory using calculated molecular properties.
18
Fig. 2 Pseudo-first order plot for OH + DMA (T = 294 K, P = 100 mbar).
3
. Computational methods
cases and determined the rate coefficients kpseudo always from 3.1. Electronic structure calculations
the initial, linear part of the semi-logarithmic plots.
The experimental conditions for the OH + MF and OH +
DMF experiments are summarized in Tables S1 and S2 of the
ESI.† Due to the multitude of parameters that influence the
uncertainty of the experiments, and which are difficult to
quantify exactly, a maximum error of 30% for the rate coeffi-
cients was estimated. We note, however, that the statistical
error is smaller (see below).
Stationary points on the potential energy surfaces for reactions of
the amides with OH were characterized using second-order
Møller–Plesset perturbation theory (MP2) as well as BHandHLYP
density functional theory. Pre- and post-reaction adducts were
located by following the reaction path (IRC) from the saddle
points. In addition, reaction enthalpies were calculated using
19
G3 model chemistry, while dipole moments and isotropic
polarizabilities (employed in calculations of ion-molecule reaction
rate coefficients, see above) were obtained in B3LYP/aug-cc-pVTZ
calculations. The G3 results are summarized in Table S3 (ESI†).
DFT and MP2 calculations are often capable of giving very
2.2. Photo-oxidation studies
OH + amide reaction product studies were conducted in a 480 l
Teflon coated reaction chamber made of glass and surrounded good geometries for minima on reactive potential energy sur-
1
5
by 18 UV/Vis lamps (l Z 300 nm) to simulate sunlight.
faces, but may be less suitable for transition states as these may
Experiments were performed in dry synthetic air at an absolute have significant multi-reference character. We have therefore
pressure of 970 mbar and temperatures in the range between re-optimized the geometries of the transition states using
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CASPT2 with an active space consisting of the bonding and anti- and also three possible product channels in the DMF + OH
bonding orbital of the bond being broken in the amide and the reaction:
unpaired electron in the OH radical, i.e. 3 electrons in 3 orbitals.
˙
CH ) NCHO + OH - (CH ) NCO + H O
3 2 3 2 2
(
Energies of the stationary points were improved using
coupled cluster singles and doubles with explicitly correlated
wave functions using the F12a approximation with perturbative
triples scaled as recommended in the MOLPRO manual,
DH1 = ꢂ98 kJ molꢂ1
(2a)
(2b)
˙
2 3 2
trans-CH N(CH )CHO + H O
-
-
2
0,21
DH1 = ꢂ100 kJ molꢂ1
denoted CCSD(T*)-F12a.
Dunning’s correlation consistent
2
2,23
basis sets were used in all calculations.
˙
2 3 2
cis-CH N(CH )CHO + H O
Unrestricted wave functions were used for the open-shell
calculations. The G3, BHandHLYP, B3LYP and MP2 calcula-
DH1 = ꢂ100 kJ molꢂ1 (2c)
2
4
tions were performed using Gaussian09, while the explicitly
correlated coupled cluster calculations and CASPT2 calcula-
The reaction enthalpies listed refer to 298 K and to the
lowest energy conformations of the species involved; they stem
2
5
tions were performed using Molpro 2012.1.
1
9
from G3 calculations. We note that the barrier for cis–trans
ꢂ1
˙
isomerization of CH N(CH )CHO is 80 kJ mol .
2 3
3.2. Statistical rate theory calculations
The reactions of amides with OH radicals proceed via pre-reaction
complexes and the overall kinetics and may therefore be governed
by forward reactions and back dissociation of the complex. Specific
rate coefficients for the unimolecular forward reactions were calcu-
lated using RRKM theory based on energies and molecular data
from CCSD(T*)-F12a/aug-cc-pVTZ//MP2/aug-cc-pVTZ calculations.
Rate coefficients for back dissociation of the complex were obtained
via detailed balancing from capture rate coefficients of complex
formation. The capture rate coefficients were calculated with long-
4
.1. Structures and energies of stationary points
Stationary points on the amide + OH potential energy surfaces
PES) were located in MP2 and BHandHLYP calculations employing
(
different basis sets. The results of the calculations are summarized
in Tables S4 and S5 (ESI†) while Fig. 3 and 4 show the energies of
the stationary points calculated using CCSD(T*)-F12a/aug-cc-pVTZ//
MP2/aug-cc-pVTZ. Cartesian coordinates of the stationary points
calculated using MP2/aug-cc-pVTZ are collected in Tables S6 and S7
32
26
(ESI†). The values from G3X-K model chemistry reported by
Borduas et al. for MF are similar to the present results. The
range transition state theory (LRTST). Spin–orbit coupling in the
6
ꢂ1 27
OH radical (139.7 cm
)
was accounted for in the model by
reactions of both amides with OH radicals are characterized by
post- and pre-reaction van der Waals adducts and submerged
barriers. To locate the pre- and post-reaction complexes, IRCs were
followed in both BHandHLYP and MP2 calculations using the aug-
cc-pVDZ basis set.
lowering the energy of the OH radical by half of the splitting and
2
2
including the P and P spin–orbit states in the electronic
3/2
1/2
partition function. It was assumed that spin–orbit coupling could be
neglected in the pre-reaction adduct and in the transition states.
Lennard-Jones parameters of MF and DMF were approximated
The transition states of H abstraction from the methyl (TSCH3)
and the carbonyl (TSCHO) groups in MF are very similar in energy,
and it is therefore difficult to calculate the branching ratio accurately
as even small changes in the barrier heights have a large impact on
the calculated branching ratio. It is intuitively clear that abstraction
from the NH group (TSNH) will constitute only a minor route.
by the corresponding values for methyl acetate (e/k
B
= 469.8 K, s =
28
4.94 Å) and for collisional energy transfer an exponential-down
model was assumed. The energy transfer parameter for collisions
ꢂ1
with He was set to hDE
i = 200 cm . Variation of this value
down
ꢂ1
between 100 and 300 cm lead only to minor changes (o2%) in
the calculated rate coefficients.
The temperature and pressure dependence of the phenomeno-
logical rate coefficients for the bimolecular overall reactions
was obtained from the eigenvalues of a corresponding master
2
9,30
equation by employing the Bartis–Widom method
mented in the MESMER code.
as imple-
3
1
4
. Results
There are three possible product channels in the MF + OH
reaction:
ꢂ1
˙
CH NHCHO + OH - CH NHCO + H O DH1 = ꢂ100 kJ mol
3
3
2
(1a)
:
ꢂ1
-
CH
3
NCHO + H
2
O
DH1 = ꢂ40 kJ mol
1b)
(
Fig. 3 Potential energy diagram (including zero-point energies) for the
OH + MF reaction from CCSD(T*)-F12a/aug-cc-pVTZ//MP2/aug-cc-pVTZ
calculations; energies are relative to reactant energies.
ꢂ1
˙
CH
-
2
NHCHO + H
2
O
DH1 = ꢂ104 kJ mol
1c)
(
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Fig. 4 Potential energy diagram (including zero-point energies) for the
OH + DMF reaction from CCSD(T*)-F12a/aug-cc-pVTZ//MP2/aug-cc-pVTZ
calculations; energies are relative to reactant energies.
For DMF, the barrier of H abstraction from the carbonyl group
Fig. 5 Arrhenius plot of the rate coefficients for MF + OH; experimental
is significantly lower than for the methyl groups (TScis and TStrans),
and it is therefore expected that it will be the dominating reaction 1000 mbar in air (empty circle, with 20% uncertainty, see footnote to
values from this work at 600 mbar in helium (full circles), from ref. 6 at
a
Table 3), and from ref. 33 at 1000 mbar in air (empty square); the solid line
channel. However, it is not possible to rule out H abstraction from
the methyl groups from the present calculations.
The results from the MP2 and BHandHLYP calculations (see
ESI,† Tables S4 and S5) are generally in good agreement, although
ꢂ12
ꢂ1
corresponds to the Arrhenius equation kbim = 1.3 ꢁ 10
exp(3.7 kJ mol
ꢂ1
, the dashed line represents the predicted rate coefficient
/
3
(
RT)) cm s
from our master equation calculations based on the CCSD(T)-F12a//MP2
calculations, and the dotted line represents the calculated rate coefficient
the methods predict different conformers of the DMF post reaction with the barrier heights adjusted to best reproduce the experimental
values (see Table 1); error bars on the individual data points from our
experiments denote two standard deviations and do not include possible
adducts as the most stable, but with very similar energies. A
comparison of important transition state geometry parameters is
shown in Fig. S1 (ESI†). The transition state geometries obtained
systematic errors.
with MP2/aug-cc-pVTZ and BHandHLYP/aug-cc-pVTZ are very simi-
the experimental values. The calculated capture rate coefficient is
lar; the largest differences in bond lengths for the bonds being
ꢂ10
ꢂ1/6
3
ꢂ1
rather large, kcap = 7.59 ꢁ 10
(T/298 K)
cm s (ESI,†
formed and broken is 5 pm, and the largest difference in the
ꢂ1
Table S8), but reducing it by a factor of 2 lowers the phenom-
enological rate coefficient only by less than 8% in the 250–300 K
range. The relative branching fractions for H abstraction at the
improved (CCSD(T*)-F12a, see above) barrier heights is 4 kJ mol
.
BHandHLYP calculations predict slightly earlier transition states
and lower barriers for all reactions. The CASPT2(3,3) calculations
give geometries very similar to the MP2 results except for abstraction
from the carbonyl group in DMF where CASPT2 gives a much
CHO and CH group hardly varies with a change in the capture
3
rate coefficient and are calculated to be 91.5% and 8.5%,
respectively, at T = 300 K and 89.8% and 10.2%, respectively,
at T = 250 K. The contribution of H abstraction from the NH
group is negligible.
later transition state, but a very similar improved barrier height
ꢂ1
(
ꢂ14.8 kJ mol for CASPT2 vs. ꢂ15.5 for MP2). The largest deviation
between the barrier heights based on the CASPT2 and the MP2 and
BHandHLYP geometries is 3.2 and 6.5 kJ mol , respectively.
ꢂ1
Lowering the threshold energy for abstraction from the NH
ꢂ1
group by 4 kJ mol and simultaneously increasing the thresh-
old energies for abstraction from the CHO and the CH group
by 4 kJ mol changes the relative branching fractions merely
3
4.2. Rate coefficients of the OH + MF and OH + DMF reactions
ꢂ1
3
Fig. 5 shows experimental and calculated rate coefficients of the to 89.7%, 10.2% and 0.1% for H abstraction from CHO, CH ,
OH + MF reaction as a function of temperature. The experimental and NH, respectively, but results in a 75% reduction of the
values exhibit a weak negative temperature dependence that can overall rate coefficient (at T = 300 K). Therefore, we conclude
ꢂ12
be parameterized as follows: kbim(OH + MF) = (1.3 ꢀ 0.4) ꢁ 10
that the overall reaction is governed by the inner transition
ꢂ1
3
ꢂ1
exp(3.7 kJ mol /(RT)) cm s . The detailed experimental condi- states and that abstraction from the NH group is not important
tions for the individual data points along with the measured rate at atmospheric conditions.
coefficients are collected in Table S1 (ESI†). We also note that our
Fig. 5, in connection with Table 1, illustrates that a very good
experiments in the pressure range 30–600 mbar seem to indicate a agreement between calculated and experimentally determined
very weak positive pressure dependence (increase of B20% in rate coefficients can be achieved with only moderate adjustment
going from 30 to 600 mbar), which is illustrated in Fig. S2 (ESI†). of the barriers. We note that with the adjusted values also the
The rate coefficients calculated using the CCSD(T*)-F12a/aug-cc- branching fractions observed in our photo-oxidation experiments
pVTZ//MP2/aug-cc-pVTZ results are about a factor of 5 higher than are very well reproduced (see below).
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Table 1 Calculated and adjusted transition state energies for OH + MF,
Table 2 Calculated and adjusted transition state energies for OH + DMF,
ꢂ1
ꢂ1
relative to the energy of the reactants (unit: kJ mol
)
relative to the energy of the reactants (unit: kJ mol
)
TSCH3
TSCHO
TSNH
TStrans
TScis
TSCHO
a
a
Calculated
Adjusted
ꢂ9.6
ꢂ4.0
ꢂ7.7
ꢂ4.1
12.6c
12.6
Calculated
Adjusted
ꢂ7.7
ꢂ7.9
ꢂ3.3
ꢂ15.5
ꢂ6.7
b
b
+5.0
a
b
a
b
CCSD(T*)-F12a/aug-cc-pVTZ//MP2/aug-cc-pVTZ results. Adjusted barrier
CCSD(T*)-F12a/aug-cc-pVTZ//MP2/aug-cc-pVTZ
results. Adjusted
heights to reproduce experimental rate coefficients and relative product barrier heights to reproduce experimental rate coefficients and relative
branching fractions from our photo-oxidation study (see below) with our product branching fractions from our photo-oxidation study (see below)
c
master equation calculations. Excluded from analysis.
with our master equation calculations.
Fig. 6 shows experimental and calculated rate coefficients of
The calculated capture rate coefficient for the DMF + OH
ꢂ10
ꢂ1/6
3
ꢂ1
the OH + DMF reaction as a function of temperature. Also for reaction, kcap = 7.55 ꢁ 10
(T/298 K)
cm s , is very similar
this reaction system the experimental values exhibit a weak to that for the MF + OH reaction (Table S8, ESI†). But since the
negative temperature dependence that can be parameterized as energy barrier for the lowest H abstraction step relative to the
ꢂ13
ꢂ1
follows: k (OH + DMF) = (5.5 ꢀ 1.7) ꢁ 10
exp(6.6 kJ mol
/
bimolecular reactants is considerably lower than in the case of
bim
ꢂ1
3
(
RT)) cm s . The detailed experimental conditions for the MF + OH, a reduction of the capture rate coefficient by a factor of
individual data points along with the measured rate coefficients two also leads to a stronger decrease of the phenomenological
are collected in Table S2 (ESI†). We note that no discernible rate coefficient by up to 27% (compared to up to 8% for MF + OH)
pressure dependence of the rate coefficient could be observed in the temperature range 250–300 K.
within our experimental uncertainty (see ESI,† Fig. S3). The
The differences in the temperature dependence between the
agreement between the calculated and experimental rate coeffi- two reaction systems can be rationalized in terms of the potential
cients is not as good as for the MF + OH reaction. This can be energy surface. The pre-reaction complexes are nearly equally
seen from a comparison of the dashed and solid lines in Fig. 5 stable with respect to the bimolecular reactants, but the lowest
and 6. Whereas for OH + MF this difference is about a factor of 5, energy barrier for hydrogen abstraction is somewhat larger for the
the difference for OH + DMF is a factor of B10. Accordingly, OH + MF reaction than for the OH + DMF reaction (cf. Tables 1
rather large adjustments of the barriers are necessary to reproduce and 2). This may lead to a somewhat stronger (negative) tempera-
the experimental rates as can be seen from Table 2.
ture dependence in the latter case. The differences in the pressure
dependence are too small compared to our experimental uncer-
tainty for a reasonable further analysis (cf. Fig. S2 and S3, ESI†).
Table 3 summarizes the currently available kinetic data for
reactions of OH radicals with different amides. The rate coeffi-
3
3
cients reported by Solignac et al. are somewhat higher than
3
4
those from both Koch et al. and the present work but agree
within the combined error bars. An interesting feature of the
data is that the values from relative measurements are consis-
tently higher than those from absolute measurements. This is
probably related to the nature of the experiments: in the relative
5
,6,33,35,36
rate measurements,
the rate of amide removal is mea-
sured relatively to the removal rate of a reference compound, and
any additional process that removes the amide will increase the
observed rate. In the absolute measurements (this work and
3
4
Koch et al. ), the measured quantity is the removal rate of OH
radicals in an excess of amide, and any process removing the
amide will give a smaller observed rate. Despite all our efforts to
detect and prevent any wall loss, we cannot completely rule out
that some of the amide has been lost by other processes before
passing the detection volume in our photolysis cell. The negative
activation energies from this work nicely agree with the other
Fig. 6 Arrhenius plot of the rate coefficients for DMF + OH; experimental
values from this work at 100 mbar in helium (full circles) and from ref. 33 at available values for N-methylacetamide, N,N-dimethylacetamide,
1
000 mbar in air (empty square); the solid line corresponds to an Arrhenius N-methylpropanamide, and N,N-dimethylpropanamide.
ꢂ13
ꢂ1 ꢂ1
exp(6.6 kJ mol /(RT)) cm s , the dashed line
3
equation kbim = 5.5 ꢁ 10
represents the predicted rate coefficient from our master equation calcu-
lations based on the CCSD(T)-F12a//MP2 calculations, and the dotted line 4.3. CH
represents the calculated rate coefficient with the barrier heights adjusted
3
NHCHO photo-oxidation studies
5
Barnes et al. carried out OH and Cl atom initiated photo-oxidation
to best reproduce the experimental values (see Table 2); error bars on the
individual data points from our experiments denote two standard devia-
tions and do not include possible systematic errors.
experiments with MF employing long-path FTIR detection and
reported (CHO) NH and CH NCO as the main products with
2 3
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Table 3 Rate coefficients and Arrhenius activation energies for the
reactions of OH with different amides
ꢂ12
3
ꢂ1
ꢂ1
Amide
k/10
cm s
E
a
/kJ mol
Ref.
a
Formamide
N-Methylformamide
4.44 ꢀ 0.46
6
6
a
10.1 ꢀ 0.6
—
—
ꢂ3.7
b
8
5
.6 ꢀ 2.4
.7 ꢀ 1.7
33
This
work
33
This
work
5
6
5
34
6
33
34
6
34
34
35
36
36
36
c
b
N,N-Dimethylformamide
14 ꢀ 3
—
ꢂ6.6
d
8
.5 ꢀ 2.6
e
Acetamide
3.5 ꢀ 1
—
—
—
a
a
a
0
.77 ꢀ 0.35
e
N-Methylacetamide
11 ꢀ 3
e
5.2
ꢂ4.1
5
.42 ꢀ 0.19
—
b
N,N-Dimethylacetamide
19 ꢀ 3
—
f
13.6
ꢂ5.2
Propanamide
N-Methylpropanamide
N,N-Dimethylpropanamide
1.78 ꢀ 0.43
—
f
7.6
ꢂ3.3
ꢂ4.4
f
20.7
Fig. 7 Time evolution of N-methylformamide (MF) and the major degradation
products during a photo-oxidation experiment. The thin dotted curve
represents the extrapolated dilution and wall loss of MF.
g
1
-Methyl-2-pyrrolidinone
21.5 ꢀ 3.6
h
2
2 ꢀ 4
h
1
2
-Methyl-2,2-pyrrolidinedione
-Oxo-1-pyrrolidinecarboxaldehyde 6 ꢀ 1
14 ꢀ 3
h
a
T = 298 K, P = 1 bar (air), the errors given represent the standard
+
+
ion signals at m/z 43.018 (C
2
H
3
O ), m/z 45.034 (C
3
H
9
O ), m/z
deviation of multiple experiments, the authors estimate a total uncertainty
b
c
+
+
+
closer to 20%. T = 298 K. P = 1 bar (air). Measurements at T = 294 K, 45.992 (NO
2
), m/z 59.049 (C
3
H
7
O ) and m/z 77.056 (C
3
H
9
O
2
).
d
P = 600 mbar (helium). Measurements at T = 294 K, P = 30–600 mbar
These signals stem from compounds (acetaldehyde, acetone
and nitric acid) typically formed in any smog chamber photo-
oxidation experiment in the presence of isopropyl nitrite and
e
f
(
helium). T = 298 K, P = 1 bar (air). T = 300 K, P = 130 mbar (helium).
g
h
T = 296 K, P = 1 bar (air). T = 300 K and P = 1 bar (air).
NO , i.e. even in the absence of MF. In our product analysis, we
x
yields of roughly 50% and 30%, respectively, and with variable only included signals with a larger than 1% increase relative to
amounts of the peroxynitrate-type compound, CH NHC(O)OONO decrease in the protonated MF signal. In spite of every effort
depending on the NO -level in the experiment. Unfortunately, the made no trace of CH NHC(O)OONO was ever detected in any
lack of calibrated reference spectra prevented the authors from of the present experiments.
making a proper quantification of the product yields. Borduas Wall loss of MF was significant in the experiments. Additional
-free OH-initiated photo- loss was inflicted by synthetic air that was constantly added to
3
2
,
x
3
2
6
et al. recently carried out NO
x
oxidation experiments with MF employing PTR-MS detection compensate for instrumental sampling flows (kdilution = 1.13 ꢁ
ꢂ4 ꢂ1
and reported a CH NCO yield around 40%; they further 10
s ); analysis of the MF loss curve before and after the 15
3
reported formamide (B15% yield) and HNCO as oxidation min photo-oxidation period suggested an MF wall loss rate
ꢂ4 ꢂ1
products. Diformamide, (CHO)
trace product and was not quantified.
2
NH, was only detected as a coefficient kwallloss E 2.6 ꢁ 10
s .
There is apparently a mismatch in Fig. 7 between the amount
Fig. 7 shows the time evolution of the main ion signals of N-methylformamide reacted and the amounts of products
observed by PTR-ToF-MS during a photo-oxidation experiment. formed in the experiment. However, the apparent mismatch is
Fig. S4 and S5 (ESI†), respectively, illustrate the difference in deceptive: N-nitro methylamine, CH NHNO , formed with a B5%
3 2
PTR-ToF-MS mass spectra taken after and just before the photo- yield in the experiment shown, can only be formed in the reaction
:
oxidation experiment, and the NO and NO2 mixing ratios between CH NH radicals and NO , and even under conditions
3
2
during the experiment. The NO level is nearly constant around with high NO levels, as in the present case, the major loss of
2
:
2
5 ppbV during the photo-oxidation, whereas the NO
increases from a single-digit value to nearly 400 ppbV in the mine, CH
same period. Reacting compounds are MF (detected in its quantify due to polymerization and surface-induced reactions.
2
level CH
3
NH radicals is due to reaction with O
2
resulting in methani-
2
QNH, which is extremely difficult to detect and to
1
3
protonated form at m/z 60.042 and its C isotopologues
It is highly significant that there is no discernable change in
at m/z 61.043) and isopropyl nitrite, which is detected at the N-nitro methylamine volume mixing ratio upon addition of
+
+
+
m/z 29.996 (NO ), m/z 43.055 (C
H
3 7
), m/z 61.066 (C
3
H
9
O ) and excess NO to titrate peroxy radicals present in the chamber. This
+
m/z 79.074 (C H O ). Signals that increased during the indicates that either very little CH NHC(O)OONO was ever
3
11
2
3
2
+
photo-oxidation experiment are m/z 58.029 (C H NO , proto- formed during the experiment or that the lifetime of the com-
2
4
+
nated methylisocyanate), m/z 74.024 (C
2
H
2
NO , protonated pound is very short, or a combination of both of the above. FTIR-
, protonated methy- spectroscopic evidence for formation of CH NHC(O)OONO in
, proto- N-methylformamide photo-oxidation experiments is rather
nated methylnitramine). We also observed an increase in the limited, and the weak bands reported as characteristic of
+
N-formylformamide), m/z 76.041 (C
lisocyanate water cluster) and m/z 77.032 (CH
2
H
2
NO
2
3
2
+
5
5 2 2
N O
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CH
3
NHC(O)OONO
2
actually have their counterparts in the IR The barrier to H transfer from the NH group is found to be far
3
7
spectrum of CH
3
NHNO
2
.
below the entrance energy of the reactants, and an IRC calcula-
Given the scarcity of well-documented mechanistic informa- tion links a post-reaction adduct between CH NCO and HO on
3
2
tion on amide photo-oxidation chemistry, a first principles the exit side. The barrier to H transfer from the CH group is
3
mapping of the reaction thermochemistry was undertaken also below the initial entrance energy of the reactants; subse-
˙
and additional mechanism elucidating calculations carried quent dissociation of the resulting CH
out. The present quantum chemistry results have many simila- CH QNH + CO + OH proceeds via another barrier below the
rities with those reported by Borduas et al.; there are, however, entrance energy of the reactants. Fig. 8 illustrates the energetics
2
NHC(O)OOH radical to
2
2
6
˙
significant differences in approach, results and, consequently, of the CH NHCO + O reaction (quantum chemistry results
3 2
in the mechanistic conclusions.
collected in Tables S11 and S12, ESI†).
The CH NHCHO reaction with OH radicals may, as already
The branching between reactions (4)–(6) at atmospheric
3
mentioned, proceed via hydrogen abstraction from either the CHO pressure was examined using a master equation model based
group, the NH group or the CH group. The N–H abstraction route is on the potential energy surface shown in Fig. 8 and including
3
:
ꢂ1
calculated to have a medium-sized barrier of around 12 kJ mol
,
the CH
3
NHC(O)OO + NO reaction as a bimolecular sink with a
ꢂ11
3
ꢂ1
while the C–H abstraction routes have barriers well below the rate coefficient of 2 ꢁ 10
cm s , which is a typical value for
:
3
8
entrance energy of the reactants. The quantum chemistry calcula- RC(O)OO + NO reactions. With a rate coefficient for the
ꢂ12
3
ꢂ1
˙
tions therefore unambiguously show that the N–H abstraction route CH
is of no importance in the CH NHCHO + OH reaction at atmo- value for RC(O) + O
spheric conditions, and it will not be addressed any further here. thermalized before reaction with O at atmospheric conditions.
Route (1a), hydrogen abstraction from the CHO group, is The calculations show that reaction (6) is of no importance and
3
NHCO + O
2
reaction of around 5 ꢁ 10
cm s (typical
3
8
˙
˙
3
2 3
reactions), the CH NHCO radicals will be
2
˙
expected to be followed by either radical dissociation leading to that ca. 90% of the CH NHCO + O reaction results in CH NCO.
3
2
3
:
ꢂ1
isocyanates or to the CH
3
NH radical, or by addition of O
2
:
Increasing the energy of the saddle point TS5 by 4 kJ mol
decreases the CH NCO yield to around 80%.
The CH NHC(O)OO* radicals formed in reaction (4) and
subsequently thermalized by collisions are expected to react
with NO resulting in the formation of a meta-stable peroxyni-
,
ꢂ1
3
˙
:
CH NHCHO + OH - CH
3
3
NHCO + H
2
O
DH1 = ꢂ100 kJ mol
1a)
3
(
ꢂ1
˙
CH NHCO - CH NCO + H DH1 = +116 kJ mol
2
3
3
:
(3a)
trate, and with NO leading to CH NH radicals:
3
:
ꢂ1
ꢂ1
˙
CH NHC(O)OO + NO # CH NHC(O)OONO
-
HNCO + CH
3
DH1 = +29 kJ mol
3
2
3
2
(
3b)
ꢂ1
DH1 = ꢂ118 kJ mol
(7)
(8)
(9)
:
:
:
-
CH NH + CO DH1 = +75 kJ mol
3
CH
3
NHC(O)OO + NO - CH
3
NHC(O)O + NO
2
(
3c)
ꢂ1
DH1 = ꢂ35 kJ mol
:
ꢂ1
˙
CH
3
NHCO + O
2
- CH
3
NHC(O)OO DH1 = ꢂ157 kJ mol
:
:
CH
3
NHC(O)O - CH
3
NH + CO
2
(4)
DH1 = ꢂ81 kJ molꢂ1
The barriers to the N–H and N–C bond scission reactions (3a) and
ꢂ1
ꢂ1
(
3b) are around 168 and 117 kJ mol , respectively, which place both The enthalpy of reaction (7) is calculated to be ꢂ118 kJ mol
,
ꢂ1
substantially above the entrance energy of the reactants in (1a); the which compares to ꢂ130 kJ mol
for the corresponding
:
ꢂ1
barrier to reaction (3c) leading to CH NH + CO is barely 1 kJ mol
3
above the entrance energy of the reactants. Considering the amount
˙
of internal energy available in the CH
formation and the energy partitioning between CH
O, it can be concluded that these routes will not be important
3
NHCO radical upon its
˙
NHCO and
3
H
2
at atmospheric conditions. CCSD(T*)-F12a/aug-cc-pVTZ//MP2/aug-
cc-pVTZ results for reactions (3a)–(3c) are illustrated in Fig. S6 and
documented in Tables S9 and S10 (ESI†).
:
The initially hot, chemically activated CH NHC(O)OO* radi-
3
cal (* denotes rovibrational activation) may initiate two H
transfer reactions followed by radical dissociations; the corres-
ponding net reactions are:
˙
3 2 3 2
CH NHCO + O - CH NCO + HO
DH1 = ꢂ93 kJ molꢂ1
(5)
(6)
Fig. 8 Relative energies of stationary points on the potential energy surface
of the CH reaction. Results from CCSD(T*)-F12a/aug-cc-pVTZ//
NH C˙ O + O
-
2 2
CH QNH + CO + OH
3
2
DH1 = ꢂ271 kJ molꢂ1
MP2/aug-cc-pVTZ calculations (including zero-point energies).
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(
(
CH
CH
3
3
)
)
2
NC(O)OONO
NC(O)OONO
2
2
reaction (see later). The lifetime of
is around 14 minutes at 298 K and 1
2
9
3
bar. Even allowing for a considerable uncertainty in the G3
results, the 12 kJ mol difference in reaction enthalpies for the
ꢂ1
two peroxynitrates in question suggests a much shorter lifetime
for CH
results imply a lifetime of CH
s at 298 K and 1 bar. In conclusion, the theoretical results
3
NHC(O)OONO
2
than for (CH
3
)
2
NC(O)OONO
2
. The G3
3
NHC(O)OONO
2
of around only
1
:
3
suggest that not only will a minor amount of CH NHC(O)OO
radicals be formed during N-methylformamide photo-
oxidation, but also that the lifetime of the corresponding
peroxynitrate, CH NHC(O)OONO , will be very short.
3
2
:
The atmospheric fate of the CH NH radical is well estab-
3
lished from experimental and theoretical studies of the
Fig. 9 Relative energies of stationary points on the potential energy surface
of the C˙ H
NHCHO + O reaction. Results from CCSD(T*)-F12a/aug-cc-
2 2
4
0–42
CH
3
NH
2
+ OH reaction:
:
ꢂ1
pVTZ//MP2/aug-cc-pVTZ calculations (including zero-point energies).
CH
3
NH + O
2
2
- CH QNH + HO
2
DH1 = ꢂ81 kJ mol
(10)
:
ꢂ1
CCSD(T*)-F12a/aug-cc-pVTZ//MP2/aug-cc-pVTZ results for reac-
tions (13) and (15) are illustrated in Fig. 9 and documented in
Tables S13 and S14 (ESI†).
The branching between reactions (13a) and (14) at atmo-
spheric pressure was examined using a master equation model
CH
3
NH + NO
2
- CH
2
QNH + HONO DH1 = ꢂ199 kJ mol
(
11a)
-
CH
3
NHNO
2
DH1 = ꢂ213 kJ molꢂ1 (11b)
The other important abstraction route (1c) leading to the
based on the potential energy surface shown in Fig. 9 and
:
˙
CH
forming a vibrationally excited peroxy radical that may initiate
internal H abstraction reactions before reacting with NO (RO
2 2
NHCHO radical is expected to be followed by O addition
including a bimolecular reaction between OOCH NHCHO and
2
ꢂ12
3
ꢂ1
NO with a rate coefficient of 9 ꢁ 10
cm s (typical value for
:
3
8
2
+
ROO + NO reactions). The results show that less than 1%
HO2 reactions can be neglected with around 25 ppbV NO
present in the chamber experiments):
CH QNCHO is formed in the reaction of O with thermalized
2
2
˙
CH
2
NHCHO radicals at 298 K (NO level assumed 41 ppbV).
˙
2
The CH NHCHO radicals may, however, not be thermalized
˙
CH NHCHO + OH - CH NHCHO + H O
3
2
2
at the time of reaction with O
04 kJ mol
available to C˙ H NHCHO in subsequent reactions. The master
equation calculations show that with an additional 10, 20 or
0 kJ mol available to reaction, the yield of CH QNCHO
increases to respectively 6.6%, 26% and 56%. The actual
amount of additional energy available to CH
tion with O will implicitly depend on the rate coefficient of the
reaction and explicitly on the energy partitioning in the pre-
ceding CH NHCHO + OH reaction. Assuming equipartitioning
of the reaction enthalpy, the CH NHCHO radical will be formed
with an additional internal energy of around 60 kJ mol
available for reaction. Assuming that the water molecule
formed in reaction (1c) has one quantum of OH stretching
vibration (B3000 cm ) and that the remaining reaction
enthalpy is divided according to the equipartitioning principle
then the CH
internal energy of around 43 kJ mol . A typical rate coefficient
for the reaction of a medium-sized alkyl radical with O is
2
; reaction (1c) is exothermic by
and part of the reaction enthalpy could be
DH1 = ꢂ104 kJ molꢂ1
(1c)
(12)
ꢂ1
1
:
2
˙
CH
2
NHCHO + O
2
- OOCH
2
NHCHO*
DH1 = ꢂ126 kJ molꢂ1
ꢂ1
3
2
:
OOCH
2
NHCHO* - CH
2
QNCHO + HO
2
˙
2
NHCHO in reac-
ꢂ1
DH1 = +119 kJ mol
(13a)
(13b)
(14)
2
˙
-
HOOCH NHCO*
2
3
ꢂ1
DH1 = +32 kJ mol
˙
2
ꢂ1
:
:
OOCH
2
NHCHO + NO - OCH
2
NHCHO + NO
2
DH1 = ꢂ53 kJ molꢂ1
ꢂ1
ꢂ1
Reaction (13a) proceeds via a saddle point 0.6 kJ mol below
the entrance energy of the reactants and a post-reaction
complex on the exit side. The competing H transfer reaction
from the CHO group has an around 18 kJ mol lower barrier
towards formation of the intermediate HOOCH NHCO radical,
which in turn has a barrier around 32 kJ mol above the
entrance energy of the reactants towards dissociation to HNCO
via another intermediate radical that can be described as a
˙
2
NHCHO radical will be formed with an additional
ꢂ1
ꢂ1
2
˙
2
ꢂ11
3
ꢂ1 38
˙
corresponding to a CH NHCHO radi-
2
ꢂ1
around 10
cal lifetime B20 ns at atmospheric conditions. Assuming an
average energy transfer in collisions with N
of hDEdowni =
NHCHO radicals
cm s
,
2
ꢂ1
ꢂ1
˙
50 cm , the initially 60 kJ mol hot CH
2
2
HNCOꢃCH
HOOCH
2
OOH complex:
will be thermalized in less than 3 ns at atmospheric conditions.
Decreasing the barrier height of reaction (13a) by 4 kJ mol
increases the CH QNCHO yield to 2.6%. A conservative
2
ꢂ1
ꢂ1
˙
2
NHCO* - HNCO + OH + CH
2
O
DH1 = ꢂ96 kJ mol
15)
(
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PCCP
˙
estimate is therefore that less than 5% of the CH
reaction leads to CH QNCHO.
The OCH NHCHO radicals may, in principle, either dissoci- upper estimate of its yield is 0.85% (0.05 ꢁ 0.17).
2
NHCHO + O
2
photo-oxidation product, CH
the present experiments. It should be noted that the conservative
3
NCO, could not be confirmed in
2
:
2
ate or undergo H abstraction by O ; the dissociation reaction is,
CH QNH was observed in traces. CH O and HCN were also
2 2
2
however, highly endothermic ruling out this route at atmo- observed in traces, but both compounds are difficult to quantify
spheric conditions. Consequently, at atmospheric conditions by PTR-ToF-MS. Our recent theoretical study on the atmospheric
the by far dominating primary product resulting from H chemistry of CH QNH concludes unambiguously that HCN is
2
4
3
abstraction from the CH
3
group will be N-formylformamide:
the main product of methanimine photo-oxidation. However,
no firm conclusion concerning CH QNH formation and sub-
:
:
2
ꢂ1
OCH
2
NHCHO - NHCHO + CH
2
O
DH1 = +90 kJ mol
sequent reaction to HCN can be made from the available
:
(16)
experimental data. Assuming branching in the CH NH radical
3
:
reactions with O and NO2 to be the same as in the corres-
2
ꢂ1
OCH NHCHO + O - CHONHCHO + HO DH1 = ꢂ179 kJ mol
:
2
2
2
40,44
ponding (CH ) N radical reactions,
the observed amounts
3
2
(17)
of CH
3
NCO, (CHO)
2
NH, CH
3
NHNO
2
and the modeled amount
2
In summary: the theoretical study shows that OH radicals of CH QNH accounts for 490% of MF reacted.
initiate MF photo-oxidation by H abstraction from the CHO
4
.4. (CH ) NCHO photo-oxidation studies
3 2
and CH groups, and that H abstraction from the NH group is
3
5
not important at atmospheric conditions. Abstraction from the Barnes et al. carried out both OH and Cl atom initiated photo-
CH group leads, almost exclusively, to N-formylformamide oxidation studies of DMF and reported CH N(CHO) as the
although minor amounts of CH QNCHO cannot be ruled major product in the degradation (B70% yield) and unquanti-
out. Abstraction from the CHO group is followed by a branch- fied amounts of (CH NC(O)OONO , depending on the NO
NHCO radical leading to level in the experiment. The lack of calibrated reference spectra
either CH NCO or via the CH NHC(O)OO radical to CH QNH prevented the authors from making a proper quantification of
3
3
2
2
3
)
2
2
x
:
2 3
ing in the O reaction with the CH
3
3
2
and CH NHNO . Each route results in different products, and the product yields.
3
2
the branching ratios can therefore be quantified from the
Fig. 10 shows the time evolution of the main compounds
product distribution shown in Fig. 7. The major routes in the detected by PTR-ToF-MS during a DMF photo-oxidation experi-
OH-initiated photo-oxidation of MF are outlined in Scheme 1 ment, while Fig. S7 and S8 (ESI†), respectively, show the
together with the branching ratios derived from the reactant difference in PTR-ToF-MS mass spectra taken after and before
and product time profiles shown in Fig. 7. The possible the photo-oxidation experiment, and the NO and NO
2
mixing
formation of CH QNCHO, which is an isomer of the major ratios during the experiment. The NO level is nearly constant
2
Scheme 1 Reaction scheme for OH initiated photo-oxidation of N-methylformamide (MF). Compounds in boldface colored type have been detected in
photo-oxidation experiments. The grey colored (CH
)
3 2
NC(O)OONO
2
is predicted to have a short lifetime and was not detected in the experiments. The
branching ratios given have estimated uncertainties ꢀ0.05.
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DMF reacted and the amount of products formed, and, again,
the apparent mismatch is deceptive. N-Nitro dimethylamine,
(
CH ) NNO , formed with a B5% yield in the experiment
3 2 2
:
shown, can only be formed in the reaction between (CH ) N
3
2
radicals and NO , and even under conditions with high NO -levels,
2
2
:
as in the present case, the major reaction of (CH
3
)
2
N radicals is
NQCH
with O
2
resulting in N-methyl methanimine, CH
3
2
,
which is difficult to quantify due to polymerization and surface
induced reactions.
Fig. 10 shows a clear change in the (CH
3 2 2
) NC(O)OONO ,
CH NQCH , (CH ) NNO and (CH ) NNO volume mixing
3
2
3 2
2
3 2
ratios upon addition of excess NO titrating peroxy radicals
present in the chamber. This will be addressed later.
3 2
Hydrogen abstraction from the CHO group in (CH ) NCHO
is expected to be followed by either O
2
addition or dissociation
˙
of the (CH ) NCO radical:
3 2
Fig. 10 Time evolution of the main ion signals observed during photo-
oxidation of N,N-dimethylformamide (DMF). The thin dotted curve repre-
sents the extrapolated dilution and wall loss of DMF.
ꢂ1
˙
(CH
3
)
2
NCHO + OH - (CH
3
)
2
NCO + H
2
O
DH1 = ꢂ98 kJ mol
2a)
(
:
ꢂ1
˙
(
CH ) NCO + O - (CH ) NC(O)OO* DH1 = ꢂ136 kJ mol
3
2
2
3 2
around 25 ppbV during the photo-oxidation, whereas the
(18)
2
NO level increases from single-digit ppbV values to nearly
5
00 ppbV in the same period. Reacting compounds are DMF
ꢂ1
˙
˙
NCO + CH
3
(CH
3
)
2
NCO - CH
3
DH1 = +49 kJ mol
1
3
(detected in its protonated form at m/z 74.059 and its
C
(
19)
isotope at m/z 75.068) and isopropyl nitrite which is detected
at m/z 29.996 (NO ), m/z 43.054 (C
and m/z 79.074 (C H O ). Signals that increased during the above the entrance energy of the reactants in (2a), and this
photo-oxidation experiment are m/z 44.049 (C H N , 8% of total route will therefore not be of importance at atmospheric
+
+
+
ꢂ1
3
H
7
), m/z 61.064 (C
3
H
9
O ) The barrier to C–C scission is high, around 23 kJ mol
+
3
11 2
+
2
6
signal from protonated N-methylmethanimine), m/z 75.055 conditions. Results from CCSD(T*)-F12a/aug-cc-pVTZ//MP2/
+
2 7 2
(C H N O , protonated N-nitroso dimethylamine), m/z 88.037 aug-cc-pVTZ calculations of reaction (19) are collected in
+
3 6 2
(C H NO , protonated N-formyl,N-methylformamide), m/z 91.050 Tables S15 and S16 (ESI†) and the energetics are illustrated in
+
2 7 2 2
(C H N O , protonated N-nitro dimethylamine) and m/z 151.035 Fig. S9 (ESI†).
+
:
(
4
C
3
H
7
N
2
O
5
, protonated (CH
3
)
2
NC(O)OONO
2
). The signals at m/z
3 2
The hot (CH ) NC(O)OO* radical may initiate H transfer
+
+
4.049 (C
2
H
6
N , B92% of total signal), m/z 72.044 (C H NO ), reactions from the CH groups followed by dissociation and OH
3 6
3
+
+
m/z 89.043 (C H NO ), and m/z 105.044 (C H NO ) are produced regeneration:
3
7
2
3
7
2
from (CH ) NC(O)OONO . Signals from N-formyl,N-methylform-
3
2
2
:
+
(CH ) NC(O)OO - C˙ H (CH )NC(O)OOH
3 2 2 3
amide oxidation are m/z 105.029 (C
HC(O)N(CH )NO ) and m/z 165.015 (C
HC(O)N(CH )C(O)OONO ). We also observed an increase in the
˙
2 3
2 3 2 3 2
ion signals at m/z 31.017 (CH O ), m/z 43.017 (C H O ), m/z 45.033 CH (CH )NC(O)OOH - CH QNCH + CO + OH
2
H
3
H
5
O
5
O
3
N
6
N
2
,
protonated
+
DH1 = ꢂ11 kJ molꢂ1
3
2
2
, protonated
(20)
(21)
3
2
+
+
3
+
+
2
+
(
(
C
3
H
9
O ), m/z 45.991 (NO
), m/z 59.049 (C
3
H
7
O ), m/z 60.049
DH1 = ꢂ125 kJ molꢂ1
1
3
12
+
+
+
2 7 2 5 3 3 9 2
C C H O ), m/z 77.022 (C H O ) and m/z 77.056 (C H O ). As
mentioned above, these signals stem from compounds (formalde- The barrier to H-transfer (20) is found well below the entrance
hyde, acetaldehyde, acetone, nitric acid and peroxyacetylnitrate) energy of the reactants. However, the path towards dissociation
˙
typically formed in any smog chamber photo-oxidation experiment of the intermediate CH (CH )NC(O)OOH radical to CH QNCH
3
2
3
2
in the presence of isopropyl nitrite and NO
of (CH NCHO. In our product analysis, we only included signals of the reactants. The internal H transfer route is consequently of
with a 41% increase relative to decrease in the protonated no importance at atmospheric conditions. Fig. 11 illustrates the
x
2
, i.e. even in the absence + CO + OH proceeds via a barrier well above the entrance energy
3 2
)
˙
N,N-dimethylformamide signal.
The wall loss of DMF was significant in the experiments. results collected in Tables S17 and S18, ESI†).
Additional loss was caused by instrumental sampling flows
The thermalized peroxy radical may react with NO and NO
energetics of the (CH ) NCO + O reaction (quantum chemistry
3 2 2
2
that were compensated for by constantly adding synthetic air resulting in the formation of a meta-stable peroxynitrate and
:
ꢂ4 ꢂ1
(k
dilution = 1.13 ꢁ 10
s
); analysis of the MF loss curve before eventually in the (CH ) N radical:
3
2
and after the photo-oxidation period gave a wall loss rate
:
ꢂ4 ꢂ1
(CH
3
)
2
NC(O)OO + NO
2
3 2 2
# (CH ) NC(O)OONO
coefficient kwall loss E 2.8 ꢁ 10
s . Also in the DMF experi-
ments there is apparently a mismatch between the amount of
DH1 = ꢂ130 kJ molꢂ1
(22)
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˙
3 2 2 3 2
(CH ) NCHO + OH - trans-CH N(CH )CHO + H O
DH1 = ꢂ100 kJ molꢂ1
(2b)
˙
cis-CH N(CH )CHO + H O
2 3 2
-
DH1 = ꢂ100 kJ molꢂ1 (2c)
:
˙
2 3 2 2 3
CH N(CH )CHO + O - OOCH N(CH )CHO*
DH1 = ꢂ129 kJ molꢂ1
(28)
:
The hot peroxy radical, OOCH N(CH )CHO*, may initiate H
2
3
transfer reactions; two routes are conceivable:
:
ꢂ1
˙
OOCH
2
NCH
3
CHO* - HOOCH
2
NCH
3
C(O)* DH1 = +34 kJ mol
29a)
(
ꢂ1
˙
-
HOOCH
2
N(CH
2
)CHO* DH1 = +36 kJ mol
29b)
Fig. 11 Relative energies of stationary points on the potential energy
(
3 2 2
surface of the (CH ) NHC(O) + O reaction. Results from CCSD(T*)-
F12a/aug-cc-pVTZ//MP2/aug-cc-pVTZ calculations.
The barriers to the H transfer reactions (29) are found well
below the entrance energy of the reactants in the O addition
2
reaction (28), but the paths towards dissociation of the inter-
mediates proceeds via barriers well above, Fig. 12 (quantum
chemistry results are documented in Tables S19 and S20, ESI†).
These H transfer initiated routes are therefore of no importance
at atmospheric conditions.
With around 25 ppbV NO present in the chamber peroxy
radical reactions with other peroxy radicals can be neglected:
the oxyradical may either undergo H abstraction by O2 or
dissociate:
:
:
(
3 2 3 2 2
CH ) NC(O)OO + NO - (CH ) NC(O)O + NO
ꢂ1
DH1 = ꢂ90 kJ mol
(23)
(24)
:
:
(CH
3
)
2
NC(O)O - (CH
3
)
2
N + CO
2
DH1 = ꢂ58 kJ molꢂ1
3
9
Kirchner et al. investigated the thermal stabilities of perox-
ꢂ3 ꢂ1
ynitrates and reported k
= 1.2 ꢁ 10
s
at 298 K and 1 bar,
ꢂ21
ꢂ1
and E
CH NC(O)OONO
see Fig. 10, gave a loss rate coefficient kꢂ21 = 2.86 ꢁ 10
at 304 K and 1 bar, which is in excellent agreement with the
a
= (111.5 ꢀ 5.8) kJ mol . An analysis of the
:
:
(
3
)
2
2
decay upon the addition of excess NO, OOCH NCH CHO + NO - OCH N(CH )CHO + NO
2 3 2 3 2
ꢂ3 ꢂ1
s
DH1 = ꢂ48 kJ molꢂ1
(30)
(31)
(32)
:
3
9
OCH N(CH )CHO + O - CHON(CH )CHO + HO
2
3
2
3
2
results of Kirchner et al. for the same temperature.
The atmospheric reactions of the (CH ) N radical are well
established from previous studies of dimethylamine photo-
:
ꢂ1
3
2
DH1 = ꢂ183 kJ mol
:
:
4
0–42
OCH
2
N(CH
3
)CHO - CH
3
NCHO + CH
2
O
oxidation:
ꢂ1
DH1 = +76 kJ mol
:
ꢂ1
(
CH ) N + O - CH NQCH + HO DH1 = ꢂ71 kJ mol
3
2
2
3
2
2
:
The rate of OCH N(CH )CHO dissociation was estimated to be
(25)
2
3
ꢂ3 ꢂ1
45
1
ꢁ 10
s
employing the M ´e reau et al. structure–activity
:
ꢂ1
(CH
3
)
2
N + NO
2
- CH
3
NQCH
2
+ HONO DH1 = ꢂ190 kJ mol
26a)
(
-
(CH
3
)
2
NNO
2
DH1 = ꢂ203 kJ molꢂ1 (26b)
:
(
CH ) N + NO - (CH ) NNO DH1 = ꢂ192 kJ molꢂ1 (27a)
3
2
3 2
hn
_
2
ꢄ
ꢂ1
ðCH3Þ NNO ƒ! ðCH3Þ N þ NO DH ¼ þ192 kJ mol
2
(27b)
Hydrogen abstraction from one of the CH
CH NCHO is expected to be followed by O
3
groups in
addition. The
CHO is quite high
(
3
)
2
2
˙
cis–trans rotational barrier in CH
2
NCH
3
ꢂ1
(
around 100 kJ mol from BHandHLYP/aug-cc-pVDZ calcula-
tions), which may prevent thermal equilibrium to be estab-
lished before further reaction takes place; the energy difference
between the two rotamers is, however, small. We will not make
a distinction between the radical cis- and trans-rotamers here as
they eventually end up as the same product.
Fig. 12 Relative energies of stationary points on the potential energy
surface of the C˙ H
(CH )NCHO + O reaction. Results from CCSD(T*)-
F12a/aug-cc-pVTZ//MP2/aug-cc-pVTZ calculations.
2
3
2
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Scheme 2 Reaction scheme for OH initiated photo-oxidation of N,N-dimethylformamide. The branching ratios given have estimated uncertainties ꢀ0.05.
:
relationship for alkoxy radical decomposition reaction. Assum- than for dimethylamine, 65% yield of (CH ) N radicals in
3
2
ꢂ14
3
ꢂ1
40,42,44
ing a typical value for k30 = 1 ꢁ 10
2
cm s , the O abstraction DMF photo-oxidation versus B40% in dimethylamine.
reaction will therefore completely dominate the atmospheric fate Considering the recent recommendation by the Norwegian
:
of the OCH
The major routes in the OH initiated photo-oxidation of and nitramines should not exceed 0.3 ng m in air and 4 ng L
N,N-dimethylformamide is outlined in Scheme 2 together with in drinking water to ensure minimal or negligible risk of cancer
2 3
N(CH )CHO radical.
Institute of Public Health that the total amount of nitrosamines
ꢂ3
ꢂ1
4
6
relative branching fractions derived from analysis of the time for the public from exposure to these substances, immediately
profiles presented in Fig. 10. The observed amounts of CH NCO, raises the question if emission regulations of MF and DMF
3
(
CHO) NCH , (CH ) NNO, (CH ) NNO , and the modelled should be implemented.
2 3 3 2 3 2 2
2 3
amount of CH QNCH accounts for 490% of DMF reacted.
Acknowledgements
5
. Summary and conclusions
This work is part of the Atmospheric Degradation of Amines
(
ADA) project supported by Masdar, Statoil, Vattenfall, Shell
and the CLIMIT program under contracts 193438, 201604 and
08122, and by the Research Council of Norway through a
The rate coefficients of the reactions of OH radicals with N-
methylformamide and N,N-dimethylformamide have been
experimentally determined at T = 294 K to be (5.7 ꢀ 1.7) ꢁ
2
Centre of Excellence Grant (Grant No. 179568/V30). A. J. C. B. is
grateful for an Erasmus Student Mobility Grant.
ꢂ12
3
ꢂ1
ꢂ12
3
ꢂ1
10
cm s and (8.5 ꢀ 2.6) ꢁ 10
cm s , respectively.
6
ꢂ3
Taking [OH]24h = 10 cm , one obtains atmospheric lifetimes
with respect OH reaction of 1.5 to 2 days. The major MF photo-
oxidation product is methyl isocyanate, CH
benign compound in the environment. Methylnitramine,
CH NHNO , is only a minor product. However, the potential
3
NCO, which is not a
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