Article pubs.acs.org/JPCA
Theoretical Study of the Gas-Phase Reactions of NO3 Radical with a Series of trans-2-Unsaturated Aldehydes: From Acrolein to trans-2Octenal Marie-Thérèse Rayez,*,† Jean-Claude Rayez,† Jamila Kerdouci,‡ and Bénédicte Picquet-Varrault‡ †
Institut des Sciences Moléculaires ISM, CNRS, UMR5255, Université de Bordeaux, 33405 Talence Cedex, France Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR CNRS 7583, Université Paris Est Créteil (UPEC) et Université Paris Diderot (UPD), Institut Pierre Simon Laplace, 61 avenue du Général de Gaulle, 94010 Créteil, France
‡
S Supporting Information *
ABSTRACT: The density functional theory with the BH&HLYP functional has been used in this work to clarify discrepancies found in the literature about the effect of the increasing carbon chain on the reactivity of trans-2-alkenals from acrolein (C3) to trans-2-octenal (C8) with nitrate radical. In this work, it was found that (i) the alkyl chain length of the unsaturated aldehydes has little or no influence on the NO3 reaction rate coefficients (ii) the abstraction of the aldehydic hydrogen from the alkenal is always dominant (83% for trans-2-butanal to trans-2-octenal). The addition channel, which mainly concerns the β addition, has a small influence (17% of the total reaction for the whole series). These results are in good agreement with the experimental studies performed by Zhao et al. in 2011 and by Kerdouci et al. in 2012. All these findings will be useful to complete or improve structure−activity relationships developed to predict the reactivity of NO3 radicals with organic compounds.
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values reported by Cabanas et al.14 reveal a strong increase in the rate constant with increasing alkyl chain length whereas the values published by Atkinson et al.10 and more recently by Zhao et al.17 do not confirm this increase in the rate constants. In addition, for several trans-2-alkenals, disagreements between experimental data can reach a factor of 5. Kerdouci et al.18 have recently revisited all these results by performing absolute kinetic and mechanistic studies of the gasphase reactions of NO3 radical with three trans-2- alkenals: trans-2-hexenal (trans-CH 3 -(CH 2 ) 2 −CHCH−C(O)H), trans-2-heptenal (trans-CH3−(CH2)3−CHCH−C(O)H) and trans-2-octenal (trans-CH3−(CH2)4−CHCH−C(O)H). Their conclusions were that (i) The carbon chain lengthening of the trans-2-alkenals does not significantly affect the rate constants for these reactions, (ii) Unsaturated peroxyacyl nitrate-type compounds, exclusively formed through the abstraction of the aldehydic hydrogen, were observed as major products. This result shows that the addition of NO3 on the double bond is a minor pathway for these compounds. (iii) The rate constants of trans-2-alkenals are lower than those the corresponding alkenes and saturated aldehydes, suggesting that the association of an aldehydic function
INTRODUCTION Aldehydes are key compounds in the chemistry of the troposphere. They are primary pollutants emitted by anthropogenic sources (incomplete combustion of fuels, cigarette, ...) and by natural sources (vegetation, forest fires, ...)1−3 In particular, trans-2-hexenal (also called leaf aldehyde) is emitted from wounded leaves. Other aldehydes such as trans-2heptenal and trans-2-octenal have also been identified in the emissions of plants.4−6 Besides these primary sources, carbonyl compounds are also produced in the atmosphere from the photooxidation of organic compounds. Hence, the oxidation of terpenes and conjugated dienes produces a number of unsaturated carbonyl compounds.7−9 Once emitted into the troposphere, aldehydes can undergo photolysis and reactions with atmospheric oxidants, mainly OH, NO3, and Cl radicals and ozone, for unsaturated aldehydes. The photolysis of aldehydes is an important source of HOx radicals1). In addition, oxidation of aldehydes leads to the formation of peroxyacyl nitrates (PANs) which are long-lived species and can act as reservoirs of NOx in the troposphere. Finally, aldehydes and PANs are toxic and damage the air quality. The nitrate radical is known to be the dominant night-time oxidant of organic species, in particular of unsaturated compounds. Previous experimental kinetic studies on the NO3-initiated oxidation of unsaturated aldehydes have been reported in the literature.10−17 They have shown that NO3 radicals significantly contribute to the degradation of these compounds. However, several kinetic studies performed on C4−C8 trans-2-alkenals exhibit large discrepancies. Indeed, the © 2014 American Chemical Society
Received: April 13, 2014 Revised: June 14, 2014 Published: July 2, 2014 5149
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Table 1. BH&HLYP/6-31+G(d,p) Relative Electronic Energy Barriers, ΔE0# (Including Zero-Point Vibrational Energy (ZPVE) Correction), Free Energy Barriers, ΔG298# (with Respect to the Free Reactants) and Reaction Energies ΔE0 in kcal mol−1a ΔE0# acrolein Abs Addα Addβ Addcyc trans-2-butenal (crotonaldehyde) Abs Addα Addβ Addcyc trans-2-pentenal Abs Addα Addβ trans-2-hexenal Abs Addα Addβ trans-2-heptenal Abs Addα Addβ trans-2-octenal Abs Addα Addβ a
ΔG298#
ΔE0
kx
kcalc
1.21 3.69 1.13 7.90
11.235 14.696 11.584 19.269
−13.83 −14.58 −26.79
1.47 × 10−15 4.26 × 10−18 8.16 × 10−16
0.40 1.28 0.30 5.58
10.217 12.144 11.359 17.073
−14.17 −12.10 −21.72
8.21 × 10−15 3.17 × 10−16 1.19 × 10−15
−14.13 −12.27 −21.79
−15
6.15 × 10 2.99 × 10−16 9.39 × 10−16
−14.12 −11.67 −21.69
−15
6.24 × 10 3.36 × 10−16 9.38 × 10−16
−14.46 −11.13 −21.52
−15
6.38 × 10 3.97 × 10−16 1.08 × 10−15
−14.15 −11.10 − 21.41
−15
0.37 1.18 0.50 0.35 1.06 0.43 0.30 0.95 0.33 0.28 0.93 0.43
10.387 12.179 11.500 10.379 12.110 11.501 10.366 12.010 11.419 10.263 12.000 11.430
8.00 × 10 4.04 × 10−16 1.06 × 10−15
Γ (%)
2.30 × 10−15 64 0 36 9.72 × 10‑15 84 4 12 7.39 × 10‑15 83 4 13 7.51 × 10‑15 83 5 12 7.86 × 10‑15 81 5 14 9.46 × 10‑15 85 4 11
Partial and overall rate coefficients kx and kcalc in cm3 molecule−1 s−1. Branching ratios Γ = kx/kcalc.
and a double bond in α position lowers the reactivity of both groups. To our knowledge, there is no theoretical work published on the reaction of NO3 radical with unsaturated aldehydes. Using quantum chemistry calculations, we have therefore decided to carry out a theoretical study of the oxidation by NO3 of a family of six trans-2-alkenal compounds for two purposes: (i) try to rationalize the experimental data already published in this field and (ii) bring some mechanistic arguments for a comprehensive behavior of this kind of oxidation processes. In this paper, six linear unsaturated aldehydes were studied: acrolein (trans-2-propenal), trans-2-butenal (crotonaldehyde), trans-2-pentenal, trans-2-hexenal, trans-2-heptenal, and trans-2octenal involved in the following reactive processes:
of NO3 on the double bond. This last mechanism was suggested to be competitive with α and β additions in a theoretical study by Cartas-Rosado19 et al. for the reactions of NO3 with alkenes. The abstraction of aliphatic hydrogen atoms by the NO3 radical was not considered here since several experimental and theoretical studies have shown that this process is a minor pathway.18,20−22
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COMPUTATIONAL DETAILS Full geometry optimizations and frequency calculations were performed with the GAUSSIAN 09 program package23 using density functional theory (DFT) with the functional BH&HLYP24,25 and the 6-31+G(d,p) basis set. The choice of this method has been guided by the reliable results already obtained for the study of open-shell systems.26−29 Unrestricted wave functions were used to describe open-shell and bond breaking processes. It has been checked that there is no significant spin contamination (the average ⟨S2⟩ value is always close to 0.75, the S2 eigenvalue for a doublet state radical). The saddle points of the potential energy surface (PES) were characterized by the existence of only one negative eigenvalue of the Hessian matrix corresponding to an imaginary frequency in the normal-mode analysis. The normal modes were calculated using the harmonic assumption for all the vibrators. The results concerning geometry, vibrational analysis and energy were subsequently used to perform conventional transition state computations to predict rate coefficients and branching ratios between the various competing abstraction and addition channels. Calculations of rate constants k(T) in
NO3 + R−CHCH−CH(O) → R−CHCH−CO + HONO2
(1a)
→ R−CH−CH(NO3)−CH(O)
(1b)
→ R−CH(NO3)−CH−CH(O)
(1c)
→ TScyc → products
(1d)
with R = H, CH3, CH3−CH2, CH3−CH2−CH2, CH3−CH2− CH2−CH2, and CH3−CH2−CH2−CH2−CH2. Aside from the abstraction of the aldehydic-H atom by NO3 1a, three addition channels of NO3 to the double bond of the alkenals have a priori to be considered: 1b and 1c NO3 addition to the two carbon atoms of the double bond in α and β positions with respect to C(O)H group and 1d a cycloaddition 5150
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Figure 1. Transition state geometries of aldehydic hydrogen abstraction from C3 to C8 unsaturated trans-2-alkenals by NO3 radical. Numbers are in Å.
terms of free energy barriers ΔGT# are then obtained by the following formula: k(T ) = (kBT /h) exp( −ΔGT # /RT )
rate coefficients and the branching ratios which will be discussed later in the paper. 1. Aldehydic H Atom Abstraction from Unsaturated Aldehydes by NO3 Radical. Figure 1 displays the geometries of the different abstraction transition states (TSabs) encountered for the reactions studied. All these TSabs form a planar eight-membered cycle with NO3. The formation of this cycle is due first to the exchange of the aldehydic hydrogen atom, labeled “a” in Figure 1, between the carbonyl group and NO3 and second to the formation of a hydrogen bond between NO3 and the ethylenic hydrogen atom, labeled “b” in Figure 1, linked to the carbon atom in beta position with respect to the carbonyl group. The aldehydic hydrogen exchange characterizes an early transition state since the length of the breaking C---H bond is dC−H = 1.14 Å (1.15 Å for acrolein) whereas this bond distance is 1.10 Å for the aldehydes alone. The length of the newly formed bond H---O(NO2), dO‑Ha is rather large: 1.62 Å for the TS of acrolein and around 1.70 Å for all the other TSabs. The hydrogen bond is characterized by a large dO‑Hb distance: 2.33 Å for acrolein and 2.37 Å for crotonaldehyde. Of course, this hydrogen bond contributes to the stabilization of these intermediate TSabs structures. The early status of these TSabs is confirmed by the small values of the electronic barriers ΔE0# as shown in Table 1, all these reactions being significantly exoergic (ΔE0 ≈ 15 kcal mol−1). The largest barrier (1.21 kcal mol−1) concerns acrolein. All the other barrier heights are much smaller. They decrease monotonously from 0.40 kcal mol−1 for crotonaldehyde, to 0.28 kcal mol−1 for trans-2-octenal. The largest energy difference (1.21−0.40 kcal mol−1) concerns the substitution of one ethylenic hydrogen atom in the β position from the carbonyl group CO in acrolein by a methyl group in crotonaldehyde. This fact can be easily explained by the hyperconjugation effect due to the methyl group which stabilizes the crotonaldehyde TSabs with respect to the acrolein TSabs. Then the subsequent substitution of aliphatic hydrogen atoms by methyl groups in alkyl chains stabilizes only slightly the TSabs since the inductive effect decreases rapidly as long as the length of the alkyl chain
(2)
where kB and h are respectively Boltzmann and Planck constants. ΔGT# = ΔHT# − T ΔST# where ΔHT# and ΔST# are respectively the activation enthalpy and the activation entropy at temperature T. We also checked that all the saddle points found from the optimization procedure are associated with a maximum value of the free energy ΔGT calculated in the vicinity of these saddle points (see the third paragraph in the Results and Discussion). In other words, saddle point structures are genuine transition states at T. Assuming no interference between the channels, the overall rate constant kcalc(T) is the sum of the partial kx(T) (where x = abs, addα, addβ or addcyc) calculated for each reaction channel. The branching ratios can then be deduced to be Γx = kx(T)/kcalc(T). The calculations have been carried out for T = 298 K. Moreover, it is noteworthy to mention that the fact that we deal with very tiny barriers lead to a reasonable expectation of negligible tunneling effect for abstraction channels.
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RESULTS AND DISCUSSION In Table 1, we report results from a detailed computational study of reactions 1a−1d concerning a series of unsaturated aldehydes having different aliphatic linear chain lengths: from acrolein (C3) to trans-2-octenal (C8). For each alkenal, three channels were considered: the abstraction of the aldehydic-H atom by NO3 and the two addition channels of NO3 to the carbon atoms of the double bond in α or in β sites with respect to C(H)O group. Results for barrier heights corresponding to cycloaddition of NO3 to the double bond of the alkenals have been added only for acrolein and crotonaldehyde. The justification of this limitation is given later. The electronic energy barriers, ΔE0#, reaction energies ΔE0 including zero point level at 0 K and free energy barriers, ΔG298# for T = 298 K (with respect to the free reactants) are listed in the first columns of Table 1. The three last columns are devoted to the 5151
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Figure 2. Transition state geometries corresponding to addition of NO3 radical to the Cα carbon atom of the double bond of unsaturated trans-2alkenals. Numbers are in Å.
Figure 3. Transition state geometries corresponding to addition of NO3 radical to the Cα carbon atom of the double bond of unsaturated trans-2alkenals. Numbers are in Å.
parameters are indicated. In a same way as shown in the transition states structures for addition of NO3 to alkenes published in refs 19 and 30, the NO3 group lies in a plane perpendicular to the CαΔCβ double bond with one of its oxygen atoms closer to one of the carbon atom Cα (TSaddα) or Cβ (TSaddβ). It can be observed that, in all cases, TSaddβ occurs earlier than TSaddα. As a matter of fact, the Cα---O distance is 2.1 Å in TSaddαΔcompared to the Cβ---O distance of 2.3 Å in TSaddβ. This is consistent with the fact that the barrier ((ΔE0# ∼ 0.35 kcal mol−1) and free energy barrier heights (ΔG298# ∼ 10.3 kcal mol−1) (Table 1) corresponding to TSaddβ are significantly lower than those corresponding to TSaddα (ΔE0# ∼ 1.1 kcal mol−1 and (ΔG298# ∼12.1 kcal mol−1). These results clearly show that β addition is predominant over αΔ addition.
increases. In addition, the influence of substituent is less important since it is not directly bonded to the carbon atom from which the hydrogen atom is abstracted. The resulting free activation energies (ΔG298#) for T = 298 K are all similar, around 10.3 kcal mol−1, with an exception for acrolein (11.2 kcal mol−1). The same arguments can be applied to the variation of ΔG298# along the series since it concerns the enthalpy, the entropy change between the reactants and the TSabs being practically the same for all these systems. 2. Addition of NO3 Radical to the Double Bond of Unsaturated Aldehydes. Addition of an oxygen atom of NO3 to one of the carbon atom Cα (channel 1-a) or CβΔ (channel 1-b) of the alkenal double bond has been considered. The transition state structures TSaddα and TSaddβ are represented respectively in Figures 2 and 3 where few relevant 5152
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without passing through pre- or postreactive molecular complexes. This study predicts that the reaction channels are direct ones and that the rate constants are consequently only determined by the characteristics of the TS of the reaction channels calculated in this work. Table 1 displays the partial rate constants kx calculated using the formula 2 for each channel. The overall rate constants, kcalc., are then calculated as the sum of these partial contributions. In Table 2, these rate constants are compared to experimental values available from literature. A good agreement exists between our calculations and the kexp measured by Cabanas et al.14 for acrolein and crotonaldehyde reactions with NO3. For bigger aldehydes, the calculated values are in good agreement with the recent work of Kerdouci et al.18 and are slightly lower than those obtained by Atkinson et al.10 and Zhao et al.17 However, it should be mentioned that these theoretical calculations do not allow providing a precise value of the rate constants. In consequence, the agreement with these two experimental studies is satisfying. Finally, calculated values significantly differ (1 order of magnitude) from the data obtained by Cabanas et al.14 for trans-2-hexenal and trans-2heptenal. Clearly, all the global rate constants are more or less the same for the series of unsaturated aldehydes except for acrolein which exhibits a smaller rate constant due larger barrier heights for the aldehydic hydrogen abstraction and the β addition. Considering the rate constants obtained in this work, we can deduce that the alkyl chain length of trans-2-alkenals has a little or no effect on the rate constants. This result confirms the conclusions of the experimental studies performed by Kerdouci et al.18 and Zhao et al.17 Furthermore, our calculations allow us to put some light on the mechanism of the reactions of alkenals with nitrate radicals (last column of Table 1). Aside from acrolein reaction, the percentage of abstraction has been found dominant in these reactions with NO3 radicals: (83 ± 2) %. This is in line with the PAN-type compounds which are characteristic products of aldehydic abstraction and were observed as major products by Kerdouci et al.18 Among the 17% concerning all the additions, 12% and 5% correspond to addition of NO3 to the carbon atom Cβ and Cα of the alkenal double bond. In the case of the acrolein reaction, the rate constant is the lowest of the series and the abstraction is less important in the overall mechanism (64%) than for the other reactions, to the benefit of the βΔ
As for the abstraction processes, we note that the role of the length of aliphatic chain linked in β position on the ethylenic double bond is maximum for a simple methyl group. Then the energy differences are the largest between acrolein (1.13 kcal mol−1) and crotonaldehyde (0.30 kcal mol−1). As the length of aliphatic chain increases, inductive effect diminishes drastically and the barrier heights are almost the same for the entire series (0.50, 0.43, 0.33, 0.43 kcal mol−1). Cartas-Rosado et al.,19 have found that cycloaddition of NO3 to the CC bond of alkenes can be competitive with the other addition mechanisms. In order to check whether this mechanism plays a noticeable role in these reactions with NO3, we have considered this mechanism for acrolein and crotonaldehyde reaction with NO3. The corresponding transition structures that we have located are presented in Figure 4. In these cyclic transition structures, two oxygen atoms
Figure 4. Transition state geometries corresponding to cyclic addition of NO3 radical to the double bond of unsaturated trans-2-alkenals. Numbers are in Å.
of NO3 form a five-center cycle with the two carbon atoms Cα, Cβ. The CαΔ−Cβ distance (1.40 Å) is between the one of a single (1.43 Å) and a double bond (1.33 Å). The barrier height corresponding to this cyclic transition state pathway is very high compared to the other channels (7.9 and 5.6 kcal mol−1 for acrolein and crotonaldehyde). In the light of these results, we do not expect that this cyclic pathway plays a significant role in the overall mechanism. Then, we have discarded this process in the kinetic study. 3. Estimation of Rate Constants and Branching Ratios. Intrinsic reaction coordinate (IRC) calculations were performed for each channel. Starting from the TS, and following backward and forward directions the energy was found to decrease monotonically toward both the products and reactants
Table 2. Comparison of the BH&HLYP/6-31+G(d,p) Calculated Rate Constants kcalc and Measured kexp for the Oxidation of a Series of trans-2-Unsaturated Aldehydes by the NO3 Radicala
a
Cn
aldehyde
kcalc × 1014 (cm3 molecule−1 s−1)
C3 C4 C5
acrolein crotonaldehyde trans-2-pentenal
0.23 0.97 0.74
C6
trans-2-hexenal
0.75
C7
trans-2-heptenal
0.79
C8
trans-2-octenal
0.95
kexp × 1014 (cm3 molecule−1 s−1)
reference
± ± ± ± ± ± ± ± ± ± ± ±
Cabañas et al.14 Cabañas et al.14 Zhao et al.17 Cabañas et al.14 Zhao et al.17 Cabañas et al.14 Atkinson et al.10 Kerdouci et al.18 Zhao et al.17 Cabañas et al.14 Kerdouci et al.18 Kerdouci et al.18
0.25 1.61 1.93 2.88 1.36 5.49 1.21 0.47 2.31 9.59 0.53 0.56
0.04 0.19 0.40 0.29 0.29 0.95 0.44 0.15 0.36 0.19 0.16 0.23
Number Cn of carbon atoms in the aldehydes. 5153
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the β addition, has a small influence (17% of the total reaction for the whole series). This result agrees well with the work of Kerdouci et al.18 in which the PAN-type compounds have been identified as major products at room temperature. In addition, it was found that the alkyl chain length of the unsaturated aldehydes has little or no influence on NO3 reaction rate coefficient. This result is also in good agreement with the experimental studies performed by Zhao et al.17 and by Kerdouci et al.18 All these findings will be useful to complete or improve structure−activity relationships developed to predict the reactivity of NO3 radicals with organic compounds.
addition reaction (36%). These results are in excellent agreement with those published by Salgado et al.16 In order to study the influence of the double bond on the reactivity of the aldehydes, the rate constant of the aldehydic H abstraction reaction has been calculated for hexanal at the same level of theory, BH&HLYP/6-31+G(d,p). The value of 2.3 × 10−14 cm3 molecule−1 s−1 was obtained, which is larger than the one of the trans-2-hexenal reaction by a factor of 3. This tendency is in agreement with the different experimental studies which measured a value of around 1.6 × 10−14 cm3 molecule−1 s−1 for the hexanal rate constant.14,31−33 This result shows that the presence of the double bond lowers the reactivity of NO3 radicals on the carbonyl group. Energy barriers, ΔH298# and activation entropies, ΔS298# have been calculated for the aldehydic H abstraction pathway for both trans-2-hexenal and hexanal (see Table 3).
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S Supporting Information *
Calculated structural data for the transition state of the H aldehydic abstraction from trans-2-hexenal and hexanal by NO3 radical. This material is available free of charge via the Internet at http://pubs.acs.org.
Table 3. BH&HLYP/6-31+G(d,p) relative electronic energy barriers, ΔH298#, activation entropies, ΔS298#, and kabs in cm3 molecule−1 s−1 of the the Aldehydic H Abstraction Channel for trans-2-Hexenal and Hexanal ΔH298# (kcal mol−1) abstraction abstraction
0.33 0.43
ΔS298# cal mol−1 K−1 trans-2-Hexenal −33.72 Hexanal −30.75
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AUTHOR INFORMATION
Corresponding Author
kabs (cm3 molecule−1 s−1)
*(M.-T.R.) E-mail:
[email protected]. Notes
0.62 × 10−14 2.30 × 10
ASSOCIATED CONTENT
The authors declare no competing financial interest.
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−14
ACKNOWLEDGMENTS This work has been supported by the French Ministry of Research through the program ANR (Project ONCEM No. ANR-12-BS06-0017-01).
Since the activation energies, ΔEact 298 are quite the same for both systems, the difference lies in the pre-exponential factors i.e. in the activation entropies, ΔS298#. As seen in Table 3, the activation entropy difference is 3 cal mol−1 K−1 (33.72−30.75 cal mol−1 K−1), which corresponds to an abstraction rate constant ratio of 3.7 between trans-2-hexenal and hexanal abstraction rate constants. This ratio is essentially due to the existence of some “rigidity” of the double bond in trans-2hexenal in the reactants and abstraction transition state which does not exist in hexanal. The coordinates of both transition states are given in the Supporting Information.
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REFERENCES
(1) Carlier, P.; Hannachi, H.; Mouvier, G. The Chemistry of Carbonyl Compounds in the atmosphere A Review. Atmos. Environ. 1986, 20, 2079−2099. (2) Graedel, T. E., Hawkins, D. T., Claxton, L. D. Atmospheric Chemical compounds: Sources, Occurence, and Bioassay; Academic Press: Orlando, FL. 1986. (3) Grosjean, D.; Grosjean, E.; Gertler, A. W. Emissions of Carbonyls from Light-Duty and Heavy-Duty Vehicles. Environ. Sci. Technol. 2000, 35, 45−53. (4) Springett, M. B.; Williams, B. M.; Barnes, R. J. The effect of packaging conditions and storage time on the volatile composition of assam black tea leaf. Food Chem. 1994, 49, 393. (5) Sinyinda, S.; Gramshaw, J. W. Volatiles of avocado fruit. Food Chem. 1998, 62, 483−487. (6) Boue, S. M.; Shih, B. Y.; Carter-Wientjes, C. H.; Cleveland, T. E. Identification of volatile compounds in soybean at various developmental stages using solid phase microextraction. J. Agric. Food Chem. 2003, 51, 4873−4876. (7) Jemma, C. A.; Shore, P. R.; Widdicombe, K. A. Analysis of C-1-C16 Hydrocarbons Using Dual-Column Capillary GC Application to Exhaust Emissions from Passenger Car and Motorcycle Engines. J. Chromatogr. Sci. 1995, 33, 34−48. (8) Calogirou, A.; Larsen, B. R.; Kotzias, D. Gas-phase terpene oxidation products: a review. Atmos. Environ. 1999, 33, 1423−1439. (9) Finlayson-Pitts, B. J., Pitts Jr., J. N. Chemistry of the Upper and Lower Atmosphere. Academic Press: San Diego, CA, 2000. (10) Atkinson, R.; Aschmann, S. M.; Goodman, M. A. Kinetics of the gas-phase reactions of NO3 radicals with a series of alkynes, haloalkenes, and α,β-unsaturated aldehydes. Int. J. Chem. Kinet. 1987, 19, 299−307. (11) Kwok, E. S. C.; Aschmann, S. M.; Arey, J.; Atkinson, R. Product formation from the reaction of the NO3 radical with isoprene and rate
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CONCLUSIONS The density functional theory with the BH&HLYP functional has been used in this work to clarify discrepancies found in the literature about the effect of the increasing carbon chain on the reactivity of trans-2-alkenals from acrolein (C3) to trans-2octenal (C8). Our study reveals that, these reactions involve several types of channels. Two channels have been found to be major pathways, the aldehydic H atom abstraction channel and the of NO3 addition on the βΔcarbon of the double bond of the aldehyde. A good agreement has been observed between our calculated values and the experimental ones measured by Zhao et al.17 and Kerdouci et al.18 The difference in free energies of activation for the addition to the α carbon atom (nearest to the CH(O) group) with respect to the β addition and the abstraction shows that it does not play a noticeable role at room temperature. The first molecule of the series, acrolein, has a reactivity slightly different from the rest of the alkenal series in which the length of the aliphatic chain increases. It has been observed that the abstraction of the aldehydic hydrogen from the alkenal is always dominant (83% for trans-2-butenal to trans-2-octenal). The addition channel, which mainly concerns 5154
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