Article pubs.acs.org/JPCA
Theoretical Study on the Formation and Photolysis of Nitrosamines (CH3CH2NHNO and (CH3CH2)2NNO) under Atmospheric Conditions Yizhen Tang*,†,‡ and Claus Jørgen Nielsen‡ †
School of Environmental and Municipal Engineering, Qingdao Technological University, Fushun Road 11, 266033 Qingdao, Shandong, P. R. China ‡ Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, 0315 Oslo, Norway S Supporting Information *
ABSTRACT: The reactions of CH3CH2NH and (CH3CH2)2N radicals with NO have been studied using quantum chemistry methods. The results show that formation of the nitrosamines CH3CH2NHNO and (CH3CH2)2NNO is similar and that both isolated molecules are thermally stable. The nitrosamine formation reaction is highly exothermic, and the hot CH3CH2NHNO may undergo isomerization and subsequent reaction with O2 to form the corresponding imine, CH3CHNH. Time-dependent density functional theory (TDDFT) calculations show little difference of the vertical excitation energy between the π* ← n transitions in CH3CH2NHNO and (CH3CH2)2NNO, and both will readily photolyze under sunlight conditions.
1. INTRODUCTION Carbon capture and storage (CCS) is among the most important contributions to mitigate climate change. One of the more promising technologies for efficient postcombustion CO2 capture is through the using of amines.1,2 A CO2 capture plant using amines will produce amine emissions to the air, with the possibility of also forming other compounds in the atmosphere after emission. A screening study showed that several toxic compounds might be formed in the degradations of amines in the atmosphere.1 Of immediate concern are toxic compounds such as nitrosamines and nitramines, which can be formed by the reactions of amino radicals with NO/NO2.1,3 The released amines may not only potentially cause health effects like mutations, cancer, and birth defects on humans; they may also have a toxic influence on vegetation, soil, fauna, the aquatic life, and ecosystems.1 Pitts et al. carried out a study of the products formed when a mixture of 500 ppbV (CH3CH2)2NH + 80 ppbV NO + 160 ppbV NO2 was subjected to natural sunlight conditions.4 The products formed in the gas phase included CH3CHO, CH3CO(OO)NO2, (CH3CH2)2NNO2, (CH3CH2)2NCHO, (CH3CH2)2NC(O)CH3, and CH3CH2NC(O)CH3. The nitrosamine, (CH3CH2)2NNO, was detected in the experiments, but © 2012 American Chemical Society
it rapidly photolyzed. Nielsen et al. recently confirmed these findings and also reported the products formed in similar experiments with CH3CH2NH2; no primary nitrosamine was detected.3 We have recently studied the reactions of primary and secondary amino radicals with NO2 by theoretical methods; the main products being N-nitrosamines, while the amounts of imines will depend upon pressure and chain length.5 In order to clarify the mechanisms and the formation of nitrosamines in these experiments, the reactions of ethylamino and diethylamino radicals with NO were studied by quantum chemistry methods. The photolysis of the corresponding nitrosamines was also investigated.
2. COMPUTATIONAL DETAILS The potential energy surfaces (PES) of the CH3CH2NH + NO and (CH3CH2)2N + NO reaction systems were studied with density functional theory (DFT) (B3LYP)6 and MP27 methods conjunct with the 6-311++G(d,p) and aug-cc-pVDZ basis sets using the Gaussian09 package.8 The same computational levels Received: August 7, 2012 Revised: November 27, 2012 Published: November 27, 2012 126
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method has been widely used to investigate radical−radical reactions with good performance and efficiency.20−22 MP2 calculations are a well established method for many radical reactions.23,24 Therefore, geometries of reactants, products, intermediates (IM), and transition states (TS) of the (CH3CH2)2N + NO and CH3CH2NH + NO reaction systems were optimized at the B3LYP and MP2 methods in conjunction with the 6-311++G(d,p) and aug-cc-pVDZ basis sets only due to resource and computational time constraints; the results are depicted in Figures S1 and S2 (Supporting Information), respectively. It can be seen that the geometrical parameters at the four levels are close to each other for most species. The largest deviations in the reacting bond lengths are the reactive C−H and N−H bonds in TS2 ((CH3CH2)2N + NO reaction) and in TS8 (CH3CH2NH + NO reaction), differing by 0.090 (0.092) and 0.142 (0.138) Å at the aug-cc-pVDZ level with B3LYP and MP2 methods in TS2 (TS8), respectively. The stationary points on the potential energy surfaces (PES) are sketched in Figures 1 and 2. The relative energies and reaction enthalpies obtained at the G4 level and various optimized levels are listed in Tables 1 and 2, respectively. The results show that the relative energies from the B3LYP calculations, with the exception of transition states involved in direct hydrogen abstraction, are slightly lower than those from the corresponding MP2 method. This is, in part, attributed to spin contamination in the MP2 calculations. For simplicity, the geometry obtained from the B3LYP/6-311+ +G(d,p) level and the relative energy from the G4 calculations are used in the following sections, unless otherwise stated. 3.1. Mechanism of the (CH3CH2)2N + NO Reaction. The initial addition reaction of reactant (CH3CH2)2N with NO to produce nitrosamine (CH3CH2)2NNO, labeled as IM1, is shown in Figures 1 and S2, Supporting Information. The newly formed N−N bond is 1.327 Å in length in IM1, and the ONN angle is 115.5°. Compared with (CH3CH2)2N, the geometrical parameters in IM1 changes little. In terms of the internal rotation of the N−C bond in IM1, there are several conformers, but IM1 is the lowest and the most stable one according to the calculations. In view of thermochemistry, the most stable conformer will be the dominant product as the initial addition reaction; thus, other conformers are out of our consideration in this study. The entrance reaction is barrierless, and about 178.9 kJ/mol of heat is released during this process. With so much internal energy available, the newly formed molecule is vibrationally hot and can, in principle, undergo further reactions. The calculations show that two channels are possible, both involving intramolecular methylene−H shift. The first route leads to CH3CH2NCHCH3 + HON via a five-membered ring transition state, TS1. The broken N−N bond is stretched to be about twice as long as that in IM1, and the C−H bond is elongated to be 1.604 Å. The formed H−O bond is about 0.115 Å longer than the equilibrium distance in product HON, and OHC angle is 155.6°. In view of structure, TS1 is a reactant-like transition state, and this step proceeds via a rather late barrier, which is anticipated because of its endothermicity (299.5 kJ/mol). The barrier of TS1 is 288.1 kJ/ mol, and TS1 is 111.7 kJ/mol higher than the initial reactant, the large activation barrier makes this product channel unlikely to occur under atmospheric conditions. The second channel takes place via a similar step to produce CH3CH2NCHCH3 + HNO. The corresponding transition state is TS2, a four-membered ring structure. As seen in Figure S2, Supporting Information, the broken N−N bond in TS2 is
were employed in calculations of zero-point energy (ZPE) corrections and harmonic vibrational frequencies for all stationary points unambiguously identified as minima (number of imaginary frequencies, NIMAG = 0) or as transition states (NIMAG = 1) on the minimum energy path of the PES. The intrinsic reaction coordinate (IRC) paths9 were calculated at the same levels to verify that the transition states connect to the correct reactants and products. To obtain more reliable energies for the stationary points of the PES, single-point energies were calculated with the G4 method.10 It is wellknown that the latest G4 method has achieved impressive accuracy with a mean absolute deviation from experimental value of 3.48 kJ/mol. In order to understand the photolysis of these nitrosamines, vertical excitation energies (Tv) were obtained for the Nnitrosamines in time-depended DFT (TDDFT) calculations employing several basis sets. For TDDFT,11−14 the methods include B3LYP, 6 BP86, 15 X3LYP, 16 PBE1PBE, 17 and PW1PW91;18,19 the basis sets range from 6 to 311++G(d,p) to aug-cc-pVQZ. Details of each method used will be given in the following discussions.
3. RESULTS AND DISCUSSIONS An appropriate method is necessary to describe the geometries and energetic of a reaction, and it is known that the B3LYP
Figure 1. Energetic reaction routes of the (CH3CH2)2N +NO reaction at the G4 level.
Figure 2. Energetic reaction routes of the CH3CH2NH + NO reaction at the G4 level.
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Table 1. Relative Energies (ΔE, Including ZPE) and Reaction Enthalpies (ΔH) at 298 K (in kJ mol−1) for the (CH3CH2)2N + NO Reaction at Various Levels B3LYP/6-311++G(d,p)
B3LYP/aug-cc-pVDZ
MP2/6-311++G(d,p)
MP2/aug-cc-pVDZ
G4
species
ΔE
ΔH
ΔE
ΔH
ΔE
ΔH
ΔE
ΔH
ΔE
ΔH
(CH3CH2)2N + NO HNO + CH3CHNCH2CH3 1 HNO + CH3CHNCH2CH3 3 HNO + CH3CHNCH2CH3 IM1 TS1 TS2 TS3 TS4
0.0 −43.1 124.9 41.1 −154.8 114.9 57.9 130.9 101.8
0.0 −43.1 123.7 39.0 −159.2 111.4 52.6 125.9 97.6
0.0 −43.0 123.8 39.1 −164.5 107.9 47.9 123.0 95.6
0.0 −43.2 124.9 41.2 −160.0 111.3 53.3 128.0 99.8
0.0 −82.8 120.7 30.6 −202.9 106.5 −1.6 123.7 196.6
0.0 −80.7 122.8 32.7 −205.7 105.1 −3.7 122.7 196.4
0.0 −88.8 114.3 23.4 −216.3 92.6 −22.9 107.2 184.0
0.0 −89.2 114.0 23.1 −221.8 88.9 −27.6 103.9 181.2
0.0 −57.0 120.6 52.2 −176.4 111.7 43.5 130.1 140.5
0.0 −57.0 120.6 52.2 −178.9 109.2 41.0 127.7 138.0
Table 2. Relative Energies (ΔE, Including ZPE) and Reaction Enthalpies (ΔH) at 298 K (in kJ mol−1) for the CH3CH2NH + NO Reaction at Various Levels B3LYP/6-311++G(d,p)
B3LYP/aug-cc-pVDZ
MP2/6-311++G(d,p)
MP2/aug-cc-pVDZ
G4
species
ΔE
ΔH
ΔE
ΔH
ΔE
ΔH
ΔE
ΔH
ΔE
ΔH
CH3CH2NH + NO HNO + cis-CH3CHNH 1 HON + trans-CH3CHNH 3 HON + trans-CH3CHNH IM2 IM3 IM4 IM5 TS5 TS6 TS7 TS8 TS9 TS10
0.0 −70.9 93.8 10.0 −177.7 −176.8 −174.5 −45.0 −78.1 −48.2 11.9 36.7 121.8 99.7
0.0 −70.8 92.8 8.1 −181.5 −180.8 −180.7 −50.5 −84.1 −57.4 1.8 31.5 116.3 95.2
0.0 −70.7 93.0 8.3 −186.0 −185.5 −185.5 −54.1 −121.6 −62.4 −5.1 27.1 113.1 92.9
0.0 −70.7 93.9 10.2 −182.2 −181.5 −179.4 −48.1 −127.9 −53.3 5.0 32.2 118.6 97.4
0.0 −103.9 95.3 5.2 −208.5 −210.5 −215.7 −77.6 −82.7 −90.4 −42.6 −5.1 122.1 197.9
0.0 −103.7 95.4 5.3 −212.9 −215.2 −220.8 −83.8 −88.6 −95.6 −49.7 −9.4 118.6 195.0
0.0 −109.8 89.9 −1.0 −216.2 −220.8 −225.0 −91.5 −130.9 −105.6 −61.3 −24.9 105.6 185.4
0.0 −109.6 90.1 −0.7 −220.6 −225.5 −229.8 −94.5 −137.0 −110.5 −68.2 −29.1 102.3 182.4
0.0 −86.5 87.8 19.3 −188.9 −191.6 −198.2 −62.2 −108.0 −67.7 −9.1 30.5 120.8 139.3
0.0 −86.5 87.8 19.3 −191.4 −194.1 −200.7 −64.7 −110.4 −70.1 −11.6 28.0 118.3 136.8
electron structure is destroyed when H atom shifts to the O atom in TS1, and it causes higher energy of TS1. All attempts to locate a transition state involving the H atom from the CH3 group shifting to the terminal O atom in IM1 to form CH2CH2NCH2CH3 + NOH were futile. A direct hydrogen abstraction route was located for the (CH3CH2)2N + NO reaction on the singlet PES, and one transition state, TS3, was found to form 1HNO and CH3CH NCH2CH3. The transition state on the triplet PES is TS4 leading to 3HNO. The C−H bonds in the two transition states are similar, taking values of 1.389 and 1.333 Å, respectively, in TS3 and TS4. It also happens to the H−O bond with the value of 1.294 and 1.285 Å in TS3 and TS4. The OHC angle in TS4 is 168.9°, about 21° larger than that in TS3. Both barriers are very high, 130.1 and 140.5 kJ/mol above the entrance energy, respectively. Also the formation of products are thermodynamically unfavorable with an endothermicity of 120.6 and 52.5 kJ/ mol. Of course, under atmospheric conditions, the direct hydrogen channels play no significant role thermodynamically or kinetically. No transition state leading to CH3CH2NCH2CH2 + NOH was found with one of the H atoms abstracted from the CH3 group in reactant either on the singlet nor triplet PES. Comparison of channels in the (CH3CH2)2N + NO reaction indicates that nitrosamine (CH3CH2)2NNO formed via the barrierless addition process is thermally very stable. Further dissociation reactions of (CH3CH2)2NNO may occur via high barriers and will only be of minor importance under
Figure 3. Energetic reaction routes of the CH3CHNHNOH + O2 reaction at the G4 level.
also very long, about 0.81 Å stretched from IM1, and the C−H bond is 1.364 Å. While the newly formed N−H bond is about 28.4% longer than that in isolated HNO molecule. Compared with TS1, CHN is more bent with a value of 111.4°. The barrier of TS2 is 219.9 kJ/mol, and CH3CH2NCHCH3 + HNO is 57 kJ/mol lower than the initial reactant. Here, the barrier of TS1 is higher than TS2, and the reason lies in the formation of a H−O or H−N bond: the saturated eight128
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Table 3. Relative Energies (ΔEelec, Including ZPE) and Enthalpies (ΔH298) at 298 K (kJ mol−1) for the CH3CH2NH + NO + O2 Reaction System at the Various Levels B3LYP/6-311++G(d,p)
B3LYP/aug-cc-pVDZ
MP2/6-311++G(d,p)
MP2/aug-cc-pVDZ
G4
species
ΔEelec
ΔH298
ΔEelec
ΔH298
ΔEelec
ΔH298
ΔEelec
ΔH298
ΔEelec
ΔH298
CH3CH2NH+NO + O2 CH3CHNHNO + HO2 CH3CHNH + NO + HO2 syn-CH3CH2NHNO + O2 CH3CHNHNOH + O2 TS7 + O2 TS11 TS12 + HO2
0 −14.2 −81.5 −177.7 −45.0 11.9 89.3 −7.3
0 −17.4 −81.3 −181.5 −50.5 1.8 85.4 −10.1
0 −20.3 −79.9 −186.0 −54.1 −5.1 83.8 −10.4
0 −23.7 −79.7 −182.2 −48.1 5.0 78.0 −13.0
0 57.9 −63.3 −208.5 −77.6 −42.6
0 52.8 −63.2 −212.9 −83.8 −49.7
0 54.0 −64.9 −216.2 −91.5 −61.3
0 50.2 −64.7 −220.6 −94.5 −68.2
83.5
80.6
66.2
62.9
0 −10.1 −94.3 −188.9 −62.2 −9.1 44.3 4.7
0 −15.0 −96.7 −191.4 −64.7 −11.6 39.4 −0.2
Table 4. Excitation Energy (in eV) and Oscillator Strength (in au) of the First Excited State (1A″) of CH3CH2NHNO, (CH3CH2)2NNO, CH3CHNHNOH, and CH3CH2NNOH at Various DFT Methods with the aug-cc-pVTZ Geometry BP86
PBE1PBE
PW1PW91
B3LYP
X3LYP
species
excitation energy
oscillator strength
excitation energy
oscillator strength
excitation energy
oscillator strength
excitation energy
oscillator strength
excitation energy
oscillator strength
CH3CH2NHNO CH3CHNHNOH CH3CH2NNOH (CH3CH2)2NNO
3.34 3.38 4.59 3.33
0.0008 0.0017 0.0001 0.0005
3.44 3.58 4.75 3.47
0.0010 0.0018 0.0001 0.0006
3.44 3.54 4.74 3.47
0.0010 0.0017 0.0000 0.0006
3.38 3.44 4.67 3.40
0.0009 0.0017 0.0001 0.0006
3.39 3.39 4.68 3.41
0.0010 0.0017 0.0001 0.0006
CH3CH2NHNO, respectively. The nitrosamine is therefore formed as a vibrationally hot molecule:
atmospheric conditions. Direct hydrogen abstraction channel can be neglected thermodynamically and kinetically because of high barriers and unstable products. Therefore, the most feasible product in the (CH3CH2)2N + NO reaction will be the nitrosamine (CH3CH2)2NNO . 3.2. Mechanism of the CH3CH2NH + NO Reaction. First, it should be noted that the CH3CH2NH radical has gaucheand trans-conformers, with gauche-CH3CH2NH about 3.9 and 3.6 kJ/mol higher than trans-CH 3 CH 2 NH in energy, respectively, at the B3LYP/6-311++G(d,p) and G4 levels. In sake of similarity, only the more stable conformer transCH3CH2NH is chosen as reference in the following. Here, it is noted that the small energy difference between the two conformers indicates CH3CH2NH can exist in either form at room temperature. However, the mechanism and reaction progress is similar as was also shown to be the case in the CH3NH + NO reaction system.25 The N atom in NO attacks the N center in the CH3CH2NH radical leading to the nitrosamine CH3CH2NHNO on the singlet PES, and this process is barrierless as indicated in Figure 2. The calculations indicate that two conformers exist with different dihedral angles of ONNH, i.e. syn-CH3CH2NHNO (0°) (IM2) and anti-CH3CH2NHNO (180°) (IM3). Other geometrical parameters such as bond lengths and bond angles are close to each other in both conformers. The newly formed N−N bond is about 1.33 Å in CH3CH2NHNO, and the N−O bond is only elongated by 0.072 Å relative to NO. For the CH3CH2NH portion, it changes very little compared with that in reactant CH3CH2NH radical. The two conformers can invert to each other easily via the rotation of the NO group around N−N bond. In the corresponding transition state TS5, the dihedral angle of ONNH is about 125.5°, and the barrier is about 81 kJ/mol from syn-CH 3 CH 2 NHNO to antiCH3CH2NHNO. However, TS5 is still 108 kJ/mol lower than the initial reactant. The formation of nitrosamine is exothermic by 191.4 and 194.1 kJ/mol at the G4 level for syn-CH3CH2NHNO and anti-
CH3CH 2NH + NO → CH3CH 2NHNO‡
With around 190 kJ/mol available as internal energy, the hot species may react further before being quenched by collisions. Three possible channels are found, involving isomerization or dissociation. The first possible scenario starts from syn-CH3CH2NHNO with the H atom in the amino group shifting to the terminal O atom, and CH3CH2NNOH (IM4) is formed through TS6. A four-membered ring transition state (TS6) is involved, in which the broken N−H bond is stretched by 0.29 Å. The formed O− H bond is 1.353 Å, about 1.38 times longer as that in product IM4. At the same time, the N−N bond becomes shorter, while the N−O bond is elongated. The barrier of TS6 is 121.2 kJ/ mol; however, TS6 is 67.7 kJ/mol lower than the initial reactant. With the energy of −198.2 kJ/mol on the PES, IM4 is rather stable, and this process is exothermic by 9.3 kJ/mol. A similar isomerization step also takes place from antiCH3CH2NHNO with one of the H atom in the methylene group shifting to the terminal oxygen, and CH3CHNHNOH (IM5) is formed. In the corresponding transition state TS7, a five-membered ring structure is formed; the newly formed O− H bond is 1.109 Å, about 15.2% longer than the equilibrium distance in product IM5. The broken C−H bond is stretched by 49.5% from that in anti-CH3CH2NHNO, and the reactive angle OHC is about 124°. The comparison among antiCH3CH2NHNO, TS7, and IM5 indicates that the N−O bond varies from a double bond to a single bond, while the C−N bond reverses. The elongation of the broken C−H bond is greater than that of the formed O−H bond, suggesting that the transition state is product-like, and this step proceeds via a late barrier. The barrier of TS7 is rather high, taking a value of 182.5 kJ/mol. IM5 is about 62.2 kJ/mol lower than the initial reactant; however, this step is endothermic by 121.2 kJ/mol, in agreement with analysis that TS7 is a product-like transition state. 129
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3.54a, 3.39b, 3.58c 0.0011 0.0016 0.0001 0.0007
exptl
The N−N bond scission in IM4 and IM5 can, in principle, take place leading to 3HON + CH3CH2N and 1HON + CH3CHNH, respectively; no transition states were found in spite of many attempts at several levels (this was also confirmed by scanning the N−N bond length along the PES). It also will be needed much energy to break the typical double N−N bond in IM4 and a kind of chemical bond between typical single and double N−N bond in IM5. Therefore, no further reactions were considered due to the geometries of IM4 and IM5. Our calculations show one feasible dissociation channel involving syn-CH 3 CH2 NHNO via a four-centered ring transition state, TS8. As seen in Figure S2 (Supporting Information), the broken N−N and C−H bonds are elongated by 52.2% and 29.2%, respectively, from their equilibrium distances in the reactant. The newly formed N−H bond is about 1.138 times longer as that in isolated HNO, and the length of the C−N bond is between the values of synCH3CH2NHNO and product cis-CH3CHNH. With a tensile ring structure, the energy of TS8 is about 30.5 kJ/mol higher than the initial reactant, and the barrier is 219.4 kJ/mol. Although the formation of cis-CH3CHNH + HNO from synCH3CH2NHNO is endothermic by 104.9 kJ/mol, the overall channel is characterized by exothermicity, with 86.5 kJ/mol heat released. Although the formation of cis-CH3CHNH+HNO is favored thermodynamically, a barrier of around 28 kJ mol−1 above the reactant entrance energy to be surmounted, the route may not be important under atmospheric conditions, and the nitrosamine CH3CH2NHNO is expected to be the predominant product. The possibility of one H atom in the CH3 group shift to the terminal O atom is also considered from IM2 and IM3 in this study; however, no transition state was obtained to produce CH2CH2NH + HON using the current methods. Finally, a transition state involving direct hydrogen abstraction was located that formed trans-CH3CHNH + 1 HON. This channel is endothermic by 87.8 kJ/mol. The transition state is TS9, with the C−H bond stretched to be 1.381 Å, and the O−H bond is 0.053 Å longer than that in 1 HON. The CHO angle is 145.7°, rather than linear. The energy of TS9 is about 120.8 kJ/mol above reactants; therefore, with so tight a barrier and unstable product, it could be neglected under atmospheric conditions. The transition states leading to cis-CH3CHNH + 1HON or cis- and transCH3CHNH+HNO were not found with any of the methods we tried nor was the transition state leading to CH2CH2NH + NOH found with one of the H atoms abstracted from the CH3 group in the reactant. Because the reaction of CH3CH2NH with NO is a radical− radical reaction, it can take place on either the singlet or triplet PES. It is expected that the energy of the triplet species is higher than the singlet ones; therefore, only the direct hydrogen abstraction channel was considered on the triplet PES. Similar to that of the singlet PES, only transition state (TS10) was located to form trans-CH3CHNH + 3HON with a barrier of about 139.3 kJ/mol above the entrance energy. In TS10, the C−H and O−H bonds are about 1.339 and 1.287 Å, respectively, and the CHO angle is 167.4°. The formation of product 3HON is endothermic by 19.3 kJ/mol. Therefore, this channel is not important under atmospheric conditions. In summary, the dominant product in the CH3CH2NH + NO reaction is from theoretical considerations expected to be the nitrosamine. Other products are negligible either for thermodynamic or kinetic reasons. Since the experiments by
3.40 3.55 4.65 3.43 0.0011 0.0008 0.0000 0.0007 3.50 3.70 4.64 3.52 0.0011 0.0008 0.0000 0.0007 3.48 3.70 4.63 3.53 0.0011 0.0008 0.0000 0.0007 3.51 3.71 4.64 3.53 0.0011 0.0008 0.0000 0.0007
oscillator strength excitation energy oscillator strength excitation energy oscillator strength excitation energy oscillator strength
methanol
CH3NO2
ethanol
excitation energy
C6H14
oscillator strength
Article
a
excitation energy
3.38 3.44 4.67 3.40
species
CH3CH2NHNO CH3CHNHNOH CH3CH2NNOH (CH3CH2)2NNO
In ethanol. bIn petroleum. cTheoretical result.
3.52 3.71 4.64 3.54 0.0009 0.0017 0.0001 0.0006
oscillator strength
excitation energy
water gas
Table 5. PCM-TDDFT Excitation Energy (in eV) and Oscillator Strength (in au) of CH3CH2NHNO, (CH3CH2)2NNO, CH3CHNHNOH, and CH3CH2NNOH in Various Solvents at the B3LYP/aug-cc-pVTZ Level
The Journal of Physical Chemistry A
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Nielsen and co-workers3 showed no trace of nitrosamine, there must exist a fast atmospheric sink for CH3CH2NHNO. A comparison of the potential energy surface of the (CH3CH2)2N + NO system with that of CH3CH2NH + NO reveals that the only real difference is linked to the additional freedom to isomerization of CH3CH2NHNO. Otherwise, the reaction enthalpies of the two systems are comparable in every aspect. The additional CH3CH2NHNO isomerization leads to a difference in the thermal quenching processes of (CH3CH2)2NNO‡ and CH3CH2NHNO‡. The latter vibrationally excited compound is in equilibrium with it structural isomer CH3CHNHNOH‡ (IM5) via the transition state TS7 that is below the entrance energy of the reactants. This compound may be envisaged to undergo H-abstraction by O2 to give HO2 + CH3CHNHNO, which, via a low barrier (TS12), dissociates to give CH3CHNH + NO. The stationary points on this reaction PES are shown in Figure 3. The geometries of all stationary points involved in the reaction are included in Figure S2, Supporting Information, while the relative energies and enthalpies at various levels are listed in Table 3. Since all barriers between anti-CH3CH2NHNO, synCH3CH2NHNO, and CH3CHNHNOH are well below the reactant entrance energy, the reaction of CH3CHNHNOH with O2 will be the rate-determining step. Assuming thermal equilibrium, a first estimate of the rate constant for this reaction can be obtained from conventional transition state theory. A value of around 2 × 10−16 cm3 molecule−1 s−1 at 298 K is obtained from the present quantum chemistry calculations. Since O2 is around 5 × 1018 molecules cm−3, the rate of reaction will be around 103 s−1. A conservative estimate of the atmospheric lifetime of N-nitroso ethylamine will therefore be
