Pressure Dependence of Butyl Nitrate Formation in ... - ACS Publications

Article pubs.acs.org/JPCA

Pressure Dependence of Butyl Nitrate Formation in the Reaction of Butylperoxy Radicals with Nitrogen Oxide N. I. Butkovskaya,†,⊥ A. Kukui,‡ G. Le Bras,*,† M.-T. Rayez,§ and J.-C. Rayez§ †

Institut de Combustion, Aérothermique, Réactivité et Environnement (ICARE), CNRS-INSIS, 1C Avenue de la Recherche Scientifique, 45071 Orléans cedex 2, France ‡ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (LPC2E), CNRS-INSU, 3A Avenue de la Recherche Scientifique, 45071 Orléans cedex 2, France § Institut des Sciences Moléculaires, CNRS/UMR5255, Université Bordeaux 1, 351 Cours de la Libération, 33405 Talence cedex, France ABSTRACT: The yield of 1- and 2-butyl nitrates in the gas-phase reactions of NO with nC4H9O2 and sec-C4H9O2, obtained from the reaction of F atoms with n-butane in the presence of O2, was determined over the pressure range of 100−600 Torr at 298 K using a high-pressure turbulent flow reactor coupled with a chemical ionization quadrupole mass spectrometer. The yield of butyl nitrates was found to increase linearly with pressure from about 3% at 100 Torr to about 8% at 600 Torr. The results obtained are compared with the available data concerning nitrate formation from NO reaction with other small alkylperoxy radicals. These results are also discussed through the topology of the lowest potential energy surface mainly obtained from DFT(B3LYP/aug-cc-pVDZ) calculations of the RO2 + NO reaction paths. The formation of alkyl nitrates, due essentially to collision processes, is analyzed through a model that points out the pertinent physical parameters of this system. In previous chamber studies of Atkinson et al.,10,11 it was revealed that at room temperature and atmospheric pressure, the yield of alkyl nitrates in the reactions of alkylperoxy radicals with NO increases from ∼4% for propyl nitrate (n = 3) to ∼30% for heavier alkyl nitrates with n ≥ 8. In particular, the value of α = kb/ (ka + kb) = (7.7 ± 0.9)% was determined for the total butyl nitrate yield from the OH-initiated oxidation of n-butane in air.10 The alkyl nitrate yield increases with pressure, as was first shown by the Atkinson’s group for C5−C7 nitrates.12−14 On the basis of these data, a general expression for the estimation of the branching ratios β = kb/ka for the nitrate-forming channels in the reactions of alkylperoxy radicals with NO, β = Y(n,P,T), was derived by Carter and Atkinson.15 The pressure and temperature dependences observed in laboratory experiments were, in general, qualitatively confirmed by RRKM-like calculations of the isomerization/dissociation branching ratio of ROONO based on the semiempirical PES. In particular, positive pressure dependence can be explained by collisional deactivation of the excited RO−NO2. However, there are still many discrepancies both within the theoretical predictions and between theory and experiment5−.9 In this context, a turbulent flow reactor (TFR) coupled with a chemical ionization mass spectrometer (CIMS) was used at CNRS-ICARE to study the formation of RO−NO2 from the RO2 + NO reactions with R = CH3,16 C2H5,17 and i-C3H7.18 Increase

1. INTRODUCTION This study completes the series of studies on alkyl nitrate formation in RO2 + NO reactions RO2 + NO → RO + NO2 → RO−NO2

(a) (b)

where R = CnH2n+1 is the alkyl radical with n ≤ 4. The aim of the series was to provide data on the formation of small alkylperoxy radicals, for whom branching fractions of channel b were very uncertain and pressure and temperature effects on these ratios were unknown. The interest in such data is explained mainly by the importance of the above reaction in atmospheric chemistry, where channel b, acting as free radical chain termination and a source of NOx reservoir, affects production rates of ozone on local and global scales.1,2 The above reaction following the photochemical oxidation of organic compounds is a major source of alkyl nitrates in the atmosphere,3 while for methyl and ethyl nitrates, emission from oceans is also important.4 The mixing ratio of alkyl nitrates with n ≤ 5 presents a significant fraction of all alkyl nitrate atmospheric contents.1 Another aspect is that the nitrate formation channel in the ROO + NO system presents a challenge for theoretical chemistry.5,6 Previous ab initio and semiempirical calculations [e.g., refs 7−9] have established that the reaction proceeds through the formation of the ROONO intermediate complexes, which can isomerize to RO−NO2. A search for the stationary points on the reaction potential energy surface (PES), including the transition state for isomerization to RO−NO2 for small R radicals, remains an intriguing part of these studies. © XXXX American Chemical Society

Special Issue: Mario Molina Festschrift Received: September 17, 2014 Revised: November 7, 2014

A

dx.doi.org/10.1021/jp509427x | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

of β = kb/ka with pressure for C1−C3 nitrates was confirmed in these experiments. The present article presents the determination of the pressure dependence for butyl nitrate formation in the C4H9O2 + NO reaction C4 H 9O2 + NO → C4 H 9O + NO2

(1a)

C4 H 9O2 + NO → C4 H 9ONO2

(1b)

passing through a trap with molecular sieves immersed into liquid nitrogen. The initial concentration of F atoms was about 2 × 1012 molecules cm−3. The distance from the injector tip to the orifice of the inlet cone of the ion−molecule reactor (IMR) was 36 cm, which corresponded to residence times in the TFR from 18 ms at 100 Torr to 35 ms at 600 Torr. Butane (n-C4H10, AlphaGaz N45) and oxygen (AlphaGaz 2) were introduced into the reactor upstream of the tip of the movable injector through CELERITY flow controllers. Nitrogen monoxide (AlphaGaz N20) was added to the main nitrogen flow through a special line passing successively through an ethanol−liquid-N2-cooled trap and an (Fe)IISO4 filter to reduce penetration of NO2 into the reactor. Typical concentrations of the reactants were [C4H10] ≈ 5 × 1014 and [O2] ≈ 1.2 × 1016, while [NO] was varied in the (2− 10) × 1015 molecules cm−3 range. Two isomers of butyl radicals formed in reactions 2a and 2b add oxygen, producing peroxy radicals

The branching ratio, β = k1b/k1a, was determined as the concentration ratio of the final products from channels 1b and 1a, as described in the Experimental Methods section. The methodology of this study is similar to that used to investigate the pressure dependence of isopropyl nitrate formation.18 However, the determination of the branching ratio in the butane system was strongly complicated by additional unimolecular isomerization/decomposition pathways for the intermediate butoxy radicals, C4H9O. The reaction products were detected using negative ion chemical ionization (NICI). The branching ratio for reaction 1 was determined over the 100−600 Torr pressure range at room temperature. The data obtained are compared with the formation yields for other small alkyl nitrates and analyzed using DFT(B3LYP/aug-cc-pVDZ) theoretical calculations presented in the Theoretical Examination section.

n‐C4 H 9 + O2 → n‐C4 H 9O2

(n3)

sec ‐C4 H 9 + O2 → sec ‐C4 H 9O2

(s3)

with kn3 = 7.5 × 10−12 and ks3 = 1.7 × 10−11 cm3 molecule−1 s−123 (rate constants are given in the text at 298 K unless specified). Addition of NO producing stable 1- and 2-C4H9NO molecules is more than 2 orders of magnitude slower than that of O2 and was not important. Formation of n-C4H9O2 and sec-C4H9O2 peroxy radicals gives rise to two parallel sequences of reactions

2. EXPERIMENTAL METHODS Chemical Reactor. Chemical reactions took place in the TFR coupled with a chemical ionization quadrupole mass spectrometer (Figure 1). The reactor was operated at room temperature (298 ± 2 K) and a pressure from P = 100 to 600 Torr (Reynolds number Re ≈ 4000 at 100 Torr and Re ≈ 12000 at 600 Torr).

n‐C4 H 9O2 + NO → n‐C4 H 9O + NO2

(n1a)

→ 1‐C4 H 9ONO2

(n1b)

n‐C4 H 9O + NO2 → n‐C4 H 9ONO2

(n4)

n‐C4 H 9O + O2 → C3H 7CHO + HO2

(n5)

n‐C4 H 9O + NO → n‐C4 H 9ONO → C3H 7CHO + HNO n‐C4 H 9O + M → HOC4 H8 + M

(n6a) (n6b) (n7)

and sec ‐C4 H 9O2 + NO → sec ‐C4 H 9O + NO2 → 2‐C4 H 9ONO2

Figure 1. Experimental setup: 1 , ion source; 2, ion−molecule reactor (IMR); 3, temperature controller; 4, turbulizer; 5, injector; 6, resistance; 7, cooling bath; 8, discharge tube; 9, microwave discharge; 10, sampling cones; 11, temperature sensor; 12, FeII(SO4) filter; 13, liquid nitrogen/ ethanol cold bath; 14, NO cylinder.

F + n‐C4 H10 → n‐C4 H 9 + HF → sec ‐C4 H 9 + HF

(s4)

sec ‐C4 H 9O + O2 → CH3CH 2C(O)CH3 + HO2

(s5)

→ CH3CH 2C(O)CH3 + HNO

(2b) (2b)

sec ‐C4 H 9O + M → C2H5 + CH3CHO

with k2 = 7.3 × 10−11 cm3 molecule−1 s−1 at 298 K19 and branching ratio k2a/k2b = 57:4320−22 was used to initiate the chemical transformations in the presence of O2 and NO. F atoms were generated by a microwave discharge in CF4 (AlphaGaz N45) in a quartz tube connected to the moveable injector. The carrier gas He (AlphaGaz 2) in the discharge tube was purified by

(s1b)

sec ‐C4 H 9O + NO2 → sec ‐C4 H 9ONO2

sec ‐C4 H 9O + NO → sec ‐C4 H 9ONO

Fast reaction of F atoms with butane

(s1a)

(s6a) (s6b) (s7)

In all the experiments, reaction 1 was completed within less than 0.1 ms, giving stable products NO2 (channel 1a) and 1- and 2butyl nitrates (channel 1b). Butoxy radicals produced in channel 1a transform to nitrites, n- and sec-C4H9ONO, butyraldehyde (BA), C 3 H 7 CHO, and methyl−ethyl ketone (MEK), CH3CH2C(O)CH3, through their reactions with NO (channels B



Article

results of the DFT-RRKM theoretical study of Mereau et al.,33 which gave kn7(760) = 1.6 × 105 s−1 and kn7(760)/ kn7∞ = 0.88. The curve was fitted over the 100−760 Torr range using a simple Lindemann’s approximation and scaled to give kn7∞ = 2.0 × 105 s−1. Reaction n7 competes with reaction with NO consuming up to 50% of n-butoxy radicals. Because the detection method used registrated total nitrate species (vide infra), attention must be paid to the possible contribution from the hydroxy nitrates that could be formed due to transformations of the isomerization product, HOC4H8 radical, in the presence of O2 and NO. Possible reactions of this radical were investigated by Morabito and Heicklen26 and Heiss and Sahetchian.34 By GC/MS detection of the final products, it was found that this δ-hydroxy butyl radical reacts with oxygen either through O2 addition or producing BA via hydrogen abstraction and rearrangement of the biradical formed

n6b and s6b, respectively). The contribution from the butoxy reactions with oxygen, reactions n5 and s5, was negligible because their rate constants, kn5 = 9.5 × 10−15 cm3 molecule−1 s−124−26 and ks5 = 7.6 × 10−15 cm3 molecule−1 s−1,24,27,28 are more than 3 orders of magnitude lower than that with NO, kn6 ≈ ks6 = 3.2 × 10−11 cm3 molecule−1 s−1.23,28,29 Under the conditions of our experiments, reactions 4, 5, and 6 are in their high-pressure limit region. The secondary reactions 4 of n- and sec-C4H9O butoxy radicals with NO2 also produce butyl nitrates with k4 = 3.6 × 10−11 cm3 molecule−1 s−1,29 which can interfere with the measurements of nitrates from reaction 1. NO2 was present in the reactor as a trace impurity in NO and a product from reaction 1a. Typically, the NO2 background in the presence of NO was less than 1 × 1012 molecules cm−3, so that the total concentration was on the order of (2−3) × 1012 molecules cm−3. Other important processes were unimolecular reactions of the butoxy radicals, isomerization for n-butoxy (reaction n7)26,30,31 and decomposition for sec-butoxy (reaction s7).28,30 Under the conditions of our study, both reactions are in the falloff region and both compete with NO reactions. Competing reactions 5, 6, and 7 served as butoxy radical scavengers with respect to reactions 4, making the contribution from reactions 4 to the nitrate concentration less than 1%, as was shown by computer simulation of the chemical system under study. To determine the yields of the stable products, ks7(P) was calculated using a conventional Troe’s expression with low- and high-pressure limit rate coefficients obtained by Falgayrac et al. from the falloff analysis of their LFP-LIF experimental results between 8 and 608 Torr.28 The calculated ks7 was scaled to give a value of ks7(760) = (2.13 ± 0.11) × 104 s−1 (error of 1 σ), which is the average of the six coefficients tabulated in the review by Atkinson et al.23 Under the conditions of our experiments, the fraction of the decomposed sec-butoxy radicals was less than 10%. The rate coefficient for isomerization of n-C4H9O is sufficiently higher, kn7(760) = 1.8 × 105 s−1, as obtained by averaging the results from the recent PLP-CRDS study of Sprague et al.31 and the studies referenced herein. It is necessary to note that the kn7 values were determined from the relative kn7/ kn5 measurements, assuming kn5 = 9.5 × 10−15 cm3 molecule−1 s−1 for the reference reaction with oxygen recommended by Atkinson,24 which seems to be more reliable than the more recent recommendation of 1.4 × 10−14 cm3 molecule−1 s−1,23 as the latter is based on the results of Morabito and Heicklen26 extracted from the relative kn5/kn6 measurements using an early generic kn6 value of 4.4 × 10−11 cm3 molecule−1 s−1, while new results give kn6 = 3.2 × 10−11 cm3 molecule−1 s−1.28 The influence of the additional complicating factor, prompt isomerization of the chemically activated n-butoxy radicals, was analyzed by Sprague et al.31 They found that the fraction of n-butoxy that promptly isomerized was 0.04 ± 0.02. In their study, n-butoxy was generated by photolysis of butyl nitrite when the n-C4H9O and NO fragments contained 40 kcal mol−1 of available energy. As well, production of “hot” n-butoxy was supposed to take place in the RO2 + NO reaction.30 The energy available for the products of reaction n1a is about 16 kcal mol−1 (vide infra), and the effect of prompt isomerization in this case is not expected to be more important. In the absence of experimental data for the pressure dependence of kn7, isomerization parameters were computed by us using the falloff curve from the ab initio-RRKM study of Somnitz and Zellner with inclusion of tunnelling.32 Their study showed that, though kn7(760) is close to its highpressure limit, it is still in the falloff region with kn7(760) = 1.4 × 105 s−1 and kn7(760)/kn7∞ = 0.89, in good agreement with the

HOC4 H8 + O2 → HOC4 H8O2 → HO2 + C3H 7CHO

(n8a) (n8b)

Morabito and Heicklen also proposed that reaction 8 competes with some fast HOC4H8 reactions (involving internal rearrangement or bond cleavage) leading to lower (C1−C2) aldehydes. Heiss and Sahetchian showed that the HOC4H8O2 adduct can undergo further isomerization, producing peroxide HOOC3H7CHO, as was identified by GC/MS at 487 K.34 However, the high activation energy (17.6 kcal mol−1 by evaluation) makes this second isomerization unimportant at room temperature. In the recent PLP-CRDS work with direct observation of HOC4H8 and HOC4H8O2 radicals in real time,31 Sprague et al. found that within 40 μs after photolysis of nbutylnitrite, the only significant products were the primary isomerization products, HOC4H8 in the absence of O2 and HOC4H8O2 with O2. This rules out the possibility of any HOC4H8 reactions competing with reaction with oxygen (reaction n8) because the characteristic reaction times for reaction 8 are expected to be on the order of 10 μs. Also, it was shown that all HOC4H8 were totally converted to HOC4H8OO peroxy, some part of which having an internal hydrogen bond. This allows one to neglect reaction n8b and to expect some hindrance of the HOC4H8OO reaction with NO compared to the title reaction due to hydrogen bonding HOC4 H8O2 + NO → HOC4 H8O + NO2 → HOC4 H8ONO2

(n9a) (n9b)

Estimations by computer simulation, assuming kn8a = kn3, kn9 = kn1, and similar branching for reactions n9 and n1, shows that the possible contribution from reaction n9b to the nitrate concentration is about 12%. Taking into account possible effects of the internal hydrogen bonding in the HOC4H8OO radical, this estimation can be considered as an upper limit. The simulation also showed that the contribution from ethyl nitrate due to transformations of C2H5 produced in the decomposition reaction s7 was less than 0.5%. Characteristic times for reactions 6 and 7 were on the order of 10−5 s, ensuring complete conversion of the butoxy radicals to the final stable products. Therefore, the branching ratio for nitrate formation, β = k1b/k1a, could be obtained by relating the measured nitrate concentration to the sum of the BA and MEK concentrations, taking into account the branching ratios of C



Article

NF3 was flowed into the IMR downstream of the ion source to generate F− ions through the dissociative electron attachment reaction. It was found that both F−-NICI spectra of C3H7CHO and C2H5C(O)CH3 consisted practically of a single peak at m/e 71, corresponding to the M−H− ion

reactions 1 and 6 and the loss of the butoxy radicals in reactions 7. The disproportionation−combination ratios for reactions n6 and s6 were obtained by the MS technique, giving the BA and MEK yields γn = kn6b/(kn6a + kn6b) = 0.29 ± 0.0535 and γs = ks6b/(ks6a + ks6b) = 0.21 ± 0.02.36 Thus, the effective branching ratio can be determined as k y·Δ[nitrate] Δ[nitrate] β = 1b = = k1a Δ[prod] Δ[butoxy]

F− + C4 H8O → C4 H 7O− + HF

These spectra confirm the previous results of Tiernan et al.39 using NF3−NICI to detect organic compounds. The major peaks in the F−-NICI mass spectrum of isobutyl nitrate acquired in the present study are m/e 46, 62, and 71 with relative intensities 78:20:2, independent of pressure. Commercially available isobutyl nitrate was chosen for calibration as similar spectra are expected for the three isomers of butyl nitrate (n-, sec-, and isobutyl nitrates) having a hydrogen atom in the β-positon with respect to the bridge oxygen atom of the NO3 group attacked by the F− ion. The following ion−molecule reactions can account for the ions observed (R = C4H9)

(E1)

where Δ[nitrate] is the sum of of n- and sec-butyl nitrate concentrations, Δ[prod] is the sum of concentrations of produced BA and MEK, and y = y(P,[NO]) is the yield factor indicating what fraction of butoxy radicals was converted into BA and MEK. The latter was calculated as follows y = yn + ys = φnγnηn + φγ η s s s

(E2)

where φn and φs are the yields of n-butyl and sec-butyl radicals in reaction 2; ηn = kn6[NO]/(kn6[NO] + kn7), and ηs = ks6[NO]/ (ks6[NO] + ks7). As the φ and γ yields do not depend on pressure and NO concentration, one can write yn = 0.165ηn and ys = 0.089ηs. Butyraldehyde, MEK (both Fluka, 99.9%), and isobutyl nitrate (Sigma-Alrdrich, 99.8%) used for calibration purposes were introduced into the reactor as preprepared ∼4% mixtures in He. Their concentrations were calculated from the rate of the pressure drop in a calibrated volume. NO2 was calibrated using commercial mixture (AlphaGaz, 0.5% in N2), whose flow rate was regulated by a Tylan controller. Detection System. Gas mixture from the TFR was sampled through a Teflon cone into the IMR. The pressure of the Ar carrier gas in the IMR was about 1 Torr. The primary Ar+ ions and electrons were generated in the ion source with a heated filament. The ions formed in the IMR entered the ion-optical zone through the 180 μm orifice in the nickel skimmer biased at a potential of several volts and presenting a first focusing element. After passing a quadrupole mass analyzer (EXTREL), the ions were registered using a Channeltron multiplier and a MTS-100 preamplifier. The signals were measured in ion counting mode in counts per second (cps) with averaging over the dwell time of 2 s. It was shown that alkyl nitrates and organic compounds such as aldehydes and ketones can be detected both in positive mode using the proton-transfer reaction (PTR)37 and in negative mode using CH4−NICI38 and NF3−NICI39 methods. Both methods were employed in our previous studies.16−18 In the present study, testing of the PTR method showed that the main peaks in the spectra of BA and MEK were “regular” C4H8OH+ and C4H8OH+· (H2O) peaks (masses 73 and 91), while the spectrum of isobutyl nitrate consisted of one intense C4H9+ peak (mass 57) with a minor C4H9O+ peak (mass 73). Similarity of the PTR spectra for n- and isobutyl nitrates was shown earlier.37 Unfortunately, the peak at mass 57 was also observed in the mass spectrum of nbutane. Though it is generally considered that alkanes cannot be detected by PTR, occurrence of the reaction H3O+ + n‐C4 H10 → C4 H 9+ + H 2O + H 2

(2i)

F− + RONO2 → NO2− + BA or MEK + HF

(3ia)

F− + RONO2 → NO3− + RF

(3ib)

F− + RONO2 → C3H 7CO− + HF + HNO2

(3ic)

The thermochemistry of analogous reactions for isopropyl nitrate was analyzed in our previous study.18 Monitoring nitrates using NO2− ions was not possible because of the interference with the NO2− ion formed in the reaction of the F− ion with nand sec-C4H9ONO from reactions n6a and s6a F− + RONO → NO2− + RF

(4i)

Thus, only observation of the mass 62 peak could be used for detection of butyl nitrates from reaction 1b. It is worth noting that the NF3−NICI method has a very low sensitivity to NO2 because the charge-transfer reaction with the F− ion F− + NO2 → NO2− + F −12

(5i) −1 −141

is slow (k5i ≤ 6 × 10 cm molecule s ), while reactions like 4i proceed through a single channel with k ≈ 3 × 10−9 cm3 molecule−1 s−1 for all nitrites containing β-hydrogens.42 Hence, the intensity of the peak at m/e 46 from the reaction system can be ascribed to the nitrites formed in reactions n6a and s6a with a small contribution from the nitrates, which could be easily taken into account. Relating the nitrate signal at m/e 62 to that of nitrites at m/e 46 gives an additional way to obtain pressure dependence. Moreover, in our study of ethyl nitrate formation using NF3−NICI17, it was found that the sensitivity to ethyl nitrate at m/e 62 was approximately the same as the sensitivity to ethyl nitrite at m/e 46, implying similar rates for reactions 3ib and 4i in the case of R = C2H5. The background NO2 concentration was determined using charge transfer from the SF6− ion 3

SF6− + NO2 → NO2− + SF6

(6i)

In this case, SF6 was introduced instead of NF3 into the IMR, where the agent SF6− ions were generated by electron attachment. During calibration, the signal intensities were linear up to 1 × 1013 molecules cm−3 of NO2, isoC4H9ONO2, C2H5COCH3, and C3H7CHO.

(1i)

with k1i ≈ 3 × 10−12 cm3 molecule−1 s−1 was reported.40 This reaction is relatively slow, but high n-butane concentrations used in our experiments made the PTR method inapplicable for detection of low butyl nitrate concentrations resulting from reaction 1b. An ionization scheme using primary F− ions (NF3−NICI) was used in this study for detection of stable products. In this scheme,

3. EXPERIMENTAL RESULTS Branching Ratio Measurements. The branching ratio of reaction 1 was determined between P = 100 and 600 Torr at T = D



Article

Table 1. Determination of the Branching Ratio β = k1b/k1a for the C4H9O2 + NO Reaction at 298 Ka P (Torr)

[C4H10] 1014

[NO] 1015

[O2] 1016

Δ[nitrate]b 1010

Δ[prod]c 1011

kn7 105 (s−1)

ks7 104 (s−1)

yn (%)

ys (%)

β (%)

100

4.5 ″ 4.8 ″ ″ ″ 6.0 5.9 ″ 6.3 ″ ″ 6.8 ″ ″

4.7 5.8 1.1 2.0 3.9 4.9 6.1 6.0 7.6 5.1 6.8 8.5 6.7 9.0 11.2

1.2 ″ 1.2 ″ ″ ″ 1.5 1.5 ″ 1.6 ″ ″ 1.7 ″ ″

7.92 8.26 8.25 6.42 5.64 5.60 4.61 5.18 4.77 3.41 3.04 2.70 5.21 6.28 6.30

4.28 4.81 1.93 2.30 2.80 2.77 1.82 1.51 1.61 0.904 0.814 0.720 0.915 1.22 1.28

1.03

0.94

1.37

1.31

1.54

1.55

1.64

1.74

1.76

1.97

7.80 8.54 2.45 4.19 6.18 6.95 7.16 7.13 7.92 6.25 7.24 8.01 6.80 7.98 8.73

7.77 7.87 5.80 6.87 7.91 8.08 7.64 7.63 7.76 7.54 7.73 7.86 7.56 7.73 7.83

2.88 ± 0.57 2.82 ± 0.56 3.55 ± 0.71 3.09 ± 0.61 2.84 ± 0.57 3.04 ± 0.61 3.76 ± 0.75 5.06 ± 1.01 4.68 ± 0.94 5.20 ± 1.04 5.59 ± 1.12 5.96 ± 1.19 8.18 ± 1.63 8.11 ± 1.62 8.12 ± 1.62

200

300

400

600

Concentrations are in units of molecules cm−3. bΔ[nitrate] is the sum of 1- and 2-butyl nitrate concentrations. cΔ[prod] is the sum of BA and MEK concentrations. Uncertainties (1σ) were estimated accounting for uncertainties in the kinetic parameters and precision of the measurements (vide infra). a

298 ± 2 K using the F−-NICI detection method to measure concentrations of the nitrate, BA, and MEK. The branching ratio defined in eq E1 was calculated using the following equation β=

nit y·(ΔI62/S62 ) y·Δ[nitrate] = corr eff Δ[prod] (ΔI71 /S71 )

(E3)

where ΔI62 and ΔI71 are the discharge on−off signal intensities at eff m/e 62 and m/e 71; Snit 62 and S71 are the apparatus sensitivity to isobutyl nitrate at m/e 62 and the effective sensitivity to BA and MEK at m/e 71, respectively. The total signal intensity at m/e 71, ΔI71, was corrected for the contribution from the nitrate nit determined from its calibration plot: ΔIcorr 71 = ΔI71• − ΔI62(S62 / −3 nit nit S71 ). The S62 in cps units/molecules cm was determined at different pressures from the intensity (cps) versus concentration (molecules cm−3) calibration plots for isobutyl nitrate. As mentioned above, similar sensitivities are expected for the n-, sec-, and isobutyl nitrates as the F− ion attacks the NO3 group, and all three isomers have β-hydrogens providing similar ion−molecule reactions. The effective sensitivity Seff 71 in the same units was calculated as a superposition of individual sensitivities, Seff 71= MEK (ynSBA 71 + ysS71 )/(yn + ys), determined from the calibration plots for BA and MEK. The yield factors yn and ys were defined above. Combining this expression with eqs E2 and E3, the desired BA branching ratio can be written as β = (ΔI62/ΔIcorr 71 )[(ynS71 + MEK nit ysS71 )/S62 ]. This expression is valid within an accuracy of the coefficient (1 − βn) for the first term and (1 − βs) for the second term, where βn and βs are the individual branching ratios βn = kn1b/kn1a and βs = ks1b/ks1a. Assuming equal ratios, this coefficient changes from 0.985 for 100 Torr to 0.96 for 600 Torrr. Because neither y factors nor sensitivities of BA and MEK differ essentially, this estimation shows the degree of overestimation of β values obtained at any β partitioning. The results are presented in Table 1. The plot of the branching ratio as a function of pressure, which is shown in Figure 2, was least-squares fitted with a linear dependence resulting in the equation

Figure 2. Pressure dependence of the branching ratio kb/ka for the sum of 1- and 2-butyl nitrate formation in the n-C4H10/F/O2/NO reaction system at 298 ± 2 K. The open triangle shows the result of Atkinson et al.10 for the C4H10/OH/O2/NO reaction system. The solid line is the linear fit. The dashed line represents the calculation using the empirical equation with the parametrization from ref 2, assuming equal branching ratios for primary and secondary nitrates.

1%, and extrapolation to atmospheric pressure gives β(760) = (9.8 ± 1.1)%, where the uncertainty corresponds to the standard deviation of the linear fitting. The uncertainty in β consists mainly of the uncertainties in kinetic data used to determine the yield factors for BA and MEK. The branching ratio of reaction 2 was taken as the average of the four k2a/k2b values measured using gas chromatography and giving k2a/k2b = 1.32 ± 0.03.20−22 The uncertainties in the γ coefficients for the branching ratio of reactions n6 and s6 are about 15 and 10%, respectively, so that the yield factors are yn = (0.165 ± 0.023)ηn and ys = (0.089 ± 0.009)ηs. The estimated accuracy of ηn and ηs is about 8%, giving the total uncertainty of about 20%. The uncertainties in β indicated in Table 1 also include experimental errors in nitrate, BA, and MEK calibration (∼5%). Additional errors in the determination of the branching ratio are connected with the possible contribution from the secondary reaction n8b producing δ-hydroxy-1-butyl nitrate.

β(P)(%) = (1.13 ± 0.10) × 10−2 · P(Torr) + (1.19 ± 0.35) (E4)

where the error limits are the standard deviations of the fitting. Extrapolation to zero pressure gives a nonzero intercept of about E



Article

ing to βn = 0.44βs in accord with the recommendation,2 are combined in proportion of our case, one obtains the total yield of 0.062 or β = 6.6%, which is substantially lower than our result β(735) = 9.5%. One can see that simple assumption of equal branching (βn = βs = 8.3%) gives better agreement. Moreover, an accurate solution of the system of two equations that satisfies both studies gives βn even higher than βs/βn = 10.5% and βs = 7.3%, or βn ≈ 1.3βsec. This contradicts the conclusion that nalkylperoxy radicals are approximately twice less efficient in producing nitrates than 2-alkylperoxy radicals.10−13 It is not the first time when such contradiction was observed. Much less difference for nitrate formation yields was obtained by Cassanelli et al.43 from n-pentyl and 2-pentyl radicals generated by photolysis of iodopentane precursors in an air−NO mixture, βn = 8.7% and βsec = 12%. The approximate relationships βpri ≈ 0.4βsec, based on the measurements of propyl and butyl nitrate isomers from propane and n-butane reactions with OH performed in the early study,10 and βter ≈ 0.3βsec for tertiary nitrates based on observations of isomers from the OH reactions with branched C 5 −C 6 alkylperoxy radicals11,13 were proposed by Carter and Atkinson as general ones.2,15 Apparently, the former was confirmed by the measurement of the C5 alkyl nitrate yield from neopentane, βpri = 5.4%,11 that could be compared to the 2- and 3-pentylnitrate yields from n-pentane10,12 or 2-methyl-3-butyl nitrate yield from 2-methylbutane,11 βsec ≈ 12−14%. The above scaling was proposed as a part of the empirical falloff equation, Y = Y(n,P,T), constructed for prediction of alkyl nitrate yields from the atmospheric reactions of secondary alkylperoxy radicals.2,15 For n = 4 (butylperoxy) and P = 735 Torr, this function gives βsec = 9.0%, which rather well agrees with our results. With allowance for a specific curvature, the full falloff curve assuming βn = βs agrees with our results as presented in Figure 2. It seems that by considering formation of nitrates, it is necessary to distinguish between linear (βn) and branched primary peroxy radicals. It is also interesting to compare the results obtained for butyl nitrate with the data available for other C1−C5 systems (Figure 4 and Table 2). Figure 4 shows that for C3 propyl nitrate

The upper limit for this contribution is about 12% as estimated by computer simulation (vide supra). Figure 3 shows the pressure dependence of the branching ratios calculated by relating the nitrate signal to the signal of

Figure 3. Comparison of the branching ratios obtained by relating the nitrate signal to the signal at m/e 46 (◇) (not calibrated nitrites) and m/e 71 (■) (calibrated BA and MEK products).

nitrites n-C4H9ONO and sec-C4H9ONO from reactions n6a and s6a, respectively β ONO =

k1b yONO Δ[nitrate] Δ[nitrate] = = k1a Δ[butoxy] Δ[nitrite]

(E5)

where y = φn(1 − γn)ηn + φs(1 − γs)ηs is the yield factor for nitrites. In the Detection System section, it was shown that the signal at m/e 46 can be attributed to nitrites, so that βONO = corr nit nit yONO(ΔI62/ΔIcorr 46 ), where ΔI46 = ΔI46 − ΔI62(S62 /S46 ), where nit nit S62 and S46 are the apparatus sensitivities to butyl nitrate at m/e 62 and m/e 46, respectively. The points presented in Figure 3 correspond to the maximal NO concentration for each pressure. The obtained values appeared to be very close to the average branching ratios obtained by normalization using BA and MEK stable products. It means that the apparatus sensitivity to butyl nitrites, SONO 46 , is approximately equal to the sensitivity to butyl nitrates Snit 62 , similarly to ethyl nitrite and ethyl nitrate. Although being in agreement with calibrated data, the normalization by nitrites was carried out only to check a general shape of the pressure dependence and was not taken into account in the further discussion. Comparison with Previous Studies. In the chamber study of Atkinson et al.10 using gas chromatography with flame ionization detection (GC-FID), the value of α = kb/(ka + kb) = (7.7 ± 0.9)% (β = kb/ka = 8.3%) was determined for the total butyl nitrate yield from the OH-initiated oxidation of n-butane in air at T = 299 K and 735 Torr of pressure. The total yield consisted of the mixture of 1- and 2-butyl nitrates with concentration ratio [1-C4H9ONO2]/ [2-C4H9ONO2] = 0.07 ± 0.21. The individual nitrate isomer yields for n-C4H9O2 + NO and sec-C4H9O2 + NO reactions derived from the data of that study were given in ref 11 as ≤4 and (9.0 ± 0.9)%, respectively. Extrapolation of our results to 735 Torr gives β(735) = (9.5 ± 1.1)%. However, having so significant of a difference for the yields from n- and sec-isomers, our results cannot be compared directly because they correspond to different ratios of isomer concentrations; OH reaction with n-butane gives 17% of n-butyl and 83% of sec-butyl radicals,10 while F reaction gives 57% and 43%, respectively. When the above individual yields, correspondONO

Figure 4. Comparison of the branching ratios kb/ka of alkyl nitrate formation in RO2 + NO reactions for small alkylperoxy radicals: ▽, ref 14; ■, this work; ●, ref 18; ▲, ref 17; ◆, ref 16; atmospheric pressure data for propyl nitrate (○) and butyl nitrate (□) are from ref 10, and those for n-pentyl (Δ) nitrate are from ref 43. The open circle at 100 Torr is the branching ratio for isopropyl nitrate from ref 44. F



Article

agreement between all of the data, though the result of Cassanelli et al.43 for 2-pentyl nitrate appears to be substantially lower than this estimation. Taking into account rather large experimental errors in the determination of the nitrate yields in all of the experiments, it can be concluded that (1) the empirical function somewhat overestimates β = kb/ka for C1−C5 secondary alkyl radicals and (2) the branching ratios of the reactions of primary and secondary alkylperoxy radicals are not very different. These results are now analyzed in the Theoretical Examination section.

Table 2. Comparison of the Measured Branching Ratios at Room Temperature and 740 Torr for Nitrate Formation in RO2 + NO Reactions with Calculated Ones Using the Scaled Empirical Function (β = kb/ka = Y) from Reference 2 alkyl (Y) C1 (0.0098) C2 (0.023) C3 (0.049)

C4 (0.089)

C5 (0.144)

βexp

R methyl ethyl 0.05 n-propyl + 0.95 isopropyl 0.31 n-propyl + 0.69 isopropyl 0.57 n-butyl + 0.43 sec-butyl 0.17 n-butyl + 0.83 sec-butyl n-pentyl 2-pentyl 0.08 n-pentyl + 0.92 (2+3)pentyl 0.08 n-pentyl + 0.92 (2+3)pentyl

ref

βcalca

scaling

0.0105 ± 0.0013

16

0.012

1.07b

0.032 ± 0.004 0.039 ± 0.002

17 18

0.028 0.040

1.39b 0.80

0.037 ± 0.005

10

0.045

0.76

0.095 ± 0.011

0.091

1.07

0.077 ± 0.010

this work 10

0.078

0.87

0.087 ± 0.021 0.120 ± 0.022 0.106 ± 0.008

43 43 14

0.172 0.120 0.117

0.60b 0.83b 0.74

0.122 ± 0.017

10

0.117

0.85

4. THEORETICAL EXAMINATION In order to discuss the Experimental Results, and specifically the relation of the branching ratio β = kb/ka with the structure of the RO2, the mechanism of the RO2 + NO reactions was analyzed on the basis of quantum chemical calculations of the PES for C1−C5 systems. The PESs of the RO2 + NO reactions have been explored using a DFT approach with the B3LYP functional and the aug-ccpVDZ basis set45 As explained in ref 46 (and references therein), the biradical character of the transition states has been taken into account properly by mixing HOMO and LUMO orbitals. Minima and transition states were fully optimized and characterized by harmonic vibrational frequency analysis. The zero-point vibrational corrected energies (ZPVEs) of the relevant points on the PESs of different C1−C5 reactants are presented in Table 3. It can be observed that the general trend of energy variation along the reaction paths is similar regardless of the system studied. Figure 5 depicts the corresponding energy diagram where the energies correspond to the n-C4H9O2 + NO reaction. As was found earlier (e.g., refs 7−9), the reaction proceeds first by the barrierless formation of two ROONO intermediate complexes, denoted ROONOcisperp and ROONOtransperp. These notations mean that the RO bond is almost perpendicular to the OONO plane, the four-atom OONO presenting either a cis or a trans configuration with respect to the mid ON bond. The two ROONO conformers are connected to the fragments RO + NO2 via two transition states TS1cis and TS1trans (ROO··· NO structure). As can be seen in Figure 5 and Table 3, only TS1cis is below the energy of the entrance valley RO2 + NO. Then, at room temperature, TS1trans is expected to play a minor role in the evolution of the reactive process, and it was neglected in this work. After having crossed the TS1cis barrier, the activated RO··· ONO systems come out into an almost flat valley of the PES

Calculation using βn = 1.2Y for linear alkyls and βs = 0.8Y for secondary alkyls. bScaling factors βexp/Y for “pure” cases devoid of alkyl isomers.

a

extrapolation of our data to 740 Torr gives β = 3.9%, practically coinciding with the result of Atkinson et al.,10 β = 3.7% (total nitrate yield of 3.6%). Similarly to n- and sec-butyl nitrates, these close values correspond to very different ratios of the initial n- to isopropyl radical concentrations, 5:95 in our experiments (initiation by H + propene reaction) and 31:69 in the chamber study (initiation by OH + n-propane reaction). It is clear that nearly equal total nitrate yields can be obtained only if individual yields are close values. Proceeding as in the case of n-butane, one finds that the results of both studies are consistent with βn = 3.4% and βs = 3.9% or βn ≈ 0.9βsec. Considering the application of the empirical falloff expression with the most recent parametrization2 to the formation of small C1−C5 alkyl nitrates, we found that the 298 K atmospheric pressure values can be fitted applying the scaling coefficient of ∼0.8 for secondary and ∼1.2 for primary alkyl radicals. Table 2 contains the experimental branching ratios for P = 740 Torr data, which are compared with the calculation using the abovementioned coefficients. Such approximation gives satisfactory

Table 3. Energies of Stationary Points (in kcal/mol) on the PES for RO2 + NO Reactionsa R

Primary Carbon CH3 C2H5 n-C3H7 n-C4H9 i-C4H9 n-C5H11 Secondary Carbon i-C3H7 sec-C4H9 Tertiary Carbon tert-C4H9 a

ΔE0 ROONO transperp

ΔE0 rxn RONO2

ΔE#0 cis RO···ONO

ΔE#0 trans RO···ONO

ΔE0 rxn RO + NO2

−19.15 −19.08 −19.13 −19.13 −18.36 −19.11

−17.92 −17.84 −17.87 −17.88 −17.11 −17.85

−49.69 −48.23 −48.41 −48.43 −47.83 −48.93

−10.30 −9.31 −9.60 −9.66 −8.85 −9.63

4.20 5.20 4.80 4.66 5.82 4.67

−15.20 −15.31 −15.66 −15.58 −15.64 −15.58

−18.39 −23.63

−17.02 −16.91

−47.30 −44.84

−8.88 −9.70

5.54 4.78

−12.83 −14.22

−17.87

−16.49

−44.00

−8.27

6.10

−12.45

ΔE0 ROONO cisperp

Data related to n- and sec-butyl nitrates are displayed in bold characters. G



Article

analogous molecular systems considered here. Then, the information concerning the discrimination between all of the compounds is now focused on the density of states ρ(E). The energy domain corresponding to the dissociation of a nitrate into RO + NO2 is between 35 and 50 kcal mol−1 (roughly 12000−17000 cm−1). Figure 6 gives the variation of the density

Figure 5. General correlation diagram for RO2 + NO reactions. Energy values displayed are ZPVE corrected energies in kcal/mol for the nC4H9O2 + NO reaction. The point noted W does not correspond to a precise minimum but indicates a rather flat region of the PES connected (i) to TS1cis, (ii) to the fragments RO + NO2, and (iii) to RO−NO2 (see text). The energy associated with this W region is slightly below the RO + NO2 one.

(zone noted W in Figure 5; also see ref 46), leading mainly to the dissociation into RO + NO2. This almost flat region (energetically slightly below the products RO + NO2) favors a roaming motion of RO and ONO parts. In some cases, when the relative orientation of the RO and ONO fragments is favorable to the formation of activated RO···NO2 structures, RONO2 molecules can then be formed via collisional stabilization. These results agree with the trajectory study of Chen et al.47 for R = H. As a matter of fact, their calculations show that once HO···ONO begins to dissociate, a few trajectories undergo a rotation of OH, causing the incipient fragments to return and form an HO−NO2 structure. Therefore, to explain the nitrate formation (described through the β = kb/ka ratio), we have been looking for a simple qualitative collisional model that can be used for a large family of RO···NO2 structures. The β = kb/ka ratio depends essentially on the relative efficiency of the collisional deactivation of the excited RO···ONO leading to the RONO2 formation. This β ratio is expected to depend upon several parameters, (i) the frequency z of the collisions per pressure unit, (ii) the total pressure P, (iii) the fraction Ω of a solid angle corresponding to a favorable orientation of RO with respect to NO2 for the creation of a bonding between these two entities (the oxygen atom of RO should be oriented toward the nitrogen atom of NO2), and (iv) a Ceff term that we call the “efficiency coefficient”, which is expected to be mainly a function of two factors, (a) the probability P(E,ΔE) to remove a given amount ΔE of energy from excited RO···NO2 during the collision event at the energy E and (b) the density of states ρ(E) at this energy level E. The term z mainly depends, at a given temperature, on the ratio d2/√μ, where d is the “diameter” of the nitrate molecule and μ the reduced mass of the nitrate/air system. Then, because z is a slightly increasing function of the molecular mass of RONO2, the collision frequency, z × P, increases with P and becomes larger for larger ROONO molecules, in agreement with experimental observations. For the fraction Ω, we estimate that its value is in the range of 10−20%, independent of the R structure. Then, the efficiency of the nitrate formation should be mainly influenced by the R structure via the efficiency coefficient Ceff, a function of P(E,ΔE) and ρ(E). For P(E,ΔE), we assume that it does not vary significantly for different molecules within the family of

Figure 6. BS density of states of C0−C5 linear aliphatic nitrates versus energy.

of states calculated using the Beyer−Swinehart (BS) method48 for a set of linear aliphatic nitrates from HO−NO2 to C5H11O− NO2 with the energy E over this range. Figure 7 shows, for the same domain, the variation of the density of states ρ(E) of the four possible butyl nitrate isomers.

Figure 7. BS density of states for the four isomers of butyl nitrate. The four schemes associated with the curves represent the aliphatic radical part R of each nitrate. The dot represents the electron bounded to the O−NO2 radical.

Because ρ(E) is always an increasing function of the number of carbon atoms, we may expect higher efficiency of the alkyl nitrate formation for larger peroxy radicals RO2. For the isomers of C4H9O−NO2 the density of states of n-nitrate (linear) is larger than that of the branched one for any value of energy. Similar behavior is also observed for propyl and pentyl nitrates. This can be explained by the fact that a branched system involves larger H



Article

Alkyl Nitrates in the Troposphere over the Pacific Ocean during PEMTropics A and B: Oceanic and Continental Sources. J. Geophys. Res. 2003, 108, 8242. (4) Ballschmiter, K. A Marine Source for Alkyl Nitrates. Science 2002, 297, 1127−1128. (5) Dibble, T. S. Failures and Limitations of Quantum Chemistry for Two Key Problems in the Atmospheric Chemistry of Peroxy Radicals. Atmos. Environ. 2008, 42, 5837−5848. (6) Stimac, P. J.; Barker, J. R. Non-RRKM Dynamics in the CH3O2 + NO Reaction System. J. Phys. Chem. A 2008, 112, 2553−2562. (7) (a) Barker, J. R.; Lohr, L. L.; Shroll, R. M.; Reading, S. Modeling the Organic Nitrate Yields in the Reaction of Alkyl Peroxy Radicals with Nitric Oxide. 2. Reaction Simulations. J. Phys. Chem. A 2003, 107, 7434− 7444. (b) Lohr, L. L.; Barker, J. R.; Shroll, R. M. Modeling the Organic Nitrate Yields in the Reaction of Alkyl Peroxy Radicals with Nitric Oxide. 1. Electronic Structure Calculations and Thermochemistry. J. Phys. Chem. A 2003, 107, 7429−7433. (8) Zhang, J.; Dransfield, T.; Donahue, N. M. On the Mechanism for Nitrate Formation via the Peroxy Radical + NO Reaction. J. Phys. Chem. A 2004, 108, 9082−9095. (9) Zhao, Y.; Houk, K. N.; Olson, L. P. Mechanisms of Peroxynitrous Acid and Methyl Peroxynitrite, ROONO (R = H, Me), Rearrangements: A Conformation-Dependent Homolytic Dissociation. J. Phys. Chem. A 2004, 108, 5864−5871. (10) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. Alkyl Nitrate Formation from the NOx−Air Photooxidations of C2−C8 n-Alkanes. J. Phys. Chem. 1982, 86, 4563−4569. (11) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. Formation of Alkyl Nitrates from the Reaction of Branched and Cyclic Alkyl Peroxy Radicals with NO. Int. J. Chem. Kinet. 1984, 16, 1085−1101. (12) Atkinson, R.; Carter, W. P. L.; Winer, A. M. Effects of Temperature and Pressure on Alkyl Nitrate Yields in the NOx Photooxidations of n-Pentane and n-Heptane. J. Phys. Chem. 1983, 87, 2012−2018. (13) Atkinson, R.; Aschmann, S. M.; Winer, A. M. Alkyl Nitrate Formation from the Reaction of a Series of Branched RO2 Radicals with NO as a Function of Temperature and Pressure. J. Atmos. Chem. 1987, 5, 91−102. (14) Aschmann, S. M.; Long, W. D.; Atkinson, R. Pressure Dependence of Pentyl Nitrate Formation from the OH RadicalInitiated Reaction of n-Pentane in the Presence of NO. J. Phys. Chem. A 2006, 110, 6617−6622. (15) Carter, W. P. L.; Atkinson, R. Alkyl Nitrate Formation from the Atmospheric Photooxidation of Alkanes; A Revised Estimation Method. J. Atmos. Chem. 1989, 8, 165−173. (16) Butkovskaya, N.; Kukui, A.; Le Bras, G. Pressure and Temperature Dependence of Methyl Nitrate Formation in the CH3O2 + NO Reaction. J. Phys. Chem. A 2012, 116, 5972−5980. (17) Butkovskaya, N.; Kukui, A.; Le Bras, G. Pressure and Temperature Dependence of Ethyl Nitrate Formation in the C2H5O2 + NO Reaction. J. Phys. Chem. A 2010, 114, 956−964. (18) Butkovskaya, N.; Kukui, A.; Le Bras, G. Pressure Dependence of Iso-Propyl Nitrate Formation in the i-C3H7O2 + NO Reaction. Z. Phys. Chem. 2010, 224, 1025−1038. (19) Pearson, R. K.; Cowles, J. O.; Hermann, G. L.; Gregg, D. W.; Creighton, J. R. Relative Perfomance of a Variety of NF3 + HydrogenDonor Transverse-Discharge HF Chemical-Laser Systems. IEEE J. Quantum Electron. 1973, 9, 879−889. (20) Fettis, G. C.; Knox, J. H.; Trotman-Dickenson, A. F. The Reactions of Fluorine Atoms with Alkanes. J. Chem. Soc. 1960, 1064− 1071. (21) Foon, R.; McAskill, N. A. Kinetics of Gas-Phase Fluorination of Halomethanes. Trans. Faraday Soc. 1969, 65, 3005−3012. (22) Foon, R.; Reid, G. P. Kinetics of the Gas Phase Fluorination of Hydrogen and Alkanes. Trans. Faraday Soc. 1971, 67, 3513−3520. (23) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J. Evaluated Kinetic

internal interactions than a linear one. Such internal effects increase the curvatures of the PES, leading to larger gaps between the energy levels and, consequently, to a smaller number of states in branched systems. All of these conclusions are in qualitative agreement with experimental findings; β increases as long as the pressure P and the size of the RO2 radical increase. Even if the density of states is expected to play an important role, the comparison between experimental β values corresponding to linear and branched alkyl radicals is totally convincing. Then, it is difficult so far to give definite conclusions for several reasons; (i) for a given family of alkyl radicals R (linear and branched), experimentally available β values are close together with relatively large uncertainties, (ii) the data on which these comparisons can be done are rather sparse, and (iii) our model presented here focuses only on the parameter density of states, but the role of other parameters (so far not fully investigated) can be nonnegligible.

5. CONCLUSIONS The summary yield of 1- and 2-butyl nitrates in the gas-phase reactions of NO with n-C4H9O2 and sec-C4H9O2 obtained from the reaction of F atoms with n-butane was found to increase linearly with pressure from about 3 to about 8% over the pressure range of 100−600 Torr at 298 K. The results agree with the only available data from the study with OH radical initiation,10 assuming approximately equal rates of nitrate formation for both isomers. The yield of primary and secondary butyl nitrate was estimated to be 10.5 and 7.3%, respectively. The experimental findings are in general agreement with performed DFT(B3LYP/aug-cc-pVDZ) calculations and the proposed mechanism. The calculations confirm experimental results about the higher efficiency of the nitrate formation for larger nitrates and positive pressure dependence of the nitrate yield. The calculations also predict a lower density of energy states for the ramified than for linear excited ROONO structures, leading to the nitrate formation via their collisional stabilization. On the basis of this result, it is predicted that the efficiency of the nitrate formation for the branched RO2 should be lower than that for linear RO2 radicals.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ⊥

N.I.B.: Institute of Chemical Physics of the Russian Academy of Sciences, Moscow, Russian Federation. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been supported by the French Ministry of Research through the ANR program (ONCEM Project No. ANR-12-BS06-0017-01).

■

REFERENCES

(1) Perring, A. E.; Pusede, S. E.; Cohen, R. C. An Observational Perspective on the Atmospheric Impact of Alkyl and Multifunctional Nitrates on Ozone and Secondary Organic Aerosol. Chem. Rev. 2013, 113, 5848−5870. (2) Atkinson, R. Gas-Phase Tropospheric Chemistry of Organic Compounds. Phys. Chem. Ref. Data 1994, Monograph No. 2, 1−216. (3) Blake, N. J.; Blake, D. R.; Swanson, A. L.; Atlas, E.; Flocke, F.; Rowland, F. S. Latitudinal, Vertical, and Seasonal Variations of C1−C4 I



Article

and Photochemical Data for Atmospheric Chemistry. Atmos. Chem. Phys. 2006, 6, 3625−4055. (24) Atkinson, R. Atmospheric Reactions of Alcoxy and βHydroxyalcoxy Radicals. Int. J. Chem. Kinet. 1997, 29, 99−111. (25) Hoffmann, A.; Mörs, V.; Zellner, R. A Novel Laser-Based Technique for the Time-Resolved Study of Integrated Hydrocarbon Oxidation Mechanisms. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 437− 440. (26) Morabito, P.; Heicklen, J. The Reactions of Alcoxyl Radicals with O2. IV. n-C4H9O Radicals. Bull. Chem. Soc. Jpn. 1987, 60, 2641−2650. (27) Deng, W.; Davis, A. J.; Zhang, L.; Katz, D. R.; Dibble, T. S. Direct Kinetic Studies of Reactions of 3-Pentoxy Radicals with NO and O2. J. Phys. Chem. Phys. A 2001, 105, 8985−8990. (28) Falgayrac, G.; Caralp, F.; Sokolowski-Gomez, N.; Devolder, P.; Fittschen, C. Rate Constants for the Decomposition of 2-Butoxy Radicals and Their Reaction with NO and O2. Phys. Chem. Chem. Phys. 2004, 6, 4127−4132. (29) Lotz, C.; Zellner, R. Fluorescence Excitation Spectrum of the 2Butoxy Radical and Kinetics of its Reactions with NO and NO2. Phys. Chem. Chem. Phys. 2001, 3, 2607−2613. (30) Cassanelli, P.; Johnson, D.; Cox, R. A. A Temperature-Dependent Relative-Rate Study of the OH Initiated Oxidation of n-Butane: The Kinetics of the Reactions of the 1- and 2-Butoxy Radicals. Phys. Chem. Chem. Phys. 2005, 7, 3702−3710. (31) Sprague, M.; Garland, E. R.; Mollner, A. K.; Bloss, C.; Bien, B. D.; Weichman, M. L.; Mertens, L. A.; Okumura, M.; Sander, S. P. Kinetics of n-Butoxy and 2-Pentoxy Isomerization and Detection of Primary Products by Infrared Cavity Ringdown Spectroscopy. J. Phys. Chem. A 2012, 116, 6327−6340. (32) Somnitz, H.; Zellner, R. Kinetics and Dynamics of Multi-Channel Unimolecular Reactions of Alcoxyl Radicals Over an Extended Range of Temperature and Pressure. A Combined Quantum Chemical/RRKM Dynamical Study. Z. Phys. Chem. 2006, 220, 1029−1048. (33) Mereau, R.; Rayez, M.-T.; Caralp, F.; Rayez, J.-C. Isomerization Reactions of Alkoxy Radicals: Theoretical Study and Structure−Activity Relationships. Phys. Chem. Chem. Phys. 2003, 5, 4828−4833. (34) Heiss, A.; Sahetchian, K. Isomerization Reactions of the n-C4H9O and n-OOC4H8OH Radicals in Oxygen. Int. J. Chem. Kinet. 1996, 28, 531−544. (35) Morabito, P.; Heicklen, J. Disproportionation to Combination Ratios of Alcoxy Radicals with Nitric Oxide. J. Phys. Chem. 1985, 89, 2914−2916. (36) Walker, R. F.; Phillips, L. The Kinetics of Disproportionation− Combination Reactions between the s-Butoxy Radical and Nitric Oxide. J. Chem. Soc. London, Ser. A 1968, 2103−2106. (37) Aoki, N.; Inomata, S.; Tanimoto, H. Detection of C1−C5 Alkyl Nitrates by Proton Transfer Reaction Time-of-Flight Mass Spectrometry. Int. J. Mass Spectrom. 2007, 263, 12−21. (38) Sato, K.; Tanimoto, H.; Imamura, T. Negative Ion Chemical Ionization Mass Spectra of C1-C6 n-Alkyl Nitrates. Chem. Lett. 2005, 34, 1200−1201. (39) Tiernan, T. O.; Chang, C.; Cheng, C. C. Formation and Reactions of Negative Ions Relevant to Chemical Ionization Mass Spectrometry. I. CI Mass Spectra of Organic Compounds Produced by F− Reactions. Environ. Health Perspect. 1980, 36, 47−62. (40) Anicich, V. G. An Index of the Literature for Bimolecular Gas Phase Cation-Molecule Reaction Kinetics, Publication 03-19; Jet Propulsion Laboratory: Pasadena, CA, 2003. (41) Ikezoe, I.; Matsuoka, S.; Takebe, M.; Viggiano, A. Gas Phase Ion− Molecule Reaction Rate Constants through 1986; Maruzen Company: Tokyo, 1987. (42) King, G. K.; Maricq, M. M.; Bierbaum, V. M.; DePuy, C. H. GasPhase Reactions of Negative Ions with Alkyl Nitrites. J. Am. Chem. Soc. 1981, 103, 7133−7140. (43) Cassanelli, P.; Fox, D. J.; Cox, R. A. Temperature Dependence of Pentyl Nitrate Formation from the Reaction of Pentyl Peroxy Radicals with NO. Phys. Chem. Chem. Phys. 2007, 9, 4332−4337. (44) Chow, J. M.; Miller, A. M.; Elrod, M. J. Kinetics of the C3H7O2 + NO Reaction: Temperature Dependence of the Overall Rate Constant

and the i-C3H7ONO2 Branching Channel. J. Phys. Chem. A 2003, 107, 3040−3047. (45) Butkovskaya, N.; Rayez, M.-T.; Rayez, J.-C.; Kukui, A.; Le Bras, G. Water Vapor Effect on the HNO3 Yield in the HO2 + NO Reaction: Experimental and Theoretical Evidence. J. Phys. Chem. A 2009, 113, 11327−11342. (46) Dunning, T. H., Jr. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (47) Chen, C.; Shepler, B. C.; Braams, B. J.; Bowman, J. M. Quasiclassical Trajectory Calculations of the HO2 + NO Reaction on a Global Potential Energy Surface. Phys. Chem. Chem. Phys. 2009, 11, 4722−4727. (48) Beyer, T.; Swinehart, D. F. Commun. ACM 1973, 16, 379.

J


Pressure Dependence of Butyl Nitrate Formation in ... - ACS Publications

Recommend Documents