Gas-phase rate constants for the reaction of NO3 radicals with a series of cyclic alkenes, 2-ethyl-1-butene and 2,3-dimethyl-1,3-butadiene
Introduction
The nitrate radical is well known as an important oxidant in the troposphere [1]. NO3 is mainly produced by the reaction of O3 with NO2 and, as it is photolysed by sunlight, it can act as a radical initiating degradation processes only at night. An important area of the NO3 radical chemistry is the reaction kinetics as well as investigations concerning product distributions from reactions with natural organic compounds such as monoterpenes. Terpenes belong to the most naturally emitted hydrocarbons in the troposphere with a global averaged emission of 550 Tg a−1[2]. The comparison of the tropospheric lifetimes, for example, α-pinene: τOH=5.2 h, τO3=4.3 h, τNO3=11 min and β-pinene: τOH=3.6 h, τO3=20 h, τNO3=27 min (calculated with the assumption [OH]=106 molecule cm−3, [O3]=7.5×1011 molecule cm−3 and [NO3]=2.5×108 molecule cm−3[3]), shows that the night-time reaction of NO3 radicals with terpenes is an important sink for these substances.
To facilitate the study of the reactions of NO3 radicals with terpenes, the use of simple model compounds containing the same reactive structural element as the terpenes themselves seems to be appropriate. For the most important terpenes the 1-methylcyclohexene structure (e.g., α-pinene, limonene and Δ3-carene) or the methylenecyclohexane structure (e.g., β-pinene and camphene) are present and therefore both these substances or their simple derivatives can serve as model substances. Similar chemical behaviour is assumed.
With the knowledge of the rate constants for a sufficient number of substances, a correlation of these rate constants with molecular data is possible. This approach allows a prediction of rate constants. Sabljic and Güsten [4]showed that this concept is also feasible for rate constants of the reaction of the NO3 radicals with organic compounds using experimentally obtained ionization energies as the molecular data. Aliphatic and aromatic compounds were discussed separately. For bromine- and chlorine-containing species Sabljic and Güsten found a systematical deviation from their model [4].
In the present study, gas-phase rate constants for the reaction of NO3 radicals with a series of cyclic alkenes have been obtained in order to find out a correlation especially for this class of organic compounds. Thus, a more detailed description of the reactivity of NO3 radicals toward cyclic alkenes should be possible.
Section snippets
Experimental
The experiments were carried out in a 2.13 cm i.d. flow-tube at T=298±2 K at a total pressure of 6.8 mbar in helium. The total pressure was measured by means of a Baratron gauge near the tube entrance and regulated using a butterfly valve (VAT) between the flow-tube and pump. Gas flows were set by mass flow controllers (MKS) in a range from 2.8 to 4.8 ms−1.
In a side arm, NO3 radicals were formed by thermal decomposition of N2O5 at ∼400 K using a thermal pre-reactor.N2O5 was
Results and discussion
In all cases the rate constant ratio k2/k3 was obtained from the slope of the plot according Eq. (I)by linear regression and is given with an error of two standard deviations. The rate constant k2 was derived from this ratio using the recommendation of the corresponding rate constant k3 for the reference substance from Ref. [12]: 1-butene, 1.25×10−14; trans-butene, 3.9×10−13; 2-methyl-2-butene, 9.37×10−12; and 2,3-dimethyl-2-butene, 5.72×10−11 cm3 molecule−1 s−1. The error of the rate constant k
Unsubstituted cyclic alkenes
Investigations have been performed for cyclopentene, -hexene and -heptene using trans-butene in each case as the reference substance. For a demonstration of the accuracy of our approach, the plots ln([reactant]0/[reactant]t) vs. ln([trans-butene]0/[trans-butene]t) are shown in Fig. 1.
A reasonable linearity was found, axis intercepts were slightly positive but within two standard deviations from zero. It can be seen that the slopes and consequently the rate constants increase slightly with
Substituted cyclic alkenes
Comprehensive investigations of the rate constant for the reaction of NO3 radicals with this class of organic compounds are only available in the literature for α-pinene. For the dimethyl-substituted cyclohexenes rate constants are presented here. A compilation of our results and literature data is given in Table 2.
From the series cyclohexene (see Table 1), 1-methylcyclohexene and 1,2-dimethyl-cyclohexene it is clearly shown that with increasing number of alkyl groups around the double bond the
Substances with an exocyclic double bond
In Table 3 our results as well as the literature data for β-pinene are summarized.
With exception of β-pinene all rate constants with respect to the NO3 radical reaction represent first determinations. Disregarding methylenecyclopentane in the series of methylenecycloalkane, the rate constants increase with increasing ring size. In this sense, for methylenecyclopentane an unexpectedly high value of the rate constant was observed. For the corresponding reaction of O(3P) as well as dichlorocarbene
Other
For further considerations, the rate constant for the reaction of NO3 radicals with 2-ethyl-1-butene was obtained to compare with the rate constants found for substances with an exocyclic double bond. Furthermore, the rate constant for the reaction of NO3 radicals with 2,3-dimethyl-1,3-butadiene was also measured to check the influence of a second conjugated double bond. The results are listed in Table 4.
The rate constant for 2-ethyl-1-butene was obtained for the first time. For
Correlation of the rate constants with EHOMO
In contrast to the use of experimentally obtained ionization energies (Ei) as demonstrated by Sabljic and Güsten [4], a more convenient way is to derive Ei from calculated HOMO energies (EHOMO). According to Koopman's theorem, EHOMO is equal in magnitude and opposite in sign to Ei. Using EHOMO for the correlation with obtained rate constants, as well for a series of NO3 radical reactions with pure or substituted alkenes this approach was successfully applied 24, 25, 26. Marston et al. [24]found
Acknowledgements
The financial support of “Deutsche Forschungsgemeinschaft” is gratefully acknowledged. The authors wish to thank Dr. C.E. Canosa-Mas for helpful discussions.
References (29)
- et al.
Atmos. Environ.
(1991) - et al.
Atmos. Environ.
(1990) - et al.
Atmos. Environ.
(1990) - et al.
Chem. Phys. Lett.
(1992) - et al.
Chem. Phys. Lett.
(1988) - et al.
Geophys. Res. Lett.
(1978) - et al.
J. Chem. Phys.
(1981) - et al.
J. Chem. Phys.
(1978) - Organikum, VEB Deutscher Verlag der Wissenschaften, Berlin,...
- et al.
J. Am. Chem. Soc.
(1954)