Article pubs.acs.org/JPCA
Detection and Characterization of Products from Photodissociation of XCH2CH2ONO (X = F, Cl, Br, OH) Rabi Chhantyal-Pun, Ming-Wei Chen, Dianping Sun,† and Terry A. Miller* Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus Ohio 43210, United States S Supporting Information *
ABSTRACT: Alkyl nitrites have been used previously to produce alkoxy radicals, which are important intermediates in the oxidation of alkanes in atmospheric and combustion processes. Substituted alkoxy radicals, particulary hydroxyalkoxy radicals, are also important intermediates in the atmospheric oxidation of alkenes and combustion of alcohols. In order to produce substituted alkoxy radicals we have photolyzed at 351 nm substituted alkyl nitrites, XCH2CH2ONO (X = F, Cl, Br, OH). Using laser-induced fluorescence only in the case of X = F do we observe the spectrum of substituted alkoxy radical, XCH2CH2O; but we always observe the electronic transitions of formaldehyde, HCHO, and vinoxy radical, CH2CHO. HCHO can be formed by the dissociation of XCH2CH2O in its ground state as the barrier to C−C bond dissociation is less than the photon energy remaining after O−NO bond breakage. However, the barrier along the reaction path directly leading from XCH2CH2O to CH2CHO + HX is much higher than the available energy remaining after O−NO bond breakage. A roaming mechanism, involving a frustrated dissociation of X followed by HX extraction, might explain the apparent paradox. Under the conditions of our observations vinoxy retains considerable vibrational excitation but the observed rotational temperatures of both HCHO and CH2CHO are ≲7 K.
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INTRODUCTION Alkoxy radicals are important intermediates in atmospheric and combustion chemistry.1 Substituted alkoxy radicals, XRO, where X is a halogen (F,Cl or Br) or a pseudohalogen (OH) are also important in atmospheric and combustion processes. For example, oxidation of ethanol, a common ingredient in fuels, produces HOCH2CH2O2, an intermediate which can react with NO to produce HOCH2CH2O.2 Hydroxy−alkoxy radicals are also important intermediates in atmospheric oxidation of alkenes.3 Chlorine and bromine atoms have been found to play important roles in arctic and marine atmospheric chemistry.4−6 Recently a precursor for chlorine atoms has been shown to be produced in abundance in midcontinental atmospheres.7 Chlorine atoms have also been shown to be primarily responsible for the destruction of ethene in highly polluted areas.8 Laser-induced fluorescence (LIF) spectroscopy coupled with supersonic jet cooling has been used previously to study many different primary,9,10 secondary,9,10 and tertiary9,10 saturated and unsaturated11,12 alkoxy radicals. Moderate resolution survey scans as well as high resolution spectra have been reported giving accurate vibrational frequencies, rotational and spin rotation constants.13,14 It was therefore reasonable to try to obtain similar LIF characterization of the XRO radicals. Alkyl nitrites (RONO) have been used extensively as precursor molecules for the production of alkoxy radicals. Different primary, secondary, and tertiary alkoxy radicals have been © 2012 American Chemical Society
generated by photolyzing corresponding RONOs under both ambient temperature and jet cooled conditions.9,10,15−21 The S1 ← S0 transition of CH3ONO has been a subject of extensive molecular dissociation dynamics studies.22−31 Here, 351 nm light excites the S1 ← S0 transition which is a π* ← n type transition involving the lone pair of electrons on the terminal oxygen atom. Similar studies haven been performed on other RONOs (R = CH3CH2 and (CH3)3C).32−35 These studies have found that the S1 state is repulsive along the RO−NO bond and produces RO and NO fragments. Ab initio calculations have shown that the S1 excitation energy is mainly absorbed in the NO bond which is then channeled to break the O−N bond.36−41 The internal energy of the NO fragment has been probed using laser-induced fluorescence (LIF) or multi photon ionization (MPI) methods. Doppler profiles and femtosecond studies of the RONO photodissociation have shown that the S1 state has a lifetime of about 125 fs irrespective of alkyl group.42−44 Substituted alkyl nitrites are logical precursors therefore for the production of hydroxy-alkoxy radicals. Recently, we have focused on a moderate resolution LIF study of the photofragments produced from substituted ethyl nitrite precursors, XCH2CH2ONO, with the goal of obtaining the B̃ − X̃ Received: August 24, 2012 Revised: November 27, 2012 Published: November 27, 2012 12032
dx.doi.org/10.1021/jp308428a | J. Phys. Chem. A 2012, 116, 12032−12040
The Journal of Physical Chemistry A
Article
excitation spectrum of the corresponding XCH2CH2O radicals. Of all the substituted precursors only for X = F has the B̃ − X̃ spectrum for the corresponding alkoxy (FCH2CH2O) been observed so far and its analysis is presently being pursued. In this paper we present the moderate resolution LIF study of the other photofragments produced by the 351 nm photodissociation of the substituted nitrite precursors.
Table 1. Calculated Enthalpy Changes and Barrier Heights for Reaction I, XCH2CH2O → CH2CHO + HX, and reaction II, XCH2CR1R2O → XCH2 + R1C(O)R2, at CBS-QB3 Level of Theorya
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EXPERIMENTAL SECTION The alkyl nitrite precursors were synthesized by addition of sulfuric acid and the appropriate alcohol to an aqueous solution of sodium nitrite. 10.7 mL of concentrated (≈97%) H2SO4 was mixed with 7 mL of H2O. An amount of 0.4 mol of XCH2CH2OH (X = F, Br, Cl) was added to the H2SO4 solution. The H2SO4 and XCH2CH2OH mixture was added dropwisely to 100 mL of 0.04 M NaNO3 solution kept at 0 °C over a period of about 30 min. Alkyl nitrite was then separated from the aqueous layer using a separatory funnel. Synthesis for HOCH2CH2ONO was more challenging as there is a possibility of formation of the dinitrite. A 3.6 M 100 mL aqueous solution of sodium nitrite was kept at 0 °C. 1.8 mol of ethylene glycol was mixed with 0.9 mol of concentrated sulfuric acid and then cooled to 0 °C. The two solutions were then mixed slowly and the reaction was allowed to proceed for 10 min. The mononitrite HOCH2CH2ONO was extracted 3 times from the mixture using 25 mL diethyl ether. Ether was then evaporated using a rotary evaporator giving a 50/50 mix of HOCH2CH2ONO and dinitrite (ONOCH2CH2ONO). This mixture was treated with an equal volume of ethlene glycol which reacts with the dinitrite to make the mononitrite giving a 90/10 mononitrite to dinitrite ratio. All the chemicals for the synthesis were purchased from Sigma-Aldrich, with the exception of FCH2CH2OH which was purchased from Alfa Aesar. Sample purity was characterized by NMR after every synthesis. There are minor impurities, mainly solvent, but we do not expect them to dissociate using 351 nm light. All observed LIF signals are dependent on the 351 nm light. The spectroscopy experimental setup is similar to the one described previously.9,10,45 Alkylnitrite precursors were expanded into a vacuum chamber, with a 80 psi helium backing pressure, through a 0.5 mm pinhole to create a supersonic jet. The 351 nm light produced from a XeF excimer laser was focused at the throat of the expansion to photolyze the nitrite samples. Probing UV light (
