Energy Barriers for the Addition of H, C ˙ H 3 , and C ˙ 2 H 5 to CH 2 dCHX [X ) H, CH 3 , OH] and for H-Atom Addition to RCHdO [R ) H, CH 3 ,C ˙ 2 H 5 , n-C 3 H 7 ]: Implications for the Gas-Phase Chemistry of Enols John M. Simmie* and Henry J. Curran Combustion Chemistry Centre, National UniVersity of Ireland, Galway, Ireland ReceiVed: April 8, 2009; ReVised Manuscript ReceiVed: May 9, 2009 Although enols have been identified in alcohol and other flames and in interstellar space and have been implicated in the formation of carboxylic acids in the urban troposphere in the past few years, the reactions that give rise to them are virtually unknown. To address this data deficit, particularly with regard to biobutanol combustion, we have carried out a number of ab initio calculations with the multilevel methods CBS-QB3 and CBS-APNO to determine the activation enthalpies for methyl addition to the CH 2 group of CH 2 dCHX where X ) H, OH, and CH 3 . These average at 26.3 ( 1.0 kJ mol -1 and are not influenced by the nature of X; addition to the CHX end is energetically costlier and does show the influence of group X ) OH and CH 3 . Replacing the attacking methyl radical by ethyl makes very little difference to addition at CH 2 and follows the same trend of a higher barrier for addition to the CH(OH) end. In the case of H-addition it is more problematic to draw general conclusions since the DFT-based methodology, CBS-QB3, struggles to locate transition states for some reactions. However, the increase in barrier heights in reaction at the CHX end in comparison to addition at the methylene end is evident. For hydrogen atom reaction with the carbonyl group in the compounds methanal, ethanal, propanal, and butanal we see that for addition at the O-center the barrier heights of ca. 38 kJ mol -1 are not influenced by the nature of the alkyl group whereas addition at the C-center is different on going from H f alkyl but seems to be invariant at 20 kJ mol -1 once alkylated. Rate constants for H-atom elimination from 1-hydroxyethyl, 1-hydroxypropyl, and 1-hydroxybutyl radicals, valid over the range 800-2000 K, are reported. These demonstrate that enols are more prevalent than previously suspected and that 1-buten-1-ol should be almost as abundant as its isomeric aldehyde 1-butanal during the combustion of 1-butanol and that this will also be the case for other alcohols provided that the appropriate structural features are present. Since the toxicity of enols is not known experiments and further theoretical studies are clearly desirable before the large-scale usage of alcohol biofuels commences. An enthalpy of formation for butanal of Δ f H(298.15 K) )-204.4 ( 1.4 kJ mol -1 [Buckley, E.; Cox, J. D. Trans. Faraday Soc. 1967, 63, 895-901] is recommended, the uncertainty surrounding that for the 2-hydroxypropyl radical has been markedly reduced, and new values for 1-buten-1-ol, 1-propen-1-ol, and 2-propen-2-ol of -171.8 ( 1.6, -151.8 ( 1.7, and -169.9 ( 1.5 kJ mol -1 , respectively, are proposed. Introduction The development of detailed chemical kinetic mechanisms 1 to both understand and predict the behavior of existing and novel biofuels is of major current interest. The present-day market leader bioethanol, no matter how it is produced, suffers from some significant drawbacks as an automotive fuel in terms of both its physical and chemical properties. The search is therefore on for novel, “next-generation”, biofuels 2 that do not impact adversely on the environment (atmosphere and hydrosphere), are not produced from animal or human foodstuffs, and have desirable performances in internal combustion engines or gas turbines. One possible candidate is biobutanol (normal or 1-butanol) for which new methods of production through the manipulation of biological systems 3 offers significant advantages over the classical fermentation route. 4,5 Consequently a number of experimental studies have emerged very recently on the combustion of butanol. 6-12 It is probable that, in comparison to hydrocarbons, the burning of this oxygenated compound will lead to increases in the formation of aldehydes and lower rates of formation of particulate matter but our understanding of the combustion chemistry of this and other oxygenates is at an early stage of development. For example, it has only very recently been recognized that enols, strictly compounds with a hydroxyl group adjacent to a CdC double bond, R 1 R 2 CdCH(OH), are implicated in the combustion of oxygenated 7,13 and nitrogenous 14 compounds as well as hydrocarbons following on from their observation in flames. 15 In addition there is a growing recognition that enols may play a role in the chemistry of the interstellar medium with syn- and anti-ethenol (vinyl alcohol) first detected by microwave emissions from Sagittarius B2N in 2001 16 and a number of other enols in cold plasma discharges of alcohols very recently. 17 These latter experiments utilized tunable synchrotron radiation in the vacuum ultraviolet to selectively photoionize both stable and transient species and then detect these qualitatively, and quantitatively in some cases, via molecular-beam mass spectro- metry. 18-22 Clearly this new technique, which has been recently incorporated into a flow reactor, 23 has provided additional capabilities in reactive flows and will generate extremely valuable data for the validation of reaction mechanisms 24 * To whom correspondence should be addressed. E-mail: john.simmie@ nuigalway.ie. J. Phys. Chem. A 2009, 113, 7834–7845 7834 10.1021/jp903244r CCC: $40.75  2009 American Chemical Society Published on Web 06/11/2009