Energy Barriers for the Addition of H, C˙ H3, and C˙ 2H5 to CH2dCHX [X ) H, CH3, OH] and for H-Atom Addition to RCHdO [R ) H, CH3, C˙ 2H5, n-C3H7]: Implications for the Gas-Phase Chemistry of Enols

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Introduction
The development of detailed chemical kinetic mechanisms1
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”,
biofuels2 that do not impact adversely on the environment
(atmosphere and hydrosphere), are not produced from animal
or human foodstuffs, and have desirable performances in inte
al
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 systems3 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 bu
ing 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, R1R2CdCH(OH), are implicated in the
combustion of oxygenated7,13 and nitrogenous14 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 200116 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 spectrometry.
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 mechanisms24
* To whom correspondence should be addressed. E-mail: john.simmie@
nuigalway.ie.
7834 J. Phys. Chem. A 2009, 113, 7834–7845
10.1021/jp903244r CCC: $40.75  2009 American Chemical Society
Published on Web 06/11/2009
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Published on June 11, 2009 on http://pubs.acs.org | doi: 10.1021/jp903244r
(however, the perturbative effect of the sampling probe on the
flame temperature is currently not known, and until this is
resolved, successful chemical kinetic simulation is not possible).
The new technique is particularly useful for the case for species
like enols, which are thermodynamically extremely stable in
comparison to their isomeric aldehydes yet nevertheless are
known to rapidly isomerize in contact with Pyrex25 and have
much shorter lifetimes in condensed phases.