We experimentally determined complete falloff curves of the rate constant for the unimolecular decomposition of ethoxy radicals. Two different techniques, laser flash photolysis and fast flow reactor were used both coupled to a detection of C₂H₅O radicals by laser induced fluorescence. Experiments were performed at total pressures between 0.001 and 60 bar of helium and in the temperature range of 391–471 K. Under these conditions the β-C–C scission (1a) CH₃CH₂O+M→CH₂O+CH₃+M is the dominating decomposition channel. From a complete analysis of the experimental falloff curves the low and the high pressure limiting rate constants of k_1a,0=[He] 3.3×10^-8 exp(-58.5 kJ mol^-1/RT) cm³ s^-1 and k_1a,∞=1.1×10¹³ exp(-70.3 kJ mol^-1/RT) s^-1 were extracted. We estimate an uncertainty for the absolute values of these rate constants of ±30%. Preexponential factor and activation energy are significantly lower than previous estimations. The rate constants are discussed in terms of statistical unimolecular rate theory. Excellent agreement between the experimental and the statistically calculated rate constants has been found. BAC-MP4, QCISD(T), or higher level of theory provide a reliable picture of the energy and the structure of the transition state of this radical bond dissociation reaction. On the same theoretical basis we predict the high pressure limiting rate constant for the β-C–H scission (1b) CH₃CH₂O+M→CH₃CHO+H+M of k_1b,∞=1.3×10¹³ exp(-84 kJ mol^-1/RT) s^-1. Atmospheric implications are discussed.

The temperature and pressure dependence of the unimolecular decomposition of t-butoxy radicals was studied by the laser photolysis/laser induced fluorescence technique. Experiments have been performed at total pressures between 0.04 and 60 bar of helium and in the temperature range 323–383 K. The low and the high pressure limiting rate constants as well as the broadening factor F_c have been extracted from a complete falloff analysis of the experimental results: k₀=[He]×1.5×10⁻⁸ exp(−38.5 kJ mol⁻¹/RT) cm³ s⁻¹, k_∞=1.0×10¹⁴ exp(−60.5 kJ mol⁻¹/RT) s⁻¹, and F_c=0.87−T/870 K. We anticipate an uncertainty for these rate constants of ±30%. Important features of the potential energy surface have been computed by ab initio methods. The Arrhenius parameters for the high pressure limiting rate constant for the β C–C bond scission of t-butoxy radicals have been computed from the properties of a transition state based on the results of G2(MP2) ab initio calculation. The results from density functional theory (DFT) with a small basis set (B3LYP/SVP) are very similar. Excellent agreement between the calculated and the experimental rate constants has been found. We suggest a common pre-exponential factor for β C–C bond scission rate constants of all alkoxy radicals of A=10^14±0.3 s⁻¹. Thus we express the high pressure limiting rate constant for ethoxy and i-propoxy radicals by k_∞=1.0×10¹⁴ exp(−78.2 kJ mol⁻¹/RT) and 1.0×10¹⁴ exp(−63.1 kJ mol⁻¹/RT) s⁻¹, respectively. For the reverse reactions, the addition of CH₃ radicals to CH₂O, CH₃CHO, and (CH₃)₂CO, we obtained activation enthalpies of 32, 42, and 52 kJ mol⁻¹, respectively.

Policy-makers and researchers have been considering a shift from conventional fossil fuels to renewable sources due to the growing concerns over global warming and diminishing oil reserves. Biodiesel, a renewable bio-driven fuel, can be derived from vegetable oils and animal fats, and is considered to be bio-degradable, non-toxic and environmentally friendly. The cetane number and calorific power of biodiesel are quite similar to those of conventional diesel. Crude glycerol of about 10–20% by volume appears as a byproduct in biodiesel production. The increasing demand for biodiesel has led to a substantial increase of glycerol supply in the global market and a dramatic fall in the price of glycerol which has warranted alternative uses of glycerol. One potential way to deal with the crude glycerol overflow is to convert it to glycerol carbonate (GC) and use GC as a fuel or fuel additive. Prior studies have indicated that carbonate esters can significantly reduce particulate emissions during engine combustion. In this work, we have explored possible reaction pathways in the initial stage of glycerol carbonate pyrolysis. Ab initio/RRKM-master equation methods are employed to differentiate various reaction pathways and to obtain the pressure- and temperature-dependence of the major channels. We have found that glycerol carbonate decomposes almost exclusively to produce CO₂ and 3-hydroxypropanal over 800–2000 K and radical forming channels are unimportant. As 3-hydroxypropanal is one of the main products of GC decomposition, and aldehydes are known to have a very high impact on soot reduction, we conclude that GC has great potential for cleaner combustion as a fuel additive.