Structures, thermochemical properties (enthalpy, entropy and heat capacity), rotation barriers, and peroxide bond energies of vinyl, allyl, ethynyl and phenyl hydroperoxidesElectronic supplementary information (ESI) available. Structures, geometry parameters, thermochemical properties, rotation barriers and peroxide bond energies of vinyl, allyl, ethynyl and phenyl hydroperoxides. See http://www.rsc.org/suppdata/cp/b1/b111303h/Presented at the Bunsen Discussion on Formation and Degradation of Hydrocarbons in High-Temperature Reactions, Bad Herrenalb,

Abstract

Alkyl hydroperoxides and peroxy radicals are important intermediates in atmospheric chemistry and in low to moderate temperature combustion processes, where they are strongly linked to knock in spark ignition engines and the observed negative temperature coefficient in thermal hydrocarbon oxidation. Enthalpy, ΔH0f298, entropy, S0298, and heat capacities, Cp (T), (300 ⩽ T/K ⩽ 1500), are determined for vinyl, allyl, ethynyl and phenyl hydroperoxides using the density functional B3LYP/6-311G(d, p) calculation method. The molecular structures and vibration frequencies are determined at the B3LYP/6-311G(d,p) level, and frequencies are scaled for zero point energies and for thermal corrections. Enthalpies of formation (ΔH0f298) are determined at the B3LYP/6-311G(d,p) level using three isodesmic working reactions for the hydroperoxides. Entropy (S) and heat capacity (Cp (T), values from vibrational, translational and external rotational contributions are calculated using the rigid-rotor-harmonic-oscillator approximation, based on the vibration frequencies and structures obtained from the density functional studies. Contribution to S and Cp(T) from analysis on the internal rotors are used in place of torsion frequencies. ΔH0f298 for vinyl hydroperoxide, CH2CHOOH, is −9.63 and for allyl hydroperoxide, CH2CHCH2OOH, −13.59 (values in kcal mol−1). Methyl substituted vinyl hydroperoxide values are CH2C(CH3)OOH, −21.80; CH3CHC(CH3)OOH, −30.03 and CH3(CH3)CCHOOH, −30.79. The cis conformation of CH3CHCHOOH, −21.66, is more stable than the trans form, −20.44. Enthalpies for ethynyl hydroperoxides are 42.25 kcal mol−1 for HCCOOH and 30.26 kcal mol−1 for CH3CCOOH. The calculated ΔH0f298 for phenyl hydroperoxide, C6H5OOH, is −2.68 kcal mol−1. The resulting hydroperoxide enthalpies allow determination of the R–OOH, RO–OH, ROO–H bond energies. The vinyl and ethynyl hydroperoxides are found to have weak RO–OH bond energies; they are unstable and their formation in reaction systems can lead to chain branching. Enthalpies of formation were also calculated for a number of unsaturated ethers and alcohols because the values were needed in the working reactions for the hydroperoxides. CH2CHCH2OCH3 (−25.68), cis and trans CH3CHCHOCH3 (−36.24, −34.33 kcal mol−1), CH2C(CH3)OCH3 (−32.55), CH3(CH3)CCHOCH3 (−43.72), CH3(CH3)CCOH (−49.31), CHC–O–CH3 (26.08), CH3–CC–OH and CH3–CC–O–CH3 (9.84, 15.93) (kcal mol−1).