Structures, thermochemical properties (enthalpy, entropy and heat capacity), rotation barriers, and peroxide bond energies of vinyl, allyl, ethynyl and phenyl hydroperoxides

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, ΔH⁰_f298, entropy, S⁰₂₉₈, and heat capacities, C_p (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 (ΔH⁰_f298) are determined at the B3LYP/6-311G(d,p) level using three isodesmic working reactions for the hydroperoxides. Entropy (S) and heat capacity (C_p (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 C_p(T) from analysis on the internal rotors are used in place of torsion frequencies. ΔH⁰_f298 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 ΔH⁰_f298 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).