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

Document Type

Article

Publication Date

1-1-2002

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, ΔHf2980, entropy, S2980, 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 (ΔHf2980) 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. ΔHf2980 for vinyl hydroperoxide, CH2=CHOOH, is -9.63 and for allyl hydroperoxide, CH2=CHCH2OOH, -13.59 (values in kcal mol-1). Methyl substituted vinyl hydroperoxide values are CH2=C(CH3)OOH, -21.80; CH3CH=C(CH3)OOH, -30.03 and CH3(CH3)C=CHOOH, -30.79. The cis conformation of CH3CH=CHOOH, -21.66, is more stable than the trans form, -20.44. Enthalpies for ethynyl hydroperoxides are 42.25 kcal mol-1 for HC≡COOH and 30.26 kcal mol-1 for CH3C≡COOH. The calculated ΔHf2980 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. CH2=CHCH2OCH3 (-25.68), cis and trans CH3CH=CHOCH3 (-36.24, -34.33 kcal mol-1), CH2=C(CH3)OCH3 (-32.55), CH3(CH3)C=CHOCH3 (-43.72), CH3(CH3)C=COH (-49.31), CH≡C-O-CH3 (26.08), CH3-C≡C-OH and CH3-C≡C-O-CH3 (9.84, 15.93) (kcal mol-1).

Identifier

0036378358 (Scopus)

Publication Title

Physical Chemistry Chemical Physics

External Full Text Location

https://doi.org/10.1039/b111303h

ISSN

14639076

First Page

3691

Last Page

3703

Issue

15

Volume

4

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