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

摘要

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, DELTAH_(1298)~0, entropy, S_(298)~0, 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 (DELTAH_(1298)~0) 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 usign 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. DELTAH_(1298)~0 for vinyl hyddroperoxide, CH_2 = CHOOH, is -9.63 and for allyl values are CH_2 = C(CH_3)OOH, -21.80; CH_3CH = C(CH_3)OOH, -30.03 and CH_3(CH_3)C=CHOOH, -30.79. The cis conformation of CH_3CH = CHOOH, -21.66, is more stable than the trans from, -20.44. Enthalpies for ethynyl hydroperoxides are 42.25 kcal mol~(-1) for HC ident to COOH and 30.26 kcal mol~(-1) for CH_3C ident to COOH. The calculated DELTAH_(1298)~0 for phenyl hydroperoxide, C_6H_5OOH, 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. CH_2 = CHCH_2OCH_3 (-25.68), cis and trans CH_3CH = CHOCH_3 (-36.24, -34.33 kcal mol~(-1)), CH_2 = C(CH_3)OCH_3 (32.55), CH_3(CH_3)C = CHOCH_3(-43.72), CH_3(CH_3)C=COH(-49.31), CH ident to C-O-CH_3 (26.08), CH_3-C ident to C - OH and CH_3 - C ident to C-O-CH_3 (9.84, 15.93) (kcal mol~(-1)).