Peroxides have relatively low stability and are often difficult to obtain at needed purity for experimental studies. This study uses computational chemistry to estimate thermochemical properties of peroxides and compares the values to properties of corresponding hydroperoxides. The combined thermochemical properties are utilized to develop groups for use in group additivity. Structures, enthalpy (∆ f H°2 98 ), entropy (S°2 98 ), and heat capacity (C p (T)) are determined for several vinyl, allyl, ethynyl, and phenyl peroxides using the density functional B3LYP/6-311G(d,p) calculations. Enthalpies of formation (∆ f H°2 98 ) are determined at the B3LYP/ 6-311G(d,p) level using four to six isodesmic working reactions. Entropy (S) and heat capacity (C p (T)) values from vibration, translational, and external rotational contributions are calculated using the rigid-rotor-harmonicoscillator approximation based on the vibration frequencies and structures obtained from the density functional studies. Contribution to ∆ f H(T), S, and C p (T) from analysis on the internal rotors is included. ∆ f H°2 98 for vinyl-methyl peroxide, CH 2 dCHOOCH 3 , is -10.04 ( 0.66 kcal mol -1 and for allyl-methyl peroxide, CH 2 dCHCH 2 OOCH 3 , is -12.08 ( 1.05 kcal mol -1 . Methyl-substituted vinyl peroxide values are CH 2 d C(CH 3 )OOCH 3 ) -20.79 ( 0.42, CH 3 CHdC(CH 3 )OOCH 3 ) -29.58 ( 0.32, and CH 3 (CH 3 )CdCHOOCH 3 ) -30.60 ( 0.90 kcal mol -1 . The cis conformation of CH 3 CHdCHOOCH 3 , -21.27 ( 0.47 kcal mol -1 , is more stable than the trans form, -19.95 ( 0.67 kcal mol -1 . Enthalpies for ethynyl peroxides are 41.31 ( 0.72 kcal mol -1 for HCtCOOCH 3 and 29.51 ( 0.27 kcal mol -1 for CH 3 CtCOOCH 3 . The calculated ∆ f H°2 98 for phenyl peroxide, C 6 H 5 OOCH 3 , is -2.19 ( 0.52 kcal mol -1 . The resulting peroxide enthalpies allow determination of the RO-OC, ROO-C bond energies. The vinyl, phenyl and ethynyl peroxides are unstable and rapidly dissociate via cleaving of the weak RO-OR′ peroxide bond with formation of a strong carbonyl bond. Enthalpy of formation was also calculated for CH 3 OOCH 3 (-30.77 ( 0.64 kcal mol -1 ) and CH 3 CH 2 -OOCH 3 (-39.0 ( 0.24 kcal mol -1 ) due to the need for these enthalpy values in the working reaction analysis. Thermodynamic properties for the oxygenated carbon groups O/C d /O, O/C b /O, C t /O, O/C t /O, and O/C/C t were also determined. The agreement between group additivity and calculated enthalpies and heat capacities for the different classes of molecules studied supports the group additivity principal for these systems.

There are two values, -26.0 and -27.7 kcal mol(-1), that are routinely reported in literature evaluations for the standard enthalpy of formation, Delta(f) H(o)(298), of formaldehyde (CH(2)=O), where error limits are less than the difference in values. In this study, we summarize the reported literature for formaldehyde enthalpy values based on evaluated measurements and on computational studies. Using experimental reaction enthalpies for a series of reactions involving formaldehyde, in conjunction with known enthalpies of formation, its enthalpy is determined to be -26.05+/-0.42 kcal mol(-1), which we believe is the most accurate enthalpy currently available. For the same reaction series, the reaction enthalpies are evaluated using six computational methods: CBS-Q, CBS-Q//B3, CBS-APNO, G2, G3, and G3B3 yield Delta(f) H(o)(298)=-25.90+/-1.17 kcal mol(-1), which is in good agreement to our experimentally derived result. Furthermore, the computational chemistry methods G3, G3MP2B3, CCSD/6-311+G(2df,p)//B3LYP/6-31G(d), CCSD(T)/6-311+G(2df,p)//B3LYP/6-31G(d), and CBS-APNO in conjunction with isodesmic and homodesmic reactions are used to determine Delta(f) H(o)(298). Results from a series of five work reactions at the higher levels of calculation are -26.30+/-0.39 kcal mol(-1) with G3, -26.45+/-0.38 kcal mol(-1) with G3MP2B3, -26.09+/-0.37 kcal mol(-1) with CBS-APNO, -26.19+/-0.48 kcal mol(-1) with CCSD, and -26.16+/-0.58 kcal mol(-1) with CCSD(T). Results from heat of atomization calculations using seven accurate ab initio methods yields an enthalpy value of -26.82+/-0.99 kcal mol(-1). The results using isodesmic reactions are found to give enthalpies more accurate than both other computational approaches and are of similar accuracy to atomization enthalpy calculations derived from computationally intensive W1 and CBS-APNO methods. Overall, our most accurate calculations provide an enthalpy of formation in the range of -26.2 to -26.7 kcal mol(-1), which is within computational error of the suggested experimental value. The relative merits of each of the three computational methods are discussed and depend upon the accuracy of experimental enthalpies of formation required in the calculations and the importance of systematic computational errors in the work reaction. Our results also calculate Delta(f) H(o)(298) for the formyl anion (HCO(-)) as 1.28+/-0.43 kcal mol(-1).

Phenyl radicals are formed in combustion and oxidation systems by abstraction of the phenyl—hydrogen from benzene or aromatics by active radical species and by oxidation and thermal reactions of the benzylic carbon on alkyl‐substituted aromatics. The reaction of phenyl with O2 leads to chain‐branching reactions and a number of unsaturated oxygenated hydrocarbon intermediates that may need to be included in detailed combustion models. Thermochemical parameters and structures on important species resulting from the phenyl radical + O2 association and reaction are reported in this study. Enthalpies, Δf H 0298, of a series of stable molecules, radicals, and transition state structures are calculated using ab initio (G3MP2B3 and G3) and density functional (DFT, B3LYP/6–311g(d,p) calculations, group additivity (GA), and literature data. The ab initio and density functional calculations are combined with isodesmic reaction analysis, whenever possible, to improve the accuracy of the enthalpy values. Entropies, S 0italicf298, and heat capacities, Cpf298 (T), are calculated using density functional calculations, group additivity, and literature data. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 583–604, 2008

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Oxidation of unsaturated and aromatic hydrocarbons in atmospheric and combustion processes results in formation of linear and cyclic unsaturated, oxygenated-hydrocarbon intermediates. The thermochemical parameters delatafH degrees 298, S degrees 298, and C(p)(f298)(T) for these intermediates are needed to understand their stability and reaction paths in further oxidation. These properties are not available for a majority of these unsaturated oxy-hydrocarbons and their corresponding radicals, even via group additivity methods. Enthalpy, entropy, and heat capacity of a series of 40 oxygenated and non-oxygenated molecules, or radicals corresponding to hydrogen atom loss from the parent stable molecules are determined in this study. Enthalpy (delatafH degrees 298 in kcal mol(-1)) is derived from the density function calculations at the B3LYP/6-311g(d,p) calculated enthalpy of reaction (delatafH degrees rxn,298) and by use of isodesmic (work) reactions. Estimation of error in enthalpy delatafH degrees 298, from use of computational chemistry coupled with work reactions analysis, is presented using comparisons between the calculated and literature enthalpies of reaction. Entropies (S degrees 298) and heat capacities (C(p)(f298)(T)) were calculated using the B3LYP/6-311G(d,p) determined frequencies and geometries. Potential barriers for internal rotors in each molecule were determined and used (in place of torsion frequencies) to calculate contributions to S and C(p)(T) from the hindered rotors. Twenty-six groups for use in group additivity (GA) are also developed.