No abstract

Ab initio calculations were performed on CH3CH2OOH, CH3CHClOOH, and CH3CCl2OOH molecules using the Gaussian92 system of programs. Geometries of stable rotational conformers and transition states for internal rotation were optimized at the RHF/6-31G* and MP2/6-31G* levels of theory. Harmonic vibrational frequencies were computed at the RHF/6-31G* level of theory. Potential barriers for internal rotations were calculated at the MP2/6-31G**/HF/6-31G* level. Parameters of the Fourier expansion of the hindrance potentials have been tabulated. Standard entropies (S°298) and heat capacities (C p (T)'s, 300 ≤ T/K ≤ 1500) were calculated using the rigid-rotor−harmonic-oscillator approximation based on the information obtained from the ab initio studies. Contributions from hindered rotors were calculated by summation over the energy levels obtained by direct diagonalization of the Hamiltonian matrix of hindered internal rotations. Enthalpies of formation for these three molecules were calculated using isodesmic reactions. Enthalpies of formation were calculated to be ΔH f°298(CH3CH2OOH) = −41.5 ± 1.5 kcal mol-1, ΔH f°298(CH3CHClOOH) = −50.9 ± 3.4 kcal mol-1, and ΔH f°298(CH3CCl2OOH) = −55.3 ± 2.2 kcal mol-1. Entropies (S°298) are calculated to be 76.1, 79.2 and 86.6 cal mol-1 K-1 for CH3CH2OOH, CH3CHClOOH, and CH3CCl2OOH, respectively.

Alkyl peroxides, trioxides, and the corresponding radicals are important in atmospheric chemistry, photochemical smog formation, and combustion processes, but accurate and widely accepted enthalpy data for these species are not available. In this work we verify enthalpy data for several compounds and derive the corresponding group values for use in the group additivity method. Isodesmic reactions and ab initio calculations (MP4SDTQ/6-31G*//MP2/6-31G* and G2) are used to determine enthalpies of formation for the following compounds (in kcal mol-1):  CH3OOH (−31.8), C2H5OOH (−39.9), iPrOOH (iPr = (CH3)2CH−, −49.0), (CH3)3COOH (tBu = (CH3)3C−, −58.4), iPrOO• (−15.1), tBuCOO• (−25.2), CH3OOCH3 (−31.0), C2H5OOC2H5 (−47.2), iPrOOiPr, (−65.4), tBuOOtBu (−84.2), HOOOH (−23.0), CH3OOOH (−22.2), CH3OOOCH3 (−21.4). Our results on isodesmic reactions indicate that group additivity is an accurate method to estimate enthalpies (ΔH f°298 ) of alkyl peroxides and trioxides. Bond enthalpies are determined as follows:  HOOO−H (82.6), CH3O2−H (86.6), C2H5O2−H (86.1), iPrO2−H (86.0), tBuO2−H (85.3), HOO−OH (35.8), CH3O−OCH3 (38.8), CH3OO−OH (34.2), CH3O−OOH (29.6), CH3O−OOCH3 (28.0), iPr−OO (36.6), tBu−OO (37.5). The recommended enthalpy group values of (O/C/O) and (O/O2) are −5.5 and 9.6 kcal mol-1, respectively.

Ab initio calculations are performed on nine fluorinated ethane compounds and thermodynamic properties (S°2 98 and C p (T)'s 300 < T/K < 1500) are calculated. Geometries of stable rotational conformers and transition states for internal rotation are optimized at the RHF/6-31G* (6-31G(d)) and MP2/6-31G* levels of theory. Harmonic vibrational frequencies are computed at the RHF/6-31G* level of theory. Potential barriers for internal rotations are calculated at the MP2/6-31G*//MP2/6-31G* level. Parameters of the Fourier expansion of the hindrance potential are tabulated. Standard entropies (S°2 98 ) and heat capacities (C p (T)'s, 300 < T/K < 1500) are calculated using the rigid-rotor-harmonic-oscillator approximation with direct integration over energy levels of the intramolecular rotation potential energy curve. Heats of formation are adopted from literature evaluation and BAC-MP4 ab initio calculations. Thermodynamic properties for fluorinated carbon groups C/C/F/H2, C/C/F2/H, and C/C/F3 are determined by existing thermodynamic group parameter of C/C/H3 and data on CH 2 FCH 3 , CHF 2 CH 3 , and CF 3 CH 3 , respectively: no fluorine or other halogen is on the methyl carbon adjacent to the carbon bonded to the fluorine(s). Six interaction terms in addition to the above groups are developed to account for repulsion and steric effects. Interaction terms are required to accurately estimate ∆H f °298 , S°2 98 , and C p (T)'s (300 < T/K < 1500) for fluoroethanes where fluorine(s) are on carbons adjacent to a carbon bonded to fluorine(s).

Reaction pathways and kinetics are analyzed as function of temperature and pressure on formation and reactions of the adduct resulting from OH addition to ethylene. Ab initio methods are used to determine thermodynamic properties of intermediate radicals, transition states (TS) and vinyl alcohol. Enthalpies of formation (ΔH f°298 in kcal/mol) are determined for C·H2CH2OH, CH3CH2O•, and CH2CHOH using CBS-q//MP2(full)/6-31G(d,p) and G2 methods with isodesmic reactions, where zero point vibrational energies (ZPVE) and thermal correction to 298.15 K are incorporated. ΔH f°298 of TS's are determined for (H atom shift), CH3CHO−H (β-scission to form acetaldehyde + H), CH3−CH2O (β-scission to form formaldehyde + methyl radical) and CH2CHOH−H (β-scission to form vinyl alcohol + H) using CBS-q//MP2(full)/6-31G(d,p) and G2 methods. Entropies (S°298 in cal/mol K) and heat capacities (C p(T) 300 ≤ T/K ≤ 1500 in cal/mol K) are determined using geometric parameters and scaled vibrational frequencies obtained at the MP2(full)/6-31G(d,p) level of theory for CBS-q calculations. Geometric parameters obtained at MP2(full)/6-31G(d) level of theory and vibrational frequencies obtained at HF/6-31G(d) are used for G2 calculations. Quantum Rice−Ramsperger−Kassel (QRRK) analysis is used to calculate energy dependent rate constants, k(E), and master equation analysis is used to account for collisional stabilization. Rate constants are compared with experimentally determined product branching ratios (C·H2CH2OH stabilization:  CH2O + CH3:CH3CHO + H). OH adds to ethylene to form an energized ethylene-OH adduct radical (C·H2CH2OH)*. This energized adduct can dissociate back to reactants, isomerize via hydrogen shift (E a ,rxn = 29.8 and 30.8 kcal/mol) to form CH3CH2O, (ΔH f°298 = −1.7 and −3.3 kcal/mol) for CBS-q and G2 calculations respectively, or be stabilized. The CH3CH2O• isomer can undergo β-scission reaction to either formaldehyde (CH2O) + methyl radical (CH3) (E a ,rxn = 13.4 and 16.0 kcal/mol) or acetaldehyde CH3CHO + H atom (E a ,rxn = 17.6 and 19.2 kcal/mol) for CBS-q and G2 calculations, respectively. Hydrogen atom tunneling is included by use of the Eckart formalism. Tunneling effect coefficients are 842, 93.1, and 21.1 for C·H2CH2OH → CH3CH2O, CH3CH2O• → C·H2CH2OH and CH3CH2O• → CH3CHO + H at 295 K, respectively. Chemical activation and falloff are determined to be of major importance in determination of the dominant reaction paths and rate constants versus pressure and temperature in this three heavy atom system.