J.S. Francisco

A comprehensive theoretical examination of primary dissociation pathways of formic acid

Primary dissociation pathways have been investigated for formic acid by ab initio molecular orbital methods. Reactant, transition state, and products were fully optimized with unrestricted Hartree–Fock and unrestricted second‐order Mo/ller–Plesset wave functions. The activation energy for decarboxylation of formic acid (CO2+H2) is 65.2 kcal mol−1, while that for the dehydration process (CO+H2O) is 63.0 kcal mol−1. These theoretical results suggest that the decarboxylation and dehydration processes are competitive. The activation energy barrier for isomerization of formic acid to yield dihydroxymethylene is 73.7 kcal mol−1 and may be a competitive process. Free radical initiation processes are predicted to be minor.

Dissociation dynamics of low‐lying electronic states of SiH2

Essential features of the potential surfaces for low‐lying electronic states of silylene, SiH2, have been characterized. Calculated transition energies between the X 1A1, a 3B1, and A 1B1 states are in agreement with previously published experimental and theoretical values. The reactions Si(1D)+H2 and SiH(2Π)+H2 leading to SiH2(X 1A1) appear to be barrierless processes. On the a 3B1 and A 1B1 surfaces, asymmetric transition states are found for SiH2 dissociation. The activation energy for dissociation, SiH2(a 3B1)→Si(3P)+H2(1Σ+g), is calculated to be 44.7 kcal/mol, and the dissociation energy for SiH2(X 1A1)→Si(1D)+H2(1Σ+g) is calculated to be 65.8 kcal/mol. Structures and vibrational frequencies are presented for the low‐lying electronic states of SiH2 and for the associated transition states. Lifetimes of individual rovibronic levels of SiH2(A 1B1) are found to decrease dramatically in v’2 =8 as compared with v2 =7. This lifetime shortening is attributed to the opening of the dissociation chann...

An examination of substituent effects on the reaction of OH radicals with HXCO (where X=H, F, and Cl)

In the present work, ab initio molecular orbital theory have been used in the study of the reaction of OH radicals with HXCO (where X=H, F, and Cl). Equilibrium geometries and transition state structures have been fully optimized at the unrestricted Hartree–Fock (UHF)/6‐31G*, unrestricted Mo/ller–Plesset (UMP2)/6‐31G* and UMP2/6‐311G** levels of theory. Activation energy barriers and heats of reaction have been estimated using fourth‐order Mo/ller–Plesset perturbation theory with spin projection including single, double, triple, and quadruple excitations with extended basis sets. Transition state theory treatment of the kinetics of these reactions is performed and is used to estimate the rate coefficient at 299.3 K. The results for the OH+H2CO reaction show reasonable agreement with experiment.

FCO: UV spectrum, self-reaction kinetics and chain reaction with F2

Abstract Flash photolysis of F 2 in the presence of CO combined with time-resolved UV and transient diode laser spectroscopies has been used to record the UV spectrum and to study the reaction kinetics of the FCO radical. The gas phase spectrum of the B 2 A′ and C 2 A″ states of FCO exhibits three vibrational progressions of 740, 340, and 670 cm −1 spacing, in agreement with previous matrix isolation spectra of FCO and with recent ab initio quantum chemical studies. Time-resolved UV spectra show the FCO radical to decay via second-order kinetics with a rate constant of (1.9 ± 0.2) × 10 −11 cm 3 s −1 at 297 K. The reaction is nearly temperature independent with k 2 = (3.7 ± 1.2) × 10 −11 exp[− (160 ± 80)/ T ] cm 3 s −1 over the range of 208 to 358 K, which is consistent with the prediction that there is no activation barrier for this reaction. The yield of CF 2 O at 297 K, probed by transient infrared absorption, is approximately 2–3 times [F] 0 . This provides evidence of chain propagation via the reaction of FCO with F 2 , the measured rate constant of which is (4.5 ± 0.8) × 10 −14 cm 3 s −1 at 297 K.

Dissociation dynamics of FCO and HCO radicals

Dissociation energies and barriers to dissociation for XCO→X+CO have been calculated for X 2A’ and A 2π states of FCO and HCO by ab initio molecular orbital methods. At the PUMP4//UMP2/6‐311G* level, D○298 (F‐CO)=22.3 kcal mol−1 and ΔH298=24.2 kcal mol−1 for dissociation of ground‐state FCO; these values are much higher than the corresponding bond energy and activation enthalpy for HCO dissociation. Calculated RRKM rate constants suggests that the lifetime of FCO under stratospheric conditions is sufficient to allow bimolecular reactions to compete with dissociation. Reaction with O2 may provide an in situ source of stratospheric CO2.

Ab initio studies of dissociation pathways on the ground state potential energy surface for HFCO and HClCO

Reaction pathways for the decomposition of HFCO and HClCO on the ground state potential energy surface have been studied by using ab initio methods. Heats of reaction and barrier heights have been computed by using Mo/ller–Plesset perturbation theory. Spin projections have been applied to free radical dissociation pathways for annihilation of spin contamination. The favorable dissociation path predicted is molecular elimination of HX to yield CO. The substitution effects on decomposition pathways of HFCO and HClCO are also examined.

Temperature dependent kinetics of the formation and self‐reactions of FC(O)O2 and FC(O)O radicals

This paper presents the ultraviolet (UV) absorption spectrum of FC(O)O2 and the temperature dependent rate constants for its formation via the addition of O2 to FCO, its self‐reaction to form FC(O)O, and for the subsequent dimerization of FC(O)O. The UV spectrum of FC(O)O2 shows two absorption bands in the 190–300 nm range. The lower energy band has a peak cross section of (3.0±0.3)×10−18 cm2 at 232 nm. A higher energy band begins at ∼210 nm and reaches an absorption cross section of 4.3×10−18 cm2 at 190 nm. The FC(O)O2 self‐reaction exhibits a negative temperature dependence with k2(T)=(2.5±0.4)×10−12 e(286±40)/T cm3 s−1 over the 213–358 K temperature range. The oxygen addition to FCO has a rate constant of k1=(8.0±0.8)×10−13 cm3 s−1 and the FC(O)O dimerization rate constant is in the range k3a=(2.3–6.5)×10−12 cm3 s−1, at 293 K and 300 Torr total pressure. Both of these rate constants show little variation over the 213–358 K temperature range.

Laser-induced fluorescence spectroscopic study of the Ã2A1X̃ 2E transition of trifluoromethoxy radical

Abstract The first observation of the laser-induced fluorescence spectrum of trifluoromethoxy radical, CF 3 O, is reported. The radical is produced by infrared multiple photon dissociation of CF 3 OOCF 3 . The origin of the A 2 A 1 X 2 E transition is found to lie at 351.15 nm. Combination bands associated with an interval of ≈ 640 cm −1 are observed. The dispersed fluorescence spectrum has also been measured following excitation at 351.15 nm. The dispersed fluorescence spectrum displayed a progression associated with an ≈ 1241 cm −1 interval, which is consistent with the CO stretching frequency of ground state CF 3 O predicted by ab initio MO predictions. The lifetime of the excited state is measured to be 30.1 ns.

Ab initio calculation of the heats of formation of CF3OH and CF2O

The heats of formation of the trifluoromethanol (CF3OH) and carbonic difluoride (CF2O) molecules are calculated with theoretical methods of demonstrated high accuracy. The results are used to assess the accuracy of the reported value of the heat of formation of CF2O and to provide an accurate estimate for CF3OH. The implications of these results for atmospheric chemistry are discussed.

An ab initio investigation of the significance of the HOOF intermediate in coupling reactions involving FOOx and HOx species

Ab initio calculations are used to investigate the stability and role of HOOF in the reaction of FO with HO radicals. The heat of formation for HOOF is estimated as 0.4±2 kcal mol−1 using an isodesmic reaction scheme. Spectroscopic properties of the HOOF intermediate is also provided in order to facilitate its identification. Decomposition pathways of the intermediate are examined. The lowest energy pathway is the formation of F atoms and HO2 radicals and requires 27.2 kcal mol−1 to proceed. Reactions leading to the formation of the HOOF intermediate are examined in regard to their importance in understanding stratospheric chemistry involving the coupling of fluorine and fluorine oxide with HOx species in catalytic cycles.