Post-CCSD(T) ab Initio Thermochemistry of Halogen Oxides and Related Hydrides XOX, XOOX, HOX, XOn, and HXOn (X = F, Cl), and Evaluation of DFT Methods for These Systems

Big data benchmarking: how do DFT methods across the rungs of Jacob's ladder perform for a dataset of 122k CCSD(T) total atomization energies?

Total atomization energies (TAEs) are a central quantity in density functional theory (DFT) benchmark studies. However, so far TAE databases obtained from experiment or high-level ab initio wavefunction theory included up to hundreds of TAEs. Here, we use the GDB-9 database of 133k CCSD(T) TAEs generated by Curtiss and co-workers [B. Narayanan, P. C. Redfern, R. S. Assary and L. A. Curtiss, Chem. Sci., 2019, 10, 7449] to evaluate the performance of 14 representative DFT methods across the rungs of Jacob's ladder (namely, PBE, BLYP, B97-D, M06-L, τ-HCTH, PBE0, B3LYP, B3PW91, ωB97X-D, τ-HCTHh, PW6B95, M06, M06-2X, and MN15). We first use the A25[PBE] diagnostic for nondynamical correlation to eliminate systems that potentially include significant multireference effects, for which the CCSD(T) TAEs might not be sufficiently reliable. The resulting database (denoted by GDB9-nonMR) includes 122k species. Of the considered functionals, B3LYP attains the best performance relative to the G4(MP2) reference TAEs, with a mean absolute deviation (MAD) of 4.09 kcal mol-1. This first-generation hybrid functional, in which the three mixing coefficients were fitted against a small set of TAEs, is one of the few functionals that are not systematically biased towards overestimating the G4(MP2) TAEs, as demonstrated by a mean-signed deviation (MSD) of 0.45 kcal mol-1. The relatively good performance of B3LYP is followed by the heavily parameterized M06-L meta-GGA functional, which attains a MAD of 6.24 kcal mol-1. The PW6B95, M06, M06-2X, and MN15 functionals tend to systematically overestimate the G4(MP2) TAEs and attain MADs ranging between 18.69 (M06) and 28.54 (MN15) kcal mol-1. However, PW6B95 and M06-2X exhibit particularly narrow error distributions. Thus, scaling their TAEs by an empirical scaling factor reduces their MADs to merely 3.38 (PW6B95) and 2.85 (M06-2X) kcal mol-1. Empirical dispersion corrections (e.g., D3 and D4) are attractive, and therefore, their inclusion worsens the performance of methods that systematically overestimate the TAEs.

Accurate determination of reaction energetics and kinetics of the HO2˙ + O3 → OH˙ + 2O2 reaction

In the present work, we have studied the HO₂˙ + O₃ → HO˙ + 2O₂ reaction using chemical kinetics and quantum chemical calculations. We have employed the post-CCSD(T) method to estimate the barrier height and reaction energy for the title reaction. In the post-CCSD(T) method, we have included zero point energy corrections, contributions from full triple excitations and partial quadratic excitations at the coupled-cluster level, and core corrections. We have also computed the reaction rate in the temperature range of 197-450 K and found good agreement with all the available experimental results. In addition, we have also fitted the computed rate constants with the Arrhenius expression and obtained an activation energy of 1.0 ± 0.1 kcal mol^-1, almost identical to the value recommended by IUPAC and JPL.

Static Electron Correlation in Anharmonic Molecular Vibrations: A Hybrid TAO-DFT Study

Hybrid thermally-assisted-occupation density functional theory is used to examine the effects of static electron correlation on the prediction of a benchmark set of experimentally observed molecular vibrational frequencies. The B3LYP and B97-1 thermally-assisted-occupation measure of static electron correlation is important for describing the vibrations of many of the molecules that make up several popular test sets of experimental data. Shifts are seen for known multireference systems and for many molecules containing atoms from the second row of the periodic table of elements. Several molecules only show significant shifts in select vibrational modes, and significant improvements are seen for the prediction of hydrogen stretching frequencies throughout the test set.

Fullerenes Pose a Strain on Hybrid Density Functional Theory

The computational modeling of fullerenes plays a fundamental role in designing low-dimension carbon nanostructures. Nevertheless, the relative energies of fullerenes larger than C20 and C24 have not been comprehensively examined by means of highly accurate ab initio methods, for example, the CCSD(T) method. Here we report such an investigation for a diverse set of 29 C40 isomers. We calculate the energies of the C40 fullerenes using the G4(MP2) composite ab initio method, which approximates the CCSD(T) energy in conjunction with a triple-ζ-quality basis set (CCSD(T)/TZ). The CCSD(T)/TZ isomerization energies span 43.1-763.3 kJ mol-1. We find a linear correlation (R2 = 0.96) between the CCSD(T)/TZ isomerization energies and the fullerene pentagon signatures (P1 index), which reflect the strain associated with fused pentagon-pentagon rings. Using the reference CCSD(T)/TZ isomerization energies, we examine the relationship between the percentage of exact Hartree-Fock (HF) exchange in hybrid density functional theory (DFT) methods and the pentagon-pentagon strain energies. We find that the performance of hybrid DFT methods deteriorates with the pentagon-pentagon strain energy. This deterioration in performance becomes more pronounced with the inclusion of high amounts of HF exchange. For example, for B3LYP (20% HF exchange), the root-mean-square deviation (RMSD) relative to G4(MP2) increases from 8.9 kJ mol-1 for the low-strain isomers (P1 = 11) to 18.0 kJ mol-1 for the high-strain isomers (P1 > 13). However, for BH&HLYP (50% HF exchange) the RMSD increases from 23.0 (P1 = 11) to 113.2 (P1 > 13) kJ mol-1. A similar trend is observed for the M06/M06-2X pair of functionals. Namely, for M06 (27% HF exchange) the RMSD increases from 0.8 (P1 = 11) to 21.0 (P1 > 13) kJ mol-1, whereas for M06-2X (54% HF exchange) the RMSD increases from 16.7 (P1 = 11) to 77.7 (P1 > 13) kJ mol-1. Overall, we find that the strain associated with pentagon adjacency is an inherently challenging problem for hybrid DFT methods involving high amounts of HF exchange and that there is an inverse relationship between the optimal percentage of HF exchange and the pentagon-pentagon strain energy. For example, for BLYP the optimal percentages of HF exchange are 13% (P1 = 11), 10% (P1 = 12), 7.5% (P1 = 13), and 6% (P1 > 13).

Oxidation of HOSO˙ by O2 (3Σg-): a key reaction deciding the fate of HOSO˙ in the atmosphere

In the present work, we have studied the oxidation of HOSO˙ by O₂ (³Σ_g^-) employing quantum chemical and kinetic calculations. The present work reveals that HOSO˙ + O₂ (³Σ_g^-) is a barrierless reaction which proceeds through a stable hydrogen-bonded complex. The estimated atmospheric lifetime of HOSO˙ in the presence of O₂ (³Σ_g^-) is found to be several orders of magnitude less compared to the other oxidation paths of HOSO˙, suggesting that the oxidation of HOSO˙ by O₂ (³Σ_g^-) might be the most dominant oxidation path of HOSO˙ in the atmosphere.

Role of post-CCSD(T) corrections in predicting the energetics and kinetics of the OH • +O 3 reaction

The present work investigates the OH • +O 3 reaction by means of chemical kinetics and quantum chemical calculations. To predict the reaction barrier height and reaction energy, we have...

Quantum Chemical Investigation of Perchloric Acid Decomposition Releasing Oxygen

The primary objectives of this study are to identify the initiation steps of perchloric acid (HClO₄) decomposition and to validate and provide insights into the reaction pathways of O₂ formation. To this end, we have performed quantum chemical calculations using the Gaussian 09 program package to identify new reaction pathways and species formed during decomposition. The thermodynamic quantities of the species, such as Gibbs free energy and enthalpy, are calculated using a double-hybrid density functional theory method, B2PLYP, with Jensen's basis set, aug-pc2. For heavy atoms, such as chlorine, the basis set is augmented by adding 2 d functions with a stride factor of 2.5. To incorporate the solvation effect, the conductor-like polarizable continuum model, which is an implicit solvation model, is used. Numerical simulations using a control-volume analysis of an experiment are also performed using the proposed mechanism. In these simulations, rates of the reactions are calculated using transition state theory, incorporating diffusion effects on the rate constants. In order to consider the nonideal behavior of a concentrated HClO₄ solution, activity coefficients are used to calculate the effective concentration of acid in solution. The activity coefficient of HClO₄ plays a critical role in the calculation of the induction period involved in the HClO₄ decomposition. A comparison of numerically predicted O₂ evolution and duration of the induction period with experimental data shows that the numerical simulation using the proposed mechanism predicts both the three-stage decomposition characteristics and the induction period observed during HClO₄ decomposition, thus validating the proposed mechanism.

Density Functional Geometries and Zero-Point Energies in Ab Initio Thermochemical Treatments of Compounds with First-Row Atoms (H, C, N, O, F)

Density functionals are often used in ab initio thermochemistry to provide optimized geometries for single-point evaluations at a high level and to supply estimates of anharmonic zero-point energies (ZPEs). Their use is motivated by relatively high accuracy at a modest computational expense, but a thorough assessment of geometry-related error seems to be lacking. We have benchmarked 53 density functionals, focusing on approximations of the first four rungs and on relatively small basis sets for computational efficiency. Optimized geometries of 279 neutral first-row molecules (H, C, N, O, F) are judged by energy penalties relative to the best available geometries, using the composite model ATOMIC/B5 as energy probe. Only hybrid functionals provide good accuracy with root-mean-square errors around 0.1 kcal/mol and maximum errors below 1.0 kcal/mol, but not all of them do. Conspicuously, first-generation hybrids with few or no empirical parameters tend to perform better than highly parameterized ones. A number of them show good accuracy already with small basis sets (6-31G(d), 6-311G(d)). As is standard practice, anharmonic ZPEs are estimated from scaled harmonic values. Statistics of the latter show less performance variation among functionals than observed for geometry-related error, but they also indicate that ZPE error will generally dominate. We have selected PBE0-D3/6-311G(d) for the next version of the ATOMIC protocol (ATOMIC-2) and studied it in more detail. Empirical expressions have been calibrated to estimate bias corrections and 95% uncertainty intervals for both geometry-related error and scaled ZPEs.

Reduced Two-Electron Interactions in Anharmonic Molecular Vibrational Calculations Involving Localized Normal Coordinates

Spatially localized vibrational normal mode coordinates are shown to reduce the importance of calculating the full set of two-electron terms in the molecular electronic Schrödinger equation. Electron correlation and dispersion interactions become less significant in (E,E)-1,3,5,7-octatetraene vibrational self-consistent field calculations when displacing remote atoms along multiple coordinates. Electron correlation interactions between spatially remote modes are also found to be less important compared to their corresponding uncorrelated interaction terms. Attenuation of the Coulomb operator indicates that the two-electron terms between remote electrons become less important for accurately describing the strongly contributing mode-coupling terms between sets of localized vibrational modes.

Effective basis set extrapolations for CCSDT, CCSDT(Q), and CCSDTQ correlation energies

It is well established that extrapolating the coupled-cluster single double triple [CCSD and (T)] correlation energies using empirically motivated extrapolation exponents can accelerate the basis set convergence. Here, we consider the extrapolation of coupled-cluster expansion terms beyond the CCSD(T) level to the complete basis set (CBS) limit. We obtain reference CCSDT-CCSD(T) [T₃-(T)], CCSDT(Q)-CCSDT [(Q)], and CCSDTQ-CCSDT(Q) [T₄-(Q)] contributions from cc-pV{5,6}Z extrapolations for a diverse set of 16 first- and second-row systems. We use these basis-set limit results to fit extrapolation exponents in conjunction with the cc-pV{D,T}Z, cc-pV{T,Q}Z, and cc-pV{Q,5}Z basis set pairs. The optimal extrapolation exponents result in noticeable improvements in performance (relative to α = 3.0) in conjunction with the cc-pV{T,Q}Z basis set pair; however, smaller improvements are obtained for the other basis sets. These results confirm that the basis sets and basis set extrapolations used for obtaining post-CCSD(T) components in composite thermochemical theories such as Weizmann-4 and HEAT are sufficiently close to the CBS limit for attaining sub-kJ/mole accuracy. The fitted extrapolation exponents demonstrate that the T₃-(T) correlation component converges more slowly to the CBS limit than the (Q) and T₄ terms. A systematic investigation of the effect of diffuse functions shows that it diminishes (i) in the order T₃-(T) > (Q) > T₄-(Q) and (ii) with the size of the basis set. Importantly, we find that diffuse functions tend to systematically reduce the T₃-(T) contribution but systematically increases the (Q) contribution. Thus, the use of the cc-pVnZ basis sets benefits from a certain degree of error cancellation between these two components.

Kinetics and Thermodynamics of Reactions Involving Criegee Intermediates: An Assessment of Density Functional Theory and Ab Initio Methods Through Comparison with CCSDT(Q)/CBS Data

Reactions involving Criegee intermediates (CIs, R₁ R₂ COO) are important in atmospheric ozonolysis models. In recent years, density functional theory (DFT) and CCSD(T)-based ab initio methods are increasingly being used for modeling reaction profiles involving CIs. We obtain highly accurate CCSDT(Q)/CBS reaction energies and barrier heights for ring-closing reactions involving atmospherically important CIs (R₁ /R₂ = H, Me, OH, OMe, F, CN, cyclopropene, ethylene, acetaldehyde, and acrolein). We use this benchmark data to evaluate the performance of DFT, double-hybrid DFT (DHDFT), and ab initio methods for the kinetics and thermodynamics of these reactions. We find that reaction energies are more challenging for approximate theoretical procedures than barrier heights. Overall, taking both reaction energies and barrier heights into account, only one of the 58 considered DFT methods (the meta-GGA MN12-L) attains near chemical accuracy, with root-mean-square deviations (RMSDs) of 3.5 (barrier heights) and 4.7 (reaction energies) kJ mol^-1 . Therefore, MN12-L is recommended for investigations where CCSD(T)-based methods are not computationally feasible. For reaction barrier heights performance does not strictly follow Jacob's Ladder, for example, DHDFT methods do not perform better than conventional DFT methods. Of the ab initio methods, the cost-effective CCSD(T)/CBS(MP2) approach gives the best performance for both reaction energies and barrier heights, with RMSDs of 1.7 and 1.4 kJ mol^-1 , respectively. All the considered Gaussian-n methods show good performance with RMSDs below the threshold of chemical accuracy for both reaction energies and barrier heights, where G4(MP2) shows the best overall performance with RMSDs of 2.9 and 1.5 kJ mol^-1 , respectively. © 2019 Wiley Periodicals, Inc.

ENTHALPY OF FORMATION OF AQUEOUS HYDROFLUORIC ACID: REVISION NEEDED?

Accurate enthalpies of formation of hydrofluoric acid in the gas and liquid states as well as in aqueous solutions are critical for reduction and interpretation of combustion calorimetry data for fluorinated compounds. Analysis of current recommendations reveals inconsistencies with the existing literature that can significantly affect experimental values derived using these recommendations. Through thorough and comprehensive analysis of available experimental data, including the sources not considered before, we provide recommendations that substantially improve consistency with these results. However, the scatter in the existing data also prevents further improvements and uncertainty reduction. New experimental data, particularly for aqueous HF solutions, are needed to advance.

Basis Set Effects in the Description of the Cl-O Bond in ClO and XClO/ClOX Isomers (X = H, O, and Cl) Using DFT and CCSD(T) Methods

The performance of a group of density functional methods of progressive complexity for the description of the ClO bond in a series of chlorine oxides was investigated. The simplest ClO radical species and the two isomeric structures XClO/ClOX for each X = H, Cl, and O were studied using the PW91, TPSS, B3LYP, PBE0, M06, M06-2X, BMK, and B2PLYP functionals. Geometry optimizations and reaction enthalpies and enthalpies of formation for each species were calculated using Pople basis sets and the (aug)-cc-pVnZ Dunning sets, with n = D, T, Q, 5, and 6. For the calculation of enthalpies of formation, atomization and isodesmic reactions were employed. Both the precision of the methods with respect to the increase of the basis sets, as well as their accuracy, were gauged by comparing the results with the more accurate CCSD(T) calculations, performed using the same basis sets as for the DFT methods. The results obtained employing composite chemical methods (G4, CBS-QB3, and W1BD) were also used for the comparisons, as well as the experimental results when they are available. The results obtained show that error compensation is the key for successful description of molecular properties (geometries and energies) by carefully selecting the method and basis sets. In general, expansion of the one-electron basis set to the limit of completeness does not improve results at the DFT level, but just the opposite. The enthalpies of formation calculated at the CCSD(T)/aug-cc-pV6Z for the species considered are generally in agreement with experimental determinations and the most accurate theoretical values. Different sources of error in the calculations are discussed in detail.

Equilibria and Speciation of Chloramines, Bromamines, and Bromochloramines in Water

The stabilities and speciation of the halamines in water are difficult to characterize experimentally. We provide theoretical estimates of aqueous standard free energies of formation for inorganic chloramines, bromamines, and bromochloramines, based on high-accuracy theoretical standard free energies of formation in gas phase combined with quantum chemical estimates of Henry's law constant. Based on comparisons between several theoretical and experimental datasets, we assign an error of 1.1-1.2 log unit for equilibrium constants of several reactions leading to halamines in water. The reactions of ammonia with HOCl or HOBr that lead to dichloramine, trichloramine, and tribromamine are found to be thermodynamically more favorable than was previously believed. The newly reported equilibrium data also allow us to propose rate constant values for some hydrolysis and disproportionation reactions of dichloramine, monobromamine, and bromochloramine. Finally, theoretical results indicate aqueous acid dissociation constant (pK_a) values of 1.5 ± 1 for NH₃Cl⁺, 0.8 ± 1 for NH₃Br⁺, 11.8 ± 1 for NHCl₂, and 12.5 ± 1 for NHBrCl. The present report provides a comprehensive data set describing the free energies of the neutral inorganic halamines, the anionic conjugate base species, and the cationic conjugate acid species, with approximately uniform uncertainty bounds assigned throughout.

An Evaluation of Gas Phase Enthalpies of Formation for Hydrogen-Oxygen (HxOy) Species

We have compiled gas phase enthalpies of formation for nine hydrogen-oxygen species (HxOy) and selected recommended values for H, O, OH, H2O, HO2, H2O2, O3, HO3, and H2O3. The compilation consists of values derived from experimental measurements, quantum chemical calculations, and prior evaluations. This work updates the recommended values in the NIST-JANAF (1985) and Gurvich et al. (1989) thermochemical tables for seven species. For two species, HO3 and H2O3 (important in atmospheric chemistry) and not found in prior thermochemical evaluations, we also provide supplementary data consisting of molecular geometries, vibrational frequencies, and torsional potentials which can be used to compute thermochemical functions. For all species, we also provide supplementary data consisting of zero point energies, vibrational frequencies, and ion reaction energetics.

Exploring the structural and conformational properties of dioxygen dihalides (halogen = F, Cl, Br)

The structural and conformational properties of dioxygen difluoride (1), dioxygen dichloride (2), and dioxygen dibromide (3) have been investigated by means of the hybrid density functional theory (B3LYP) and the hybrid meta exchange-correlation functional (M06-2X) with the aug-cc-pVQZ, aug-cc-pVTZ, cc-pVTZ, and 6-311++G** basis sets and natural bond orbital interpretation. The results obtained showed that the rotation at the O–O bond by passing from the plane symmetrical trans-(C2h) (or cis-(C2v)) form leads to the O–F bonds breaking. The natural bond order analysis revealed that the O–O bonds in the C2h and C2v forms of compound 1 possess double bond characters and the small bond orders between the oxygen and fluorine atoms results from the strong electron delocalization between the lone pairs of the fluorine atoms (LP4F) and the antibonding orbitals of the adjacent O–O double bond (π*O–O). The lengthening and shortening of the O–F and O–O bonds, respectively, in the trans-(C2h) or cis-(C2v) forms of compound 1 can be interpreted by the decrease of the bonding interactions between the fluorine and oxygen orbitals. The profiles of the orbital amplitudes (or electron densities) of O–O and O–F bonds in the C2h and C2v forms of compound 1 revealed that there are strong electronic repulsions between the πO–O and the lone pairs of oxygen atoms with the lone pairs of fluorine atoms, leading to the dissociation of O–F bonds. It is worth noting that there are strong hyperconjugative interactions between LP2O and the antibonding orbitals of adjacent O–F bonds (σ*O–F) in the skew (C2) form of compound 1, leading to the decrease of the O–O bond length and the increase of the F–O bond length by increasing the πO-O bonding and the σ*O–F antibonding orbital occupations. The increase of the electron delocalization from LP3F to σ*O–O with the decrease of the σ*O–O–σO–O energy gap (results from the increase of the O–O bond length) justifies the similarity between the adiabatic O–O bond dissociation energies in compound 1 and HOOH. The electron delocaizations from LP2O2 to σ*O3–X (X = F (1), Cl (2), Br (3)) decrease drastically from the skew ground state (C2) forms of compound 1 to compound 2 but increase slightly from compound 2 to compound 3, causing the significant increase of the O–O bond length ongoing from compound 1 to compound 2 and the slight decrease from compound 2 to compound 3. Effectively, the conformational properties of compounds 1–3 can be interpreted with the principle of maximum softness.

Molecular mechanism of NDMA formation from N,N-dimethylsulfamide during ozonation: quantum chemical insights into a bromide-catalyzed pathway

During ozonation of drinking water, the fungicide metabolite N,N-dimethylsulfamide (DMS) can be transformed into a highly toxic product, N-nitrosodimethylamine (NDMA). We used quantum chemical computations and stopped-flow experiments to evaluate a chemical mechanism proposed previously to describe this transformation. Stopped-flow experiments indicate a pK(a) = 10.4 for DMS. Experiments show that hypobromous acid (HOBr), generated by ozone oxidation of naturally occurring bromide, brominates the deprotonated DMS(-) anion with a near-diffusion controlled rate constant (7.1 ± 0.6 × 10(8) M(-1) s(-1)), forming Br-DMS(-) anion. According to quantum chemical calculations, Br-DMS has a pK(a) ∼ 9.0 and thus remains partially deprotonated at neutral pH. The anionic Br-DMS(-) bromamine can react with ozone with a high rate constant (10(5 ± 2.5) M(-1) s(-1)), forming the reaction intermediate (BrNO)(SO2)N(CH3)2(-). This intermediate resembles a loosely bound complex between an electrophilic nitrosyl bromide (BrNO) molecule and an electron-rich dimethylaminosulfinate ((SO2)N(CH3)2(-)) fragment, based on inspection of computed natural charges and geometric parameters. This fragile complex undergoes immediate (10(10 ± 2.5) s(-1)) reaction by two branches: an exothermic channel that produces NDMA, and an entropy-driven channel giving non-NDMA products. Computational results bring new insights into the electronic nature, chemical equilibria, and kinetics of the elementary reactions of this pathway, enabled by computed energies of structures that are not possible to access experimentally.

Benchmark thermochemistry of chloramines, bromamines, and bromochloramines: halogen oxidants stabilized by electron correlation

The free energy of the formation of NH₂Br at 298 K can be estimated by taking into account the total atomization energy of NH₂Br and the atomic and molecular contributions to the enthalpy and the entropy of formation of NH₂Br at 0 K and 298 K.

Gas phase structures of peroxides: experiments and computational problems

Gas-phase structures of several organic and inorganic peroxides X-O-O-X and X-O-O-X', which have been determined experimentally by gas electron diffraction and/or microwave spectroscopy, are discussed. The OO bond length in these peroxides varies from 1.481(8) Å in Me3 SiOOSiMe3 to 1.214(2) Å in FOOF and the dihedral angle ϕ(XO-OX) between 0° in HC(O)O-OH and near 180° in Bu(t) O-OBu(t) . Some of the peroxides cause problems for quantum chemistry, since several computational methods fail to reproduce the experimental structures. Extreme examples are MeO-OMe and FO-OF. In the case of MeO-OMe only about half of the more than 100 computational methods reported in the literature reproduce the experimentally determined double-minimum shape of the torsional potential around the OO bond correctly. For FO-OF only a small number of close to 200 computational methods reproduce the OO and OF bond lengths better than ±0.02 Å.

Accurate bond energies of biodiesel methyl esters from multireference averaged coupled-pair functional calculations

Accurate bond dissociation energies (BDEs) are important for characterizing combustion chemistry, particularly the initial stages of pyrolysis. Here we contribute to evaluating the thermochemistry of biodiesel methyl ester molecules using ab initio BDEs derived from a multireference averaged coupled-pair functional (MRACPF2)-based scheme. Having previously validated this approach for hydrocarbons and a variety of oxygenates, herein we provide further validation for bonds within carboxylic acids and methyl esters, finding our scheme predicts BDEs within chemical accuracy (i.e., within 1 kcal/mol) for these molecules. Insights into BDE trends with ester size are then analyzed for methyl formate through methyl crotonate. We find that the carbonyl group in the ester moiety has only a local effect on BDEs. C═C double bonds in ester alkyl chains are found to increase the strengths of bonds adjacent to the double bond. An important exception are bonds beta to C═C or C═O bonds, which produce allylic-like radicals upon dissociation. The observed trends arise from different degrees of geometric relaxation and resonance stabilization in the radicals produced. We also compute BDEs in various small alkanes and alkenes as models for the long hydrocarbon chain of actual biodiesel methyl esters. We again show that allylic bonds in the alkenes are much weaker than those in the small methyl esters, indicating that hydrogen abstractions are more likely at the allylic site and even more likely at bis-allylic sites of alkyl chains due to more electrons involved in π-resonance in the latter. Lastly, we use the BDEs in small surrogates to estimate heretofore unknown BDEs in large methyl esters of biodiesel fuels.

Properties and thermal decomposition of the hydro-fluoro-peroxide CH₃OC(O)OOC(O)F

The thermal decomposition of methyl fluoroformyl peroxycarbonate CH3OC(O)OOC(O)F was studied in the range of 30- 96 °C using FTIR spectroscopy to follow the course of the reaction in the presence of either N2, O2, or CO as bath gases. The rate constants of the homogeneous first-order process fit the Arrhenius equation k(exp) = (5.4 ± 0.2) × 10(14) exp[-(27.1 ± 0.6 kcal mol(-1)/RT)] (in units of s(-1)). A complete mechanism of decomposition is presented. An experimental O-O bond energy of 27 ± 1 kcal mol(-1) was obtained. The products observed when N2 or O2 are used as bath gases were CO2, CO, HF, and CH3OC(O)H, while in the presence of CO, CH3OC(O)F was also observed. Transition state ab initio calculations were carried out to understand the dynamics of the decomposition. Additionally, thermodynamic properties of the atmospherically relevant CH3OCO2• radical were calculated. The heat of formation, ΔH°(f 298), obtained for CH3OCO2• and CH3OC(O)OOC(O)F, were 78 ± 3 kcal mol(-1) and 191 ± 5 kcal mol(-1), respectively.

Reactivity of BrCl, Br₂, BrOCl, Br₂O, and HOBr toward dimethenamid in solutions of bromide + aqueous free chlorine

HOBr, formed via oxidation of bromide by free available chlorine (FAC), is frequently assumed to be the sole species responsible for generating brominated disinfection byproducts (DBPs). Our studies reveal that BrCl, Br(2), BrOCl, and Br(2)O can also serve as brominating agents of the herbicide dimethenamid in solutions of bromide to which FAC was added. Conditions affecting bromine speciation (pH, total free bromine concentration ([HOBr](T)), [Cl(-)], and [FAC](o)) were systematically varied, and rates of dimethenamid bromination were measured. Reaction orders in [HOBr](T) ranged from 1.09 (±0.17) to 1.67 (±0.16), reaching a maximum near the pK(a) of HOBr. This complex dependence on [HOBr](T) implicates Br(2)O as an active brominating agent. That bromination rates increased with increasing [Cl(-)], [FAC](o) (at constant [HOBr](T)), and excess bromide (where [Br(-)](o)>[FAC](o)) implicate BrCl, BrOCl, and Br(2), respectively, as brominating agents. As equilibrium constants for the formation of Br(2)O and BrOCl (aq) have not been previously reported, we have calculated these values (and their gas-phase analogues) using benchmark-quality quantum chemical methods [CCSD(T) up to CCSDTQ calculations plus solvation effects]. The results allow us to compute bromine speciation and hence second-order rate constants. Intrinsic brominating reactivity increased in the order: HOBr ≪ Br(2)O < BrOCl ≈ Br(2) < BrCl. Our results indicate that species other than HOBr can influence bromination rates under conditions typical of drinking water and wastewater chlorination.

Substituent effects on O-H bond dissociation enthalpies: a computational study

Bond dissociation enthalpies (BDEs) can exhibit dramatic variations resulting from substituent effects. The remarkable range of experimental OH bond dissociation enthalpies have been reproduced using CBS-APNO calculations with very good accuracy, so we have employed these calculations to extend the available BDE data. The effect on these BDEs of lone pairs on the atom adjacent to oxygen shows that conjugation in the product radicals is the most important interaction leading to the wide range of values. The BDE's were found to be linearly related to both the spin density at the radical center and to the change in X-O bond order in going from X-O-H to X-O·.

Benchmarking Experimental and Computational Thermochemical Data: A Case Study of the Butane Conformers

Due to its crucial importance, numerous studies have been conducted to determine the enthalpy difference between the conformers of butane. However, it is shown here that the most reliable experimental values are biased due to the statistical model utilized during the evaluation of the raw experimental data. In this study, using the appropriate statistical model, both the experimental expectation values and the associated uncertainties are revised. For the 133-196 and 223-297 K temperature ranges, 668 ± 20 and 653 ± 125 cal mol(-1), respectively, are recommended as reference values. Furthermore, to show that present-day quantum chemistry is a favorable alternative to experimental techniques in the determination of enthalpy differences of conformers, a focal-point analysis, based on coupled-cluster electronic structure computations, has been performed that included contributions of up to perturbative quadruple excitations as well as small correction terms beyond the Born-Oppenheimer and nonrelativistic approximations. For the 133-196 and 223-297 K temperature ranges, in exceptional agreement with the corresponding revised experimental data, our computations yielded 668 ± 3 and 650 ± 6 cal mol(-1), respectively. The most reliable enthalpy difference values for 0 and 298.15 K are also provided by the computational approach, 680.9 ± 2.5 and 647.4 ± 7.0 cal mol(-1), respectively.

Polarized Molecular Orbital Model Chemistry. II. The PMO Method

We present a new semiempirical molecular orbital method based on neglect of diatomic differential overlap. This method differs from previous NDDO-based methods in that we include p orbitals on hydrogen atoms to provide a more realistic modeling of polarizability. As in AM1-D and PM3-D, we also include damped dispersion. The formalism is based on the original MNDO one, but in the process of parameterization we make some specific changes to some of the functional forms. The present article is a demonstration of the capability of the new approach, and it presents a successful parametrization for compounds composed only of hydrogen and oxygen atoms, including the important case of water clusters.