Isogyric Reactions Research Articles

Thermochemistry of Gas-Phase Thermal Oxidation of C2 to C8 Perfluorinated Sulfonic Acids with Extrapolation to C16.

New ideal-gas thermochemistry Cp°(T), H°(T), S°(T), and G°(T) are predicted for 53 species involved in the thermal destruction of perfluorinated sulfonic acids (PFSAs) ranging from C2 to C8 in perfluorinated alkyl chain length. Species were selected by considering both the pyrolytic and oxidative pathways of PFSA destruction. After the sulfur-containing moieties are removed, subsequent reactions largely involve species from a prior set of thermochemistry for the thermal destruction of perfluorinated carboxylic acids (Ram et al., J. Phys. Chem. A, 2024, 128, 7, 1313-1326). Enthalpies of formation at 0 K are computed using a new isogyric reaction scheme. Rigid-rotor harmonic-oscillator partition functions were calculated over a 200-2500 K temperature range using rovibrational properties at G4 (≤C3S1 species) and M06-2X-D3(0)/def2-QZVPP (≥C4S1 species), employing the 1D hindered rotor approximation to correct for torsional modes. Seven-coefficient NASA polynomial fits are reported in standardized formats. Bond dissociation energies and important reaction equilibria are examined to provide insights into the reactivity of potentially persistent species. Extrapolated NASA polynomials are also systematically predicted for 126 species larger than C8/C8S1 in size, allowing reasonably accurate estimates of thermochemistry without the need for expensive electronic structure calculations.

Thermochemistry of per- and polyfluoroalkyl substances.

The determination of gas phase thermochemical properties of per- and polyfluoroalkyl substances (PFAS) is central to understanding the long-range transport behavior of PFAS in the atmosphere. Prior gas-phase studies have reported the properties of perfluorinated sulfonic acid (PFOS) and perfluorinated octanoic acid (PFOA). Here, this study reports the gas phase enthalpies of formation of short- and long-chain PFAS and their precursor molecules determined using density functional theory (DFT) and ab initio approaches. Two density functionals, two ab initio methods and an empirical method were used to compute enthalpies of formation with the total atomization approach and an isogyric reaction. The performance of the computational methods employed in this work were validated against the experimental enthalpies of linear alkanoic acids and perfluoroalkanes. The gas-phase determinations will be useful for future studies of PFAS in the atmosphere, and the methodological choices will be helpful in the study of other PFAS.

Reactive orbital energy theory serving a theoretical foundation for the electronic theory of organic chemistry.

It is established that the reactive orbital energy theory (ROET) theoretically reproduces the rule-based electronic theory diagrams of organic chemistry by a comparative study on the charge transfer natures of typical organic carbon-carbon and carbon-heteroatom bond formation reactions: aldol, Mannich, α-aminooxylation, and isogyric reactions. The ROET, which is an expansion of the reaction electronic theories (e.g., the frontier orbital theory) in terms of orbital energies, elucidates the reactive orbitals driving reactions and the charge transferability indices of the reactions. Performing the ROET analyses of these reactions shows that the charge transfer directions given in the rule-based diagrams of the electronic theory are reproduced even for the functional groups of charge transfer destinations in all but only two processes for 38 reaction processes. The ROET analyses also make clear the detailed orbital-based pictures of these bond formation reactions: that is, the use of the out-of-plane antibonding π orbitals in acidic conditions (enol-mode) and in-plane antibonding π orbitals in basic conditions (enolate-mode), which explain the experimentally assumed mechanisms such as the π-bond formations in acidic conditions and σ-bond formations at α-carbons in basic conditions. Furthermore, the ROET analyses explicate that the methyl group initially accepts electrons and then donates them to the bond formations in the target reactions. It is, consequently, suggested that the ROET serves a theoretical foundation for the electronic theory of organic chemistry.

Thermodynamic Properties: Enthalpy, Entropy, Heat Capacity, and Bond Energies of Fluorinated Carboxylic Acids.

Fluorinated carboxylic acids and their radicals are becoming more prevalent in environmental waters and soils as they have been produced and used for numerous commercial applications. Understanding the thermochemical properties of fluorinated carboxylic acids will provide insights into the stability and reaction paths of these molecules in the environment, in body fluids, and in biological and biochemical processes. Structures and thermodynamic properties for over 50 species related to fluorinated carboxylic acids with two and three carbons are determined with density functional computational calculations B3LYP, M06-2X, and MN15 and higher ab initio levels CBS-QB3, CBS-APNO, and G4 of theory. The lowest energy structures, moments of inertia, vibrational frequencies, and internal rotor potentials of each target species are determined. Standard enthalpies of formation, ΔfH298°, from CBS-APNO calculations show the smallest standard deviation among methods used in this work. ΔfH298° values are determined via several series of isodesmic and/or isogyric reactions. Enthalpies of formation are determined for fluorinated acetic and propionic acids and their respective radicals corresponding to the loss of hydrogen and fluorine atoms. Heat capacities as a function of temperature, Cp(T), and entropy at 298 K, S298°, are determined. Thermochemical properties for the fluorinated carbon groups used in group additivity are also developed. Bond dissociation energies (BDEs) for the carbon-hydrogen, carbon-fluorine, and oxygen-hydrogen (C-H, C-F, and O-H BDEs) in the acids are reported. The C-H, C-F, and O-H bond energies of the fluorinated carboxylic acids are in the range of 89-104, 101-125, and 109-113 kcal mol-1, respectively. General trends show that the O-H bond energies on the acid group increase with the increase in the fluorine substitution. The strong carbon fluorine bonds in a fluorinated acid support the higher stability of the perfluorinated acids in the environment.

Accurate estimation of enthalpies of formation for C-, H-, O-, and N-containing compounds using DLPNO-CCSD(T1)/CBS method

The DLPNO-CCSD(T1)/CBS and G4 methods combined with isogyric, isodesmic, and homodesmotic working reactions were applied to calculate gas-phase enthalpies of formation for thirty-three C–H–N–O compounds with well-established experimental values. Unlike the G4 method, the DLPNO calculations, even together with isogyric reactions with small reference species, reproduce the experimental enthalpies of formation within chemical accuracy (≈ 4 kJ/mol). For more than half of the compounds, a better accuracy, within 2 kJ/mol, was achieved. These results make the DLPNO-CCSD(T1)/CBS method a promising tool for accurate prediction of enthalpy of formation of medium-sized organic compounds. Being convinced of its reliability, the DLPNO-CCSD(T1)/CBS method was also used to estimate the enthalpies of formation for some challenging compounds with conflicting values reported in the literature (glycerol, nitromethane, nitrobenzene, 1-aminoadamantane, piperazine, and others).

Thermochemistry of Fluorinated Dimethyl and Ethyl Methyl Ethers and Corresponding Radical Species

Standard enthalpies of formation (ΔfH°298), standard entropies (S°(T)), and heat capacities (Cp(T)) are calculated for dimethyl and ethyl methyl fluorinated ethers, both the parent and related radical species. The parent and radical species are utilized to determine carbon–hydrogen, carbon–fluorine, carbon–carbon, and carbon–oxygen bond dissociation energies (C–H, C–F, C–C, and C–O BDEs). The ΔfH°298 and BDEs are calculated using a variety of error-canceling isogyric and/or isodesmic reactions at the MN15/cc-pVTZ, CBS-QB3, and CBS-APNO levels of theory. ΔfH°298 calculations from the MN15 functional for these ether species are shown to have consistency with the CBS-QB3 and CBS-APNO composite methods with our recommended ideal gas phase ΔfH°298 values from the average of these two composite methods. C–F BDE values are shown to range overall between 111 and 128 kcal mol–1, the C–H BDEs are in the 93–106 kcal mol–1 range, the C–O BDEs are in the 83–107 kcal mol–1 range, and the C–C BDEs are in the 88–101 kcal...

Triplet-State Structures, Energies, and Antiaromaticity of BN Analogues of Benzene and Their Benzo-Fused Derivatives.

It is well known that benzene is aromatic in the ground state (the Hückel's rule) and antiaromatic in the first triplet (T1) excited state (the Baird's rule). Whereas its BN analogues, the three isomeric dihydro-azaborines, have been shown to have various degrees of aromaticity in their ground state, almost no data are available for their T1 states. Thus, the purpose of this work is to theoretically [B3LYP/6-311+G(d,p)] predict structures, energies, and antiaromaticity of T1 dihydro-azaborines and some benzo-fused derivatives. Conclusions are based on spin density analysis, isogyric and hydrogenation reactions, HOMA, NICS, and ACID calculations. The results suggest that singlet-triplet energy gaps, antiaromaticity, and related excited-state properties of benzene, naphthalene, and anthracene could be tuned and controlled by the BN substitution pattern. While all studied compounds remain (nearly) planar upon excitation, the spin density distribution in T1 1,4-dihydro-azaborine induces a conformational change by which the two co-planar C-H bonds in the ground state become perpendicular to each other in the excited state. This predicted change in geometry could be of interest for the design of new photomechanical materials. Excitation of B-CN/N-NH2 1,4-azaborine would have a few effects: intramolecular charge transfer, aromaticity reversal, rotation, and stereoelectronic Umpolung of the amino group.

Thermochemistry of Intermediates and Products in the Oxidation Reaction of 1,1,2-Trifluoroethene via OH Radical.

Density functional theory (DFT) and composite ab initio based calculations are performed on trifluoroethane along with intermediate radicals, parent molecules of the radicals, and products related to the reaction of hydroxyl radical with 1,1,2-trifluoroethene, as a reference for hydrofluoroolefins (HFO). Potential energy barriers for internal rotations have been computed. Calculated torsional potentials are incorporated into the determination of entropy, S°298, and heat capacities as a function of temperature, Cp(T), for each target molecule. Six isodesmic or isogyric reactions and five calculation methods are used to determine heats of formation at 298 K (ΔfH298) in kcal mol-1 of each target species. The CBS-APNO method shows the best agreement with experimental data in comparisons from 16 reference reactions on ΔrxnH of each method. The lowest configuration structures of each target species are reported. Intramolecular hydrogen bonds between the hydroxyl hydrogen atom and the fluorine atom on the adjacent carbon can stabilize molecules by up to 3 kcal mol-1. R-OH bond dissociation energies are observed to increase with the number of fluorine atoms on the carbon connected to hydroxy group. Recommended ΔfH298 values in kcal mol-1 derived from the most stable conformers are CF2(OH)CH2F (-213.0), CF2(O•)CH2F (-148.6), CF2(OH)C•FH (-162.4), CHF2CHFOH (-207.5), CHF2C•FOH (-158.3), C•F2CHFOH (-155.5), CHF2CHFO• (-150.4), CF3CH2OH (-212.5), and CF3C•HOH (-167.9).

Thermodynamics of OHgX, XHgOH, XHgOCl, XHgOBr, and HOHgY Gaseous Oxidized Mercury Molecules from Isodesmic, Isogyric, and Atomization Work Reactions (X = Halogen, Y = OH, OCl, OBr)

Isogyric, isodesmic, and atomization reactions were used to determine the standard enthalpies of formation (Δ H°f) for a series of OHgX, XHgOH, XHgOCl, XHgOBr, and HOHgY molecules (X = halogen, Y = OH, OCl, OBr) from single-point CCSD(T)/AV nZ//M06-2X/AVTZ ( n = D, T, Q) energies extrapolated to the complete basis set limit using a three-point mixed Gaussian exponential scheme. Standard enthalpies of formation from isogyric and isodesmic reactions are in good agreement with experimental data and values from higher level calculations but at significantly reduced computation time. The established template work reaction scheme and methodology can potentially be extended for Hg species outside the scope of this work. The standard error for the Δ H°f for the studied molecules averaged over several isodesmic/isogyric reactions is 1.67 kcal/mol or less. The X-Hg-Y bond dissociation energies (BDEs) were determined from the Δ H°f and used as a metric of the molecular stability. The BDE decreases from F to I for the X-HgOH and OHg-X series. The trend is reversed for X-HgOCl and X-HgOBr where the BDE increases from F to Br but decreases with I. All studied molecules have significant BDEs in the range of 50-90 kcal/mol. Assuming that the concentrations of halogens OH, OCl, or OBr are reasonably high near the vicinity of a mercury emission source, the studied molecules can potentially exist long enough to affect the mercury oxidation-reduction rate and impact the global mercury budget.

Re-evaluation of the standard thermochemical properties of the Al 2 cluster on the basis of CBS thermochemistry of isogyric reactions and correlation dependencies

Abstract In the present study, the value of ΔfHo298.15(3Al2(3Πu))atom = 518.4, 521, 521.4, 521.3 and 517.7 kJ/mol have been calculated using, respectively, CBS-QB3, ROCBS-QB3, W1U, W1RO and CBS-Q approaches. The slightly lower values of ΔfHo298.15(3Al2)isog = 495 ± 15, 513 ± 7, 503 ± 17, 513 ± 9 and 507 ± 8 kJ/mol have been determined, respectively, by the same quantum mechanical approaches, but on the basis of thermochemistry of the isogyric reactions. The higher values of ΔfHo298.15(3Al2)CORR = 522.8 (CBS-QB3), 521 (ROCBS-QB3), 520.4 (W1U), 521.3 (W1RO) and 519.7 (CBS-Q) kJ/mol have been determined on the basis of the correlation dependence between tabulated and calculated values of the bond dissociation energy (BDE) of compounds contained double and triple bonds. The close value of ΔfHo298.15(3Al2) = 525 kJ/mol have also been determined within ROCBS-QB3 approach, on the basis of the perfectly linear correlation dependence between tabulated and calculated values of the BDE of dimers, formed by elements of the third row of the periodic table of elements. In contrast, the significantly higher value of ΔfHo298.15(3Al2)CORR = 549.9 kJ/mol have been determined on the basis of the correlation dependence between tabulated and theoretical values of BDE calculated for the lowest triplet states of B2, Al2 and Ga2. Such high difference, observed in the last case, has been attributed to the high uncertainty in the tabulated/experimental values of ΔfHo298.15(3B2) and ΔfHo298.15(3Ga2). As a result, the lowest limit of the experimental values of ΔfHo298.15(3Al2) = 525 kJ/mol and ΔfHo298.15(3Ga2) = 430 kJ/mol, as well as the upper limit of the value of ΔfHo298.15(3B2) = 851.2 kJ/mol have been accepted as the best available estimate. The temperature dependence of ΔfHo298.15(3Al2), has been calculated using the ROCBS-QB3 approach. The value of energy gap E(S-T) = 31.8, 29.6 and 29.1 kJ/mol between the lowest triplet (3Πu) and singlet (1Σg+) states of Al2 have been determined using, respectively, CBS-Q, ROCBS-QB3 and W1RO calculations. The slightly different values of E(S-T) = 32.1 ± 6, 28.9 ± 7 and 25.6 ± 4 kJ/mol have also been determined using correlation dependencies. The last of these values has been considered as the most accurate. The values of So298.15(3Al2) = 236.6 ± 0.1 J/(mol K) and its temperature dependence, calculated in the present work, agree with those reported previously.

Enthalpies of Formation of Hydrazine and Its Derivatives.

Enthalpies of formation, ΔfH298°, in both the gas and condensed phase, and enthalpies of sublimation or vaporization have been estimated for hydrazine, NH2NH2, and its 36 various derivatives using quantum chemical calculations. The composite G4 method has been used along with isodesmic reaction schemes to derive a set of self-consistent high-accuracy gas-phase enthalpies of formation. To estimate the enthalpies of sublimation and vaporization with reasonable accuracy (5-20 kJ/mol), the method of molecular electrostatic potential (MEP) has been used. The value of ΔfH298°(NH2NH2,g) = 97.0 ± 3.0 kJ/mol was determined from 75 isogyric reactions involving about 50 reference species; for most of these species, the accurate ΔfH298°(g) values are available in Active Thermochemical Tables (ATcT). The calculated value is in excellent agreement with the reported results of the most accurate models based on coupled cluster theory (97.3 kJ/mol, the average of six calculations). Thus, the difference between the values predicted by high-level theoretical calculations and the experimental value of ΔfH298°(NH2NH2,g) = 95.55 ± 0.19 kJ/mol recommended in the ATcT and other comprehensive reference sources is sufficiently large and requires further investigation. Different hydrazine derivatives have been also considered in this work. For some of them, both the enthalpy of formation in the condensed phase and the enthalpy of sublimation or vaporization are available; for other compounds, experimental data for only one of these properties exist. Evidence of accuracy of experimental data for the first group of compounds was provided by the agreement with theoretical ΔfH298°(g) value. The unknown property for the second group of compounds was predicted using the MEP model. This paper presents a systematic comparison of experimentally determined enthalpies of formation and enthalpies of sublimation or vaporization with the results of calculations. Because of relatively large uncertainty in the estimated enthalpies of sublimation, it was not always possible to evaluate the accuracy of the experimental values; however, this model allowed us to detect large errors in the experimental data, as in the case of 5,5'-hydrazinebistetrazole. The enthalpies of formation and enthalpies of sublimation or vaporization have been predicted for the first time for ten hydrazine derivatives with no experimental data. A recommended set of self-consistent experimental and calculated gas-phase enthalpies of formation of hydrazine derivatives can be used as reference ΔfH298°(g) values to predict the enthalpies of formation of various hydrazines by means of isodesmic reactions.

Performance of DFT, MP2, and composite ab initio methods for the prediction of enthalpies of formations of CHON compounds using isodesmic reactions

This paper assesses the performance of quantum chemical models with regard to the calculation of enthalpy of formation of CHON molecules using isodesmic reactions. The high accuracy of prediction of enthalpy of formation of CHON compounds can be achieved by the combination of isodesmic reaction scheme with composite ab initio methods. The best composite methods, such as G4, G4MP2, CBS-QB3, and CBS-APNO, can attain near chemical accuracy or better depending on reaction type. Other composite methods yield the same accuracy only in conjunction with isodesmic or homodesmotic reactions. The DFT and MP2 methods lead to good results if they are used with homodesmotic reactions. For CHO compounds, all composite methods demonstrate the high accuracy not only with isodesmic and homodesmotic reactions, but also with isogyric reactions. Moreover, the DFT and MP2 methods can also yield a high accuracy estimate of the enthalpy of formation when they are used with isodesmic or homodesmotic reactions. Four composite methods (G4, G3, CBS-APNO, and CBS-QB3) employed in conjunction with 45 isogyric reactions yield the enthalpy of formation of hydrazine (97.7±2.0kJ/mol) in agreement with the best approximating to CCSD(T)/CBS energy, thus calling into question the usually quoted experimental value of 95.5kJ/mol.

Calculated bond dissociation energies and enthalpy of formation of α-amino acid radicals

Bond dissociation energies (BDEs), radical stabilization energies (RSEs) and the enthalpies of formation (Δf H°( $${\text{AA}}^{\bullet}$$ )) for the Cα-centered radical of 20 common α-amino acids were calculated at the G3MP2 level of theory. In addition, BDEs and Δf H°( $${\text{AA}}^{\bullet}$$ ) for the Cα-centered radicals were calculated via atomization, homolytic, isogyric and isodesmic reactions using 31 reference compounds. RSEs for the α-amino acid radicals reveal a strong captodative effect due to the resonance stabilization. Intramolecular H-bonding and steric repulsion between the side chains and –NH2 and ‒COOH also have an effect on the stability of α-carbon-centered radicals. For the key small reference compounds investigated, the BDEs (Δf H°( $${\text{AA}}^{\bullet}$$ )) computed with isodesmic reactions were found to be quite consistent with each other in terms of mean absolute deviations (MADs) and root-mean-square deviations between 2.0 (2.6) and 3.2 (4.4) kJ mol−1 from experimental data.

Construction and application of a new dual-hybrid random phase approximation.

The direct random phase approximation (dRPA) combined with Kohn-Sham reference orbitals is among the most promising tools in computational chemistry and applicable in many areas of chemistry and physics. The reason for this is that it scales as N(4) with the system size, which is a considerable advantage over the accurate ab initio wave function methods like standard coupled-cluster. dRPA also yields a considerably more accurate description of thermodynamic and electronic properties than standard density-functional theory methods. It is also able to describe strong static electron correlation effects even in large systems with a small or vanishing band gap missed by common single-reference methods. However, dRPA has several flaws due to its self-correlation error. In order to obtain accurate and precise reaction energies, barriers and noncovalent intra- and intermolecular interactions, we construct a new dual-hybrid dRPA (hybridization of exact and semilocal exchange in both the energy and the orbitals) and test the performance of this new functional on isogyric, isodesmic, hypohomodesmotic, homodesmotic, and hyperhomodesmotic reaction classes. We also use a test set of 14 Diels-Alder reactions, six atomization energies (AE6), 38 hydrocarbon atomization energies, and 100 reaction barrier heights (DBH24, HT-BH38, and NHT-BH38). For noncovalent complexes, we use the NCCE31 and S22 test sets. To test the intramolecular interactions, we use a set of alkane, cysteine, phenylalanine-glycine-glycine tripeptide, and monosaccharide conformers. We also discuss the delocalization and static correlation errors. We show that a universally accurate description of chemical properties can be provided by a large, 75% exact exchange mixing both in the calculation of the reference orbitals and the final energy.

Assessment of the second-order perturbative corrections to PNOF5

In a recent paper (J. Chem. Phys. 139, 064111, 2013), an antisymmetrised product of strongly orthogonal geminals with the expansion coefficients explicitly expressed by means of the occupation numbers was used to generate the Piris natural orbital functional 5 (PNOF5). This functional describes most of the non-dynamical effects, but also an important part of the intrapair (intrageminal) electron correlation. Second-order corrections to the generating PNOF5 wave function were derived using the multiconfigurational perturbation theory size consistent at the second order (SC2-MCPT) to include the missing interpair (intergeminal) electron correlation. A modified version of the SC2-MCPT involving double excitations only from different geminals was introduced and denoted as PNOF5-PT2. In this paper, the ground-state energies of 36 closed-shell species belonging to the G2/97 test set of molecules are studied by the PNOF5-PT2 and PNOF5-SC2-MCPT methods. The numerical performance of both methods on eight dimers, with different strength of hydrogen bonds, and 13 isogyric reactions is also assessed. The results are in reasonable agreement with those obtained using the complete active space second-order perturbation theory and coupled-cluster method with singles, doubles, and noniterative triples corrections.

Ab Initio Chemical Kinetics for H + NCN: Prediction of NCN Heat of Formation and Reaction Product Branching via Doublet and Quartet Surfaces

The reaction of NCN with H atoms has been investigated by ab initio MO and RRKM theory calculations. The mechanisms for formation of major products on the doublet and quartet potential energy surfaces have been predicted at the CCSD(T) level of theory with the complete basis set limit. In addition, the heat of formation for NCN predicted at this rigorous level and those from five isogyric reactions are in close agreement with the best value based on the isodesmic process, (3)CCO + N2 = (3)NCN + CO, 109.4 kcal/mol, which lies within the two existing experimental values. The rate constants for the three possible reaction channels, H + NCN → CH + N2 (k(P1)), HCN + (4)N (k(QP1)), and HNC + (4)N (k(QP2)), have been calculated in the temperature range 298-3000 K. The results show that k(P1) is significantly higher than k(QP1) and k(QP2) and that the total rate constant agrees well with available experimental values in the whole temperature range studied. The kinetics of the reverse CH + N2 reaction has also been revisited at the CCSD(T)/CBS level; the predicted total rate constants at 760 Torr Ar pressure can be represented by kr = 4.01 × 10(-15) T(0.90) exp(-17.42 kcal mol(-1)/RT) cm(3) molecule(-1) s(-1) at T = 800-4000 K. The result agrees closely with the most recent experimental data and the best theoretical result of Harding et al. (J. Phys. Chem. A 2008, 112, 522) as well as that of Moskaleva and Lin (Proc. Combust. Inst. 2000, 28, 2393) evaluated with a steady-state approximation after a coding error correction made in this study.

Enthalpies of formation of nitromethane and nitrobenzene: Theory vs experiment

Enthalpies of formation of gaseous nitromethane and nitrobenzene are calculated by Gaussian-4 method with the use of isogyric and isodesmic reactions. Of the two widely-used experimental values for nitromethane (−74.5 and −81.0kJ·mol−1), the calculations provide strong support for the first one. The cited experimental values of enthalpy of formation of nitrobenzene (67.5 and 68.5kJ·mol−1) are based on the only calorimetric investigation; the both these values look to be a somewhat overestimated when compared with the average result (62.8kJ·mol−1) obtained from 95 isogyric and isodesmic reactions with 26 reference nitro compounds involved. The purpose of the present work is to attract attention of experimentalists to new experimental investigation of nitrobenzene, and probably nitromethane as well.

Prediction of Reaction Barriers and Thermochemical Properties with Explicitly Correlated Coupled-Cluster Methods: A Basis Set Assessment

We assessed the performance of our perturbative explicitly correlated coupled-cluster method, CCSD(T)F12, for accurate prediction of chemical reactivity. The reference data included reaction barrier heights, electronic reaction energies, atomization energies, and enthalpies of formation from the following sources: (1) the DBH24/08 database of 22 reaction barriers (Truhlar et al.), (2) the HJO12 set of isogyric reaction energies (Helgaker et al.), and (3) a HEAT set of atomization energies and heats of formation (Stanton et al.). We performed two types of analyses targeting the two distinct uses of explicitly correlated CCSD(T) models: as a replacement for basis-set-extrapolated CCSD(T) in highly accurate composite methods like HEAT and as a distinct model chemistry for standalone applications. Hence, we analyzed in detail (1) the basis set error of each component of the CCSD(T)F12 contribution to the chemical energy difference in question and (2) the total error of the CCSD(T)F12 model chemistry relative to the benchmark values. Two basis set families were utilized in the calculations: the standard aug-cc-p(C)VXZ-F12 (X = D, T, Q) basis sets for the conventional correlation methods and the cc-p(C)VXZ-F12 (X = D, T, Q) basis sets of Peterson and co-workers that are specifically designed for explicitly correlated methods. Our conclusion is that the performance of the two families for CCSD correlation contributions (which are the only components affected by the explicitly correlated terms in our formation) are nearly identical with triple- and quadruple-ζ quality basis sets, with some differences at the double-ζ level. Chemical accuracy (∼4.18 kJ/mol) for reaction barrier heights, electronic reaction energies, atomization energies, and enthalpies of formation is attained on average with the aug-cc-pVDZ, aug-cc-pVTZ, cc-pCVTZ-F12/aug-cc-pCVTZ, and cc-pCVDZ-F12 basis sets, respectively, at the CCSD(T)F12 level of theory. The corresponding mean unsigned errors are 1.72 kJ/mol, 1.5 kJ/mol, ∼2 kJ/mol, and 2.17 kJ/mol, and the corresponding maximum unsigned errors are 4.44 kJ/mol, 3.6 kJ/mol, ∼5 kJ/mol, and 5.75 kJ/mol.

Explicitly correlated second-order Møller-Plesset perturbation theory employing pseudospectral numerical quadratures

We implemented explicitly correlated second-order Møller-Plesset perturbation theory with numerical quadratures using pseudospectral construction of grids. Introduction of pseudospectral approach for the calculation of many-electron integrals gives a possibility to use coarse grids without significant loss of precision in correlation energies, while the number of points in the grid is reduced about nine times. The use of complementary auxiliary basis sets as the sets of dealiasing functions is justified at both theoretical and computational levels. Benchmark calculations for a set of 16 molecules have shown the possibility to keep an error of second-order correlation energies within 1 milihartree (mH) with respect to MP2-F12 method with dense grids. Numerical tests for a set of 13 isogyric reactions are also performed.

An ab initio chemical kinetic study on the reactions of H, OH, and Cl with HOClO3

AbstractThe mechanisms for reactions of H, HO, and Cl with HOClO3, important elementary processes in the early stages of the ammonium perchlorate (AP) combustion reaction, have been investigated at the CCSD(T)/6‐311+G(3df,2p)//PW91PW91/6‐311+G(3df) level of theory. The rate constants for the low‐energy channels have been calculated by statistical theory. For the reaction of H and HOClO3, the main channels are the production of H2 + ClO4 (k1a) and HO + HOClO2 (k1b); k1a and k1b can be represented as 1.07 × 10−17 T1.97 exp(−7484/T) and 6.08 × 10−17T1.96 exp(−7729/T) cm3 molecule−1 s−1, respectively. For the HO + HOClO3 reaction, the main pathway is the H2O + ClO4 (k2a) production process, with the predicted rate constant k2a = 1.24 × 10 −8 T−2.99 exp(1664/T) for 300–500 K and k2a = 1.27×10−19 T2.12 exp(−1474/T) for 500–3000 K. For the Cl + HOClO3 reaction, the formation of HOCl + ClO3 (k3a) and HCl + ClO4 (k3b) is dominant, with k3a = 1.33×10−12 T0.67 exp(−9658/T) and k3b = 1.75×1016 T1.63 exp(−11156/T) cm3 molecules−1 in the range of 300–3000 K. In addition, the heats of formation of ClO3 and HOClO3 have been predicted based on several isodesmic and/or isogyric reactions with ΔfHo0 (ClO3) = 47.0 ± 1.0 and ΔfHo0 (HOClO3) = 5.5 ± 1.5 kcal/mol, respectively. These data may be used for kinetic simulation of the AP decomposition and combustion reaction. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 253–261, 2010