First principles based group additive values for the gas phase standard entropy and heat capacity of hydrocarbons and hydrocarbon radicals

In this work a complete and consistent set of 95 Benson group additive values (GAVs) for standard entropies S(o) and heat capacities C(p)(o) of hydrocarbons and hydrocarbon radicals is presented. These GAVs include 46 groups, among which 25 radical groups, which, to the best of our knowledge, have not been reported before. The GAVs have been determined from a set of B3LYP/6-311G(d,p) ideal gas statistical thermodynamics values for 265 species, consistently with previously reported GAVs for standard enthalpies of formation. One-dimensional hindered rotor corrections for all internal rotations are included. The computational methodology has been compared to experimental entropies (298 K) for 39 species, with a mean absolute deviation (MAD) between experiment and calculation of 1.2 J mol(-1) K(-1), and to 46 experimental heat capacities (298 K) with a resulting MAD = 1.8 J mol(-1) K(-1). The constructed database allowed evaluation of corrections on S(o) and C(p)(o) for non-nearest-neighbor effects, which have not been determined previously. The group additive model predicts the S(o) and C(p)(o) within approximately 5 J mol(-1) K(-1) of the ab initio values for 11 of the 14 molecules of the test set, corresponding to an acceptable maximal deviation of a factor of 1.6 on the equilibrium coefficient. The obtained GAVs can be applied for the prediction of S(o) and C(p)(o) for a wide range of hydrocarbons and hydrocarbon radicals. The constructed database also allowed determination of a large set of hydrogen bond increments, which can be useful for the prediction of radical thermochemistry.     

DFT and AIM study of the protonation of nitrous acid and the pKa of nitrous acidium ion

The gas phase and aqueous thermochemistry, NMR chemical shifts, and the topology of chemical bonding of nitrous acid (HONO) and nitrous acidium ion (H(2)ONO(+)) have been investigated by ab initio methods using density functional theory. By the same methods, the dissociation of H(2)ONO(+) to give the nitrosonium ion (NO(+)) and water has also been investigated. We have used Becke's hybrid functional (B3LYP), and geometry optimizations were performed with the 6-311++G(d,p) basis set. In addition, highly accurate ab initio composite methods (G3 and CBS-Q) were used. Solvation energies were calculated using the conductor-like polarizable continuum model, CPCM, at the B3LYP/6-311++G(d,p) level of theory, with the UAKS cavity model. The pK(a) value of H(2)ONO(+) was calculated using two different schemes: the direct method and the proton exchange method. The calculated pK(a) values at different levels of theory range from -9.4 to -15.6, showing that H(2)ONO(+) is a strong acid (i.e., HONO is only a weak base). The equilibrium constant, K(R), for protonation of nitrous acid followed by dissociation to give NO(+) and H(2)O has also been calculated using the same methodologies. The pK(R) value calculated by the G3 and CBS-QB3 methods is in best (and satisfactory) agreement with experimental results, which allows us to narrow down the likely value of the pK(a) of H(2)ONO(+) to about -10, a value appreciably more acidic than literature values.     

Quantum chemical computations of 1,3-phenylenediacetic acid

Molecular geometry, vibrational wavenumbers and gauge including atomic orbital (GIAO) 13C NMR and 1H NMR chemical shift values of 1,3-phenylenediacetic acid (C10H10O4), in the ground state have been calculated by using ab initio Hartree-Fock (HF) and density functional theory (DFT/B3LYP) methods with 6-311++G(d,p) basis set for the first time. Comparison of the observed fundamental vibrational modes of 1,3-phenylenediacetic acid and calculated results by density functional B3LYP and Hartree-Fock methods indicate that B3LYP is superior to the scaled Hartree-Fock approach for quantum chemical studies. Geometric parameters (bond lengths and bond angles) and vibrational wavenumbers obtained by the HF and DFT/B3LYP methods are in good agreement with the experimental data. Furthermore, this is the first time the results of the calculated JCH and JCC coupling constants of the C10H10O4 molecule are presented in this study.     

Group Additive Values for the Gas‐Phase Standard Enthalpy of Formation,Entropy and Heat Capacity of Oxygenates

A complete and consistent set of 60 Benson group additive values (GAVs) for oxygenate molecules and 97 GAVs for oxygenate radicals is provided, which allow to describe their standard enthalpies of formation, entropies and heat capacities. Approximately half of the GAVs for oxygenate molecules and the majority of the GAVs for oxygenate radicals have not been reported before. The values are derived from an extensive and accurate database of thermochemical data obtained by ab initio calculations at the CBS‐QB3 level of theory for 202 molecules and 248 radicals. These compounds include saturated and unsaturated, α‐ and β‐branched, mono‐ and bifunctional oxygenates. Internal rotations were accounted for by using one‐dimensional hindered rotor corrections. The accuracy of the database was further improved by adding bond additive corrections to the CBS‐QB3 standard enthalpies of formation. Furthermore, 14 corrections for non‐nearest‐neighbor interactions (NNI) were introduced for molecules and 12 for radicals. The validity of the constructed group additive model was established by comparing the predicted values with both ab initio calculated values and experimental data for oxygenates and oxygenate radicals. The group additive method predicts standard enthalpies of formation, entropies, and heat capacities with chemical accuracy, respectively, within 4 kJ mol^?1 and 4 J mol^?1 K^?1 for both ab initio calculated and experimental values. As an alternative, the hydrogen bond increment (HBI) method developed by Lay et al. (T. H. Lay, J. W. Bozzelli, A. M. Dean, E. R. Ritter, J. Phys. Chem.­ 1995 , 99, 14514) was used to introduce 77 new HBI structures and to calculate their thermodynamic parameters (Δ_fH°, S°, C_p°). The GAVs reported in this work can be reliably used for the prediction of thermochemical data for large oxygenate compounds, combining rapid prediction with wide‐ranging application.     

Quantum chemical study of silicon pentacoordination in (Aroyloxymethyl)trifluorosilanes and their analogs

Pentacoordinated silicon compounds of the series 4-XC₆H₄C(O)O(CH₂)_mSi(CH₃)_3-nF_n (m = 1, 2; n= 1,2,3) with an intramolecular 0→Si bond are studied by ab initia and semiempirical (AMI) quantum chemical methods. The results are compared with published experimental data. The C₆H₅C(O)OCH₂SiF₃ molecule is calculated in an RHF approximation using the 6–31G*basis set. The total energy of the molecule for its geometry optimization is calculated by the MP2 method including electron correlation. This leads to considerably improved agreement between the calculated coordination energy (25.3 kJ/mole) and the experimental value (28.5 kJ/mole). The geometry and the dipole moment calculated by both ab initio (HF/6-31G*//HF/6-31G*, MP2/6-31G*//MP2/6-31G*) methods and by the AMI method are in satisfactory agreement with the experimental data.     

Geometry optimization and electronic structures of molecules H_3AXAH_3

The geometries of molecules H_3AXAH_3(X=O,S,Se and A=C,Si)have been optimizedusing STO-3G ab initio calculations and gradient method and the results are in good agreement withreported experimental values.From the STO-3G optimized geometries,we have also calculated theelectronic structures of these molecules using 4-31G and 6-31G basis sets to obtain the MO energies.atomic net charges and dipole moments.The ionization potentials calculated by 6-31G basis set are ingood agreement with experimental values.     

The effect of stoichiometry on ab initio-based thermochemistry predictions

The effect of various reaction schemes used to convert total reaction enthalpies obtained via ab initio methods into enthalpies of formation has been studied employing a new optimization technique. The reaction schemes, conventional, isostoichiometric, and isodesmic have been defined and generated for 47 species for which highly accurate experimental enthalpies of formation in addition to total reaction enthalpies at 298 K at G2 and G3 levels of theory are known. The main finding is that the effect of reaction schemes in ab initio-based thermochemistry predictions is small, especially, for enthalpy predictions involving high-level ab initio methods.     

Theoretically Calculated Deformation Density of Small Molecules

Analysis of the theoretical electron deformation density based on EHMO and ab initio calculations has been applied to the simple molecules F₂, H₂O and SO₂ The effects from varied basis sets for such deformation density were sought. The accumulation of electron density between the bonded atoms calculated from EHMO and ab initio methods with STO-3G is generally under-estimated. Such phenomena are significantly improved by using split-valence basis sets e.g. 3–21G and 4–31G. The addition of d polarization functions is apparently important for the sulfur atom in sulfur-related bonding. 3–21G or 3–21G^* basis sets were found to provide not only valuable deformation density distributions of molecules but also comparable orbital energy states with respect to the experimental values.     

The computation of the potential energy surface for H2 + OH → H2O + H using ab initio and density functional theory methods

The reaction energy profile for H₂ + OH → H + H₂O was computed using HF, MP2, MP4, QCISD, G1, G2, and G2MP2 ab initio methods. In addition, the B3LYP, B3P86, B3PW91, BLYP, BP291, and SVWN density functional theory (DFT) methods were also used. All the ab initio methods, with the exception of the G series, produced much higher activation barriers and heats of reaction than the experimental values. On the other hand, the DFT methods produced negative forward and reverse barriers which were too low, with the exception of the hybrid DFT methods. The G2 ab initio method generated energies which deviated from the experimental values by ∼ 1 kcal/mol and therefore should be considered a very accurate computational method. The hybrid DFT methods produced positive forward reaction barriers with energies that were 2–4 kcal/mol lower than the experimental values. The geometries of the transition state and energies computed by the ab initio and DFT methods were compared. These results suggest that, in the hybrid exchange functional, the portion of the Slater exchange term should be increased. This may be the reason why the computed energies were too low. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62: 639–644, 1997     

Computational study of the thermochemistry of organophosphorus(III) compounds

The enthalpies of formation of organophosphorus(III) compounds have been calculated at the G3X, G3X(MP2), and B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) levels of theory using the atomization energy procedure and the method of isodesmic reactions. The Delta f H298 degree values for 50 relatively large molecules with up to 10 non-hydrogen atoms, such as P(CH3)3, P(C2H5)3, P(OCH3)3, n-C4H9OPCl2, [(CH3)2N]2PCl, (C2H5)2NPCl2, and [(CH3)2N]2PCN, have been calculated directly from the G3X atomization energies. A good agreement between the known experimental values and G3X results for 14 compounds provides support to our predictions for remaining species whose experimental enthalpies of formation are unknown or known with relatively large uncertainties. On the basis of our calculations and sometimes conflicting experimental data a set of internally consistent enthalpies of formation has been recommended for organophosphorus(III) compounds. Our computational results call into question the experimental enthalpies of formation of P(C2H5)3 and P(n-C4H9)3. From comparison with most reliable experimental data, the accuracy of the theoretical enthalpies of formation is estimated as ranging from 5 to 10 kJ/mol. The recommended Delta f H298 degree values were used to derive the group additivity values (GAVs) for 45 groups involving the phosphorus(III) atom. These GAVs significantly extend the applicability of Benson's group additivity method and may be used to estimate the enthalpies of formation of larger organophosphorus(III) compounds, where high level quantum chemical calculations are impracticable.     

The C2H3 + O2 reaction revisited: is multireference treatment of the wave function really critical?

This letter revisits critical intermediates and transition states of the C2H3 + O2 reaction. To obtain their accurate relative energies, ab initio calculations are performed using sophisticated single and multireference theoretical methods with various basis sets. The energy difference between two crucial transition states, for ring opening in dioxiranylmethyl radical and its isomerization to C2H3OO, is calculated as approximately 2 kcal/mol both at multireference MRCI and at single-reference CCSD(T) levels extrapolated to the complete basis set limit. The deviation from the earlier G2M(RCC,MP2) value (approximately 7 kcal/mol) is caused by a deficiency of the 6-311+G(3df,2p) basis set as compared to correlation-consistent Dunning's basis sets.     

Hydroxyl-radical-initiated oxidation mechanism of bromopropane

Bromopropane has been considered as a replacement for chlorofluorocarbons used as the active component of industrial cleaning solvents, more specifically for HCFC-141b. The proposed mechanism for the atmospheric oxidation of bromopropane is studied via ab initio methodology. Ab initio molecular orbital methods at the CCSD(T)/6-311++G(2df,2p)//MP2/6-31G(d) level of theory have been used to determine the structure and energetics of the 58 species and transition states involved in the atmospheric oxidation of bromopropane. The calculations show that the major oxidation species is bromoacetone. Other brominated species that result from the oxidation are BrCH 2CH 2C(O)H, BrC(O)CH 2CH 3, and BrC(O)H, potential new bromine reservoir species that result from bromopropane in the atmosphere.     

Ab initio thermochemistry of some halogenated cyclopropanes

The standard enthalpies of formation for a series of chloro- and fluoro-substituted cyclopropanes have been calculated by using high-level ab initio G3/B3LYP methods. The relative stabilities of isomers and the influence of substituents on thermochemistry in several classes of substituted derivatives are discussed.     

双氮桥联1,10-菲咯啉大环化合物分子构型的从头算研究

应用规范不变原子轨道法(GIAO)在RHF/6-31G**和B3LYP/6-31G**水平上计算了质子化双氮桥联1,10-菲咯啉大环化合物(H4HAPP2+)C2h和C2h构型的1HNMR,并用TDDFT法计算了H4HAPP2+电子光谱.结果表明,B3LYP/6-31G*优化的C2h构型为较优构型,经谐振频率验证无虚频,C2h构型是H4HAPP2+合理的对称性构型.     

Adsorption of NH(3) and H(2)O in acidic chabazite. Comparison of ONIOM approach with periodic calculations

The adsorption of NH(3) and H(2)O in acidic chabazite has been studied with the B3LYP method within the cluster approach (5T, 48T clusters) and the periodic approach adopting a Si/Al = 11/1 chabazite and a basis set of polarized double-zeta quality. The 5T cluster has been treated fully ab initio at the B3LYP level whereas the 48T cluster has been treated with the ONIOM2 scheme using B3LYP as the high level of theory and the MNDO, AM1, and HF/3-21G methods as low levels of theory. Periodic calculations show that the adsorption of NH(3) in acidic chabazite takes place through an ion pair (NH(4)(+)-CHA(-)) structure, the computed adsorption energy being -32 kcal/mol. The adsorption of H(2)O leads to a hydrogen bonded (H(2)O-HCHA) complex with the computed adsorption energy of -20 kcal/mol. All ONIOM combinations provide similar structures to those obtained with periodic calculations. Adsorption energies, however, are sensitive to the low level used, especially for NH(3). The ONIOM B3LYP:HF/3-21G method is the one that provides more satisfactory results. Present results show that, for larger zeolites, the ONIOM scheme can be successfully applied while drastically reducing the cost of a fully ab initio treatment.     

Thermochemistry of aqueous hydroxyl radical from advances in photoacoustic calorimetry and ab initio continuum solvation theory

Photoacoustic signals from dilute ( approximately 30 mM) solutions of H2O2 were measured over the temperature range from 10 to 45 degrees C to obtain the reaction enthalpy and volume change for H2O2(aq) --> 2 OH(aq) from which we ultimately determined DeltafG degrees , DeltafH degrees and partial molal volume, v degrees , of OH (aq). We find DeltarH = 46.8 +/- 1.4 kcal/mol, which is 4 kcal/mol smaller than the gas-phase bond energy, and DeltaVr = 6.5 +/- 0.4 mL/mol. The v degrees for OH in water is 14.4 +/- 0.4 mL/ml: smaller than the v degrees of water. Using ab intio continuum theory, the hydration free energy is calculated to be -3.9 +/- 0.3 kcal/mol (for standard states in number density concentration units) by a novel approach devised to capture in the definition of the solute cavity the strength and specific interactions of the solute with a water solvent molecule. The shape of the cavity is defined by "rolling" a three-dimensional electron density isocontour of water on the ab initio water-OH minimum interaction surface. The value of the contour is selected to reproduce the volume of OH in water. We obtain for OH(aq): DeltafH degrees = -0.2 +/- 1.4 and DeltafG degrees = 5.8 +/- 0.4 kcal/mol that are in agreement with literature values. The results provide confidence in the pulsed PAC technique for measuring aqueous thermochemistry of radicals and open the way to obtaining thermochemistry for most radicals that can be formed by reaction of OH with aqueous substrates while advancing the field of continuum solvation theory toward ab initio-defined solute cavities.     

Thermochemistry of chlorine‐containing hydrocarbons related to waste combustion

The thermochemistry of 25 chlorinated compounds potentially involved in chemical reactions related to waste combustion has been calculated using ab initio methods. Some of these species have never been reported in the literature. To check the validity of our calculations, 11 additional brominated analogues were also calculated. The thermochemical properties of these compounds were used in reaction kinetic models designed to estimate the performance of waste incinerators. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 113–122, 2009     

Highly accurate first-principles benchmark data sets for the parametrization and validation of density functional and other approximate methods. Derivation of a robust, generally applicable, double-hybrid functional for thermochemistry and thermochemical kinetics

We present a number of near-exact, nonrelativistic, Born-Oppenheimer reference data sets for the parametrization of more approximate methods (such as DFT functionals). The data were obtained by means of the W4 ab initio computational thermochemistry protocol, which has a 95% confidence interval well below 1 kJ/mol. Our data sets include W4-08, which are total atomization energies of over 100 small molecules that cover varying degrees of nondynamical correlations, and DBH24-W4, which are W4 theory values for Truhlar's set of 24 representative barrier heights. The usual procedure of comparing calculated DFT values with experimental atomization energies is hampered by comparatively large experimental uncertainties in many experimental values and compounds errors due to deficiencies in the DFT functional with those resulting from neglect of relativity and finite nuclear mass. Comparison with accurate, explicitly nonrelativistic, ab initio data avoids these issues. We then proceed to explore the performance of B2x-PLYP-type double hybrid functionals for atomization energies and barrier heights. We find that the optimum hybrids for hydrogen-transfer reactions, heavy-atoms transfers, nucleophilic substitutions, and unimolecular and recombination reactions are quite different from one another: out of these subsets, the heavy-atom transfer reactions are by far the most sensitive to the percentages of Hartree-Fock-type exchange y and MP2-type correlation x in an (x, y) double hybrid. The (42,72) hybrid B2K-PLYP, as reported in a preliminary communication, represents the best compromise between thermochemistry and hydrogen-transfer barriers, while also yielding excellent performance for nucleophilic substitutions. By optimizing for best overall performance on both thermochemistry and the DBH24-W4 data set, however, we find a new (36,65) hybrid which we term B2GP-PLYP. At a slight expense in performance for hydrogen-transfer barrier heights and nucleophilic substitutions, we obtain substantially better performance for the other reaction types. Although both B2K-PLYP and B2GP-PLYP are capable of 2 kcal/mol quality thermochemistry, B2GP-PLYP appears to be the more robust toward nondynamical correlation and strongly polar character. We additionally find that double-hybrid functionals display excellent performance for such problems as hydrogen bonding, prototype late transition metal reactions, pericyclic reactions, prototype cumulene-polyacetylene system, and weak interactions.     

Vibrational mode frequencies of silica species in SiO2-H2O liquids and glasses from ab initio molecular dynamics

Vibrational spectroscopy techniques are commonly used to probe the atomic-scale structure of silica species in aqueous solution and hydrous silica glasses. However, unequivocal assignment of individual spectroscopic features to specific vibrational modes is challenging. In this contribution, we establish a connection between experimentally observed vibrational bands and ab initio molecular dynamics (MD) of silica species in solution and in hydrous silica glass. Using the mode-projection approach, we decompose the vibrations of silica species into subspectra resulting from several fundamental structural subunits: The SiO(4) tetrahedron of symmetry T(d), the bridging oxygen (BO) Si-O-Si of symmetry C(2v), the geminal oxygen O-Si-O of symmetry C(2v), the individual Si-OH stretching, and the specific ethane-like symmetric stretching contribution of the H(6)Si(2)O(7) dimer. This allows us to study relevant vibrations of these subunits in any degree of polymerization, from the Q(0) monomer up to the fully polymerized Q(4) tetrahedra. Demonstrating the potential of this approach for supplementing the interpretation of experimental spectra, we compare the calculated frequencies to those extracted from experimental Raman spectra of hydrous silica glasses and silica species in aqueous solution. We discuss observed features such as the double-peaked contribution of the Q(2) tetrahedral symmetric stretch, the individual Si-OH stretching vibrations, the origin of the experimentally observed band at 970 cm(-1) and the ethane-like vibrational contribution of the H(6)Si(2)O(7) dimer at 870 cm(-1).     

Sensitivity of the H + CH3 → CH4 recombination rate constant to the shape of the CH stretching potential

Using a potential-energy surface obtained in part from ab initio calculations, the H + CH₃ → CH₄ bimolecular rate constant at T = 300 K is determined from a Monte Carlo classical trajectory study. Representing the CH stretching potential with a standard Morse function instead ofthe ab initio curve increases the calculated rate constant by an order of magnitude. The experimental recombination rate constant is intermediate of the rate constants calculated with the Morse and ab initio stretching potentials.Two properties of the H + CH₃ α CH₄ potential-energy surface which significantly affect the recombination rate constant are the shape of the CH stretching potential and the attenuation of the H₃CH bending frequencies. Ab initio calculations with a hierarchy of basis sets and treatment of electron correlation indicate the latter is properly described [13]. The exact shape of the CH stretching potential is not delineated by the ab initio calculations, since the ab initio calculations are not converged for bond lengths of 2.0–3.0 Å [12]. However, the form of this stretching potential deduced from the highest-level ab initio calculations, and fit analytically by eq. (2), is significantly different from a Morse function. The experimental recombination rate constant is intermediate of the rate constants calculated with the Morse and ab initio CH stretching potentials. This indicates that the actual CH potential energy curve lies between the Morse and ab initio curves. This is consistent with the finding that potential energy curves for diatomics are not well described by a Morse function [12].