Kinetic Modeling of α‐Hydrogen Abstractions from Unsaturated and Saturated Oxygenate Compounds by Carbon‐Centered Radicals

Hydrogen abstractions are important elementary reactions in a variety of reacting media at high temperatures in which oxygenates and hydrocarbon radicals are present. Accurate kinetic data are obtained from CBS‐QB3 ab initio (AI) calculations by using conventional transition‐state theory within the high‐pressure limit, including corrections for hindered rotation and tunneling. From the obtained results, a group‐additive (GA) model is developed that allows the Arrhenius parameters and rate coefficients for abstraction of the α‐hydrogen from a wide range of oxygenate compounds to be predicted at temperatures ranging from 300 to 1500 K. From a training set of 60 hydrogen abstractions from oxygenates by carbon‐centered radicals, 15 GA values (ΔGAV^os) are obtained for both the forward and reverse reactions. Among them, four ΔGAV^os refer to primary contributions, and the remaining 11 ΔGAV^os refer to secondary ones. The accuracy of the model is further improved by introducing seven corrections for cross‐resonance stabilization of the transition state from an additional set of 43 reactions. The determined ΔGAV^os are validated upon a test set of AI data for 17 reactions. The mean absolute deviation of the pre‐exponential factors (log A) and activation energies (E_a) for the forward reaction at 300 K are 0.238 log(m³ mol^?1 s^?1) and 1.5 kJ mol^?1, respectively, whereas the mean factor of deviation <ρ> between the GA‐predicted and the AI‐calculated rate coefficients is 1.6. In comparison with a compilation of 33 experimental rate coefficients, the <ρ> between the GA‐predicted values and these experimental values is only 2.2. Hence, the constructed GA model can be reliably used in the prediction of the kinetics of α‐hydrogen‐abstraction reactions between a broad range of oxygenates and oxygenate radicals.     

Modeling the gas-phase thermochemistry of organosulfur compounds

Key to understanding the involvement of organosulfur compounds in a variety of radical chemistries, such as atmospheric chemistry, polymerization, pyrolysis, and so forth, is knowledge of their thermochemical properties. For organosulfur compounds and radicals, thermochemical data are, however, much less well documented than for hydrocarbons. The traditional recourse to the Benson group additivity method offers no solace since only a very limited number of group additivity values (GAVs) is available. In this work, CBS‐QB3 calculations augmented with 1D hindered rotor corrections for 122 organosulfur compounds and 45 organosulfur radicals were used to derive 93 Benson group additivity values, 18 ring‐strain corrections, 2 non‐nearest‐neighbor interactions, and 3 resonance corrections for standard enthalpies of formation, standard molar entropies, and heat capacities for organosulfur compounds and organosulfur radicals. The reported GAVs are consistent with previously reported GAVs for hydrocarbons and hydrocarbon radicals and include 77 contributions, among which 26 radical contributions, which, to the best of our knowledge, have not been reported before. The GAVs allow one to estimate the standard enthalpies of formation at 298 K, the standard entropies at 298 K, and standard heat capacities in the temperature range 300–1500 K for a large set of organosulfur compounds, that is, thiols, thioketons, polysulfides, alkylsulfides, thials, dithioates, and cyclic sulfur compounds. For a validation set of 26 organosulfur compounds, the mean absolute deviation between experimental and group additively modeled enthalpies of formation amounts to 1.9 kJ mol^?1. For an additional set of 14 organosulfur compounds, it was shown that the mean absolute deviations between calculated and group additively modeled standard entropies and heat capacities are restricted to 4 and 2 J mol^?1 K^?1, respectively. As an alternative to Benson GAVs, 26 new hydrogen‐bond increments are reported, which can also be useful for the prediction of radical thermochemistry.     

Group additive values for the gas phase standard enthalpy of formation of hydrocarbons and hydrocarbon radicals

A complete and consistent set of 95 Benson group additive values (GAV) for the standard enthalpy of formation of hydrocarbons and hydrocarbon radicals at 298 K and 1 bar is derived from an extensive and accurate database of 233 ab initio standard enthalpies of formation, calculated at the CBS-QB3 level of theory. The accuracy of the database was further improved by adding newly determined bond additive corrections (BAC) to the CBS-QB3 enthalpies. The mean absolute deviation (MAD) for a training set of 51 hydrocarbons is better than 2 kJ mol(-1). GAVs for 16 hydrocarbon groups, i.e., C(C(d))(3)(C), C-(C(d))(4), C-(C(t))(C(d))(C)(2), C-(C(t))(C(d))(2)(C), C-(C(t))(C(d))(3), C-(C(t))(2)(C)(2), C-(C(t))(2)(C(d))(C), C-(C(t))(2)(C(d))(2), C-(C(t))(3)(C), C-(C(t))(3)(C(d)), C-(C(t))(4), C-(C(b))(C(d))(C)(H), C-(C(b))(C(t))(H)(2), C-(C(b))(C(t))(C)(H), C-(C(b))(C(t))(C)(2), C(d)-(C(b))(C(t)), for 25 hydrocarbon radical groups, and several ring strain corrections (RSC) are determined for the first time. The new parameters significantly extend the applicability of Benson's group additivity method. The extensive database allowed an evaluation of previously proposed methods to account for non-next-nearest neighbor interactions (NNI). Here, a novel consistent scheme is proposed to account for NNIs in radicals. In addition, hydrogen bond increments (HBI) are determined for the calculation of radical standard enthalpies of formation. In particular for resonance stabilized radicals, the HBI method provides an improvement over Benson's group additivity method.     

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.     

Hydrogen Radical Additions to Unsaturated Hydrocarbons and the Reverse β‐Scission Reactions: Modeling of Activation Energies and Pre‐Exponential Factors

The group additivity method for Arrhenius parameters is applied to hydrogen addition to alkenes and alkynes and the reverse β‐scission reactions, an important family of reactions in thermal processes based on radical chemistry. A consistent set of group additive values for 33 groups is derived to calculate the activation energy and pre‐exponential factor for a broad range of hydrogen addition reactions. The group additive values are determined from CBS‐QB3 ab‐initio‐calculated rate coefficients. A mean factor of deviation of only two between CBS‐QB3 and experimental rate coefficients for seven reactions in the range 300–1000 K is found. Tunneling coefficients for these reactions were found to be significant below 400 K and a correlation accounting for tunneling is presented. Application of the obtained group additive values to predict the kinetics for a set of 11 additions and β‐scissions yields rate coefficients within a factor of 3.5 of the CBS‐QB3 results except for two β‐scissions with severe steric effects. The mean factor of deviation with respect to experimental rate coefficients of 2.0 shows that the group additive method with tunneling corrections can accurately predict the kinetics and is at least as accurate as the most commonly used density functional methods. The constructed group additive model can hence be applied to predict the kinetics of hydrogen radical additions for a broad range of unsaturated compounds.     

From a Network of Computed Reaction Enthalpies to Atom‐Based Thermochemistry (NEAT)

A simple and fast, weighted, linear least‐squares refinement protocol and code is presented for inverting the information contained in a network of quantum chemically computed 0 K reaction enthalpies. This inversion yields internally consistent 0 K enthalpies of formation for the species of the network. The refinement takes advantage of the fact that the accuracy of computed enthalpies depends strongly on the quantum‐chemical protocol employed for their determination. Different protocols suffer from different sources of error; thus, the reaction enthalpies computed by them have “random” residual errors. Since it is much more natural for quantum‐chemical energy and enthalpy results, including reaction enthalpies, to be based on the electronic ground states of the atoms and not on the historically preferred elemental states, and since these two possible protocols can be converted into each other straightforwardly, it is proposed that first‐principles thermochemistry should employ the ground electronic states of atoms. In this scheme, called atom‐based thermochemistry (AT), the enthalpy of formation of a gaseous compound corresponds simply to the total atomization energy of the species; it is always positive, and it reflects the bonding strength within the molecule. The inversion protocol developed and based on AT is termed NEAT, which represents the fact that the protocol proceeds from a network of computed reaction enthalpies toward atom‐based thermochemistry, most directly to atom‐based enthalpies of formation. After assembling a database that consisted of 361 ab initio reactions and reaction enthalpies involving 188 species, collected from 31 literature sources, the following dependable 0 K atom‐based enthalpies of formation, Δ_f${H{{{\rm AT}\hfill \atop 0\hfill}}}$ , all in kJ mol^?1, have been obtained by means of NEAT: H₂=432.07(0), CH=334.61(15), NH=327.69(25), OH=425.93(21), HF=566.13(31), CO=1072.08(28), O₂=493.51(34), CH₂=752.40(21), H₂O=918.05(20), HO₂=694.53(32), CO₂=1597.77(40), CH₃=1209.64(29), NH₃=1157.44(33), C₂H₂=1625.78(40), and CH₄=1641.68(40), in which the uncertainty values given in parentheses represent 95 % confidence intervals. The average deviation of these values from the well‐established active thermochemical tables (ATcT) values is a mere 0.25 kJ mol^?1, with a maximum deviation of 0.7 kJ mol^?1. This shows that the use of a large number of ab initio reaction enthalpies within a NEAT‐type protocol has considerable advantages over the sequential utilization of the ab initio information.     

A group additivity methodology for predicting the thermochemistry of oxygen-containing organosilanes

A combinatorial approach was applied to devise a set of reference Si–C–O–H species that is used to derive group-additivity values (GAVs) for this class of molecules. The reference species include 62 stable single-bonded, 19 cyclic, and nine double-bonded Si–C–O–H species. The thermochemistry of these reference species, that is, the standard enthalpy of formation, entropy, and heat capacities covering the temperature range from 298 to 2000 K was obtained from quantum chemical calculations using several composite methods, including G4, G4MP2, and CBSQB3, and the isodesmic reaction approach. To calculate the GAVs from the ab initio based thermochemistry of the compounds in the training set, a multivariable linear regression analysis is performed. The sensitivity of GAVs to the different composite methods is discussed, and thermodynamics properties calculated via group additivity are compared with available ab initio calculated values from the literature.     

Ab initio and density functional theory study of the enthalpies of formation of F2SOx and FClSOx (x=1, 2)

Enthalpies of formation of F₂SO, F₂SO₂, FClSO and FClSO₂ molecules have been determined using ab initio molecular orbital theory and density functional theory (DFT) calculations. Different DFT approaches and levels of the Gaussian-3 and the complete basis set (CBS) ab initio model chemistries have been employed to calculate enthalpies of formation from both total atomization energies and isodesmic reaction schemes. The best values at 298 K for F₂SO, F₂SO₂, FClSO and FClSO₂ as derived from an average of G3, G3B3, CBS-Q and CBS-QB3 isodesmic energies are −140.6, −181.1, −92.6 and −132.3 kcal mol⁻¹, respectively. The results obtained suggest that the accumulated small component errors found in the DFT-based methods are significantly reduced at the ab initio levels employed. Structural properties, harmonic vibrational frequencies, mode assignations and infrared intensities derived from B3LYP and mPW1PW91 functional with the 6-311+G(3df) basis set are presented.     

Accurate reaction barrier heights of pericyclic reactions: Surprisingly large deviations for the CBS‐QB3 composite method and their consequences in DFT benchmark studies

Accurate barrier heights are obtained for the 26 pericyclic reactions in the BHPERI dataset by means of the high‐level Wn‐F12 thermochemical protocols. Very often, the complete basis set (CBS)‐type composite methods are used in similar situations, but herein it is shown that they in fact result in surprisingly large errors with root mean square deviations (RMSDs) of about 2.5 kcal mol^?1. In comparison, other composite methods, particularly G4‐type and estimated coupled cluster with singles, doubles, and quasiperturbative triple excitations [CCSD(T)/CBS] approaches, show deviations well below the chemical‐accuracy threshold of 1 kcal mol^?1. With the exception of SCS‐MP2 and the herein newly introduced MP3.5 approach, all other tested Møller‐Plesset perturbative procedures give poor performance with RMSDs of up to 8.0 kcal mol^?1. The finding that CBS‐type methods fail for barrier heights of these reactions is unexpected and it is particularly troublesome given that they are often used to obtain reference values for benchmark studies. Significant differences are identified in the interpretation and final ranking of density functional theory (DFT) methods when using the original CBS‐QB3 rather than the new Wn‐F12 reference values for BHPERI. In particular, it is observed that the more accurate Wn‐F12 benchmark results in lower statistical errors for those methods that are generally considered to be robust and accurate. Two examples are the PW6B95‐D3(BJ) hybrid‐meta‐general‐gradient approximation and the PWPB95‐D3(BJ) double‐hybrid functionals, which result in the lowest RMSDs of the entire DFT study (1.3 and 1.0 kcal mol^?1, respectively). These results indicate that CBS‐QB3 should be applied with caution in computational modeling and benchmark studies involving related systems. © 2015 Wiley Periodicals, Inc.     

Analysis of Nascent Rotational Energy Distributions and Reaction Mechanisms of the Gas‐Phase Radical–Radical Reaction O(3P)+(CH3)2CH→C3H6+OH

This paper reports on the gas‐phase radical–radical dynamics of the reaction of ground‐state atomic oxygen [O(³P), from the photodissociation of NO₂] with secondary isopropyl radicals [(CH₃)₂CH, from the supersonic flash pyrolysis of isopropyl bromide]. The major reaction channel, O(³P)+(CH₃)₂CH→C₃H₆ (propene)+OH, is examined by high‐resolution laser‐induced fluorescence spectroscopy in crossed‐beam configuration. Population analysis shows bimodal nascent rotational distributions of OH (X²Π) products with low‐ and high‐N′′ components in a ratio of 1.25:1. No significant spin–orbit or Λ‐doublet propensities are exhibited in the ground vibrational state. Ab initio computations at the CBS‐QB3 theory level and comparison with prior theory show that the statistical method is not suitable for describing the main reaction channel at the molecular level. Two competing mechanisms are predicted to exist on the lowest doublet potential‐energy surface: direct abstraction, giving the dominant low‐N′′ components, and formation of short‐lived addition complexes that result in hot rotational distributions, giving the high‐N′′ components. The observed competing mechanisms contrast with previous bulk kinetic experiments conducted in a fast‐flow system with photoionization mass spectrometry, which suggested a single abstraction pathway. In addition, comparison of the reactions of O(³P) with primary and tertiary hydrocarbon radicals allows molecular‐level discussion of the reactivity and mechanism of the title reaction.     

Ab initio chemical kinetics for reactions of H atoms with SiHx (x = 1–3) radicals and related unimolecular decomposition processes

Hydrogen atoms and SiH_x (x = 1–3) radicals coexist during the chemical vapor deposition (CVD) of hydrogenated amorphous silicon (a‐Si:H) thin films for Si‐solar cell fabrication, a technology necessitated recently by the need for energy and material conservation. The kinetics and mechanisms for H‐atom reactions with SiH_x radicals and the thermal decomposition of their intermediates have been investigated by using a high high‐level ab initio molecular‐orbital CCSD (Coupled Cluster with Single and Double)(T)/CBS (complete basis set extrapolation) method. These reactions occurring primarily by association producing excited intermediates, ¹SiH₂, ³SiH₂, SiH₃, and SiH₄, with no intrinsic barriers were computed to have 75.6, 55.0, 68.5, and 90.2 kcal/mol association energies for x = 1–3, respectively, based on the computed heats of formation of these radicals. The excited intermediates can further fragment by H₂ elimination with 62.5, 44.3, 47.5, and 56.7 kcal/mol barriers giving ¹Si, ³Si, SiH, and ¹SiH₂ from the above respective intermediates. The predicted heats of reaction and enthalpies of formation of the radicals at 0 K, including the latter evaluated by the isodesmic reactions, SiH_x + CH₄ = SiH₄ + CH_x, are in good agreement with available experimental data within reported errors. Furthermore, the rate constants for the forward and unimolecular reactions have been predicted with tunneling corrections using transition state theory (for direct abstraction) and variational Rice–Ramsperger–Kassel–Marcus theory (for association/decomposition) by solving the master equation covering the P,T‐conditions commonly employed used in industrial CVD processes. The predicted results compare well experimental and/or computational data available in the literature. © 2013 Wiley Periodicals, Inc.     

Accurate Quantum‐Chemical Prediction of Enthalpies of Formation of Small Molecules in the Gas Phase

The coupled‐cluster approach, including single and double excitations and perturbative corrections for triple excitations, is capable of predicting molecular electronic energies and enthalpies of formation of small molecules in the gas phase with very high accuracy (specifically, with error bars less than 5 kJ mol^?1), provided that the electronic wavefunction is dominated by the Hartree–Fock configuration. This capability is illustrated by calculations on molecules containing O–H and O–F bonds, namely OH, FO, H₂O, HOF, and F₂O. To achieve this very high accuracy, it is imperative to account for electron‐correlation effects in a quantitative manner, either by using explicitly correlated two‐particle basis functions (R12 functions) or by extrapolating to the limit of a complete basis. Besides taking into account harmonic zero‐point vibrational energies, it is also necessary to account for anharmonic corrections to the zero‐point vibrational energies, to include the core orbitals into the coupled‐cluster calculations, and to account for spin–orbit corrections and scalar relativistic effects. These additional corrections constitute small but significant contributions in the range of 1–4 kJ mol^?1 to the enthalpies of formation of the aforementioned molecules. The highly accurate coupled‐cluster results, obtained by employing R12 functions and by including various corrections, are compared with standard Kohn–Sham density‐functional calculations as well as with the Gaussian‐2 and complete‐basis‐set model chemistries.     

Ab initio quantum-mechanical calculations of the propagation enthalpies for the radical and anionic polymerizations of formaldehyde and methanimine

The gas phase enthalpies of formation for oligomeric radicals and anions H(CH₂NH)_n^* and H(CH₂O)_n^* were theoretically determined by ab initio quantum-mechanical calculations with n in the range 1 to 6. From these results, the reaction enthalpies for each of the first five propagation steps of the polymerization were estimated for methanimine (H₂C = NH) and formaldehyde (H₂C = O). At the same step of oligomerization, enthalpies associated with anionic polymerizations are always more negative than enthalpies corresponding to radical polymerizations, but the difference between them decreases with increasing n. Both Delta;H (propagation) vs. n curves tend rapidly, particularly for radical polymerizations, towards an asymptotic value independent of the mode of polymerization and equal to - 12 kcal/mol for formaldehyde and - 14 kcal/mol for methanimine. Experimental data for the gas phase polymerization of formaldehyde are in good agreement with our theoretical value. These results demonstrate that heats of polymerization can be reasonably estimated by intensive calculation methods if a careful choice of the reaction mimicking the propagation step is done.     

Thermochemistry for enthalpies and reaction paths of nitrous acid isomers

Recent studies show that nitrous acid, HONO, a significant precursor of the hydroxyl radical in the atmosphere, is formed during the photolysis of nitrogen dioxide in soils. The term nitrous acid is largely used interchangeably in the atmospheric literature, and the analytical methods employed do not often distinguish between the HONO structure (nitrous acid) and HNO₂ (nitryl hydride or isonitrous acid). The objective of this study is to determine the thermochemistry of the HNO₂ isomer, which has not been determined experimentally, and to evaluate its thermal and atmospheric stability relative to HONO. The thermochemistry of these isomers is also needed for reference and internal consistency in the calculation of larger nitrite and nitryl systems. We review, evaluate, and compare the thermochemical properties of several small nitric oxide and hydrogen nitrogen oxide molecules. The enthalpies of HONO and HNO₂ are calculated using computational chemistry with the following methods of analysis for the atomization, isomerization, and work reactions using closed‐ and open‐shell reference molecules. Three high‐level composite methods G3, CBS‐QB3, and CBS‐APNO are used for the computation of enthalpy. The enthalpy of formation, ΔH^o_f(298 K), for HONO is determined as ?18.90 ± 0.05 kcal mol^?1 (?79.08 ± 0.2 kJ mol^?1) and as ?10.90 ± 0.05 kcal mol^?1 (?45.61 ± 0.2 kJ mol^?1) for nitryl hydride (HNO₂), which is significantly higher than values used in recent NO_x combustion mechanisms. H‐NO₂ is the weakest bond in isonitrous acid; but HNO₂ will isomerize to HONO with a similar barrier to the HO? NO bond energy; thus, it also serves as a source of OH in atmospheric chemistry. Kinetics of the isomerization is determined; a potential energy diagram of H/N/O₂ system is presented, and an analysis of the triplet surface is initiated. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 378–398, 2007     

W4‐17: A diverse and high‐confidence dataset of atomization energies for benchmarking high‐level electronic structure methods

Atomization reactions are among the most challenging tests for electronic structure methods. We use the first‐principles Weizmann‐4 (W4) computational thermochemistry protocol to generate the W4‐17 dataset of 200 total atomization energies (TAEs) with 3σ confidence intervals of 1 kJ mol⁻¹. W4‐17 is an extension of the earlier W4‐11 dataset; it includes first‐ and second‐row molecules and radicals with up to eight non‐hydrogen atoms. These cover a broad spectrum of bonding situations and multireference character, and as such are an excellent benchmark for the parameterization and validation of highly accurate ab initio methods (e.g., CCSD(T) composite procedures) and double‐hybrid density functional theory (DHDFT) methods. The W4‐17 dataset contains two subsets (i) a non‐multireference subset of 183 systems characterized by dynamical or moderate nondynamical correlation effects (denoted W4‐17‐nonMR) and (ii) a highly multireference subset of 17 systems (W4‐17‐MR). We use these databases to evaluate the performance of a wide range of CCSD(T) composite procedures (e.g., G4, G4(MP2), G4(MP2)‐6X, ROG4(MP2)‐6X, CBS‐QB3, ROCBS‐QB3, CBS‐APNO, ccCA‐PS3, W1, W2, W1‐F12, W2‐F12, W1X‐1, and W2X) and DHDFT methods (e.g., B2‐PLYP, B2GP‐PLYP, B2K‐PLYP, DSD‐BLYP, DSD‐PBEP86, PWPB95, ωB97X‐2(LP), and ωB97X‐2(TQZ)). © 2017 Wiley Periodicals, Inc.     

C?X⋅⋅⋅O Halogen Bonding: Interactions of Trifluoromethyl Halides with Dimethyl Ether

The formation of weakly bound molecular complexes between dimethyl ether (DME) and the trifluoromethyl halides CF₃Cl, CF₃Br and CF₃I dissolved in liquid argon and in liquid krypton is investigated, using Raman and FTIR spectroscopy. For all halides evidence is found for the formation of C? X???O halogen‐bonded 1:1 complexes. At higher concentrations of CF₃Br, a weak absorption due to a 1:2 complex is also observed. Using spectra recorded at temperatures between 87 and 125 K, the complexation enthalpies for the complexes are determined to be ?6.8(3) kJ mol^?1 (DME?CF₃Cl), ?10.2(1) kJ mol^?1 (DME?CF₃Br), ?15.5(1) kJ mol^?1 (DME?CF₃I), and ?17.8(5) kJ mol^?1 [DME(?CF₃Br)₂]. Structural and spectral information on the complexes is obtained from ab initio calculations at the MP2/ 6‐311++G(d,p) and MP2/6‐311++G(d,p)+LanL2DZ* levels. By applying Monte Carlo free energy perturbation calculations to account for the solvent influences, and statistical thermodynamics to estimate the zero‐point vibrational and thermal influences, the ab initio complexation energies are converted into complexation enthalpies for the solutions in liquid argon. The resulting values are compared with the experimental data deduced from the cryosolutions.     

Thermochemical properties for n‐propyl,iso‐propyl,and tert‐butyl nitroalkanes,alkyl nitrites,and their carbon‐centered radicals

Density functional theory (DFT) based calculations are performed on a series of alkyl nitrites and nitroalkanes representing large‐scale primary, secondary, and tertiary nitro compounds and their radicals resulting from the loss of their skeletal hydrogen atoms. Geometries, vibration frequencies, and thermochemical properties [S°(T) and C°_p(T) (10 K ? T ? 5000 K)] are calculated at the B3LYP/6‐31G(d,p) DFT level. Δ_fH°₂₉₈ values are from B3LYP/6‐31G(d,p), B3LYP/6‐31+G(2d,2p), and the composite CBS‐QB3 levels. Potential energy barriers for the internal rotations have been computed at the B3LYP/6‐31G(d,p) level of theory, and the lower barrier contributions are incorporated into entropy and heat capacity data. The standard enthalpies of formation at 298 K are evaluated using isodesmic reaction schemes with several work reactions for each species. Recommended values derived from the most stable conformers of respective nitro‐ and nitrite isomers include ?30.57 and ?28.44 kcal mol^?1 for n‐propane‐, ?33.89 and ?32.32 kcal mol^?1 for iso‐propane‐, ?42.78 and ?41.36 kcal mol^?1 for tert‐butane‐nitro compounds and nitrites, respectively. Entropy and heat capacity values are also reported for the lower homologues: nitromethane, nitroethane, and corresponding nitrites. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 181–199, 2010     

Equilibrium structure and spectroscopic constants of maleic anhydride

The quadratic, cubic, and semi-diagonal quartic force fields of maleic anhydride have been calculated at the MP2 level of theory employing the cc-pVTZ basis set. The spectroscopic constants derived from the force field are in excellent agreement with the corresponding experimental values. The semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational corrections calculated from the cubic force field. This semi-experimental equilibrium structure is in excellent agreement with the ab initio structures computed at the CCSD(T) level of theory and it is closer to the ab initio structure than the purely experimental (or empirical) structures r ₀, r _m⁽¹⁾, and r _m⁽²⁾ obtained by microwave spectroscopy as well as the equilibrium structure derived from gas-phase electron diffraction data.     

Relationship between Electronic Charges at Nitrogen Atoms of Nitro Groups and Thermal Reactivity of Nitramines

Thermal reactivity of eleven nitramines has been examined by means of non-isothermal differential thermal analysis, and the data were analyzed according to the Kissinger method. The reactivity was expressed as the E _a R ⁻¹ slopes of the Kissinger relationship. Electronic charges, q ^N, at nitrogen atoms of the nitramine molecules were calculated by means of ab initio DFT B3LYP/6-31G** method. The relationships were confirmed between the slopes E _a R ⁻¹ and the q values for the nitro groups that are primarily split off. Conclusions are made in relation to the mechanism of initiation of polynitro compounds in general. This revised version was published online in July 2006 with corrections to the Cover Date.     

Activation Energies and Reaction Energetics for 1,3‐Dipolar Cycloadditions of Hydrazoic Acid with C?C and C?N Multiple Bonds from High‐Accuracy and Density Functional Quantum Mechanical Calculations

The reactions of hydrazoic acid (HN₃) with ethene, acetylene, formaldimine (H₂CNH), and HCN were explored with the high‐accuracy CBS‐QB3 method, as well as with the B3LYP and mPW1K density functionals. CBS‐QB3 predicts that the activation energies for the reactions of hydrazoic acid with ethylene, acetylene, formaldimine, and HCN have remarkably similar activation enthalpies of 19.0, 19.0, 21.6, and 25.2 kcal/mol, respectively. The reactions are calculated to have reaction enthalpies of −21.5 for triazoline formation from ethene, and −63.7 kcal/mol for formation of the aromatic triazole from acetylene. The reaction to form tetrazoline from formaldimine has a reaction enthalpy of −8 kcal/mol (ΔG_rxn=+5.6 kcal/mol), and the formation of tetrazole from HCN has a reaction enthalpy of −23.0 kcal/mol. The trends in the energetics of these processes are rationalized by differences in σ‐bond energies in the transition states and adducts, and the energy required to distort hydrazoic acid to its transition‐state geometry. The density functionals predict activation enthalpies that are in relatively good agreement with CBS‐QB3, the results differing from CBS‐QB3 results by ca. 1–2 kcal/mol. Significant errors are revealed for mPW1K in predicting the reaction enthalpies for all reactions.