Extension of the composite CBS-QB3 method to singlet diradical calculations

The composite CBS-QB3 method is widely used to obtain accurate energies of molecules and radicals although its use in the case of singlet diradicals gives rise to some difficulties. The problem is related to the parameterized correction this method introduces to account for spin-contamination. We report a new term specifically designed to describe singlet diradicals separated by at least one CH2 unit. As a test case, we have computed the formation enthalpy of a series of diradicals that includes hydrocarbons as well as systems involving heteroatoms (nitrogen, oxygen). The resulting CBS-QB3 energies are very close to experiment

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 beta-scission reactions, an important famliy 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. Thee;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 beta-scissions yields rate coefficients within a factor of 3.5 of the CBS-QB3 results except for two beta-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

A G4/W1BD theoretical study into the gas phase enthalpies of formation for potential high energy materials

Enthalpies of formation (Δ~f~H~(g)~) at 298.15 K and 0 K were calculated for various potential high energy materials (HEMs) using the high-level Gaussian-4 (G4) and W1BD methods with the atomization approach. Where prior high level estimates are available in the literature, the G4 and W1BD Δ~f~H~(g)~ are in good agreement. The results presented herein represent the highest level calculations performed to date on this suite of HEMs. These G4/W1BD enthalpies of formation should provide utility among the research community as a benchmark set of values against which to assess future experimental and/or theoretical data

Direct Detection of Products from the Pyrolysis of 2-Phenethyl Phenyl Ether

The pyrolysis of 2-phenethyl phenyl ether (PPE, C_6H_5C_2H_4OC_6H_5) in a hyperthermal nozzle (300-1350 °C) was studied to determine the importance of concerted and homolytic unimolecular decomposition pathways. Short residence times (<100 μs) and low concentrations in this reactor allowed the direct detection of the initial reaction products from thermolysis. Reactants, radicals, and most products were detected with photoionization (10.5 eV) time-of-flight mass spectrometry (PIMS). Detection of phenoxy radical, cyclopentadienyl radical, benzyl radical, and benzene suggest the formation of product by the homolytic scission of the C_6H_5C_2H_4-OC_6H_5 and C_6H_5CH_2-CH_2OC_6H_5 bonds. The detection of phenol and styrene suggests decomposition by a concerted reaction mechanism. Phenyl ethyl ether (PEE, C_6H_5OC_2H_5) pyrolysis was also studied using PIMS and using cryogenic matrix-isolated infrared spectroscopy (matrix-IR). The results for PEE also indicate the presence of both homolytic bond breaking and concerted decomposition reactions. Quantum mechanical calculations using CBS-QB3 were conducted, and the results were used with transition state theory (TST) to estimate the rate constants for the different reaction pathways. The results are consistent with the experimental measurements and suggest that the concerted retro-ene and Maccoll reactions are dominant at low temperatures (below 1000 °C), whereas the contribution of the C_6H_5C_2H_4-OC_6H_5 homolytic bond scission reaction increases at higher temperatures (above 1000 °C)

Photoionization mass spectrometry of ω-phenylalkylamines: Role of radical cation-π interaction

Linear ω-phenylalkylamines of increasing alkyl chain length have been investigated employing synchrotron radiation in the photon energy range from 7 to 15 eV. These molecules have received considerable interest because they bear the skeleton of biologically relevant compounds including neurotransmitters and because of the possible interaction between the amino moiety and the phenyl ring. Recently, the contribution of this interaction has been assayed in both neutral and protonated species, pointing to a role of the polymethylene chain length. In this work, the ionization energy (IE) values of benzylamine (BA), 2-phenylethylamine (2-PEA), 3-phenylpropylamine (3-PPA), and 4-phenylbutylamine (4-PBA) were investigated in order to ascertain the impact of the different alkyl chain lengths and to verify an amino radical cation-π interaction. The IEs obtained experimentally, 8.54, 8.37, 8.29, and 8.31 eV for BA, 2-PEA, 3-PPA and 4-PBA, respectively, show a decreasing trend that is discussed employing calculations at the CBS-QB3 level. Moreover, the appearance energy values for major fragments produced by the photofragmentation process are reported

Fully ab initio atomization energy of benzene via W2 theory

The total atomization energy at absolute zero, (TAE$_0$) of benzene, C$_6$H$_6$, was computed fully {\em ab initio} by means of W2h theory as 1306.6 kcal/mol, to be compared with the experimentally derived value 1305.7+/-0.7 kcal/mol. The computed result includes contributions from inner-shell correlation (7.1 kcal/mol), scalar relativistic effects (-1.0 kcal/mol), atomic spin-orbit splitting (-0.5 kcal/mol), and the anharmonic zero-point vibrational energy (62.1 kcal/mol). The largest-scale calculations involved are CCSD/cc-pV5Z and CCSD(T)/cc-pVQZ; basis set extrapolations account for 6.3 kcal/mol of the final result. Performance of more approximate methods has been analyzed. Our results suggest that, even for systems the size of benzene, chemically accurate molecular atomization energies can be obtained from fully first-principles calculations, without resorting to corrections or parameters derived from experiment.Comment: J. Chem. Phys., accepted. RevTeX, 12 page

Gas phase formation of the prebiotic molecule formamide: insights from new quantum computations

New insights into the formation of interstellar formamide, a species of great relevance in prebiotic chemistry, are provided by electronic structure and kinetic calculations for the reaction NH2 + H2CO -> NH2CHO + H. Contrarily to what previously suggested, this reaction is essentially barrierless and can, therefore, occur under the low temperature conditions of interstellar objects thus providing a facile formation route of formamide. The rate coefficient parameters for the reaction channel leading to NH2CHO + H have been calculated to be A = 2.6x10^{-12} cm^3 s^{-1}, beta = -2.1 and gamma = 26.9 K in the range of temperatures 10-300 K. Including these new kinetic data in a refined astrochemical model, we show that the proposed mechanism can well reproduce the abundances of formamide observed in two very different interstellar objects: the cold envelope of the Sun-like protostar IRAS16293-2422 and the molecular shock L1157-B2. Therefore, the major conclusion of this Letter is that there is no need to invoke grain-surface chemistry to explain the presence of formamide provided that its precursors, NH2 and H2CO, are available in the gas-phase.Comment: MNRAS Letters, in pres

The heats of formation of the haloacetylenes XCCY [X, Y = H, F, Cl]: basis set limit ab initio results and thermochemical analysis

The heats of formation of haloacetylenes are evaluated using the recent W1 and W2 ab initio computational thermochemistry methods. These calculations involve CCSD and CCSD(T) coupled cluster methods, basis sets of up to spdfgh quality, extrapolations to the one-particle basis set limit, and contributions of inner-shell correlation, scalar relativistic effects, and (where relevant) first-order spin-orbit coupling. The heats of formation determined using W2 theory are: \hof(HCCH) = 54.48 kcal/mol, \hof(HCCF) = 25.15 kcal/mol, \hof(FCCF) = 1.38 kcal/mol, \hof(HCCCl) = 54.83 kcal/mol, \hof(ClCCCl) = 56.21 kcal/mol, and \hof(FCCCl) = 28.47 kcal/mol. Enthalpies of hydrogenation and destabilization energies relative to acetylene were obtained at the W1 level of theory. So doing we find the following destabilization order for acetylenes: FCCF $>$ ClCCF $>$ HCCF $>$ ClCCCl $>$ HCCCl $>$ HCCH. By a combination of W1 theory and isodesmic reactions, we show that the generally accepted heat of formation of 1,2-dichloroethane should be revised to -31.8$\pm$0.6 kcal/mol, in excellent agreement with a very recent critically evaluated review. The performance of compound thermochemistry schemes such as G2, G3, G3X and CBS-QB3 theories has been analyzed.Comment: Mol. Phys., in press (E. R. Davidson issue