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.     

New Reactions of N‐tert‐Butylimines; Formation of N‐Heterocycles by Methyl Radical Elimination on Flash Vacuum Thermolysis of N‐Benzylidene‐ and N‐(2‐Pyridylmethylidene)‐tert‐butylamines

Thermal reactions of N‐benzylidene‐ and N‐(2‐pyridylmethylidene)‐tert‐butylamines ( 5 and 13 ) under FVT conditions have been investigated. Unexpectedly, at 800 °C, compound 5 yields 1,2‐dimethylindole and 3‐methylisoquinoline. In the reaction of 13 at 800 °C, 3‐methylimidazo[1,5‐a]pyridine was obtained as the major product. Mechanisms of these reactions have been proposed on the basis of DFT calculations. Furthermore, UV‐photoelectron spectroscopy combined with FVT has been applied for direct monitoring and characterization of the thermolysis products in situ.     

An Ab initio theoretical investigation on the geometrical and electronic structures of BAun−/0 (n = 1–4) clusters

An ab initio theoretical investigation on the geometrical and electronic structures and photoelectron spectroscopies (PES) of BAu_n^?/0 (n = 1–4) auroboranes has been performed in this work. Density functional theory and coupled cluster method (CCSD(T)) calculations indicate that BAu (n = 1–4) clusters with n‐Au terminals possess similar geometrical structures and bonding patterns with the corresponding boron hydrides BH. The PES spectra of BAu (n = 1–4) anions have been simulated computationally to facilitate their future experimental characterizations. In this series, the T_d BAu anion appears to be unique and particularly interesting: it possesses a perfect tetrahedral geometry and has the highest vertical electron detachment energy (VDE = 3.69 eV), largest HOMO‐LUMO gap (ΔE_gap = 3.0 eV), and the highest first excitation energy (E_ex = 2.18 eV). The possibility to use the tetrahedral BAu unit as the building block of Li⁺[BAu₄]^? ion‐pair and other [BAu₄]^?‐containing inorganic solids is discussed. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

In Situ Analysis of the Zn(S,O) Buffer Layer Preparation for Chalcopyrite Solar Cells by Zn L‐edge X‐Ray Absorption Spectroscopy

Bridging the gap between high‐vacuum soft X‐ray absorption spectroscopy and real systems under ambient conditions probes chemical reactions in situ during deposition and annealing processes. The origin of highly efficient buffer layers in Zn(S,O) is the complex formation between Zn²⁺ and the S?C group of thiourea (see schematic), which allows ligand‐to‐metal and metal‐to‐ligand charge transfer (LMCT and MLCT).

     

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.     

Electronic Structure,Chemical Bonding,and Solid‐State NMR Spectroscopy of the Digallides of Ca,Sr, and Ba

Delving into digallides : The characteristics of the chemical bonding of the digallides of the alkaline‐earth metals (see figure) have been studied by application of experimental methods, such as single‐crystal X‐ray diffraction and solid‐state NMR spectroscopy, in combination with quantum mechanical calculations.

     

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.     

A Valence Bond Model for Electron‐Rich Hypervalent Species: Application to SFn (n=1, 2, 4), PF5, and ClF3

Some typical hypervalent molecules, SF₄, PF₅, and ClF₃, as well as precursors SF (⁴Σ^? state) and SF₂ (³B₁ state), are studied by means of the breathing‐orbital valence bond (BOVB) method, chosen for its capability of combining compactness with accuracy of energetics. A unique feature of this study is that for the first time, the method used to gain insight into the bonding modes is the same as that used to calculate the bonding energies, so as to guarantee that the qualitative picture obtained captures the essential physics of the bonding system. The ⁴Σ^? state of SF is shown to be bonded by a three‐electron σ bond assisted by strong π back‐donation of dynamic nature. The linear ³B₁ state of SF₂, as well as the ground states of SF₄, PF₅ and ClF₃, are described in terms of four VB structures that all have significant weights in the range 0.17–0.31, with exceptionally large resonance energies arising from their mixing. It is concluded that the bonding mode of these hypervalent species and isoelectronic ones complies with Coulson’s version of the Rundle–Pimentel model, but assisted by charge‐shift bonding. The conditions for hypervalence to occur are stated.     

Cooperativity between the Halogen Bond and the Hydrogen Bond in H3N⋅⋅⋅XY⋅⋅⋅HF Complexes (X,Y=F,Cl, Br)

Ab initio calculations are used to provide information on H₃N???XY???HF triads (X, Y=F, Cl, Br) each having a halogen bond and a hydrogen bond. The investigated triads include H₃N???Br₂‐HF, H₃N???Cl₂???HF, H₃N???BrCI???HF, H₃N???BrF???HF, and H₃N???ClF???HF. To understand the properties of the systems better, the corresponding dyads are also investigated. Molecular geometries, binding energies, and infrared spectra of monomers, dyads, and triads are studied at the MP2 level of theory with the 6‐311++G(d,p) basis set. Because the primary aim of this study is to examine cooperative effects, particular attention is given to parameters such as cooperative energies, many‐body interaction energies, and cooperativity factors. The cooperative energy ranges from ?1.45 to ?4.64 kcal mol^?1, the three‐body interaction energy from ?2.17 to ?6.71 kcal mol^?1, and the cooperativity factor from 1.27 to 4.35. These results indicate significant cooperativity between the halogen and hydrogen bonds in these complexes. This cooperativity is much greater than that between hydrogen bonds. The effect of a halogen bond on a hydrogen bond is more pronounced than that of a hydrogen bond on a halogen bond.     

Spin‐Resolved Rotational Energy Transfer for the CH B 2Σ−(v=0, N,F) State by Collisions with Ar

An experimental and theoretical investigation of rotational energy transfers (RET) of CH involving the B ²Σ^? (v=0, 0≤N≤5, F) state by collisions with Ar is undertaken, using the photolysis‐probe technique. Time‐resolved laser‐induced fluorescence resulting from an initially prepared fine‐structure label is dispersed using a step‐scan Fourier transform spectrometer. The spin‐resolved RET rate constants are evaluated with the simulation of a kinetic model. The quantum‐scattering method is used for the calculation of the fine‐structure‐resolved cross sections and rate constants in the rotationally inelastic collisions. The theoretical values are generally consistent with our experimental findings, both in the order of magnitude and trend of N and ΔN dependence. The propensity rules obtained from the experiments are essentially obeyed by theoretical calculations, and are also in accordance with those reported by Kind and Stuhl. The RET rate constants obtained for the v=0 level are smaller than those obtained previously for v=1. The discrepancy in the RET behavior may be caused by an anisotropy difference of the interaction potential resulting from vibrational excitation.