Complete basis set, the MP2 ab initio, and hybrid density functional theory evaluation of ionization potential and electron affinity for PH, PH2, PHF, PF, and PF2

A systematic study was performed on the small molecular systems built from phosphor, hydrogen and fluorine with the target being to evaluate accurately their ionization potentials and electron affinities, as well as influence fluorine on the ionization potential of phosphor as a central atom. To determine the accuracy of hybrid density functional methods for computing those energies, ionization energies for hydrogen, fluorine and phosphor were calculated and compared with the experimental and CBSQ values. To demonstrate the accuracy of this method, both the ionization potential and the electron affinity for phosphorus and fluorine atoms were calculated and compared with the experimental data. For both PF and PF₂, an identical electron affinity of 0.72 eV and for PH and PHF 1.0 eV were suggested.     

A Gaussian-2 ab initio study of [C2H5S] ions: III. H2 and CH4 eliminations from CH3CHSH and CH3SCH2

Gaussian-2 ab initio calculations were performed to examine the six modes of unimolecular dissociation of cis-CH₃CHSH⁺ (1⁺), trans-CH₃CHSH⁺ (2⁺), and CH₃SCH₂⁺ (3⁺): 1⁺→CH₃⁺+trans-HCSH (1); 1⁺→CH₃+trans-HCSH⁺ (2); 1⁺→CH₄+HCS⁺ (3); 1⁺→H₂+c-CH₂CHS⁺ (4); 2⁺→H₂+CH₃CS⁺ (5); and 3⁺→H₂+c-CH₂CHS⁺ (6). Reactions (1) and (2) have endothermicities of 584 and 496 kJ mol⁻¹, respectively. Loss of CH₄ from 1⁺ (reaction (3)) proceeds through proton transfer from the S atom to the methyl group, followed by cleavage of the C–C bond. The reaction pathway has an energy barrier of 292 kJ mol⁻¹ and a transition state with a wide spectrum of nonclassical structures. Reaction (4) has a critical energy of 296 kJ mol⁻¹ and it also proceeds through the same proton transfer step as reaction (3), followed by elimination of H_2. Formation of CH₃CS⁺ from 2⁺ (reaction (5)) by loss of H₂ proceeds through protonation of the methine (CH) group, followed by dissociation of the H₂ moiety. Its energy barrier is 276 kJ mol⁻¹. On both the MP2/6-31G* and QCISD/6-31G* potential-energy surfaces, the H₂ 1,1-elimination from 3⁺ (reaction (6)) proceeds via a nonclassical intermediate resembling c-CH₃SCH₂⁺ and has a critical energy of 269 kJ mol⁻¹.     

Complete basis set ab initio computational study of ionization potential, electron affinity and the C-F bond dissociation energy for perfluorinated methane derivatives

A computational study of perfluorinated methane derivatives was performed with complete basis set ab initio methods. The total energies for their neutral, cation, and anionic states were computed. From these values, the energy gaps between different electronic states, ionization potentials, electron affinities, and C-F bond dissociation energies were calculated. The computed values are compared with experimental data and the reliability of complete basis set ab initio methods is discussed. New values for C-F bond dissociation energies are suggested. Received: 12 January 1998 / Accepted: 2 April 1998 / Published online: 29 July 1998     

An ab initio study of the structures and energetics of CH3OCl and CH3ClO

The equilibrium structures, vibrational spectra, and heats of formation for CH₃OCl and CH₃ClO have been estimated using high levels of ab initio molecular orbital theory. The lowest energy isomer is found to be CH₃OCl, and its heat of formation is estimated to be −13.5±2 kcal mol⁻¹, in good agreement with bond additivity estimates. Results for the CH₃ClO isomer are presented for the first time, and it is found to be 40.5 kcal mol⁻¹ higher in energy relative to CH₃OCl. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 29–35, 1999     

Crystal structures and magnetic properties of CeAu4Si2 and CeAu2Si2

Single crystals of CeAu₄Si₂ and CeAu₂Si₂ have been grown out of ternary fluxes rich in Au, and the former, also by sintering the stoichiometric composition at 750 °C. The single-crystal X-ray refinement result for CeAu₄Si₂ is orthorhombic, Cmmm (No. 65, Z=2), different from a tetragonal result found from an X-ray powder diffraction refinement [H. Nakashima, et al., J. Alloys Compds. 424 (2006) 7]. For CeAu₂Si₂, this is the first report of the stoichiometric crystalline phase, in the known tetragonal I4/mmm structure. The anisotropic field- and temperature-dependent magnetizations, as well as specific heat and resistivity data are compared. Although both compounds have related structural packing, they present unique magnetic features. CeAu₂Si₂ is a typical antiferromagnet with T_N=8.8(1) K and CeAu₄Si₂ features a ferromagnetic component below T_c=3.3(1) K. Both phases have effective moments close in value to that of free Ce³⁺.     

CH4-CO2 reforming to syngas over Pt-CeO2-ZrO2/MgO catalysts: Modification of support using ion exchange resin method

Pt-CeO2-ZrO2/MgO (Pt-CZ/MgO) catalysts with 0.8 wt% Pt, 3.0 wt% CeO2 and 3.0 wt% ZrO2 were prepared by wet impregnation method. Support MgO was obtained using ion exchange resin method or using commercial MgO. XRD, BET, SEM, TEM, DTA-TG and CO2-TPD were used to characterize the catalysts. CH4-CO2 reforming to synthesis gas (syngas) was performed to test the catalytic behavior of the catalysts. The catalyst Pt-CZ/MgO-IE(D) prepared using ion exchange resin exhibits more regular structure, smaller and more unique particle sizes, and stronger basicity than the catalyst Pt-CZ/MgO prepared from commercial MgO. At 1073 K and atmospheric pressure, Pt-CZ/MgO-IE(D) catalyst has a higher activity and greater stability than Pt-CZ/MgO catalyst for CH4-CO2 reforming reaction at high gas hourly space velocity of 36000 mL/(g·h) with a stoichiometric feed of CH4 and CO2. Activity measurement and characterization results demonstrate that modification of the support using ion exchange resin method can promote the surface structural property and stability, therefore enhancing the activity and stability for CH4-CO2 reforming reaction.     

Competitive bromination kinetics of CH3Br and CH2ClBr

The relative-rate method with gas-chromatographic product analysis was applied to study the kinetics of the reactions Br + CH₃Br → CH₂Br + HBr (1) and Br + CH₂ClBr → CHClBr + HBr (2) The rate coefficient ratio of k ₁/ k ₂ = (1.6 ± 0.2) exp[(-15.2 ± 0.3) kJ mol^-1/ RT] was determined in the temperature range of 353 - 410 K. This revised version was published online in August 2006 with corrections to the Cover Date.     

Carbon nitride formation in gas-phase reactions of CH4, NH3 and H2: an ab initio study

Ab initio QCISD(T)/6-311++(2d,2p) calculations have been carried out for an extensive study of gas-phase reactions among CH₄, NH₃ and their radicals. Our study shows that stable HCN molecules are readily formed by successive H abstraction reactions. Some of the reactions are strongly exothermic and have negligible energy barriers. In agreement with some recent experiments, our results indicate that H abstraction reactions, which make the chemical vapor deposition of diamond thin films successful, do not favor the formation of carbon nitride thin films.     

Vibrational spectra of [(CH3)3NH]3Sb2Cl9, [(CH3)3NH]3Bi2Cl9 and their mixed crystals

The IR and Raman spectra of [(CH₃)₃NH]₃Sb₂Cl₉ (A), [(CH₃)₃NH]₃Bi₂Cl₉ (B) and two of their mixed crystals containing respectively 33% (AB.33) and 42% Bi (AB.42) are analyzed and compared. A and AB.33 show ferroelectric–paraelectric phase transition at 364 K and 344 K, respectively. AB.42 and B are paraelectric in the temperature range between 90 and 365 K. Most of the vibrational modes show continuous changes, with the temperature, in the IR frequencies or intensities with no soft mode behavior. However, characteristic ν(NHCl) and δ(NHCl) vibrations of weakly hydrogen-bonded species are only observed in A and AB.33 below the temperature of the phase transition and are related to the ferroelectricity. The evolution of the IR spectra with the temperature suggests that the ferroelectric properties are connected with the reorientation of the cations which needs a breaking of the weak NHCl hydrogen bonds in the paraelectric phase.     

An ab initio SCF study of conformational isomerism in HNO2

The structures and complete force fields of cis and trans nitrous acid have been calculated with a (7, 3) basis set. The differences between the two stable isomers are reproduced well. The dipole moments, centrifugal distortion constants, vibrational frequencies and isotope shifts also agree satisfactorily with observed values. The fully optimized structures of rotamers corresponding to intermediate values of internal rotation around the N-O bond have been calculated to investigate structural changes during internal rotation. The considerable changes show the strong influence of π electron delocalization in the planar forms. The barrier is calculated to be 8.7 kcal mol⁻¹ in reasonable agreement with experimental values. Both the cis and trans barriers are calculated to be attractive dominant in contradiction to earlier work.     

An ab initio and density functional theory study of the dimethylzinc-hydrogen selenide adduct: (CH3)2Zn:SeH2

The molecular structure (equilibrium geometry) and binding energy of the dimethylzinc (DMZn)-hydrogen selenide (H₂Se) adduct, (CH₃)₂Zn:SeH₂, have been computed with ab initio molecular orbital and density functional theory (DFT) methods and, where possible, compared with experimental results. The structure of the precursors DMZn and H₂Se are perturbed to only a small extent upon adduct formation. (CH₃)₂Zn:SeH₂ was found to be 3 kcal mol⁻¹ less stable than the precursors at the B3LYP/6-311 + G(2d,p)//B3LYP/6-311 + G(2d,p) level of computation, indicating that the (CH₃)₂Zn:SeH₂ adduct is unlikely to be a stable gas-phase species under chemical vapour deposition conditions. Further calculations at the B3LYP/6-311 + G(2d,p)//B3LYP/6-311 + G(2d,p) level of computation suggest that the 1:2 adduct species, (CH₃)₂Zn:(SeH₂)₂, is much less stable than the 1:1 adduct and consequently the precursors by 19 kcal mol⁻¹.     

O2+O2 electron transfer reactivity in the quartet state from ab initio calculation including electron correlation

The structures and properties of the O₂+O₂⁻ electron transfer system in the quartet state, both in the gaseous phase and in solution, were studied at the UMP2(full)/6-311+G* basis set level for the five selected coupling structures: two T-type, collinear, parallel, and crossing. The stabilities of these encounter complexes were compared. The activation barriers, coupling matrix elements, and the electron transfer rate at two theoretical levels (semiclassical and quantum mechanical) were also calculated for the quartet state, and the effect of the solvent medium evaluated at the self-consistent reaction field level. Results indicate that the structures and properties of the encounter complexes directly affect the mechanism and rate of the electron transfer reaction, the contact distances for this O₂…O₂⁻ were generally large (3 Å), the interaction between the donor and the acceptor was weak, and the structures are floppy. The electronic transmission factor for the reacting system, O₂+O₂⁻, was less than unity (ca. 001–0.6), thus the electron transfer reaction was non-adiabatic in nature. Analysis of the dependence of relevant kinetic parameters on various influencing factors showed that the effect of the solvent medium on the coupling matrix element was small, but that on the electron transfer rate was very large, and the gaseous phase results for the molecular geometrical parameters and their contributions can directly transfer to solution. Among the five selected transition state structures, the electron transfer was more likely to take place via the T-type and the P-type structures, the rate values from two theoretical levels were in good agreement with each other and were also very close to the experimental findings. If the various anharmonic vibrational contributions, the effect of the solvent molecular electronic structures and the interaction between the reacting species and the solvent medium are taken into account, the results can be improved.     

An ab initio study of the CO–N2 complex

The interaction energy and van der Waals intermolecule bond length of several structures of the CO–N₂ complex are calculated by the supermolecule CCSD(T) and MP4 methods using aug-cc-pVXZ (X=D,T,Q) basis sets extended by a set of midbond functions centered in the middle of the vdW bond. The most stable structures are found to be two distorted T-shaped configurations with the N atom pointing towards the C–O bond. This conclusion is compatible with the results of high-resolution infrared, microwave and millimeter studies.     

Distance dependence analysis of the electron transfer reactivity for the metastable FeOH2/FeOH2 system: an ab initio investigation

A novel theoretical scheme and ab initio application in discussing the electron transfer (ET) reactivity are presented in this paper, and are also calibrated in terms of the mono-hydrated iron ion system, Fe²⁺–OH₂/Fe³⁺–OH₂. The detailed geometry optimizations have been made at UMP2(full)/6-311+G^* level, and the activation geometrical configuration and the energy have been obtained at this level of theory using the activation model and the ab initio potential energy surface fitted from MP2(full)/6-311+G^* single point energies. The corresponding energy quantities (such as the activation energy, and dissociation energy) have also been obtained at different levels of theory (HF, MP2, MP3, MP4, QCISD and PUHF, PMP2 and PMP3 with the spin-projection) and a same basis set (6-311+G^*). The electron correlation calculations include the all electron correlation and the valence electron correlation. The electronic transmission coefficient is calculated using the ab initio potential energy surface slopes and the coupling matrix element determined from the two-state model and the Slater-type d-electron wave functions. The pair distribution function is calculated using two different schemes. Taking the pair distribution function and the local ET rate into account, a statistically averaged overall observed ET rate scheme and a spherically averaged local ET rate scheme are proposed. The relevant kinetic parameters are obtained in terms of these new schemes at different ab initio calculational levels. The contact distance dependence of these parameters and the applicability of the presented models and ab initio calculational method are also discussed.     

Kinetic study of the reaction of CH3O with Br and Br2

The title reactions have been studied at room temperature by applying the discharge flow method coupled with laser induced fluorescence detection of methoxy radicals and resonance fluorescence detection of bromine atoms. The following rate constants were determined: CH₃O + Br Õ products (1) k ₁ (298 K) = (3.4 ± 0.4 (1)) × 10¹³ cm³ mol^-1 s^-1, CH₃O + Br₂ Õ products (2) k ₂ (298 K) £ 5 × 10⁸ cm³ mol^-1 s^-1.     

An ab initio MO study of allene episulfide, cyclopropanethione and thioxyallyl

The electronic structures of allene episulfide, cyclopropanethione and thioxyallyl were examined by ab initio MO calculations and were compared with those of the corresponding oxygen compounds, allene oxide, cyclopropanone and oxyallyl. The difference in reactivities of allene episulfide and allene oxide was also speculatively estimated from the calculated electronic structures. The lowest singlet state of thioxyallyl was predicted to be the B₂ state, which corresponds to the σ, π-diradical. A small activation energy is required for the cyclization of the B₂ state to give allene episulfide. The A₁ singlet state lies 11 kcal mol⁻¹ higher than the B₂ singlet state and undergoes the disrotatory rotation of methylene groups to give cyclopropanethione with no activation energy.     

An ab initio study of the electronic structure of SiH4, SiH3F and SiH2F2

Changes in molecular properties and in the electronic charge distribution of the molecules SiH₄, SiH₃F and SiH₂F₂ are studied within the framework of the ab initio Hartree—Fock SCF—LCAO—MO method. The ionisation potentials, calculated with the use of Koopmans' theorem, correlate well with the experimental vertical ionisation potentials. The combined s and p electronic populations of the Si atom are not substantially altered when adding Si 3d functions to the basis set.     

An ab initio MO study on the disulfide bond: properties concerning the characteristic S-S dihedral angle

The characteristics of disulfide groups concerning the S-S dihedral angle are represented by ab initio SCF calculations using the split-valence 6-31G(^*) basis set. It is shown that the hyperconjugation between the S-H bond and the electron pair on the other sulfur plays an important role in determining the characteristic S-S dihedral angle. The S 3d orbitals do not participate in such characteristics. The nature of the S-S bond is compared with that of the O-O bond. The S-S bond length varies largely depending on the S-S dihedral angle. This is related to the frequency-conformation correlation of the disulfide group.     

Activity and physicochemical properties of Fe-zeolite catalysts for SCR of N2o with CH4

Summary Various supported Fe catalysts prepared by ion-exchange and impregnation have been examined for the reduction of N₂O by CH₄and the most active is ion-exchanged Fe-ZSM-5 in which Fe species are highly dispersed in the form of Fe³⁺with tetrahedral coordination.     

Ionization and protonation of NaC3: a theoretical study

A theoretical study of the NaC₃⁺ and NaC₃H⁺ systems has been carried out. Predictions have been made for some of the molecular properties, which could help in their possible experimental detection. The predicted global minimum for NaC₃⁺ is the linear isomer 1s (¹Σ). The lowest-lying triplet state is a three-membered ring 3t (³B₂), lying about 27.1 kcal/mol higher in energy than the predicted global minimum at the G2(P) level. In the case of NaC₃H⁺, there are two isomers that lie close in energy: a linear species, 1d (²Π), and a three-membered ring, 4d (²A′). The most reliable levels of theory employed predict that 1d (²Π) is the global minimum, whereas 4d (²A′) is predicted to lie 5.3 kcal/mol higher in energy at the G2(P) level. In any case it seems that both structures could be accessible to experimental detection. Low ionization potential and high proton affinities are obtained for the most stable NaC₃ isomers. Therefore, if present in the interstellar medium, NaC₃ should be easily ionized and would react quite easily to give the protonated species.