An ab initio study of the structure and torsional modes of the H2SO4 molecule

Ab initio calculations at the STO—3G and 4—31G levels have been carried out for the H₂SO₄ molecule as a function of the pair of twist angles of the HO bonds about the respective OS bonds. Values for the remaining bond angles and lengths were taken from the recent microwave structural determination by Kuczkowski et al. The results indicate a minimum energy for a structure with a (sc, sc) conformation and C₂ symmetry, where sc denotes synclinal, or gauche. This structure corresponds to that observed. At a higher energy of 11.5 kJ mol^?1 (4—31G) there is a structure with a (+sc, ?sc) conformation and C_s symmetry. The torsional modes corresponding to the a and b irreducible representations of the C₂ point group are estimated to have frequencies of 280 and 265 cm^?1, respectively.     

Die gas-chromatographische Analyse von Fluormethanen (CF4, CHF3, CH2F2, CH3F)

Ohne Zusammenfassung     

The synthesis and vibrational spectra of the antimony(V)fluoride-fluorosulfates SbF3(SO3F)2, SbF4(SO3F) and Sb2F9(SO3F)

The synthesis of the title compounds by a variety of routes is described. These routes involve the oxidation of antimony(III)fluoride by bis(fluorosulfuryl)peroxide and the use of non-statistical ligand redistribution reactions. Complex formation with ClO₂SO₃F leads to ClO₂[SbF_n(SO₃F)_6-n], with n=3 or 4. Polarized Raman data and low temperature infrared data are reported. All antimony(V)fluoride-fluorosulfates are found to have fluorosulfate bridges with SbF₃(SO₃F)₂ and SbF₄(SO₃F) being polymers. Ionic formulations for the ClO⁺₂ complexes are again based on vibrational spectra.     

An AB initio molecular orbital study of the NF2 and N2F4 molecules,and ESR hyperfine coupling constants in NF2

In this publication, we present the results of gaussian type orbital calculations of ESR hyperfine coupling constants in NF₂. We also present electron density maps for the molecule, and the results of a calculation of ΔH⁰₂₉₈ for the reaction N₂F₄ → 2NF₂.     

An ab initio study of the electronic structure of diimide

Ab initio calculations on the structure and geometry of the three isomers of N₂H₂ (trans-diimide, cis-diimide, and 1,1-dihydrodiazine) were performed both on HF and CI level using gaussian basis sets with polarization functions. The trans and cis isomers have singlet ground states; the trans isomer is found to be lower in energy than the cis isomer by 6.9 kcal/mol (HF) and 5.8 kcal/mol (CI), respectively. The barrier for the trans-cis isomerization is predicted to be 56 (HF) and 55 (CI) kcal/mol. H₂ N=N has a triplet ground state with a non-planar equilibrium geometry and a rather long NN bond of 1.34 Å. Its lowest singlet state, however, is planar with an NN double bond of 1.22 Å; it is found to lie about 3 kcal/mol above the triplet and 26 kcal/mol above the singlet ground state of trans-diimide.     

X-ray/neutron diffraction studies and ab initio electronic structure of CeMgNi4 and its hydride

The crystal structure of CeMgNi₄ intermetallic compound has been studied by both X-ray and neutron diffraction. Rietveld refinement shows that both 4a and 4c sites are occupied by Ce and Mg. The exchange has been evaluated to be about 15%. The hydrogenation of the sample leads to a decomposition and to the formation of CeH_2.52. Ab initio calculations using pseudo-potential and all-electron DFT methods are performed to explain such an unexpected behaviour. They predict a larger stability of the hydride system in the orthorhombic structure rather than in the cubic one. Anti-bonding Ce–H interactions within the hydride are proposed to assess the observed easy decomposition. Moreover, the metastability introduced by mechanosynthesis (i.e. exchange between Ce and Mg) was also evaluated.     

Gas phase electron diffraction study of SiH3N3 and (SiH3)2 NCN

The molecular structures of SiH₃N₃ and (SiH₃)₂ NCN have been determined by electron diffraction : SiH₃N₃ has a non-linear SiN₃ skeleton, and (SiH₃)₂ NCN is a carbodiimide with SiNCN Si probably linear. A new model is proposed which rationalises the observed geometry of a number of molecular pseudohalides.     

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.     

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.     

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.     

Reaction path dynamics and theoretical rate constants for the SiH3Cl+H→SiH2Cl+H2 reaction by ab initio direct dynamics method

Ab initio direct dynamics method has been used to study the title reaction. Electronic structure information including geometries, gradients and force constants (Hessians) are calculated at the UQCISD/6-311+G^** level. Energies along the minimum energy path are improved by a series of single-point G2//QCISD calculations. The changes of the geometries, vibratioanal frequencies, potential energies and total curvature along the reaction path are discussed. The rate constants in the temperature range 200–3000 K are calculated by canonical variational transition state theory with small-curvature tunneling correction (CVT/SCT) method. The results show that the variational effect is small and in the lower temperature range, the small curvature tunneling effect is important for the reaction.     

AB initio SCF study of the electronic structure and spectrum of CuF2

MC SCF and contracted CI calculations have been performed for the three ligand-field states of CuF₂ and also for two charge-transfer states. With the most extensive basis set the calculated d-d transition energies, including a Davidson correctior for cluster effects, are 4150 cm^?1 (²11g) and 10560 cm^?1 (²Δg). These calculations were made with 98 basis functions, including of orbitals on Cu and d orbitals on F. To check the charge distribution in the molecule, calculations of the ESR g factors were also made at the SCF and CI levels of approximation. Resulting CI values are g_| = 1.93 (1.91) and g₁ = 2.76 (2.60). with corresponding experimental numbers in parentheses.     

Ab Initio calculation of molecular, thermodynamic, and kinetic parameters of CF, CF2, CF3, and C2F5 radicals and CF4, CF3I, C2F4, and C2F6 molecules in gas

Enthalpies of formation of ground states of the gaseous particles CF, CF₂, C₂F₅, CF₄, CF₃I, C₂F₄, and C₂F₆ were calculated by ab initio method in the CCSD(T) approximation with extrapolation to the full basis and regard to the correlation energy. Their equilibrium geometrics, frequencies of normal vibrations, and other values were found by the B3LYP/aug-cc-pvdz method, from which thermodynamic functions within the range of 0–6000 K were calculated. Equilibrium constants were calculated from these functions, and then the information on the rate constants in the limit of high pressures was obtained.     

An ab initio MO study on structures and energetics of CH2Si2, CH2Si2, and CH2Si2

The geometric structures and isomeric stabilities of various stationary points in CH₂Si₂ neutral, cation and anion are investigated at the coupled-cluster singles, doubles (triples) (CCSD(T)) level of theory. For the geometrical survey, the basis sets used are of the cc-pVTZ for the neutral and cation. The final energies are calculated by the use of the CCSD(T) level of theory with the aug-cc-pVTZ basis set at their optimized geometries. To the competitive two-anion isomers, the aug-cc-pVTZ basis sets are applied. The global minimum (N-1) of the CH₂Si₂ neutral has a quite different framework from those of the C₃H₂ (cyclopropenylidene) and Si₃H₂ (trisilacyclopropenylidene) neutrals. No competitive low-lying isomers are found in the CH₂Si₂ neutral. The attractive conformer (C-1) is predicted for the most stable cation, where its framework is quite different from that of the neutral N-1. Both H atoms are connected to the same C atom, but each C–H bond length is different from each other. Two competitive anion isomers with positive (real) electron affinities are predicted. The framework of the most stable anion A-1 is quite similar to that of the cation C-1, whereas both H atoms are equally connected to the same C atom. The framework of the anion isomer A-2 is the same as that in the neutral N-1. The vertical and adiabatic ionization potentials from the most stable neutral N-1 are 9.02 and 8.71 eV, respectively. The adiabatic electron affinity of the lowest lying isomer N-1 is only 0.43 eV and the vertical electron detachment energy form the global minimum anion (A-1) is 2.02 eV. The multi-centered Si–H–Si bonds are found in the neutral, cation, and anion.     

Thermal behaviour of (NH4)3VO2F4 and Na(NH4)2VO2F4

The thermal behaviour of (NH₄)₃VO₂F₄ and Na(NH₄)₂VO₂F₄ was investigated using TG, DTA and DSC techniques. The occurrence of a first order phase transition with the onset of decomposition in both the compounds is confirmed. The temperature, energetics and hysteresis of the transition are obtained. A possible path for the thermal degradation is given for both the compounds, and the residues are identified.     

An ab initio study of possible pathways in the thermal decomposition of NaAlH4

Density functional theory (DFT) has been used to study the structural stability of possible intermediate alanate structures, Na₅Al₃H₁₄ and Na₂AlH₅, in the thermal decomposition of NaAlH₄. Na₅Al₃H₁₄ crystallizes in the space group P4/mnc with lattice constants , and c/a=1.52. It is shown that both Na₅Al₃H₁₄ and Na₂AlH₅ have the right thermodynamics and can fit in as an intermediate state during the thermal decomposition process of NaAlH₄. The heat of formation of Na₅Al₃H₁₄ is −60 kJ/mol H₂, which is intermediate between that of NaAlH₄ (−51 kJ/mol H₂) and Na₃AlH₆ (−69.7 kJ/mol H₂). An alternative decomposition pathway based on Na₂AlH₅ has also been discussed. Frequency analysis showed that the least energetic Na₂AlH₅ structure has imaginary frequencies, implying that it is unstable. The presence of soft phonon modes also shows that Na₅Al₃H₁₄ is mechanically metastable. These results are consistent with the notion that they are the intermediate states that lead to the formation of AlH₃. This facilitates the mass transport of aluminum atoms in the decomposition pathway of NaAlH₄.     

An ab initio study of the electronic structure of BCl3(2+) and its decomposition pathways

Ab initio methods have been used to characterise the lowest energy potential energy surfaces of 1BCl3(2+) and 3BCl3(2+). The methods used are MP2, CCSD, CCSD(T) and MCSCF with 6-311G(d), cc-pVTZ and aug-cc-pVTZ basis sets. While the singlet surface is relatively straight-forward, the triplet surface is very complicated, with many stationary points in close energetic proximity. The singlet surface can fragment to the following products (1BCl + 1Cl+ + 1Cl+), (1Cl+ + 1B+ + 1Cl2), (2BCl+ + 2Cl), while the triplet surface can fragment to (1BCl2+ + 3Cl+) and (2BCl2(2+) + 2Cl). 2BCl2(2+) can further fragment to (1Cl+ + 2BCl+). These results are in good agreement with previous experimental data from coincidence mass spectroscopy. [symbol: see text]1 diagnostic values have been calculated for all of the stationary points of BCl3(2+), using the method of Lee and Taylor. These data, together with CCSD/CCSD(T) energy differences and MCSCF calculations, are used to conclude that most of the stationary points on the singlet surface are well represented using single reference methods. The stationary points of the triplet system have [symbol: see text]1 diagnostic values greater than those for the singlet system, as expected when using the closed-shell [symbol: see text]1 diagnostic method of Lee and Taylor. However, all of the structures have acceptable single reference wavefunctions if the open-shell upper limit of Rienstra-Kiracofe et al. (0.045) is used, a conclusion fully supported by CCSD/CCSD(T) energy differences. CCSD(T) energies determined for the fragmentation asymptotes have been compared with experimental data collated from the NIST Theoretical and Scientific Data website, and the generally very good agreement between theory and experiment reinforces the reliability of the CCSD(T) method.     

Hydroboration; An ab initio study of the reaction of BH3 with ethylene

Ab initio molecular orbital calculations indicate the reaction of BH₃ with ethylene to proceed exothermically via an intermediate π-complex, but without an overall activation barrier. The mechanism of the reaction in the gas phase is indicated to proceed in two facile stages: the formation of the π-complex and its rearrangement to ethyl borane product. The progress of the reaction is shown pictorially by drawings of the interacting orbitals at various stages.     

A study of the structure of fluoroethylenes ab-initio calculations on C2F4

Ab-initio calculations on C₂F₄, with full geometry optimization in a ($\text{7}\text{3}$) and a ($\text{9}\text{5}$) basis are reported. The predicted geometries, orbital energies and populations are discussed in view of the different basis sets and previous calculations of the other fluoroethylenes. The effects of substituting a hydrogen atom with a fluorine atom are rather local, the effects on the populations being additive in all fluoroethylenes except C₂F₄. A short comparison between the basis sets is presented.     

An ab initio LCAO-MO-SCF study of the electronic structure of phosphirane and thiirane

Wave functions have been determined for the C₂H₄PH and C₂H₄S cyclic molecules, using (951/52/3) and (95/52/3) uncontracted Gaussian basis sets for each molecule. From Mulliken population analyses and electron-density plots, it is shown that the valence orbitals of C₂H₄PH and C₂H₄S are closely related and that these are similar to the respective orbitals of cyclopropane.

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Zusammenfassung Für die zyklischen Moleküle C₂H₄PH und C₂H₄S wurden mit den nichtkontrahierten Basissätzen ((951/52/3) und (95/52/3)) von Gaußfunktionen Wellenfunktionen bestimmt. Die Mullikenschen Populationsanalysen sowie Diagramme der Elektronendichte zeigen, daß die Valenzorbitale von C₂H₄PH und C₂H₄S in enger Beziehung stehen und daß diese den entsprechenden Orbitalen des Cyclopropans ähnlich sind.