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A number of molecular species on the H3CO(+) energy hypersurface is examined. Ab initio molecular orbital theory is used to determine the structures and relative energetics of the two isomers of HCO(+) and HOC(+) together with the affinity of CO for protonation at either end. The proton affinities of H2 and H2CO are also examined. The calculations are performed using large basis sets and include the effects of electron correlation. The calculated vibrational frequencies are used to correct for zero point energy differences. The results show that the proton affinities of H2 and CO to form HOC(+) are within 1 kcal of each other. The calculations demonstrate that there is no thermodynamic driving force to form HOC(+) in collisions of H3(+) with CO, and that the formation of HCO(+) in such collisions is very exoergic. A plausible mechanism is suggested to explain the differences observed between the laboratory and the interstellar medium.

Investigation of mixed ferrites La1−xCaxFeIII1−τFeIVτO3−y (0 ≤ x ≤ 0.05, 0 ≤ y ≤ 0.07) has been carried out by magnetic, electrical and Mossbauer resonance measurements. Mossbauer spectra at low temperature can be interpreted assuming a disproportionation of tetravalent iron according to a scheme previously proposed for CaFeO3: 2 FeIV → FeIII + FeV. Electrical measurements show an increasing electronic localization when the temperature is decreased. This result corroborates the proposed model which moreover corresponds to a stabilization of the isotropic configurations t32ge2g of FeIII and t32ge0g of FeV in regular octahedral sites. The fit of the Mossbauer spectra at 4.2 K is also discussed in relation to the distribution of FeIII and FeV in the lattice.

Analytical first and second derivatives for effective core potentials are reported. The computational implementation of the derivative formulas makes use of new integral routines which take advantage of the shell concept. Test calculations for H 3 SnBr and F 3 AsS demonstrate the efficiency of the analytical determination of harmonic force fields using effective core potentials. The spectroscopic constants of the unknown molecule F 3 AsS are predicted.

The semiclassical theory of electronically nonadiabatic transitions in atom–diatom collisions is modified by the introduction of a decoupling approach to the nuclear dynamics. The probability of switching surfaces is calculated locally by integrating in the complex plane over a nuclear coordinate which couples most strongly to the electronic transition. A substantial computational simplification results when this approach is implemented. The power of the method lies in the fact that the potential energy surfaces need be analytically continued locally rather than globally. Thus the trajectories can be maintained on the real axis except for the one point where the surfaces are switched. Computational results are presented and discussed for two model collinear nonreactive systems, F+H2 and Br+H2. It is shown that when the possibility of an E–V resonant energy transfer exists, the vibrational as well as the electronic degrees of freedom must be treated quantum mechanically.

Analytic derivative techniques are utilized to determine molecular geometries, vibration spectra, molecular force field, proton affinity, and energetics of clustering of protonated methane. It is observed that the geometries are gradient optimized, while the harmonic force fields and IR intensities are also determined analytically at the SCF level. It is determined that the frequency of the torsional model for rotation of the H2 about the pseudothreefold axis of the CH3(+) group is extremely low, and the proton affinity of CH4 is calculated as 129.0 kcal/mol.

The Li/sup +/ and Na/sup +/ affinities of H/sub 2/, N/sub 2/, and CO have been calculated by ab initio molecular orbital theory with extended basis sets. Geometries were gradient optimized at self-consistent field (SCF) level. Force fields were calculated at the SCF level at stationary points as were correlation corrections at the Cl(SD) level with a correction for higher order excitations where appropriate. The optimum geometries are C/sub 2v/ (bent) for MH/sub 2//sup +/ and C/sub infinity v/ (linear) for MCO/sup +/, MOC/sup +/, and MN/sub 2//sup +/. The Li/sup +/ affinities (..delta..H at 300 K) for H/sub 2/, CO (at O), CO (at C), and N/sub 2/ are 4.8, 12.0, 14.6, and 11.8 kcalmol, respectively, at the correlated level. The Na/sup +/ affinities (..delta..H at 300 K) for H/sub 2/, CO (at O), CO (at C), and N/sub 2/ are 2.8, 7.6, 9.6, and 7.4 kcalmol, respectively, at the correlated level.

A set of large-scale ab initio molecular orbital calculations on the N 2 H + and N 2 H + … H 2 potential energy surfaces is presented. The calculations of energies, geometries and vibrational force fields were done with large polarized basis sets at both the SCF and the correlated wavefunction levels. The correlation corrections were incorporated using standard configuration interaction techniques including a correction for quadruples. The infrared band intensities were also determined analytically at the SCF level. Our most reliable value for the proton affinity of N 2 is calculated to be 119.7 kcal/mol which is somewhat higher than the currently accepted experimental value of 118.2 kcal/mol. Our value is based on CI SDQ calculations which include zero-point as well as thermodynamic corrections. We discuss possible sources of error in both the calculated as well as the experimentally accepted values. We have investigated the formation of a molecular complex between H 2 and N 2 H + where we find that the H 2 binds preferentially to the protonated end of the ion. The well depth at the CI SDQ level, excluding zero-point corrections, is 6.8 kcal/mol. The well depths of the complexes of H 2 with the protonated species, HN + 2 , HOC + and HCO + are discussed in terms of the proton affinities of N 2 , CO and H 2 .

Calculs de champ auto-coherent au niveau 6-31G* pour les composes du titre et les phosphines correspondantes. Determination des constantes de rotation, des structures geometriques, des frequences vibrationnelles et comparaison avec les donnees experimentales. Suggestion d'attributions detaillees pour les spectres vibrationnels des composes trimethyles

The structure, torsional potentials, vibrational spectra, and harmonic force fields for s-cis and s-trans isomers of methacryloyl fluoride are examined to understand the conformational properties of the molecules and their relationship to macroscopic polymer properties. The structure is found to be in good agreement with experiment. It is shown by calculations that the energy difference between the cis and the transisomers is less than 1 kcal/mol at both the split valence and the split valence polarized levels, with the trans form favored. Analysis of the torsional potentials indicates that a rigid rotor model provides a reasonable description of the motion of the COF group in the molecule. The torsional barrier to interconvert the s-trans to the s-cis form is found to be 7.0 kcal/mol. A fit of the data to a three-term Fourier series shows that it is possible to reproduce the experimentally derived barrier, even though a direct determination indicates that the barrier is higher.

A practical method of calculating the intrinsic reaction coordinate starting at a saddle point is proposed. The method has been used in combination with the analytical evaluation of the energy gradient for the calculation of the reaction coordinate on an ab initio potential energy surface. The reaction coordinates are obtained for the HNC to HCN isomerization and the SN2 exchange reaction involving H−+CH4→CH4+H−.

Determination par une methode ab initio des constantes de force, frequences de vibration et conformation d'une serie de composes cycliques satures a 3 chainons

The mechanism of electronic‐to‐vibrational (E–V) energy transfer in Hg(3P) + CO collisions has been studied theoretically. The configuration interaction (CI) method was employed to calculate potential energy surfaces of the collision system. A simplified theoretical model, based on the reaction coordinate concept and the calculated potential energy characteristics, was used to discuss the mechanism of the singlet–triplet transition and the energy disposal in the collision. The results obtained were that: (a) the quenching process proceeds via a collision complex mechanism; and that (b) the triplet–singlet transition occurs near the collinear geometry. A model classical trajectory calculation gives a product CO vibrational distribution in good agreement with the experimental result.

A method is outlined for the calculation of photoionization cross sections representing the initial state by a minimal basis set STO N G wavefunction and the final state by the plane wave approximation. Numerical results for several small molecules compare well with the band intensities in published ESCA spectra, and are superior to ESCA band intensities obtained from valence electron wavefunctions. For the UV region, ab-initio and valence electron calculations lead to comparable results.

The gradient of the potential energy with respect to nuclear coordinates has been calculated using ab-initio single determinant molecular orbital methods. The calculated gradient is used together with very efficient minimization methods to locate and characterize transition states on many-dimensional potential energy surfaces. Previously such methods have only been applied to semi-empirical potential functions. Although the calculation of the gradient in addition to the energy increases the computational time by about a factor of four, we have demonstrated the feasibility of these calculations by locating the transition state for the model rearrangement of HNC to HCN using both minimal (STO-3G) and split valence shell (4-31G) basis sets. Further use of such methods in the direct application of ab-initio wavefunctions to dynamical investigations is discussed.

Phases between Ca 2 Fe 2 O 5 and LaFeO 3 with La 1−2y Ca 2y Fe 3+ O 3−y formulation have been prepared in order to study the oxygen defects in perovskite-related ferrites. X-ray-diffraction analysis and magnetic measurements have confirmed the previous results i.e. the existence of a critical concentration of vacancies above which long range ordering appears. A detailed Mossbauer resonance study shows a continuous evolution of the iron environment, tetrahedra being formed even for low values of y. This result has been discussed and compared with previous ones for homologous CaTi 1−2y Fe 2y O −y phases.

Formulas are presented relating Taylor series expansion coefficients of three functions of several variables, the energy of the trial wave function (W), the energy computed using the optimized variational wave function (E), and the response function (lambda), under certain conditions. Partial derivatives of lambda are obtained through solution of a recursive system of linear equations, and solution through order n yields derivatives of E through order 2n + 1, extending Puley's application of Wigner's 2n + 1 rule to partial derivatives in couple perturbation theory. An examination of numerical accuracy shows that the usual two-term second derivative formula is less stable than an alternative four-term formula, and that previous claims that energy derivatives are stationary properties of the wave function are fallacious. The results have application to quantum theoretical methods for the computation of derivative properties such as infrared frequencies and intensities.

The kinetics of reduction of vanadium pentoxide with ammonia has been studied by thermogravimetric method in the temperature range 664–700 K. It has been found that the experimental curves obtained can be described by Belkevitsch-Erofeev equation: [(1 − α) −1 3 − 1]= kt n . The results obtained suggest that the reduction rate is limited by diffusion processes in solid phase.

The measurements of the aerodynamic properties and the convective heat transfer coefficients in the model of a highly circulating reheating furnace are reported. Two methods of measurement of the convective heat transfer coefficient are applied to verify the Chilton-Colburn analogy for the presented geometry. The measurements are compared with the results of the calculation procedure incorporating a two equation turbulence model. The calculation results of the local heat transfer coefficients are presented. The comparison indicates that the calculation procedure qualitatively represents the measurements but that quantitative differences exist.

The role of electron transitions in collisions is studied for the F + H2 reaction by combining quasi-classical Monte Carlo trajectories with a semiclassical decoupling approximation for the electron transitions. Attention is directed at the reaction of excited state F atoms reacting to form ground state products; the reactants are initiated in either of two spin-orbit states of the atom with the diatom in the ground vibrational state and the lowest four rotational states, at relative translational energies of 0.1, 0.2 and 0.3 eV. Even if the reactants are initiated on the excited state surface, the reactive cross sections (which are classically forbidden) are significant. The major dynamical effects of the excited state reaction are the flow of reactant electronic energy into product internal energy.

For semi-empirical molecular orbital methods, the gradient of the potential energy can be calculated with negligible additional computational expense. This allows powerful minimization methods to be used to calculate the geometries of large molecules. The particular minimization method used is shown to conserve molecular symmetry and its efficiency is compared to that of a method which does not use the gradient.

The proton affinities of LiH, NaH, Li/sub 2/, and Na/sub 2/ have been calculated by ab initio molecular orbital theory with extended basis sets. Geometries were gradient optimized, and force fields were calculated at the SCF level. Correlation corrections were calculated at the CI-SD level for the valence electrons. The proton affinities are calculated to be 244.2, 261.8, 277.8, and 274.1 kcal/mol for LiH, NaH, Li/sub 2/, and Na/sub 2/. The Li/sup +/ affinity of LiH is calculated to be 56.4 kcal/mol, and the Na/sup +/ affinity of NaH is calculated to be 49.7 kcal/mol.

Semiclassical calculations are carried out for the quenching of excited-state fluorine atom by collinear collisions with hydrogen molecule. The overall quenching probability is the sum of two contributions: the reactive quenching probability associated with the formation of hydrogen fluoride and the nonreactive quenching probability leading to ground-state fluorine atom and hydrogen molecule. The reactive probability is greater in the threshold region of the collision energy, whereas the nonreactive probability dominates for energies above the threshold region.

It has been known since the early days of quantum mechanics that with existing, well-understood, physical and mathematical concepts, it is in principle possible to accurately predict and account for all “chemical” behavior. Realization of this exciting and challenging goal requires only the development of an adequate computational technology and there have been significant advances toward this end in the last ten years. One of these has been the evolution of approximate, semiempirical molecular orbital (MO) methods. These methods can be used to study properties of molecules large enough to be interesting to the organic chemist and often to the biochemist. Although it is perhaps unduly optimistic to say that these methods have brought us to the state in which the computer effectively competes with existing laboratory measurement techniques(1) they do have a number of unique and valuable features. In addition to their computational economy and broad applicability, the interpretation of computed results can be couched in the simple and familiar vocabulary of LCAO molecular orbital theory.

A new and very efficient method for the calculation of the infrared and Raman spectral intensities of polyatomic molecules using ab initio Hartree–Fock theory is described. The utility of the method is exemplified by an evaluation of the dipole and polarizability derivatives of the ethylene molecule employing a Gaussian basis set of double zeta quality, augmented by two sets of polarization functions. The predicted values of the vibrational intensities and the depolarization ratios are nearly within the experimental uncertainty which indicates that Hartree–Fock theory is capable of correctly describing these phenomena. The new method enables the efficient calculation of these properties by evaluating the dipole and polarizability derivatives as the first and second order contributions to the potential energy gradient of a molecule in the presence of a finite perturbing electric field. The calculations are thus performed at a single nuclear geometry and the molecular integrals are evaluated only once. The ...