Showing papers on "Ab initio quantum chemistry methods published in 1972"

Energy distribution among reaction products. VI - F + H2, D2.

Abstract: Study of the F + H2 reaction, which is of special theoretical interest since it is one of the simplest examples of an exothermic chemical reaction. The FH2 system involves only 11 electrons, and the computation of a potential-energy hypersurface to chemical accuracy may now be within the reach of ab initio calculations. The 'arrested relaxation' variant of the infrared chemiluminescence method is used to obtain the initial vibrational, rotational and translational energy distributions in the products of exothermic reactions.

Ab Initio Calculations on Large Molecules

Abstract: Publisher Summary This chapter illustrates the efforts that have been made to develop procedures for treating large molecules in a practical manner. It considers an arbitrary and myopic definition of a large molecule as one in which there are ≥ 36 electrons and containing atoms including hydrogen and others from the first row of the periodic table. The chapter is limited to the procedures that are based upon an ab initio formalism. While several of these approaches such as CNDO (complete neglect of differential overlap), INDO (intermediate neglect of differential overlap), MCZDO (multi-center zero differential overlap), and NEMO (nonempirical molecular orbital theory) have provided useful and informative results, it seems clear that an ab initio approach is desired for several reasons. First, the identification and improvement of inadequacies is more straightforward in an ab initio approach, since only a model plus a basis set need to be examined. Second, the parametrization techniques are developed typically for the determination of the total energy and are not applicable in general to other properties. Third, the evaluation of integrals over Slater-type orbitals for properties other than the energy presents formidable problems, which are solved usually by invoking additional parameterization procedures. Consequently, these and other theoretical and practical difficulties associated with the use of semiempirical techniques have led to the search for alternative formulations of an ab initio nature.

Theoretical Potential Energy Curves for OH, HF+, HF, HF−, NeH+, and NeH

Abstract: Ab initio calculations have been carried out on the ground states of OH, HF+, HF, HF−, NeH+, and NeH. Extended basis sets were used and electron correlation was included by way of first‐order wavefunctions. Dissociation energies and other spectroscopic constants are in good agreement with available experimental data except for the bond distance of HF+. Electron detachment in collisions between H and F− is discussed on the basis of the calculated potential curves. Potential curves were also obtained ab initio for the three lowest excited states of NeH. These curves are qualitatively similar to those reported earlier by Slocomb, Miller, and Schaefer for HeH. The C 3Σ+ state of NeH is predicted to have a potential maximum of 0.87 eV at internuclear separation ∼ 4 bohr.

Internal rotation in some organic molecules containing methyl, amino, hydroxyl, and formyl groups

Abstract: Ab initio molecular orbital theory is used to study internal rotation in 20 organic molecules of the types X-Y, X-CH2-Y, X-SH-Y, X-O-Y, and X-CO-Y-where X and Y are methyl, amino, hydroxy, or formyl groups. In some of these molecules, internal rotation about two bonds is possible. The theoretical results are generally in moderate agreement with available experimental data and, in addition, lead to a number of predictions for molecules for which experimental information is lacking.

Vibrational analysis of the electronic spectrum of ethylene based on ab initio SCF-CI calculations

Abstract: Ab initio calculations for CH2 twisting and CC stretching vibrational wavefunctions and energy levels are reported for various electronic states of ethylene C2H4. Electronic transition moments between these states are also obtained to allow a calculation of the oscillator strengths for vibrational transitions involved in various electronic band systems; from this study it is concluded that thevertical electronic energy differenceΔE e may differ significantly from the energy of the absorption maximumΔE max with which it is often equated. In particular it is found in the case of theπ→π * singlet-singlet excitation of ethylene that theΔE e value overestimates the most probable vibrational transition energy (7.89 eV) by some 0.4 eV, thereby offering an explanation for the fact that previous attempts to predict the location of theV-N Franck-Condon absorption maximum have consistently obtained substantially higher results than the 7.66 eV value actually observed. Similar calculations for various Rydberg species and for theN-T transition are also found to obtain a quite consistent representation of the electronic spectrum of this system.

Photoelectron Spectroscopy of High-Temperature Vapors. II. Chemical Bonding in the Group III Monohalides

Abstract: The He(I) photoelectron spectra of InCl, InBr, and InI are presented. A general description of bonding in all Group III monohalides is given on the basis of these new data, earlier data on thallium monohalides, ab initio calculations on BF, AlCl, and GaCl, and semiempirical calculations on all of the Group III monohalides. It is found that a molecular orbital localized on the electronegative atom in an ionic diatomic molecule is appropriately classified as ``bonding''; whereas a molecular orbital localized on the positive ion has an ``antibonding'' character. This classification is qualitatively different from that used to discuss covalently bound molecules.

Ab initio calculations on the silicate ion, orthosilicic acid and their L2,3 X-ray spectra

Abstract: Ab initio calculations using small basis sets have been carried out for the silicate ion and orthosilicic acid. The results indicate that there is considerable d-orbital involvement in the wavefunctions. The L2,3 X-ray fluorescence spectra have been calculated approximately for these species and compared with the experimental spectrum for silica. The addition of d orbitals to the basis set produces calculated spectra which are similar to the experimental.

Accurate Potential Curves and Properties for the X2π and A2Σ+ States of LiO

Abstract: Ab initio calculations have been performed to determine potential curves and molecular properties for the X 2π and A 2Σ+ states of the LiO molecule. The calculations use a conventional configuration interaction (CI) method in which the Hartree‐Fock configuration is taken as reference configuration and only valence shells are correlated. The molecular orbital (MO) basis set used in the CI calculations is composed of the Hartree‐Fock orbitals and additional MO's. These additional MO's are formed by truncating a set of pseudonatural orbitals (PSNO's) obtained as the natural orbitals of a CI calculation on a single pair of valence electrons. The main results are Re=1.695 A, De0=3.37 eV, ωe=851.5 cm−1 (7Li16O), μ=6.76 D for the X 2π state; and Re=1.599 A, De0=4.90 eV, ωe=866.8 cm−1 (7Li16O), μ=5.96 D for the A 2Σ+ state. The computed De0(X 2Π)=3.37 eV is in good agreement with the observed value of 3.39± 0.26 eV. The other results are also believed to be accurate to within a few percent. The computed term ener...

SCF Calculations of the Electronic States of Magnesium Monoxide

Abstract: Ab initio calculations based on Nesbet's symmetry and equivalence restriction version of the LCAO—MO—SCF method, using an extended double‐zeta basis set, have been performed at a number of internuclear separations for six configurations, in order to account for the features of the spectrum of MgO and to ascertain the nature of the ground state. Most arguments coming from both experimental data and theoretical calculations corrected by semiempirical estimates of correlation energy differences between the configurations, are in favor of a closed‐shell 1Σ+ ground state, although the lowest Hartree—Fock state is a less correlated 3II open‐shell state. Higher‐lying observed states (D1Δ and C1Σ−) as well as unobserved ones up to the first ionization limit are accounted for by variational or reliable virtual orbital calculations, and they are presented in an energy‐level diagram. In this connection, the properties of the variational 2II and 2Σ+ low‐lying levels of MgO+ are discussed. It is then suggested that, i...

Linear Symmetric H4

Abstract: Ab initio calculations have been carried out for the lowest 1Σg+ state of H4. A contracted Gaussian basis set of two s and one p functions centered on each atom was used. Self‐consistent field (SCF), SCF plus all singly and doubly excited configurations, and full configuration interaction (2172 configurations) calculations were carried out. The results may be pertinent both to the H2+D2 reaction and the problem of the linear antiferromagnetic chain. It is predicted that two H2 molecules may approach to within 1.6 bohr with an energy only 43 kcal above that of the separated molecules. A van der Waals attraction of 22°K is predicted at H2 – H2 center of mass separation 7.1 bohr. Equidistant H4 is predicted to have lowest energy at 1.67 bohr, lying 7.1 eV below the exact energy of four H atoms but 44 kcal above two H2 molecules. The electronic structure of linear equidistant H4 is discussed as a function of H – H separation and no evidence of a Mott transition is found.

Use of Ab Initio G1 Effective Potentials for Calculations of Molecular Excited States

Abstract: The validity of using ab initio G1 effective potentials for excited states of molecules is tested for a number of excited states of LiH. We find that the effective potentials lead to wavefunctions and properties which are in excellent agreement with those obtained from full‐electron ab initio calculations.

The electronic spectrum of binuclear copper acetate monohydrate

Abstract: The near-ultraviolet spectrum of copper(11) acetate monohydrate consists of at least two bands centred at 360 and 325 nm, and polarized parallel and perpendicular to the copper-copper axis, respectively. From the temperature dependence of intensity the two bands are identified as singlet-singlet transitions. Their polarizations and intensities are consistent with the charge transfer assignments, lAlg → lA2u [b2u(py) → bIg(x2-y2)] and lAlg → lEu [eg(px) → bu2(x2-y2)], where b2u(py) and eg(px) are nonbonding ligand molecular orbitals composed of out-of-plane and in-plane oxygen 2p-orbitals, respectively.

Molecular Orbital Theory of the Hydrogen Bond. III. Dimers Containing NH2OH, H2O, HOF, and H2O

Abstract: Ab initio calculations using a minimal STO‐3G basis set have been performed on the pairs of mixed dimers NH2OH–H2O and HOF–H2O, and on (NH2OH)2 and (HOF)2. Equilibrium structures and energies are reported and analyzed for each. It is found that all dimers have equilibrium open chain structures except (NH2OH)2, which exists as a cyclic dimer. Two stable dimer structures of comparable energies are found for the mixed dimer HOF–H2O which has HOF as the proton donor molecule, and for (HOF)2. In the entire series of dimers ROH–H2O and (ROH)2, calculated dimer properties are analyzed as a function of the substituent R, and correlations are made where possible.

Ab initio and semiempirical calculations of the static potential for electron scattering off the nitrogen molecule

Abstract: Six calculations, three ab initio and three semiempirical, are presented for the static potential (i.e., the interaction potential energy of an electron with the unperturbed charge distribution) of a nitrogen molecule. The ab initio calculations are Hartree‐Fock calculations using contracted Gaussian basis sets capable of yielding wavefunctions (1) near the Hartree‐Fock limit, (2) near the sp‐limit, and (3) at the minimum basis set level. The semiempirical calculations are all within the INDO formalism. We find that the inclusion of d functions in the ab initio calculations is necessary for an accurate description of the anisotropy of the potential. We also find that the INDO calculations give the correct form of the potential at small electron‐molecule separation distances but are poor at large distances.

Calculation of Hyperfine Coupling Constants in Inorganic Radicals

Abstract: Hyperfine coupling constants have been calculated for the well‐known radicals HCO, NO2, NF2, ClO2, and CF3 as well as a large number of related species. An SCF Hartree‐Fock‐type approach was used with a minimum basis set of Slater‐type orbitals to obtain MO coefficients of the singly occupied orbital. Some of the coupling constants were obtained directly from the ab initio calculations; others were estimated by use of semiempirical relations. Comparison with experimental data shows fair agreement. In addition, the structures of NF2, O3+,O3−, O3, and CF3 were computed.

Ab Initio Calculations on Large Molecules Using Molecular Fragments. Oxygen‐Containing Molecules

Abstract: An ab initio procedure for the investigation of large molecules is applied to a series of oxygen‐containing molecules. Prototype molecules used to characterize the procedure include H2O, H2O2, CH3OH, CH3OCH3, furan, H2CO, CH3CHO, (CH3)2CO, HCOOH, CH3COOH, and HCOOCH3. The method is found to provide interesting and useful information concerning electronic structure and molecular geometry in spite of the restricted sets of basis orbitals employed. When possible, comparisions are made with other theoretical results as well as experimental values.

Ab Initio Calculations on the H2 + D2 → 2HD Four‐Center Exchange Reaction. II. Orbitals, Contragradience, and the Reaction Surface

Abstract: We discuss the reaction surface for the four‐center exchange reaction H2 + D2 → 2HD in terms of the orbitals from spin‐coupling optimized GI(SOGI) calculations. The surface is comparable to previously published surfaces determined by configuration interaction. The shape of the surface is discussed in terms of the exchange kinetic energy and contragradience, which have previously been used to discuss the bonding or nonbonding of molecules.

Magnetic behaviour and structure of polymeric oxovanadium carboxylates

Abstract: The synthesis and properties of a series of oxovanadium(1V) carboxylates (i.e. salts of organic acids) are described. The complexes closely resemble each other in magnetic properties, the observed paramagnetism being abnormally low for oxo-vanadium(1V) in the temperature range 90-350 K, with a broad maximum around 200-250 K. After testing of all reasonable models, the magnetic properties are interpreted in terms of anisotropic (Ising) antiferromagnetic exchange along infinite linear chains of interacting spins. On the basis of this model and with the aid of infrared, e.s.r., and electronic spectra, the most probable molecular structure for the series of complexes can be given. In our interpretation of the evidence, the interactions are of the super-exchange type.

Quantum mechanical calculations of the HCN-HNC isomerization

Abstract: Ab initio calculations using a minimum basis of Slater orbitals have been carried out on the molecules HCN and HNC with optimization of geometry. The predicted geometry of HNC is compatible with an observed radio-emission line which has been attributed to this molecule. The reaction profile for the internal migration of the proton has been calculated. The transition state is approximately T shaped with an activation energy from HNC of 251 kJ/mole.