Showing papers on "Ab initio quantum chemistry methods published in 1987"

Structure, stability, and fragmentation of small carbon clusters

Abstract: Accurate ab initio calculations are performed to study the structures and energies of small carbon clusters (Cn, n=2–10). The effects of polarization functions and electron correlation are included in these calculations. Significant odd–even alternation is found in the nature of the cluster geometries with the odd‐numbered clusters having linear structures and many of the even‐numbered clusters preferring cyclic structures. Energetically, odd‐numbered clusters (up to C7) are found to be more stable than the adjacent even‐numbered clusters. Ionization potentials are calculated and used in conjunction with the cluster energies to explain the fragmentation behavior of small carbon cluster ions.

A double many‐body expansion of the two lowest‐energy potential surfaces and nonadiabatic coupling for H3

Abstract: We present a consistent analytic representation of the two lowest potential energy surfaces for H3 and their nonadiabatic coupling. The surfaces are fits to ab initio calculations published previously by Liu and Siegbahn and also to new ab initio calculations reported here. The analytic representations are especially designed to be valid in the vicinity of the conical intersection of the two lowest surfaces, at geometries important for the H+H2 reaction, and in the van der Waals regions.

Spectral and chemical properties of dimethyldioxirane as determined by experiment and ab initio calculations

Abstract: It seemed to us, therefore, advisable to check our previous oxygen-transfer results' with the thianthrene 5-oxide (SSO) probe by employing the recently isolated6 dimethyl- dioxirane (2b). We used this opportunity to reinvestigate the spectroscopic properties of 2b and to compare these with the results of ab initio calculations carried out for 1, 2a,b, and appropriate reference compounds.

The ab initio model potential method. Main group elements

Abstract: In this paper we present the ab initio core model potential method, in which the Coulomb and exchange core operators Jc and Kc are represented as accurately as possible using adequate local and nonlocal potentials, and the valence basis set is optimized in atomic MP SCF calculations following the variational principle. Nonrelativistic model potential parameters and valence basis sets are presented for the main group elements from Li to Xe. The pilot SCF molecular calculations on the ground states of N2, P2, As2, Sb2, and CaO show a good agreement between all‐electron and model potential results, in particular the changes observed in molecular results due to improvements of the valence part of the all‐electron basis set are reproduced by the model potential calculations.

Ab initio pseudopotential study of the first row transition metal monoxides and iron monohydride

Abstract: Spectroscopic constants for the ground states of the first row transition metal monoxides and iron monohydride have been determined from SCF and CI(SD) calculations using energy‐adjusted ab initio pseudopotentials representing the Ne‐like X(Z−10)+ cores of the transition metals. For iron monoxide and iron monohydride excited states have also been investigated and electron affinities have been determined.

Structure/Reactivity and Thermochemistry of Ions

Abstract: Capture Collision Theory.- Collision Theory: Summary of the Panel Discussion.- Optical Studies of Product State Distributions in Thermal Energy Ion-Molecule Reactions.- Crossed-Molecular Beam Studies of Charge Transfer Reactions at Low and Intermediate Energy.- Production, Quenching and Reaction of Vibrationally Excited Ions in Collisions with Neutrals in Drift Tubes.- Kinetic Energy Dependence of Ion-Molecule Reactions: From Triatomics to Transition Metals.- Reactions of Transition Metal Ions with Cycloalkanes and Metal Carbonyls.- Gas Phase Metal Ion Chemistry: Summary of the Panel Discussion.- Dynamics of Dissociation and Reactions of Cluster Ions.- Growing Molecules with Ion/Molecule Reactions.- AB Initio Studies of Interstellar Molecular Ions.- Structures and Spectroscopic Properties of Small Negative Molecular Ions - Theory and Experiment.- AB Initio Calculations on Organic Ion Structures.- Formation of Anions in the Gas Phase.- Proton Transfer Reactions of Anions.- Assignment of Absolute Gas Phase Basicities of Small Molecules.- Kinetics and Equilibria of Electron Transfer Reactions: A? + B = A + B?. Determinations of Electron Affinities of A and B and Stabilities of.Adducts A2? and (A * B)?.- Ion Thermochemistry: Summary of the Panel Discussion.- Entropy-Driven Reactions: Summary of the Panel Discussion.- Organic Ion/Molecule Reactions: Summary of the Panel Discussion.- Experimental and Theoretical Studies of Small Organic Dications, Molecules with Highly Remarkable Properties.- Structure and Reactivity of Gaseous Ions Studied by a Combination of Mass Spectrometric, Nuclear Decay and Radiolytic Techniques.

Thermal Rate Constants for H + CH3 CH4 Recombination. II. Comparison of Experiment and Canonical Variational Transition State Theory

Abstract: Canonical variational transition state theory is used to calculate bimolecular rate constants for H + CH3 and D + CH3 recombination. The calculations are performed on an analytic potential energy surface derived from recent ab initio calculations. Rate constants calculated for this surface are in very good agreement with the experimental values. The H(D)---CH3 transitional rocking modes are treated as quantum harmonic oscillators or classical hindered rotors in the calculations. These two treatments give rate constants which agree to within 15%. The variational transition states become tighter as the temperature is increased.

Charge-transfer effects in graphite intercalates: Ab initio calculations and neutron-diffraction experiment.

Abstract: First-principles, total-energy electronic structure calculations have been used to study the effects of charge transfer in graphite intercalation compounds. Together with an in situ diffraction study on graphite undergoing intercalation with sulphuric acid, the results strongly support the idea of a universal relationship between charge transfer and the C---C bond length.

Dynamics of the endothermic reaction He+H+2 →HeH++H on an accurate ab initio potential‐energy surface

Abstract: We report herein a successful analytic fit of the ab initio potential‐energy surface of McLaughlin and Thompson for the ground state HeH+2 system and also the results of a three‐dimensional quasiclassical trajectory study of the exchange reaction over a wide range of vibrational states and relative translational energies of the reactants. While there is good agreement between theory and experiment in many respects, there are some quantitative discrepancies remaining with respect to some of the experimental results.

Ab initio calculation of harmonic force fields and vibrational spectra for the methyl, silyl, germyl, and stannyl halides

Abstract: Theoretical harmonic force fields are reported for 16 symmetric tops H3MX (M=C, Si, Ge, Sn; X=F, Cl, Br, I). Based on the evaluation of systematic test calculations for H3CF, H3CCl, H3SiF, and H3SiCl, all molecules are treated uniformly at the Hartree–Fock level using effective core potentials and polarized double‐zeta basis sets for the valence electrons. For each molecule the calculated geometries, rotational constants, frequencies, centrifugal distortion constants, Coriolis coupling constants, and infrared band intensities are compared with the available experimental data. The agreement is satisfactory and generally of similar quality as in analogous all‐electron Hartree–Fock calculations of molecules with first‐row atoms. It is stressed that theory may reliably provide data which are hard to derive from experiment, e.g., the off‐diagonal symmetry force constants and the signs of the dipole moment derivatives. The effects of scaling the theoretical force fields are investigated carefully including the transferability of scaling factors between related molecules, and the vibrational spectrum of the as yet unknown H3SnF molecule is predicted. Our present results indicate that the chosen theoretical approach is generally suitable for studying the harmonic force fields of inorganic molecules with heavy main‐group elements.

Theoretical study of the structures and stabilities of the carbon dioxide dimer ((CO2)2-) ions

Abstract: Ab initio calculations using flexible basis sets and including the effects of electron correlation are used to examine various possible structures for the (CO/sub 2/)/sub 2//sup -/ species. These calculations reveal that the symmetrical D/sub 2d/ and the asymmetrical CO/sub 2/ x CO/sub 2//sup -/ forms of the anion are close in energy, with the former being about 0.2 eV more stable. The two forms of the anion give very different vertical detachment energies, with that calculated for the D/sub 2d/ structure being in agreement with the experimental results.

The quartic force field of H2O determined by many‐body methods. II. Effects of triple excitations

Abstract: Ab initio coupled cluster and many‐body perturbation theory methods that include triple excitation effects are applied to the determination of the quartic force field of the water molecule using an extended Slater‐type basis set. Predictions of fundamental, overtone, and combination vibrational frequencies, rotational constants, and vibration–rotation coupling constants are reported for H2O and its isotopomers. The best predicted harmonic frequencies for the stretching modes of H2O are accurate to 3 cm−1, while the bending mode has an error of 28 cm−1. The mean absolute error for all frequencies reached by two quanta is 0.6%, while the anharmonic constants xi j have a mean absolute error of less than 3%. The important role of triple excitation effects in the surface determination is discussed, and is compared with the effects of quadruple excitations.

ESR and ab initio theoretical studies of the cation radicals 14N+4 and 15N+4: The trapping of ion–neutral reaction products in neon matrices at 4 K

Abstract: The 14N+4 and 15N+4 molecular cation radicals have been generated by the ion–neutral reaction N+2 +N2 and isolated in solid neon matrices at 4 K for detailed ESR (electron spin resonance) investigation. Both photonionization at 16.8 eV and electron bombardment (50 eV) were used in conjunction with the neon matrix trapping technique to produce the N+4 dimer cation. The ESR results clearly show that N+4 is linear and has a 2Σμ ground electronic state. The magnetic parameters in neon are: g∥=2.0016(4) and g⊥=1.9998(2); A∥(14N)=311(1) MHz and A⊥(14N)=264(1) MHz for the central atoms and ‖A∥‖=10.4(5) MHz and ‖A⊥‖=20.4(1) MHz for the outer or terminal 14N atoms. Electronic structure information for N+4 was obtained from the ESR results and compared with ab initio CI calculations. The unpaired electron resides primarily on the inner nitrogen atoms with significant 2pσ and 2s character. Orbital characters obtained from the commonly applied free atom comparison method (FACM) were compared with the results of a Mul...

Ab initio studies on hydrogen-bonded clusters. I. Linear and cyclic oligomers of hydrogen cyanide

Abstract: Large-scale ab initio calculations have been performed on linear and cyclic oligomers of hydrogen cyanide molecules applying basis sets ranging from double-zeta to near-Hartree-Fock quality. Equilibrium geometries of linear (HCN) n clusters with n = 1 to 5 and of cyclic clusters with n = 3, 4 are reported. For most of the complexes complete vibrational analysis has been carried out. In agreement with recent experimental data the linear HCN trimer was found to be more stable than the cyclic trimer. In case of the tetramer linear and cyclic structures are of comparable stability. The structural changes taking place upon polymerization of linear HCN clusters and the convergence of various stage properties to those of the infinite polymer (HCN) ∞ are discussed in detail. The evolution from vibrational spectra of small oligomers to phonon dispersion curves of the infinite polymer is illustrated too.

The electronic structure of α-quartz: A periodic Hartree-Fock calculation

Abstract: The electronic structure of α‐quartz has been calculated using the periodic Hartree–Fock method. Charge density maps, band structure, and density of states have been computed and are discussed. An overall agreement with available experimental results and with previous calculations performed within different theoretical approaches is found. In particular it is shown that ‘‘cluster type’’ calculations on molecules such as H6Si2O7 accurately reproduce the charge density in the neighborhood of the bridging oxygen. Geometry optimizations yield realistic values of the SiO bond length and of the SiOSi angle.

Double photoionization of H 2 : An experimental test of electronic-correlation models in molecules

Abstract: The double-photoionization cross sections of molecular hydrogen (H/sub 2/) and molecular deuterium (D/sub 2/) were measured by using the photoion-photoion coincidence method for photon energies ranging from the threshold energy around 50 eV up to, respectively, 140 and 98 eV. The comparison with the recent ab initio calculations of Le Rouzo (J. Phys. B 19, L677 (1986)) indicates that an important part of the double-photoionization process is accounted for by a rigorous description of the electron-electron interaction in the initial state. As a by-product of this work, it was also concluded that double photoionization of hydrogen can be considered as a vertical process and that Franck-Condon approximations are quite valid to calculate the kinetic energy of the resulting H/sup +/+H/sup +/ fragments.

Theoretical studies of transition-metal hydrides. 2. Calcium monohydride(1+) through zinc monohydride(1+)

Abstract: The authors present consistent ab initio calculations (generalized valence bond plus configuration interaction) of the spectroscopic parameters for the ground and low-lying electronic states of the diatomic transition-metal hydrides CaH/sup +/ through ZnH/sup +/. They examine, in detail, the competing factors affecting metal hydride bonding: (1) the relative energies of the metal low-lying electronic states; (2) the intrinsic bond strength of H to various size 4s, 3d, or hybridized metal orbitals; and (3) the loss of high-spin metal exchange energy on bonding.

Analysis of torsional spectra of molecules with two internal C3v rotors. XXIV. High resolution far infrared spectra of acetone‐d0, ‐d3, and ‐d6

Abstract: The high resolution (0.10 cm−1) far infrared spectra of gaseous acetone‐d0, ‐d3, and ‐d6 were measured between 370 and 40 cm−1. The region of the methyl torsional transitions below 130 cm−1 was analyzed in terms of the semirigid rotor model for molecules with two internal C3v rotors and the two‐dimensional torsional potential functions were determined. Effective barriers of 291 cm−1 (832 cal/mol) and sine–sine interaction terms of −l08 cm−1 (−309 cal/mol) were found for all three isotopic species. The results are compared with those from ab initio calculations and from the analysis of splittings in the microwave spectrum.

Protonation of azines: An ab initio molecular orbital study

Abstract: The structure, protonation energies, isomer stabilities and vertical ionization potentials of a wide set of azines were investigated by ab initio calculations using different basis sets ranging from minimal to split-valence plus polarization. Minimal basis tends to overestimate NN bond lengths. CN and CC bond lengths are also overestimated but to a lesser degree. In general 6-31G structures are very close to the experimental ones, while 3-21G basis does not always correct the deficiencies of the minimal basis. For those azines with basic centers in contact with the one which undergoes protonation, there are strong interactions between the acidic hydrogen and the second basic center which affect the gas-phase basicity of these compounds. 3-21G and 6-31G yield almost identical absolute protonation energies, about 30 kcal mol −1 smaller than those obtained at the minimal basis set level. The inclusion of polarization functions in the basis leads to a further decrease, this effect decreasing as the number of basic centers is increased. Our results illustrate the necessity of employing fully optimized structures to obtain reliable relative proton affinities. Basis set effects on relative proton affinities are analyzed as they are quite important. The effects of successive nitrogen substitutions on the gas-phase basicities are not additive. Koopmans' theorem predicts the wrong ionization for pyridine and diazines. In general N-lone-pair interactions are well reproduced at the 6-31G*/6-31G level.

Ab initio study of the unimolecular pyrolysis mechanisms of formic acid: additional comments based on refined calculations

Abstract: Refined ab initio calculations on the dehydration and decarboxylation mechanisms of formic acid in the gaseous state above 670/sup 0/C are presented, taking into account the effect of electron correlation on both the geometries and energies of the different stationary points. The classical barrier heights of the two processes are much more affected by the quality of the basis set used at the SCF level and by the correlation corrections evaluated in a single-point energy calculations than by the reoptimization of the structures of the MP level.

Improvement of the hydrogen bonding correction to MNDO for calculations of biochemical interest

Abstract: Recent suggestions for correcting HAcceptor interactions within MNDO, together with results of crystallographic analysis, were used to modify this SCF semiempirical calculation for multiple hydrogen bonded associations. Thermodynamic profiles for model systems of biochemical interest such as H2OH2O, hydration of neutral and charged molecules, dimerizations and proton transfers show the advantages of this method. Its treatment of charges, bonding, geometries, energetics and vibrational frequencies is shown to be comparable to ab initio calculations with various basis sets. However, basic MNDO deficiencies and criteria applied for H-bonding result in some too high barriers for proton transfers.

The prediction of nuclear quadrupole moments from ab initio quantum chemical studies on small molecules. II. The electric field gradients at the 17O, 35Cl, and 2H nuclei in CO, NO+, OH−, H2O, CH2O, HCl, LiCl, and FCl

Abstract: The electric field gradients (efg’s) at the oxygen and hydrogen nuclei in CO,NO+, OH−, H2O, and CH2O, and at the chlorine, lithium, and hydrogen nuclei in HCl, LiCl, and FCl, calculated using ab initio quantum chemical methods, are reported. Using extended Gaussian basis sets, the efg’s at the oxygen and chlorine nuclei were computed at the self‐consistent field (SCF), singles and doubles configuration interaction [CI(SD)], and coupled pair functional (CPF) levels of theory as the expectation values of the efg operator and also as the energy derivatives of the appropriate perturbed Hamiltonian using the finite field method. The efg’s at the hydrogen and lithium nuclei were computed as expectation values. Corrections due to zero point vibrational motions were also calculated. The effect of basis set incompleteness on the calculated efg’s is discussed and, where possible, corrected for. The calculated efg’s, together with the experimental nuclear quadrupole coupling constants, are used to estimate the 17O, ...

An abinitio calculation of ν1 and ν3 for triplet methylene (X̃ 3B1 CH2) and the determination of the vibrationless singlet–triplet splitting Te(ã 1A1)

Abstract: Ab initio values of ν1, ν3 and the zero point energy for triplet methylene (X 3B1 CH2) have been calculated. Complete second order configuration interaction calculations for the six valence electrons, with a Davidson‐type unlinked cluster correction, were done at 37 selected nuclear geometries. We have fitted an analytic potential through these points and used the nonrigid bender Hamiltonian, and a variational procedure, to calculate the rotation–vibration energies. We have obtained ν1=2985±20 cm−1, ν3=3205±20 cm−1, and the zero point energy as 3710±20 cm−1. The uncertainties represent our best estimate of the full uncertainties in the calculations. From the experimental data for singlet methylene (a 1A1) we have determined the zero point energy of that state to be 3620±20 cm−1. Combining these two zero point energies with the experimental value of T0(a 1A1)=3156±5 cm−1 [Bunker et al. J. Chem. Phys. 85, 3724 (1986)] we obtain the vibrationless singlet–triplet splitting Te(a 1A1) =3246±30 cm−1 (9.28±0....

Theoretical studies of electron transfer in metal dimers: XY+→X+Y, where X, Y=Be, Mg, Ca, Zn, Cd

Abstract: The electronic matrix element responsible for electron exchange in a series of metal dimers was calculated using ab initio wave functions. The distance dependence is approximately exponential for a large range of internuclear separations. A localized description, where the two nonorthogonal structures characterizing the electron localized at the left and right sites are each obtained self‐consistently, is found to provide the best description of the electron exchange process. We find that Gaussian basis sets are capable of predicting the expected exponential decay of the electronic interactions even at quite large internuclear distances.

Ab initio calculations on the effect of polarization functions on disiloxane

Abstract: The effect of polarization functions for ab initio molecular orbital calculations at the 3‐21G* level has been studied for disiloxane. Calculated molecular geometry, dipole moment, and the linearization barrier variation were analyzed for different uncontracted polarization functions. It was concluded that variation of the polarization function on oxygen has only a minor influence on the molecular properties of disiloxane, but its presence is required to obtain a bent geometry for the disiloxane bond. The calculated molecular properties of disiloxane are greatly influenced when the polarization function on silicon is varied. Two different values (0.3 and 0.9) for the exponent of the silicon polarization function provide results comparable to the experimental values for disiloxane. The only significant differences between the results obtained from ab initio calculations using the two polarization functions are in net atomic charges. The uncontracted polarization function of silicon with a value of 0.3 for its exponent is transferable to other organosilicon compounds. Calculated molecular geometries of flexible or rigid structures are in very good agreement with the experimental values.