Robert A. Eades

The electronic structure of the lithium trimer anion and cation

SCF and SCF–CI calculations have been performed on Li3, its anion and cation. We predict a value of 1.1 eV for the adiabatic electron affinity. The Li−3 bond energy is found to be 0.9 eV versus the 0.4 eV energy required to dissociate Li3 to dimer and atom. Thus, the bond strength of the anion considerably exceeds that of the neutral parent. The difference in the nature of electron binding to Li2 and Li3 can be understood through modification of simple MO concepts. The energy required to dissociate Li3+ to Li2++Li is found to be 1.28 eV. Hence, the bond strength of the cation also greatly exceeds that of the neutral. The Li3+ bond energy is comparable to the Li2+ dissociation energy (1.24 eV). The adiabatic ionization potential is found to be 3.95 eV. The combination of the present study and those of previous researchers indicates that the Li3 surface is weakly varying with bond angle and may be characterized by multiple minima. The current results for Li3 and Li3+ are discussed in the light of recent mas...

The electronic structure of the germyl anion GeH3−. A comparison with other AH3− species

The electronic structure of the germyl anion, GeH3−, has been investigated by ab initio SCF calculations using large Gaussian basis sets. Various levels of basis set contraction for Ge are discussed. The optimum geometry of the ion is found to be C3V with r (Ge–H) =1.61 A and ϑ (H–Ge–H) =95.9°. The barrier to inversion for GeH3− is 30.0 kcal/mole obtained with a (16,13,7/6,1)/[12,10,4/4,1] basis set. The inversion barrier in SiH3− was also studied and was found to be 26.0 kcal/mole. Vertical electron affinities for GeH3 and SiH3 have been determined from the negative ion wave functions using Koopmans’ theorem. The values are 1.94 eV for GeH3 and 1.65 eV for SiH3. A comparison with the experimental photodetachment results is also presented.

The electronic structure of alkali trimer anions and cations

Vibrational frequencies for the alkali trimer cations have been calculated. The frequencies of the trimer cations provide good estimates for the frequencies of the neutral trimers in the absence of affects due to the conical intersection characteristic of the trimer potentials. The frequencies decrease with increasing atomic number, this variation resulting from both mass effects and a decrease in the force constant. The electron affinities for the alkali trimers have been estimated from Koopman’s theorem based upon comparisons with SCF–CI calculations. The trimer electron affinities decrease with increasing atomic number. They are found to be much larger than the known dimer electron affinities exhibiting a trend opposite to that of the dimer and trimer ionization potentials.

Electron scattering by methane: Elastic scattering and rotational excitation cross sections calculated with ab initio interaction potentials

We calculate ab initio interaction potentials for electron‐methane scattering and use them to perform converged scattering calculations for the electronically and vibrationally elastic rotational‐state‐to‐rotational‐ state cross sections at 10 eV impact energy. The effective potential has static, local exchange, and polarization terms calculated from extended‐basis‐set Hartree–Fock wave functions for both unperturbed and polarized methane molecules. The polarization potential includes nonadiabatic effects in the semiclassical local kinetic energy approximation, and for comparison we also perform calculations based on the adiabatic polarization potentials. Five to 12 terms are retained in the angular expansion of the various parts of the interaction potential and the coupled channels calculations involved 41 total angular momenta, with 1–33 coupled channels for each. The resulting rotationally summed integral cross sections are in excellent agreement with recent experiments for scattering angles 40° and larger, but are larger than the experiment at small scattering angles. The rotationally inelastic cross sections for the full potential are smaller than those for the adiabatic potential by about a factor of 2.

Ab initio SCF probabilities and electron-molecule adiabatic polarisation potentials. I. H2

Ab initio calculations of the static electric-dipole polarisability and the adiabatic polarisation potential including all multipole terms for electron scattering are reported for H2. The authors particularly examine the dependence of the polarisation potential on distance to the electron, orientation of the molecule, and internuclear distance.

Insights into the mechanisms of chemical reactions. Reaction paths for chemical reactions

The concept of a reaction path which continuously describes the transformation of reactants to products is firmly embedded in the lore of chemistry. A formal definition of the reaction path for a general molecular system was proposed by Fukui in 1970 as the path of steepest descent in mass-weighted cartesian coordinates from the transition state to reactants in one direction and to products in the other. In 1980 Miller, Handy and Adams advanced a formal theory of reaction dynamics based on harmonic fluctuations about the reaction path. We report reaction paths for two prototypical chemical reactions: Li + HF, an electron-transfer reaction, and OH + H2, an abstraction reaction. In the first we consider the connection between the energetic terms in the reactions path Hamiltonian and the electronic changes which occur upon reaction. In the second reaction we consider the treatment of vibrational effects in chemical reactions in the reaction-path formalism.

Adiabatic Polarization Potentials for the Water and Nitrogen Molecules. A Comparison of Large and Small Basis Sets

Electrostatic potentials play an important role in a wide range of chemical applications ranging from biochemistry, where they are used in modeling large-molecule interactions and reactivity, to molecular physics, where model potentials are employed in quantum mechanical studies of electron scattering. In both of these applications, the electrostatic potential is only an approximation to the true interaction potential. A better approximation to the interaction potential is obtained by allowing for charge polarization of the target molecule by the reactive partner or scattering particle. In this chapter we focus on this charge-polarization aspect, and, in particular, we study how adiabatic charge polarization affects the interaction potentials for electron-molecule scattering. The general topic of electron-molecule interaction potentials is discussed in detail in the previous chapter in this book.

Polarization Potentials for Electron Scattering

Charge polarization effects (due to polarization of the target charge distribution by the incident electron) are important for low and intermediate-energy electron scattering (these energy ranges corresponds to roughly E >~ IP and IP >~ E >~ 10 IP, where E is the impact energy and IP is the target ionization potential). There are two approaches to the inclusion of such polarization effects in electron scattering. In the many-body approach, the scattering wavefunction for the whole system (incident electron plus target) is represented explicitly by basis functions or products of basis functions and numerically determined radial functions. Algebraic variational methods2 and R matrix3 methods are some particularly powerful variants of this approach. In this approach charge polarization effects enter by configuration mixing. Because of this and because polarization effects are of long range, basis sets are required to be large and the scattering wavefunction must be represented over a big region. To avoid the associated computational problems, most electron-molecule scattering calculations using basis functions have been restricted to the single-configuration level, i.e., the static-exchange approximation, in which polarization effects are neglected.4

An ab initio calculation of first and second derivatives of the electric dipole moment function for certain infrared-active modes of SF6

Abstract Certain first and second derivatives of the dipole moment function for SF 6 have been calculated from ab initio molecular orbital theory in the finite difference approximation. All calculations were carried out in normal coordinates with an experimental force field and geometry. A double-zeta basis with d -functions on the sulfur was employed. Calculated harmonic frequencies in cm −1 are 1084 (ω 3 ) and (648(ω 4 ) and calculated intensities in km/mole are 1612 (ν 3 ) and 209 (ν 4 ). Second derivatives of the dipole moment function for ν 1 + ν 3 x and ν 2 a +ν 3 x in units of e-amu −1 A are 0.0552 and −0.0119, respectively. A comparison with the values determined from an STO-3G basis is presented together with a comparison of the derived quantities from experiment.

Ab initio adiabatic polarisation potentials for Be and Mg

The authors have calculated adiabatic polarisation potentials for e--Be and e--Mg scattering using matrix Hartree-Fock calculations with extended Gaussian basis sets including diffuse functions. They find that the adiabatic polarisation potentials are much more attractive than the potentials that have been used recently to study low-energy shape resonances in these systems.