Gordon A. Gallup

Vibrational Feshbach resonances in uracil and thymine

Sharp peaks in the dissociative electron attachment (DEA) cross sections of uracil and thymine at energies below 3 eV are assigned to vibrational Feshbach resonances (VFRs) arising from coupling between the dipole bound state and the temporary anion state associated with occupation of the lowest sigma* orbital. Three distinct vibrational modes are identified, and their presence as VFRs is consistent with the amplitudes and bonding characteristics of the sigma* orbital wave function. A deconvolution method is also employed to yield higher effective energy resolution in the DEA spectra. The site dependence of DEA cross sections is evaluated using methyl substituted uracil and thymine to block H atom loss selectively. Implications for the broader issue of DNA damage are briefly discussed.

Temporary anion states of selected amino acids

Vertical attachment energies for the formation of low-lying temporary anion states of glycine, alanine, phenylalanine, tryptophan, and proline in the gas phase are reported using electron transmission spectroscopy. Electron attachment into the empty π* orbital of the –COOH group was observed in all the compounds. Temporary anion states associated with the side groups in phenylalanine and tryptophan are found to be stabilized with respect to those in the reference compounds toluene and indole, respectively, by approximately 0.2 eV. We attribute this to electrostatic effects and explore, using simple theoretical models, the extent to which such anion states could be further stabilized if these amino acids were in zwitterionic form.

Practical Valence-Bond Calculations

Publisher Summary This chapter describes that the valence-bond method for calculating molecular structures and energies has had an enduring popularity among chemists as a conceptual tool for interpreting chemical reactivities and processes at the qualitative level. It discusses procedures that treat problems involving a number of systems to the level of accuracy expected with other computational methods and with no greater level of difficulty. It is suggested that using these procedures, valence-bond calculations are not too difficult to be practical. It is suggested that using these procedures, valence-bond calculations are not too difficult to be practical. Many specific calculations are used to illustrate characteristics of the valence-bond approach. One of these is a π-only (frozen σ core) calculation of planar butadiene. It describes the minimal basis calculation and shows how a double-zeta treatment may be used to obtain better energies without relinquishing the easy interpretability of the simple wave function. Other system consists of an ethylene molecule and a methylene radical. Some pathways are considered for the insertion of methylene into C 2 H 4 , to form cyclopropane, with particular emphasis on the symmetries of the interacting states and the single-triplet crossing points in the process.

The relationship of the virtual orbitals of self-consistent-field theory to temporary negative ions in electron scattering from molecules

The theory given here shows how certain of the virtual orbitals of self‐consistent field (SCF) theory provide a natural ingredient for constructing basis functions to study the shape resonances associated with temporary molecular ions. We give specific calculations of resonant behavior in H2, N2, ethylene, cyclopropene, and cyclobutene. In addition, we show how the theory can rationalize the relationship between Koopmans’ theorem values of the energy of a resonance and the experimental values.

The full versus the virtual counterpoise correction for basis set superposition error in self-consistent field calculations

Abstract In He-He, He-H2, and H2-H2 we have calculated the contribution to the interaction potential from polarization and charge transfer using the SCF supermolecule approach with two versions of the counterpoise correction. We used the original Boys and Bernardi prescription and we used only virtual SCF orbitais from the partner of each subsystem as suggested by Daudey, Malrieu and Rojas. We give results for various geometries using various bases. From our results we conclude that the original version definitely can overcorrect for the basis set superposition error in some cases and possibly always does. According to our criterion, the modified counterpoise correction does not overcorrect in the cases tested, and it appears, therefore, to provide a more reasonable procedure.

Population analyses of valence-bond wavefunctions and BeH2

Abstract A method for calculating occupation numbers and making a population analysis is proposed. The method is designed particularly for use with wavefunctions written in terms of non-orthogonal bases. An application to the valence-bond wavefunction of BeH 2 is given and the results are compared with other population analysis schemes which have been proposed.

Total dissociative electron attachment cross sections of selected amino acids.

Total dissociative electron attachment cross sections are presented for the amino acids, glycine, alanine, proline, phenylalanine, and tryptophan, at energies below the first ionization energy. Cross section magnitudes were determined by observation of positive ion production and normalization to ionization cross sections calculated using the binary-encounter-Bethe method. The prominent 1.2 eV feature in the cross sections of the amino acids and the closely related HCOOH molecule is widely attributed to the attachment into the -COOH pi* orbital. The authors discuss evidence that direct attachment to the lowest sigma* orbital may instead be responsible. A close correlation between the energies of the core-excited anion states of glycine, alanine, and proline and the ionization energies of the neutral molecules is found. A prominent feature in the total dissociative electron attachment cross section of these compounds is absent in previous studies using mass analysis, suggesting that the missing fragment is energetic H-.

Dissociative electron attachment near threshold, thermal attachment rates, and vertical attachment energies of chloroalkanes

The peaks appearing near zero energy in the dissociative electron attachment cross section of 18 chloroalkanes are studied by electron beam methods. Fits to the experimental data are made using model cross sections having appropriate energy dependences and inclusion of the broadening due to the electron energy distribution. The magnitudes of the zero peaks are found to be well correlated with the vertical attachment energies (VAE) associated with occupation of the lowest empty orbitals of the compounds. The magnitudes rise exponentially by more than five orders of magnitude as VAE decreases from 2 eV to a slightly negative value. This dependence is a consequence not only of the thermal population of vibrational levels, but also of an approximately linear relationship between VAE and the energy of the crossing between the neutral and anion potential curves. Franck–Condon factors for the transition to the anion curve are computed for model potential curves, and the nature of the attachment from vibrational ...

The intermolecular potential and its angular dependence for two H2 molecules

The potential energy surface in the non-reactive region has been determined theoretically for the H2-H2 system. The procedure consists of a non-orthogonal configuration interaction calculation using the individual SCF orbitals of the separate molecules. The potential function is expressed as a 5-term sum of Legendre functions, and analytical expressions are given for the R dependence of the terms. The calculated depth of the spherically averaged Van der Waals well is -2·96 meV, which is in essentially complete agreement with the experimental value of -3·00 meV. the position of the minimum is at 3·49 A both theoretically and experimentally. The value of C 6 for dispersion forces obtained in this calculation is 12·97 a.u.

Projected Hartree Product Wavefunctions. II. General Considerations of Young Operators

A discussion is given of the forms of two specific and one general Young operator for the irreducible representations of Sn important for fermion space functions. Comparisons are made of the projected Hartree product version of Lowdons projected Hartree–Fock method with CI calculations.