Jeffrey A. Sheehy

On a Quantitative Scale for Lewis Acidity and Recent Progress in Polynitrogen Chemistry

Abstract A quantitative scale for Lewis acidities based on fluoride ion affinities is discussed. It uses pF − values which represent the fluoride ion affinities in kcal/mol divided by 10. These values were obtained for 106 Lewis acids in a self-consistent manner using ab initio calculations at the MP2/PDZ level of theory. In the area of polynitrogen chemistry, the synthesis and characterization of the novel N 5 + cation from N 2 F + and HN 3 is described.

On the angular distributions of electrons photoejected from fixed-in-space and randomly oriented molecules

Abstract New theoretical expressions are devised from a dynamical perspective for molecular photoionization cross sections differential in electron ejection angles which facilitate comparisons between theory and experiment and provide a convenient basis for ab initio calculations. The cross sections obtained for fixed-in-space molecules, including the lowest-order (nondipole) effects of retardation, are given in terms of invariant molecular body-frame transition moments and related normalized angular-distribution amplitudes which can be calculated employing interaction-prepared states without reference to specific scattering boundary conditions. Corresponding expressions for molecular dipole and nondipole anisotropy factors appropriate for randomly oriented molecules are obtained in closed forms involving expectation values of harmonic polynomials over the fixed-in-space body-frame angular-distribution amplitudes. The expressions are seen to be in the spirit of corresponding results for atomic photoionization anisotropy factors, to which they reduce in appropriate limits. Interpretations of recently measured angular distributions of photoelectrons from the K-shell of molecular nitrogen illustrate how the development relates measurements on randomly oriented molecules to those performed on fixed-in-space molecules. The theoretical formalism provides results in excellent accord with measurements of the molecular nitrogen K-shell dipole anisotropy factor, and accounts for the origins of large nondipole effects observed at relatively low photon energies (ℏω≤500 eV) in the measured angular distributions.

CRYSTAL STRUCTURE OF HEXAMETHYLGUANIDINIUM HEXAFLUOROSILICATE HEXAHYDRATE

Abstract Single crystals of the hexamethylguanidinium hexafluorosilicate hexahydrate salt, [C(NMe2)3+]2SiF62−·6H2O, were isolated when a product, obtained by water removal from aqueous [C(NMe2)3]F in a glass vessel, was recrystallized from CH3OH. The crystal structure of this salt was determined, and the geometry of the free hexamethylguanidinium cation was calculated, showing that its observed propeller-shaped arrangement is not caused by solid state effects but is the true minimum energy structure of the free ion.

Angular distribution of molecular-field- and spin-orbit-split sulfur 2p photoemission in OCS: a sensitive probe of the molecular environment

Angle-resolved sulfur 2p photoelectron spectra from the OCS molecule are reported in very high electron and photon energy resolution. The measurements reveal significant differences in the angular distributions of ejected electrons associated with different core-level ionic multiplet states. A concomitant theoretical analysis relates these data to the perturbing effects of spin-orbit and molecular-field splittings, and indicates that the measured angular distributions in high resolution provide a new and sensitive probe of the molecular body-frame environment in the absence of explicit sample alignment.

Spectral theory of the chemical bond

New theoretical methods are reported for obtaining the binding energies of molecules and other atomic aggregates employing the spectral eigenstates and related properties of the constituent atoms in the absence of prior wave function antisymmetry. Calculations of the lowest-lying attractive and repulsive states of the electron pair bond as functions of atomic separation from chemical (exchange) to physical (van der Waals) binding regions illustrate the nature of the formalism and its convergence to values in accord with results obtained employing conventional methods.

Dynamical theory of molecular photoionization: electron ejection kinematics and angular distributions from molecules fixed in space ☆

Abstract Photoionization of molecules and other atomic aggregates fixed in space is studied from a dynamical perspective employing solutions of the time-dependent Schrodinger equation. The wave functions and associated kinematical behaviors of excited and ejected electrons are determined explicitly in short- and long-time limits, generalizing early results of Bethe for central potentials to anisotropic targets. Ehrenfest’s theorem is employed to clarify the origins of the transient forces and consequent kinematical behaviors of excited and ionized K-shell electrons obtained from explicit solutions of the time-dependent Schrodinger equation, revealing hybrid classical-quantal behaviors in wave functions and expectation values of position and momentum operators. In addition to providing pedagogical insights into time-resolved aspects of these fundamental photoprocesses, and a corresponding basis for theoretical studies of dynamical target relaxation, associated photoelectron-ion correlation, and other specifically time-dependent post-collision interaction phenomena, the new development resolves technical issues associated with the irreducibly infinite degeneracy of the scattering continuum for the anisotropic potentials characteristic of polyatomic molecules. Photoionization cross sections differential in electron ejection angles are derived from the formalism for fixed-in-space molecules of arbitrary complexity which are applicable in both time-resolved and steady-state situations, and an invariant subspace of excitation and ionization functions required in computational applications of the approach is constructed employing Lanczos–Krylov sequences of L 2 vectors and previously devised Stieltjes–Akhiezer methods, without reference to or calculations of the underlying individual discrete or continuum eigenstates. Electron angular-distribution data for fixed-in-space molecules measured over the full (4 π ) range of emission angles for all molecular orientations are shown to be highly redundant, and to provide invariant physical information in the form of no more than three irreducible-tensor body-frame complex angular emission amplitudes at a given photon energy. These issues are illustrated with applications to ionization of fixed-in-space diatomic molecules for linear and circularly polarized incident radiation, in which cases the minimal set of molecular configurations and polarizations required to determine the invariant body-frame emission amplitudes is described in detail.

In Pursuit of the PO2+ Cation. The Reaction of KPO2F2 and SbF5 leads to an Eight‐membered Antimony‐Oxygen‐Phosphorus‐bridged Ring

Abstract : The reaction of KPO2F2 with the strong Lewis acid SbF3 was studied as a potential pathway to the unknown PO2(+) cation. The resulting product has the desired PO2SbF6 composition but consists of an eight-membered, oxygen-bridged ring that was characterized by vibrational and NMR spectroscopy, ab initio methods, and a single crystal x-ray diffraction study. The formation of the oxygen-bridged ring and its mechanism are discussed.

Beyond the dipole approximation: Angular distribution effects in the 1s photoemission from small molecules

Over the past two decades, the dipole approximation has facilitated a basic understanding of the photoionization process in atoms and molecules. Recent experiments on the 1s inner shells of small molecules at relatively low photon energies (⩽ 1000 eV) show strong nondipole effects. They are significant and measurable at energies close to threshold, in conflict with a common assumption that the dipole approximation is valid for photon energies below 1 keV.

New insights into molecular structure and dynamics using soft x-ray electron spectroscopy

The combination of high-resolution electron energy analyzers and 3rd generation synchrotron radiation sources opens up possibilities to study molecular inner-shell photo- and Auger electron emission in a new level of detail. Even weak molecular perturbations of the energy levels and angular emission patterns can be studied. In this report, some examples of such studies are given based on recent experiments at the Advanced Light Source, using a Scienta SES-200 electron spectrometer. Examples of carbon 1s photoemission of ethyne, sulphur 2p photoemission of carbonyl sulphide, and resonant Auger electron emission of carbon dioxide are presented.

On the angular distributions of molecular photoelectrons: dipole cross-sections for fixed-in-space and randomly oriented molecules

New theoretical expressions are devised employing a dynamical perspective for photoionization cross-sections differential in electron ejection angles for both fixed-in-space and randomly oriented molecules, and comparisons made with K-shell ionization measurements in molecular nitrogen. Closed-form cross-sectional expressions are obtained in the dipole limit in terms of molecular body-frame transition moments and related normalized angular-distribution amplitudes which can be calculated employing interaction-prepared states without reference to specific scattering boundary conditions, and which reduce to more familiar atomic expressions in appropriate limits.