Studies of the Temporary Anion States of Unsaturated Hydrocarbons by Electron Transmission Spectroscopy

The concept of occupied and unoccupied orbitals has provided a useful means for visualizing many of the most important properties of molecular systems. Yet, there is a curious imbalance in our experimental knowledge of the energies of occupied and unoccupied orbitals. Whereas photoelectron spectroscopy has provided a wealth of data on positive ion states and has established that they can be associated, within the context of Koopmans’ theorem, with the occupied orbitals of the neutral molecule, the corresponding information for the negative ion states, associated with the normally unoccupied orbitals, is sparse. In part this reflects the experimental difficulties connected with measuring the electron affinities of molecules which possess stable anions. A more interesting aspect, in our view, is that for many molecules even the ground state of the anion lies above the ground state of the neutral molecule and thus is unstable with respect to autodetachment of the additional electron. The temporary anions which we will discuss in this Account possess lifetimes in the range from 10–12 to 10–15 s in the gas phase. Ephemeral as these anions might appear, their existence is amply demonstrated in electron-scattering experiments. At energies in which an impacting electron may be temporarily captured into a normally unoccupied orbital, the electron-scattering cross section undergoes a pronounced change in magnitude, commonly referred to as a “resonance” by those in the scattering field. A number of electron-scattering techniques may be used to investigate temporary anion formation. In this article we will describe only one, the electron transmission method in the format devised by Sanche and Schulz. This technique has numerous assets: it is both conceptually and experimentally straightforward, and it is likely the most sensitive means for detection of temporary anions. During the last few years, electron transmission spectroscopy has seen increasing application to organic molecules. In the following sections, we discuss our results and those of other workers which bear on the characteristics of the temporary anion states associated with low-lying π* orbitals in a number of classes of hydrocarbons. In addition we will describe briefly the relationship of this work to other chemical studies, such as optical absorption measurements on anions in solution, molecular orbital theories of reactivity, and studies of low-lying electronic states of neutral molecules

Complete atrial-specific knockout of sodium-calcium exchange eliminates sinoatrial node pacemaker activity.

The origin of sinoatrial node (SAN) pacemaker activity in the heart is controversial. The leading candidates are diastolic depolarization by "funny" current (If) through HCN4 channels (the "Membrane Clock" hypothesis), depolarization by cardiac Na-Ca exchange (NCX1) in response to intracellular Ca cycling (the "Calcium Clock" hypothesis), and a combination of the two ("Coupled Clock"). To address this controversy, we used Cre/loxP technology to generate atrial-specific NCX1 KO mice. NCX1 protein was undetectable in KO atrial tissue, including the SAN. Surface ECG and intracardiac electrograms showed no atrial depolarization and a slow junctional escape rhythm in KO that responded appropriately to β-adrenergic and muscarinic stimulation. Although KO atria were quiescent they could be stimulated by external pacing suggesting that electrical coupling between cells remained intact. Despite normal electrophysiological properties of If in isolated patch clamped KO SAN cells, pacemaker activity was absent. Recurring Ca sparks were present in all KO SAN cells, suggesting that Ca cycling persists but is uncoupled from the sarcolemma. We conclude that NCX1 is required for normal pacemaker activity in murine SAN

Choosing fans for your livestock ventilation system

1 online resource (PDF, 2 pages)This archival publication may not reflect current scientific knowledge or recommendations. Current information available from the University of Minnesota Extension: https://www.extension.umn.edu.Jacobson, Larry D.; Jordan, Kenneth A.. (1977). Choosing fans for your livestock ventilation system. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/206773

Effect of N Atom Substitution on Electronic Resonances: A 2D Photoelectron Spectroscopic and Computational Study of Anthracene, Acridine, and Phenazine Anions

The accommodation of an excess electron by polycyclic aromatic hydrocarbons (PAHs) has important chemical and technological implications ranging from molecular electronics to charge balance in interstellar molecular clouds. Here, we use two-dimensional photoelectron spectroscopy and equation-of-motion coupled-cluster calculations of the radical anions of acridine (C13H9N–) and phenazine (C12H8N2 –) and compare our results for these species to those for the anthracene anion (C14H10 –). The calculations predict the observed resonances and additionally find low-energy two-particle-one-hole states, which are not immediately apparent in the spectra, and offer a slightly revised interpretation of the resonances in anthracene. The study of acridine and phenazine allows us to understand how N atom substitution affects electron accommodation. While the electron affinity associated with the ground state anion undergoes a sizable increase with the successive substitution of N atoms, the two lowest energy excited anion states are not affected significantly by the substitution. The net result is that there is an increase in the energy gap between the two lowest energy resonances and the bound ground electronic state of the radical anion from anthracene to acridine to phenazine. Based on an energy gap law for the rate of internal conversion, this increased gap makes ground state formation progressively less likely, as evidenced by the photoelectron spectra

QMCPACK: Advances in the development, efficiency, and application of auxiliary field and real-space variational and diffusion Quantum Monte Carlo

We review recent advances in the capabilities of the open source ab initio Quantum Monte Carlo (QMC) package QMCPACK and the workflow tool Nexus used for greater efficiency and reproducibility. The auxiliary field QMC (AFQMC) implementation has been greatly expanded to include k-point symmetries, tensor-hypercontraction, and accelerated graphical processing unit (GPU) support. These scaling and memory reductions greatly increase the number of orbitals that can practically be included in AFQMC calculations, increasing accuracy. Advances in real space methods include techniques for accurate computation of band gaps and for systematically improving the nodal surface of ground state wavefunctions. Results of these calculations can be used to validate application of more approximate electronic structure methods including GW and density functional based techniques. To provide an improved foundation for these calculations we utilize a new set of correlation-consistent effective core potentials (pseudopotentials) that are more accurate than previous sets; these can also be applied in quantum-chemical and other many-body applications, not only QMC. These advances increase the efficiency, accuracy, and range of properties that can be studied in both molecules and materials with QMC and QMCPACK