Session P04: Physical Chemistry

Astrochemistry Does Not (Always) Need Carbon

The elemental abundances of rocky bodies are vastly different from the Universe at large. Our Earth is a rocky planet largely composed of oxygen followed by notable amounts of silicon, magnesium, iron, and some aluminum. However, before these atoms can form rocks which form planets, they likely must first form small molecules which can form nanocrystals and then rocks. This class of small, inorganic molecules has largely been unexplored in the chemical literature. This quantum chemical study analyses small inorganic oxides and hydrides and has found a notable similarity between bond strengths of inorganic hydrides and elemental abundances. Additionally, hydrogenated monomers of the mineral corundum (ruby and sapphire are part of this geomorphological family) have been shown to form with only submerged barriers from the association of water with alane (AlH 3 ). This has also been shown for periclase monomers and dimers from water and magnesium hydride (MgH 2 ). In order to support such reactions, the rovibrational spectral properties of the minima along these pathways are being computed using quartic force fields and state-of-the-art electronic structure methods. Hence, these or related molecules may help to explain portions of unattributed IR spectra as well as provide evidence for the early stages of nanocrystal formation in protoplanetary disks leading to planet formation. Conversely, the presence of these molecules in planetary nebulae, supernova remnants, or other stellar corpses may indicate the previous existence of rocky planets in former planetary systems.   

Exploring perturbative density-functional-based approaches for simulating resonant-inelastic x-ray scattering maps in transition-metal complexes

Resonant inelastic x-ray scattering (RIXS) is a two-photon process that provides valuable information on the electronic structure of molecules and materials that, due to selection rule restrictions, is not easily accessible by one-photon spectroscopies. With the continuing development of light source technologies, RIXS is rapidly becoming an important technique for the study of gas- and solution-phase molecular systems, and the need for reliable yet inexpensive electronic structure methods to aid in the prediction and interpretation of complicated spectral features is becoming apparent. In this talk, I will present simplified approaches based on linear-response time-dependent density functional theory and perturbative treatments of excited-state transition moments and spin-orbit couplings. I will demonstrate that an approach based on linear-response time-dependent density functional theory and the zeroth-order regular approximation, is able to describe the RIXS maps of ruthenium complexes with sufficiently good accuracy without the need to solve expensive quadratic-response or fully relativistic equations.   

The C3H3O Potential Surface: Reactions of c-C3H2 with OH and its Interstellar Implications

Formation mechanisms for the novel astromolecule hydroxy cyclopropenylidene and the previously-detected cyclopropenone are reported, representing new ways to create the elusive carbon-oxygen linkage in space. The recent detection of ethynyl cyclopropenylidene in the cold molecular cloud TMC-1 makes way for the study of other functionalized cyclopropenylidenes, but may also be linked to a distant relative, cyclopropenone, in Sagittarius B2. High-accuracy CCSD(T)-F12b/cc-pVTZ-F12 calculations are used to optimize structures and generate harmonic frequencies for minima along the reaction pathway. Ground and excited-state potential energy scans are used to connect structures along the pathway, and molecular orbital analyses help to explain unusual structure formation and incentives for dissociation. All products are found to have multiple kinetically-barrierless paths to reaction. The antiperiplanar isomer of hydroxy cyclopropenylidene is found to have far greater opportunity for product formation than its synperiplanar rotamer. However, cyclopropenone is found to be far lower in energy than either isomer of hydroxy cyclopropenylidene, resulting in a strongly competing pathway for functionalized cyclopropenylidene formation. The branching ratio of this reaction will be largely dictated by impact of well depth on association and dissociation rates.   

Characterization of Streaming Potential in Electrical Double Layers

Non-equilibrium thermodynamics is a very active research field due to significant progresses in theoretical understanding, new research tools, and important applications. Electrokinetic phenomena are those originate from tangential fluid motion adjacent to a charge surface such as electric double layers. Such EDL has been studied extensively due to its importance ranging from technology such as supercapacitors, to geology, and to biology. In nature, the liquid could undergo flow as in ground water and rock, micro- and nano-fluidic devices, cells and membranes. One of the interesting phenomena is called the streaming potential where fluid flow generates electrical potential, called streaming potential.

The goal of this ongoing project is to characterize experimentally the streaming potential of several important materials. We carried out electrical measurements, including impedance spectroscopy, under various flow conditions, pH values, salinity, and different types of ions of the electrolytes. The streaming potential relaxation characteristics are important signatures for recognizing and detecting the contrast agents. This project investigates systematically the dependences of streaming potential characteristics on materials properties and the electrolytes properties. From these relationships I will propose mechanisms and processes responsible for such characteristics such as relaxation times and try to provide theoretical and numerical validations of the proposed mechanisms.   

F12c: A New Methodology for Highly- Accurate Quartic Force Fields

The F12c-TZ-cCR quartic force field (QFF) methodology, defined here as CCSD(T)-F12c/cc-pCVTZ-F12 with further corrections for relativity, is introduced as an alternative to calculate highly accurate theoretical rovibrational spectral data for application to observations from both ground-based radio telescopes and space-based missions like JWST.  F12c-TZ-cCR QFFs produce B₀ and C₀ vibrationally-averaged principal rotational constants within 21.4 MHz (0.13%) of gas-phase experimental values for tetratomic and larger molecules and anharmonic vibrational frequencies within 11.3 cm^-1  (0.82%) of experiment.  The faster F12c-DZ-cCR QFFs produce B₀ and C₀ vibrationally-averaged principal rotational constants within 14.6 MHz (0.15%) and anharmonic vibrational frequencies within 13.6 cm^-1 (1.54 %) of experiment. Comparatively F12b-DZ-cCR QFFs, which is 23 percent faster than F12c-DZ-cCR, produce rotational constants within 21.9 MHz (0.18%) and anharmonic vibrational frequencies withing 20.0 cm^-1 (2.37 %) of experiment. The present work demonstrates that highly accurate theoretical rovibrational spectral data can be obtained through F12c QFFs.