12 research outputs found
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To Be or Not To Be a Molecular Ion: The Role of the Solvent in Photoionization of Arginine.
Application of photoionization mass spectroscopy, a technique capable of assessing protonation states in complex molecules in the gas phase, is challenging for arginine due to its fragility. We report photoionization efficiencies in the valence region of aqueous aerosol particles produced from arginine solutions under various pH and vaporization conditions. By using ab initio calculations, we investigate the stability of different conformers. Our results show that neutral arginine fragments upon ionization in the gas phase but solvation stabilizes the molecular ion, resulting in different photoionization dynamics. We also report the valence-band photoelectron spectra of the aerosol solutions obtained at different pH values
Recent Developments in the General Atomic and Molecular Electronic Structure System
A discussion of many of the recently implemented features of GAMESS (General Atomic and Molecular Electronic Structure System) and LibCChem (the C++ CPU/GPU library associated with GAMESS) is presented. These features include fragmentation methods such as the fragment molecular orbital, effective fragment potential and effective fragment molecular orbital methods, hybrid MPI/OpenMP approaches to Hartree-Fock, and resolution of the identity second order perturbation theory. Many new coupled cluster theory methods have been implemented in GAMESS, as have multiple levels of density functional/tight binding theory. The role of accelerators, especially graphical processing units, is discussed in the context of the new features of LibCChem, as it is the associated problem of power consumption as the power of computers increases dramatically. The process by which a complex program suite such as GAMESS is maintained and developed is considered. Future developments are briefly summarized
Software for the frontiers of quantum chemistry:An overview of developments in the Q-Chem 5 package
This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design
Probing Electronic Wave Functions of Sodium-Doped Clusters: Dyson Orbitals, Anisotropy Parameters, and Ionization Cross-Sections
We
apply high-level ab initio methods to describe the electronic
structure of small clusters of ammonia and dimethyl ether (DME) doped
with sodium, which provide a model for solvated electrons. We investigate
the effect of the solvent and cluster size on the electronic states.
We consider both energies and properties, with a focus on the shape
of the electronic wave function and the related experimental observables
such as photoelectron angular distributions. The central quantity
in modeling photoionization experiments is the Dyson orbital, which
describes the difference between the initial <i>N</i>-electron
and final (<i>N</i>–1)-electron states of a system.
Dyson orbitals enter the expression of the photoelectron matrix element,
which determines total and partial photoionization cross-sections.
We compute Dyson orbitals for the Na(NH<sub>3</sub>)<sub><i>n</i></sub> and Na(DME)<sub><i>m</i></sub> clusters using correlated
wave functions (obtained with equation-of-motion coupled-cluster model
for electron attachment with single and double substitutions) and
compare them with more approximate Hartree-Fock and Kohn-Sham orbitals.
We also analyze the effect of correlation and basis sets on the shapes
of Dyson orbitals and the experimental observables
Gaseous Vanadium Molybdate and Tungstates: Thermodynamic Properties and Structures
The stability of gaseous vanadium molybdate and vanadium
tungstates
was confirmed by high-temperature mass spectrometry. A number of gas-phase
reactions involving vanadium-containing salts were studied. On the
basis of equilibrium constants, the standard formation enthalpies
of gaseous VMoO<sub>4</sub> (−676 ± 27 kJ/mol), VWO<sub>3</sub> (−331 ± 29 kJ/mol), and VWO<sub>4</sub> (−706
± 23 kJ/mol) at 298 K were determined. A theoretical study of
these salts revealed the structure with bidentate binding of the vanadium
cation to the anion part to be the lowest-lying isomer, with a quartet
spin state for VMoO<sub>4</sub> and VWO<sub>4</sub> molecules as well
as a sextet spin state for the VWO<sub>3</sub> molecule. On the basis
of critical analysis of the literature data concerning standard formation
enthalpies of gaseous VO and VO<sub>2</sub>, we adopted new values
of Δ<sub>f</sub><i>H</i>°(298) = 135 ± 10
kJ/mol for VO(g) and −185 ± 15.0 kJ/mol for VO<sub>2</sub>(g). Overall, the results obtained allowed us to estimate the standard
formation enthalpy of VMoO<sub>3</sub> to be −318 kJ/mol with
an accuracy near 40 kJ/mol
Recommended from our members
To Be or Not To Be a Molecular Ion: The Role of the Solvent in Photoionization of Arginine.
Application of photoionization mass spectroscopy, a technique capable of assessing protonation states in complex molecules in the gas phase, is challenging for arginine due to its fragility. We report photoionization efficiencies in the valence region of aqueous aerosol particles produced from arginine solutions under various pH and vaporization conditions. By using ab initio calculations, we investigate the stability of different conformers. Our results show that neutral arginine fragments upon ionization in the gas phase but solvation stabilizes the molecular ion, resulting in different photoionization dynamics. We also report the valence-band photoelectron spectra of the aerosol solutions obtained at different pH values
Photoelectron Wave Function in Photoionization: Plane Wave or Coulomb Wave?
The calculation of absolute total
cross sections requires accurate
wave functions of the photoelectron and of the initial and final states
of the system. The essential information contained in the latter two
can be condensed into a Dyson orbital. We employ correlated Dyson
orbitals and test approximate treatments of the photoelectron wave
function, that is, plane and Coulomb waves, by comparing computed
and experimental photoionization and photodetachment spectra. We find
that in anions, a plane wave treatment of the photoelectron provides
a good description of photodetachment spectra. For photoionization
of neutral atoms or molecules with one heavy atom, the photoelectron
wave function must be treated as a Coulomb wave to account for the
interaction of the photoelectron with the +1 charge of the ionized
core. For larger molecules, the best agreement with experiment is
often achieved by using a Coulomb wave with a partial (effective)
charge smaller than unity. This likely derives from the fact that
the effective charge at the centroid of the Dyson orbital, which serves
as the origin of the spherical wave expansion, is smaller than the
total charge of a polyatomic cation. The results suggest that accurate
molecular photoionization cross sections can be computed with a modified
central potential model that accounts for the nonspherical charge
distribution of the core by adjusting the charge in the center of
the expansion
Photoelectron Wave Function in Photoionization: Plane Wave or Coulomb Wave?
The calculation of absolute total
cross sections requires accurate
wave functions of the photoelectron and of the initial and final states
of the system. The essential information contained in the latter two
can be condensed into a Dyson orbital. We employ correlated Dyson
orbitals and test approximate treatments of the photoelectron wave
function, that is, plane and Coulomb waves, by comparing computed
and experimental photoionization and photodetachment spectra. We find
that in anions, a plane wave treatment of the photoelectron provides
a good description of photodetachment spectra. For photoionization
of neutral atoms or molecules with one heavy atom, the photoelectron
wave function must be treated as a Coulomb wave to account for the
interaction of the photoelectron with the +1 charge of the ionized
core. For larger molecules, the best agreement with experiment is
often achieved by using a Coulomb wave with a partial (effective)
charge smaller than unity. This likely derives from the fact that
the effective charge at the centroid of the Dyson orbital, which serves
as the origin of the spherical wave expansion, is smaller than the
total charge of a polyatomic cation. The results suggest that accurate
molecular photoionization cross sections can be computed with a modified
central potential model that accounts for the nonspherical charge
distribution of the core by adjusting the charge in the center of
the expansion