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Search Publications by: Karl K. Irikura (Fed)

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Displaying 1 - 25 of 54

Partial Ionization Cross Sections of Organic Molecules

June 13, 2017
Author(s)
Karl K. Irikura
Partial ionization cross sections are the absolute yields of specific ions from an electron- molecule collision. They are necessary for modeling plasmas and for determining the sensitivity of mass spectrometers, among other applications. One mass

To Predict Vibrational Spectra, Are Anharmonic Calculations Worthwhile?

January 22, 2013
Author(s)
Ruth L. Jacobsen, Karl K. Irikura, Russell D. Johnson III, Raghu N. Kacker
Anharmonic calculations using vibrational perturbation theory are known to provide near-spectroscopic accuracy when combined with high-level ab initio potential energy functions. However, performance with economical, popular electronic structure methods is

Electron ionization and gas-phase ion molecule reactions of methylcyclohexane

November 9, 2010
Author(s)
C. Q. Jiao, Karl Irikura, S, F. Adams, A Garscadden
Absolute cross sections for electron impact ionization of methylcyclohexane (MCH, C7H14) are measured as a function of the electron energy in a range of 10 to 200 eV. The reactions between selected hydrocarbon ions with MCH, mainly via hydride transfer

Thermochemistry of Ammonium Nitrate, NH4NO3, in the Gas Phase

October 7, 2010
Author(s)
Karl K. Irikura
Hildenbrand and coworkers have shown recently that the vapor above solid ammonium nitrate includes molecules of NH4NO3, not only NH3 and HNO3 as previously believed. Their measurements led to thermochemical values that imply an enthalpy change of D298 = 98

Scaling Factors and Uncertainties for ab Initio Anharmonic Vibrational Frequencies

August 21, 2009
Author(s)
Russell D. Johnson III, Karl K. Irikura, Raghu N. Kacker, Ruediger Kessel
To predict the vibrational spectra of molecules, ab initio calculations are often used to compute harmonic frequencies, which are usually scaled by empirical factors as an approximate correction for errors in the force constants and for anharmonic effects

Uncertainties in Scaling Factors for ab Initio Vibrational Zero-Point Energies

March 21, 2009
Author(s)
Karl K. Irikura, Russell D. Johnson III, Raghu N. Kacker, Ruediger Kessel
Vibrational zero-point energies (ZPEs) determined from ab initio calculations are often scaled by empirical factors. An empirical scaling factor partially compensates for the effects arising from vibrational anharmonicity and incomplete treatment of

How Good Are Force Fields for Gas-Phase Protonated Peptides?

October 16, 2008
Author(s)
Christopher Kinsinger, Karl K. Irikura
With the increase in mass-spectrometry-based proteomics, many research projects depend on the chemistry of peptide fragmentation. Elucidating fragmentation mechanisms requires a thorough exploration of the potential energy surface. Conformational searching

in-Adamantane, a Small Inside-Out Molecule

September 25, 2008
Author(s)
Karl K. Irikura
The hydrocarbon in-adamantane, an adamantane isomer in which one methine hydrogen atom is inside the cage, is predicted by ab initio calculations to be isolable at dry-ice temperature. It is 440 kJ/mol less stable than ordinary adamantane but has a barrier

Aminoxyl (nitroxyl)Radicals in the Decomposition of RDX

May 13, 2008
Author(s)
Karl K. Irikura
The explosive RDX (1,3,5-trinitrohexahydro-s-triazine) is thought to decompose by homolytic N-N bond cleavage, among other possible initiation reactions. Quantum chemistry calculations show that the resulting aminyl radical can abstract an oxygen atom from

Sigma Stellation: A Design Strategy for Electron Boxes

January 10, 2008
Author(s)
Karl K. Irikura
The carbon-fluorine antibonding (sigma*) orbitals in a fluorocarbon cage are directed toward a central, common point. If the cage is not too large or too small, the sigma* orbitals will overlap at that point. An added electron can occupy the resulting

Obtaining Molecular Thermochemistry From Calculations

September 30, 2007
Author(s)
Karl K. Irikura
There are several computational approaches for predicting gas-phase, molecular thermochemistry. For common organic compounds, empirical methods are quick to compute and have been effectively parameterized. For uncommon compounds, the relative robustness of

Experimental Vibrational Zero-Point Energies: Diatomic Molecules

April 18, 2007
Author(s)
Karl K. Irikura
Vibrational zero-point energies (ZPEs), as determined from published spectroscopic constants, are derived for 85 diatomic molecules. Standard uncertainties are also provided, including estimated contributions from bias as well as the statistical

Is NO 3 Formed During the Decomposition of Nitramine Explosives?

December 21, 2006
Author(s)
Karl K. Irikura, Russell D. Johnson III
Quantum chemistry calculations reveal that it is both thermodynamically and kinetically reasonable to produce NO3 (nitrate radical) during the thermal decomposition of RDX (1,3,5-trinitrohexahydro-s-triazine) and other nitramines. However, experimental

Uncertainties in Scaling Factors for ab Initio Vibrational Frequencies

September 1, 2005
Author(s)
Karl K. Irikura, Russell D. Johnson III, Raghu N. Kacker
Vibrational frequencies determined from ab initio calculations are often scaled by empirical factors. An empirical scaling factor partially compensates for the errors arising from vibrational anharmonicity and incomplete treatment of electron correlation

Ab Initio Characterization of the Weakly Bound Anions ClOO- and ArCl-

August 1, 2003
Author(s)
Karl K. Irikura
Experimental studies of the energetics and vibrational spectrum of the chloroperoxyl radical, ClOO, are contradictory. The most recent experiment accessed the radical by photodetachment of the corresponding anion, ClOO-. The present study uses both multi-

Electron-Impact Total Ionization Cross-Sections of the Chlorofluoromethanes

January 1, 2003
Author(s)
Karl K. Irikura, M A. Ali, Yong Sik Kim
Electron-impact total ionization cross sections for CCl3F (Freon 11), CCl2F2 (Freon 12), CClF3 (Freon 13), CHCl2F (Freon 21), CHClF2 (Freon 22), and CH2ClF (Freon 31) are calculated using the Binary-Encounter-Bethe (BEB) theoretical model. The BEB model

New Empirical Procedures for Improving Ab Initio Energetics

October 1, 2002
Author(s)
Karl K. Irikura
Empirical schemes are developed for correcting the energies resulting from a moderately high-level abinitio calculation. Each method involves only a few adjustable parameters, which are determined by fitting a set of 31 high-spin atomization energies

Electron-Impact Total Ionization Cross Sections of Hydrocarbon Ions

February 1, 2002
Author(s)
Karl K. Irikura, Yong Sik Kim, M A. Ali
The Binary-Encounter-Bethe (BEB) model for electron-impact total ionization cross sections has been applied to CH2+, CH3+, CH4+, C2H2+, C2H4+, C2H6+, and H3O+. The cross sections for the hydrocarbon ions are needed for modeling cool plasmas in fusion