Browsing by Author "Mebel, Alexander M."

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Absorption cross sections of NH3, NH2D, NHD2, and ND3 in the spectral range 140-220 nm and implications for planetary isotopic fractionation
(2006-08-20)
Cheng, Bing-Ming
;
Lu, Hsiao-Chi
;
Chen, Hong-Kai
;
Bahou, Mohammed
;
Lee, Yuan-Pern
Cross sections for photoabsorption of NH3, NH2D, NHD2, and ND3 in the spectral region 140-220 nm were determined at similar to 298 K using synchrotron radiation. Absorption spectra of NH2D and NHD2 were deduced from spectra of mixtures of NH3 and ND3, of which the equilibrium concentrations for all four isotopologues obey statistical distributions. Cross sections of NH2D, NHD2, and ND3 are new. Oscillator strengths, an integration of absorption cross sections over the spectral lines, for both A X and B X systems of NH3 agree satisfactorily with previous reports; values for NH2D, NHD2, and ND3 agree with quantum chemical predictions. The photolysis of NH3 provides a major source of reactive hydrogen in the lower stratosphere and upper troposphere of giant planets such as Jupiter. Incorporating the measured photoabsorption cross sections of NH3 and NH2D into the Caltech/JPL photochemical diffusive model for the atmosphere of Jupiter, we find that the photolysis efficiency of NH2D is lower than that of NH3 by as much as 30%. The D/H ratio in NH2D/NH3 for tracing the microphysics in the troposphere of Jupiter is also discussed.
Effect of the Medium on Intramolecular H-Atom Tunneling: Cis-Trans Conversion of Formic Acid in Solid Matrixes of Noble Gases
(2010-12-30)
Trakhtenberg, Leonid I.
;
Fokeyev, Anatoly A.
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Zyubin, Alexander S.
;
Mebel, Alexander M.
;
Lin, S. H.
Intramolecular tunneling of a hydrogen atom in formic acid at low temperatures has been studied theoretically on the basis of quantum-chemical modeling of HCOOH@Nb(12) clusters. Three noble matrixes (Ar, Kr, and Xe) are considered. Energetic and geometric parameters as well as vibrational frequencies for the formic acid in cis and trans configurations surrounded by 12 Nb atoms are calculated within the frame of the MP2 approach with extended basis sets. The rate constant of HCOOH cis-trans conversion is analyzed by taking into account matrix reorganization and the change of HCOOH position in the cluster. The matrix reorganization is considered within the Debye model of lattice vibrations, whereas the external motion of HCOOH in the cluster is treated using the Einstein model of solids. It has been shown that the literature experimental data on the cis to trans tunneling reaction in the formic acid can be accounted for within the proposed mechanism, which describes the matrix reorganization and the change of the HCOOH position in the noble gas matrix, with fitting parameters of the suggested theoretical model attaining reasonable values.
Matrix reorganization with intramolecular tunneling of H atom: Formic acid in Ar matrix
(2009-04-14)
Trakhtenberg, Leonid I.
;
Fokeyev, Anatoly A.
;
Zyubin, Alexander S.
;
Mebel, Alexander M.
;
Lin, S. H.
The intramolecular tunneling of hydrogen atom in formic acid HCOOH at low temperatures is considered based on literature experimental data on C-O internal rotation. The energetic and geometric parameters as well as vibrational frequencies for formic acid in cis and trans configurations surrounded by 12 Ar atoms are calculated in the frame of the MP2 approach with extended basis sets. The temperature and pressure dependence of the rate constant is analyzed taking into consideration the matrix reorganization for the Debye model of lattice motion. It has been shown that the available experimental data can be explained by the suggested matrix reorganization mechanism. Theoretical expressions for the temperature dependence of the rate constant agree well with the experimental data on the cis to trans tunneling reactions in formic acid with fitting parameters attaining reasonable values. A mechanism describing pressure dependence of the rate constant for H-atom intramolecular tunneling reactions is also proposed.