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The Anharmonic Force Field of Ethylene, C2H4, by Means of Accurate Ab Initio CalculationsThe quartic force field of ethylene, C2H4, has been calculated ab initio using augmented coupled cluster, CCSD(T), methods and correlation consistent basis sets of spdf quality. For the C-12 isotopomers C2H4, C2H3D, H2CCD2, cis-C2H2D2, trans-C2H2D2, C2HD3, and C2D4, all fundamentals could be reproduced to better than 10 per centimeter, except for three cases of severe Fermi type 1 resonance. The problem with these three bands is identified as a systematic overestimate of the Kiij Fermi resonance constants by a factor of two or more; if this is corrected for, the predicted fundamentals come into excellent agreement with experiment. No such systematic overestimate is seen for Fermi type 2 resonances. Our computed harmonic frequencies suggest a thorough revision of the accepted experimentally derived values. Our computed and empirically corrected re geometry differs substantially from experimentally derived values: both the predicted rz geometry and the ground-state rotational constants are, however, in excellent agreement with experiment, suggesting revision of the older values. Anharmonicity constants agree well with experiment for stretches, but differ substantially for stretch-bend interaction constants, due to equality constraints in the experimental analysis that do not hold. Improved criteria for detecting Fermi and Coriolis resonances are proposed and found to work well, contrary to the established method based on harmonic frequency differences that fails to detect several important resonances for C2H4 and its isotopomers. Surprisingly good results are obtained with a small spd basis at the CCSD(T) level. The well-documented strong basis set effect on the v8 out-of-plane motion is present to a much lesser extent when correlation-optimized polarization functions are used. Complete sets of anharmonic, rovibrational coupling, and centrifugal distortion constants for the isotopomers are available as supplementary material to the paper.
Document ID
20020023600
Acquisition Source
Ames Research Center
Document Type
Preprint (Draft being sent to journal)
Authors
Martin, Jan M. L.
(Limburgs Univ. Diepenbeek, Belgium)
Lee, Timothy J.
(NASA Ames Research Center Moffett Field, CA United States)
Taylor, Peter R.
(San Diego Supercomputer Center San Diego, CA United States)
Francois, Jean-Pierre
(Limburgs Univ. Diepenbeek, Belgium)
Langhoff, Stephen R.
Date Acquired
August 20, 2013
Publication Date
January 1, 1995
Subject Category
Chemistry And Materials (General)
Funding Number(s)
PROJECT: RTOP 232-01-04
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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