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The isomers of ionized ethane
Author(s) -
Hudson Charles E.,
McAdoo David J.,
Giam C. S.
Publication year - 1996
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/(sici)1096-987x(199610)17:13<1532::aid-jcc4>3.0.co;2-t
Subject(s) - chemistry , bond length , bond order , single bond , diborane , sextuple bond , ionization energy , bond energy , atomic physics , potential energy surface , ionization , computational chemistry , crystallography , molecule , ion , physics , alkyl , organic chemistry , crystal structure , boron
The previously reported 2 A g , 2 A 1 g , and 2 B g states of ionized ethane are characterized at several levels of theory. The diborane‐like 2 A g state, which gives rise to the observed ESR spectrum, is predicted by SCF and CCD calculations not to exist in a separate minimum from the 2 A 1 g state formed by ionization of the C(SINGLE BOND)C bond. However, as reported by Lunell and Huang, second‐order Moller‐Plesset theory places the 2 A g lowest, provided polarization functions are included on carbon. QCISD theory predicts that both A states correspond to potential energy minima, but places the long‐bond 2 A 1 g state lower, at least with moderately large basis sets. F orbitals on carbon stabilize the diborane structure more than the long‐bond one. When a potential energy surface is generated for a series of fixed C(SINGLE BOND)C bond lengths by optimizing all variables except for the C(SINGLE BOND)C bond length with MP2 theory and calculating the energy with QCISD(T), the 2 A g state is predicted to be the lowest energy state with the 2 A 1 g state 1.83 kJ/mol above it. The two A states are predicted to be separated by a barrier 2.79 kJ/mol above the lower state. This barrier is above the zero‐point energy in the C(SINGLE BOND)C stretch for the lower state but below the ZPE for this stretch in the upper state, which is therefore predicted not to exist as a stable species. A single quantum of vibrational excitation in the low frequency C(SINGLE BOND)C stretch is predicted to yield an ion with a poorly defined C(SINGLE BOND)C bond length. The highest levels of theory employed give poor agreement with the experimental hyperfine coupling constants. The discrepancy could either be due to neglect of vibrational effects, to poor inherent accuracy of the calculation, as one author has concluded, or to compression of the ion by the matrix as suggested by another. The 2 B g state is found to be higher in energy than the A states at all theoretical levels and is predicted to have a large (160.2–177.4 G) hyperfine coupling from four hydrogens. The transition state for simultaneous exchange of two hydrogen atoms between the carbons by a diborane structure is predicted to lie above the lowest energy fragmentation threshold, in agreement with experiment. © 1996 by John Wiley & Sons, Inc.