Isomerization and unimolecular dissociation channels of the oxalic acid monomer
The results of molecular orbital calculations performed for the oxalic acid monomer using the 6-31G, 6-31G*(5d), 6-31G**(5d), and MP2/6-31G*(5d) levels are reported. At the latter three levels of approximation the geometry with intramolecular hydrogen bonds is calculated to have the lowest energy, a...
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Veröffentlicht in: | The Journal of chemical physics 1986-11, Vol.85 (10), p.5391-5400 |
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Format: | Artikel |
Sprache: | eng |
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Zusammenfassung: | The results of molecular orbital calculations performed for the oxalic acid monomer using the 6-31G, 6-31G*(5d), 6-31G**(5d), and MP2/6-31G*(5d) levels are reported. At the latter three levels of approximation the geometry with intramolecular hydrogen bonds is calculated to have the lowest energy, and the energies calculated for planar rotational conformers fall within about 2 kcal/mol of one another. The barrier hindering internal rotation of the carboxyl groups about the C–C bond for the nonhydrogen bonded conformer is calculated to be less than 1 kcal/mol, and secondary potential energy wells for gauche, rather than for the expected cis, carbonyl orientation are found about 300 cal/mol above the trans conformer at the 6-31G level. Vibrational frequencies calculated at the 6-31G level for the H-bonded conformer average about 9% higher than the observed values. On including electron correlation at the MP2 level the barrier calculated for the concerted, symmetrical, transfer of two protons between equivalent potential energy minima is 31.9 kcal/mol. This value is probably an upper bound. When electron correlation is ignored, calculations for the transfer of a single proton lead to a potential energy well that stabilizes an (HO)2CCO2 configuration with C2v symmetry at an energy 26–28 kcal/mol above that of the H-bonded ground state conformer. When electron-correlation is included at the MP2 level there is a saddle point in the PES instead. This is the transition state for proton exchange by successive transfers, and a search for its position at the MP2 level was not made. However, energy values of 25.7 and 36.6 kcal/mol are calculated using MP2/6-31G*(5d) at optimized 6-31G critical point geometries, and these values are taken as temporary estimates for the MP2 level transition state energies for two-step proton exchange and for decarboxylation, respectively. It remains to be determined whether the stepwise proton exchange channel will be present for higher level calculations. The results of these MO calculations are consistent with the experimental observations of Lapidus, Barton, and Yankwich
yielding first order kinetics, a low activation energy (31.5 kcal/mol), and complex kinetic isotope effects for the thermal decarboxylation of the oxalic acid monomer. Because of its importance in this reaction, calculations of the 1,2 H atom shift of dihydroxycarbene are also reported. |
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ISSN: | 0021-9606 1089-7690 |
DOI: | 10.1063/1.451603 |