for methane elimination, whereas rearrangement via 6, 2, and 4 would produce C H 2 D C H 0 , which would not. It would be of interest to see whether experimental isotope effects support the lower energy two-step pathway or the higher energy one-step rearrangement of 3 to 1.Our calculations indicate that elimination of molecular hydrogen from acetaldehyde30 via transition structure 9 has a marginally lower energy requirement (339 kJ mol-') than the methane elimination via 7 (347 kJ mol-').
Concluding Remarks(i) We find that vinyl alcohol (2) lies 47 kJ mol-' higher and hydroxyethylidene (3) 213 kJ mol-' higher in energy than acetaldehyde.(ii) Hydroxyethylidene (3), although a high-energy species, is predicted to be separated by significant barriers (1 18 and 98 kJ mol-', respectively) from its lower energy isomers 1 and 2 it should thus be observable, consistent with experiment.(iii) The energy ordering of the transition structures (4, 5, and 6, respectively) for the rearrangements 1 -2, 1 -3, and 2 -3 is 4 C 6 C 5. This is in apparent conflict with conclusions based on experimental observations.(iv) The rate-determining step for the production of CO from any of the three isomers 1-3 is predicted to correspond to the elimination of methane from acetaldehyde (1) (preceded in the case of both 2 and 3 by lower energy rearrangements to l), and this is consistent with isotope effects observed in N R M S studies. Acknowledgment. We thank Professor Fred McLafferty for helpful discussions, Professor J. D. Goddard for providing us with results prior to publication, and gratefully acknowledge a generous allocation of time on the Fujitsu FACOM VP-100 of the Australian National University Supercomputer Facility.
Abstract:The molecular mechanics simulation of infrared absorption spectra utilizing the effective atomic charges and charge fluxes as both the potential and the intensity parameters has been applied to formic acid monomer and dimer. The thermodynamic properties, optimized geometries, vibrational frequencies, and infrared absorption intensities are consistently derived from the potential models in which the difference between the monomer and the dimer is empirically taken into account. The large splitting between the A, and the B, cabonyl stretching frequencies of the dimer and the extension of the carbonyl bond on the dimerization are simultaneously reproduced by introducing two charge fluxes aq,--ar-and aq6,,/ar-(4) CODATA recommended key values for thermodynamics, 1977: J. Chem. Thermodyn. 1978, 10, 903.
