Extended molecular mechanics calculations of thermodynamic quantities, structures, vibrational frequencies, and infrared absorption intensities of formic acid monomer and dimer

Abstract

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.

Coherent Proton Tunneling in Hydrogen Bonds of Isolated Molecules: Carboxylic Dimers

“…The same argument holds for the results of the electron diffraction measurements which obtained an O Á Á Á O distance of 2.696 . These values are in reasonable agreement with the rotational constants as predicted by the potential of Yokoyama et al [43]. They postulated a semiempirical model potential which was set up to reproduce the experimental vibrational frequencies and the bond lengths in the monomers.…”

Origin of the CO Stretch Mode Splitting in the Formic Acid Dimer

“…Of course, these insights are made possible in part by a determination of the correct experimental value of ∆ν(CdO), which we believe to be 71 cm -1 and not the 84 cm -1 accepted by others 3,28 , and by a recognition that a significant anharmonic correction is necessary in order to obtain the harmonic value of the splitting to be compared to predictions of an electrostatic model of the intermonomer interactions.…”

“…With this assurance that they will also reproduce infrared intensities, we can ask whether they also reproduce the observed ∆ν(CdO). Third, none of the electrostatic model treatments [2][3][4] has examined the question of whether the charge-charge flux model is consistent with the intermonomer Coulomb energy. This quantity can be obtained from ab initio calculation, and we should expect it to be reproduced by the Coulombic interactions of the model.…”

Transient HDO rovibrational satellite peaks in solid parahydrogen: Evidence of hydrogen atoms or vacancies?

“…This optical setup permits FTIR spectra to be recorded within the photolyzed region of the crystal either during or immediately after 193 nm irradiation. The DCOOD concentration is the average value determined from the ν 1 (OD stretch) and ν 6 (deformation) peaks with integrated absorption intensities of 36.9 and 56.8 km/mol, respectively [23]. Fig.…”