Two unimolecular dissociation reactions of formic acid have been investigated theoretically. The decarboxylation reaction yields molecular hydrogen and carbon dioxide, while the dehydration reaction produces water and carbon monoxide. The 1,2‐hydrogen shift rearrangement from formic acid to dihydroxymethylene has also been considered. Methods ranged from double zeta plus polarization self‐consistent field to triple zeta plus double polarization coupled cluster singles and doubles. For certain key structures, the coupled cluster method including single, double, and linearized triple excitations (CCSDT‐1) was applied as well with the double zeta plus polarization basis set. A barrier height of ∼71 kcal mol−1 with zero point vibrational energy correction is predicted for the dissociation to molecular hydrogen and carbon dioxide. A rather comparable value of ∼68 kcal mol−1 is predicted for the barrier to the dehydration reaction. The 1,2‐hydrogen shift transition state is somewhat higher in energy at ∼79 kcal...

The decarboxylation and dehydration reactions of monomeric formic acid

Infrared laser chemistry and the dynamics of molecular multiphoton excitation

Primary dissociation pathways have been investigated for formic acid by ab initio molecular orbital methods. Reactant, transition state, and products were fully optimized with unrestricted Hartree–Fock and unrestricted second‐order Mo/ller–Plesset wave functions. The activation energy for decarboxylation of formic acid (CO2+H2) is 65.2 kcal mol−1, while that for the dehydration process (CO+H2O) is 63.0 kcal mol−1. These theoretical results suggest that the decarboxylation and dehydration processes are competitive. The activation energy barrier for isomerization of formic acid to yield dihydroxymethylene is 73.7 kcal mol−1 and may be a competitive process. Free radical initiation processes are predicted to be minor.

http://wwwuser.gwdg.de/~ggroenh/assignments/JChemPhys_96_1167.pdf

A comprehensive theoretical examination of primary dissociation pathways of formic acid

Areas of particular current interest in the field of unimolecular reactions are reviewed. New developments and experimental applications of the theory of thermal unimolecular reactions are discussed, with particular stress on simple descriptions of both the low and high-pressure limiting ranges and the intermediate fall-off. Recent experimental studies of elementary steps in unimolecular reactions after optical (one photon) and molecular beam (chemical) activation are reviewed in relation to theory. The kinetic and mechanistic aspects of the new field of unimolecular reactions induced by monochromatic infrared radiation (URIMIR, with multiphoton excitation) are developed in some detail. Experiments in this area are summarized and their role for the interpretation of URIMIR is discussed.

Current Aspects of Unimolecular Reactions

The oxidation mechanism of dimethyl ether is investigated using ab initio methods. The structure and energetics of reactants, products, and transition structures are determined for all pathways involved in the oxidation mechanism. The detailed pathways leading to the experimentally observed products of dimethyl ether oxidation are presented. The energetics of over 50 species and transition structures involved in the oxidation process are calculated with G2 and G2(MP2) energies. The principal pathway following the initial attack of dimethyl ether (CH3OCH3) by the OH radical is the formation of the methoxymethyl radical (CH2OCH3). Oxidation steps lead to the formation of methyl formate, which is consistent with the experimentally observed products. Oxidation pathways of methyl formate are also considered.

Tropospheric Oxidation Mechanism of Dimethyl Ether and Methyl Formate

The focussed beam from a single line [P 1 (6)] pulsed HF laser has been used to decompose CH 3 OH (for pressures between 0.169 and 14.95 kPa) and isotopic mixtures of methanol. The normalized yields (number of product molecules Pulse/ P METHANOL ) of the non-condensable products H 2 , CO and CH 4 increased linearly with pressure (for the range ≈ 1 - 7 kPa). For sufficiently low pressures, selective excitation of one component of an isotopic mixture gives an isotopically enriched product. For example, selective excitation of CH 3 OH in equimolar of mixture CH 3 OH/CH 3 OD at a total pressure of 269 Pa gives hydrogen which is enriched 60-fold in H versus D. The degree of isotopic enrichment decreases with increasing mixture pressure. The efficiency of conversion of photon energy to reaction product has been observed to increase linearly with pressure. Decomposition studies have been performed in the presence of additives. These imply that the decomposition of methanol to H 2 involves mainly molecular rather than free radical steps.

Laser isotope separation and the multiphoton decomposition of methanol using a pulsed HF or DF laser

Laser isotope separation and the multiphoton dissociation of formic acid using a pulsed HF laser

Multiphoton absorption (MPA) of HF laser radiation has been studied, as a function of pressure (15 Pa to 1.3 kPa) and fluence (2 mJ/cm2 to 75 J/cm2) for the series: water, methanol, methan‐d 3‐ol, ethanol, and 2,2,2‐trifluoroethanol. As the group attached to the –OH is made more complex, the quasicontinuum occurs after fewer excitation steps, and under ’’collisionless’’ conditions, the same degree of multiphoton excitation is found to require a lower fluence. For water, at pressures between 73 Pa and 1.3 kPa, the cross sections are considerably lower than those for the other molecules, and MPA requires fluences in excess of ∼75 J/cm2. The remaining molecules divide into two groups, the ’’small’’ molecules (CH3OH and CD3OH) and the ’’large’’ molecules (C2H5OH and CF3CH2OH). For the small molecules at low pressures, the cross sections decrease with increasing fluence, an effect which is thought to be due to anharmonic bottlenecking. As pressure increases, the fluence dependence of the cross sections disappear. For the large molecules, anharmonic bottlenecking appears to be reduced, due to the greater density of states, and cross sections increase with increasing fluence according to the empirical form: σ(E, P)=K′E b′ P a (where P is pressure, E is fluence and b′, a, and K′ are constants). The facility of HF laser‐induced collisionless multiphotondissociation of the –OH containing molecules is discussed in light of these results.

F.K. McClusky

Papers

Laser isotope separation and the multiphoton decomposition of methanol using a pulsed HF or DF laser

Laser isotope separation and the multiphoton dissociation of formic acid using a pulsed HF laser

Multiphoton absorption of HF laser photons by molecules containing a hydroxyl group

Laser isotope separation and the multiphoton decompotions of formaldehyde using a focused DF laser: the effect of single- or multi-line irradiation

Multiphoton absorption of intense HF laser radiation by methanol