HCO dissociation

The HCO dissociation reaction is fast for a dissociation reaction, because the H-CO bond in the formyl (HCO) is very weak. These reactions are sufficiently fast to compete with the H + O2 OH + O chain-branching reaction, and thus produce the mole-number overshoot. Under some circumstances pressure-dependent dissociation of small hydrocarbon free radicals can also contribute to the mole-number overshoot. These reactions can include... 

This model is clearly incomplete, since it does not account for vague tori [355] and the complex Arnold web [357, 358] structure of a multidimensional phase space with both chaotic and quasi-periodic trajectories. However, Eq. (74) does properly describe that, with non-ergodic dynamics, the lifetime distribution will have an initial component that decays faster than the RRKM prediction as found in the simulations by Bunker [323,324] and the more recent study of HCO dissociation [51]. Additionally, there will be a component to the classical rate, which is slower than /srrkm, for example, in the dissociations of NO2 and O3 this component cannot be described by an expression as simple as the one in Eq. (74). 

At higher pH values (> 6) HCOs" dissociates further to + COs . The pH value is usually lower, so the latter dissociation occurs normally to a very small extent. The cathodic reaction is assumed primarily to follow the equation... 

Similar behaviour is observed in both experiments and calculations for HCO—>H+CO dissociation [88, 90... 

Tobiason J D, Dunlap J R and Rohifing E A 1995 The unimolecular dissociation of HCO a spectroscopic study of resonance energies and widths J. Cham. Phys. 103 1448-69... 

Keller H-M and Schinke R 1999 The unimolecular dissociation of HCO. IV. Variational calculation of Slegert states J. Chem. Phys. 110 9887-97... 

By contrast, the dissociation channel leading to H + HCO (the radical channel) has no barrier and a dissociation threshold [49] of 30,328.5 cm. In 1993, van Zee et al. [50] found that excitation above this threshold led to CO rotational distributions that were bimodal. In addihon to the high-/ CO... 

Because the pathway to H + HCO on So is barrierless (with a loose TS), whereas the pathway on Ti has an exit barrier (tight TS), the dissociation dynamics of the two pathways can be expected to differ markedly. Measuring the translational energy release and the product state distributions of the HCO fragment are therefore appropriate experimental techniques for exploring this competition. 

Because dissociation on So is barrierless, the product state distributions should be well approximated by statistical theories, especially when the excess energy is small, as in the Valachovic study. Product state distributions arising from the So pathway should be characterized by small translational energy release, but significant rovibrational excitation of HCO. This signature is demonstrated in the top panel of Fig. 17, which shows a HRTOF spectrum with... 

Rearrangements must be involved also in radical eliminations in which the open shell species X is not present as a structural unity of M+ but has to be created prior to the eventual dissociation step. Examples of such processes are the loss of HC = O from ionized methylvinylsulfide 6- 7i) or the elimination of both OH and HC = O from the cation radical of 8S,6> (the arrows in (2) indicate the atoms of 8 which are involved in the formation of the radicals OH and HCO. Both radicals are obviously not present as structural functions in the precursor. For HCO loss from 6 no labelling work has been reported). 

The above examples should suffice to show how ion-molecule, dissociative recombination, and neutral-neutral reactions combine to form a variety of small species. Once neutral species are produced, they are destroyed by ion-molecule and neutral-neutral reactions. Stable species such as water and ammonia are depleted only via ion-molecule reactions. The dominant reactive ions in model calculations are the species HCO+, H3, H30+, He+, C+, and H+ many of then-reactions have been studied in the laboratory.41 Radicals such as OH can also be depleted via neutral-neutral reactions with atoms (see reactions 13, 15, 16) and, according to recent measurements, by selected reactions with stable species as well.18 Another loss mechanism in interstellar clouds is adsorption onto dust particles. Still another is photodestruction caused by ultraviolet photons produced when secondary electrons from cosmic ray-induced ionization excite H2, which subsequently fluoresces.42... 

Figure 5. Potential energy surface for the C2HsO+ system, showing stable structures, transition states and dissociation limits. The information is predominantly from the theoretical calculations of Fairley et al.71 with the region of the HCO+ + CH4 and CH3 + H2CO dissociation limits from Audier et al.75...

Most likely the cobalt catalyst is HCo(CO)2(L), which has a very electron rich metal centre and dissociation of CO does not occur under the reaction conditions. The first step is a reaction of the cobalt hydride with ethylene oxide forming a hydroxyethylcobalt species, which does not require dissociation of... 

The primary products are methyl and formyl radicals [36, 37] because potential energy surface crossing leads to a H shift at combustion temperatures [35], It is rather interesting that the decomposition of cyclic ethylene oxide proceeds through a route in which it isomerizes to acetaldehyde and readily dissociates into CH3 and HCO. Thus two primary addition reactions that can be written are... 

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