Formaldehyde dissociation

In structure II (numbered 13 in the IRC output), the C-H bond has lengthened with respect to the transition structure (1.23 versus 1.09A), while theC-O bond length has contracted slightly. Both changes are what would be expected as formaldehyde dissociates to form carbon monoxide and hydrogen molecule. ... 

Table 16.10. The leading Rumer tableaux in the wave function for the saddle point state of formaldehyde dissociation.

Laser-induced formaldehyde dissociation has been intensively studied experimentally and theoretically in the last decade.51-54 Detailed calculations of the potential energy surface have been reported and various studies have been undertaken regarding the possibility of mode selective decay, The low density of vibrational states at the typical energies studied make H2CO an interesting candidate for possible non-RRKM effects, a point to which we return later. 

Polik W F, Guyer D R and Moore C B 1990 Stark level-crossing spectroscopy of Sq formaldehyde eigenstates at the dissociation threshold J. Chem. Phys. 92 3453-70... 

Addition of pyrazole to C—X double bonds is also common. Formaldehyde gives stable adducts (260) and (261) (69BSF2064), but in the addition to ketones, (262) is only observed at low temperatures (Section 4.04.1.3.3(i)). However, hexafluoroacetone forms a stable adduct (262 R = Cp3) that has been used as a chelating agent (Section 4.04.2.1.3(iv)). Addition of pyrazoles to aryl isocyanates affords (263) the addition is also reversible, but it requires high temperatures to dissociate the adduct (Section 4.04.1.5.1). 

These values suggest that the two hydroxycarbene isomers convert into one another very easily. The barrier to molecular dissociation of the cis form is significant, however, and so this structure probably does not dissociate directly, but rather first converts to the trans isomer, which is subsequently transformed into formaldehyde, which dissociates to carbon monoxide and hydrogen gas. The article from which this study was drawn computes the activation energy for the trans to cis reaction as 28.6 kcal- moT at RMP4(SDQ)/6-31G(d,p) (it does not consider the other reactions). 

Recently, we [61] applied this method to model the (HOCO) photodetachment experiments of Continetti and co-workers [62]. We are currently using the method to study the dissociation dynamics other four-atom systems, including hydrogen peroxide and formaldehyde. 

Nakabayashi, S., Sugiyama, N., Yagi, 1. and Uosaki, K. (1996) Dissociative adsorption dynamics of formaldehyde on a platinum electrode surface onedimensional domino Chem. Phys., 205, 269-275. 

Chen Y-X, Heinen M, Jusys Z, Behm RJ. Dissociative adsorption and oxidation of formaldehyde on a Pt film electrode under controlled mass-transport conditions, an in-situ spectro-electrochemical flow-cell study. To he published. 

Comparable patterns are followed by other organic substances such as formaldehyde and formic acid. All these substances are dissociatively adsorbed on platinum [4] and it was suggested that they build the same adsorption product [35]. 

Fahey (16) suggests that intermediate 3 dissociates formaldehyde he finds supportive evidence in the rhodium-based system by observation of minor yields of 1,3-dioxolane, the ethylene glycol trapped acetal of formaldehyde. For reasons to be discussed later, we believe the formation of free formaldehyde is not on the principal reaction pathway. (c) We have also rejected two aspects of the reaction mechanism proposed by Keim, Berger, and Schlupp (15a) (i) the production of formates via alcoholysis of a formyl-cobalt bond, and (ii) the production of ethylene glycol via the cooperation of two cobalt centers. Neither of these proposals accords with the observed kinetic orders and the time invariant ratios of primary products. 

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. 

A dissociative adsorption of methanol forming surface methoxy groups is suggested as the initial step. This is followed by the slow step, the formation of some form of adsorbed formaldehyde species. Evidence.for the bridged species is not available, experiments with °0 labeled methanol are expected to clarify this. Continued surface oxidation leads to a surface formate group and to carbon monoxide. All the byproducts can be obtained by combination of the appropriate surface species. 

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