Dissociation of Formaldehyde

We ll consider the molecular dissociation reaction first (upper illustration). We want to determine the transition structure and to predict the activation energy for the reaction. In order to do so, we ll need the following information  

Zero-point-corrected energies for formaldehyde, hydrogen molecule, and carbon monoxide. 

We will model this reaction at the HF/6-3IG(d) level. We can draw upon previous calculations for some of this data. The following table lists these results  

Exploring Chemistry with Electronic Structure Methods 175 

To compute the energy of the transition structure, we ll need to perform the following set of calculations  

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The detailed mechanism of the dissociation of formaldehyde (H2CO) has been investigated in molecular beam experiments. The reaction itself may appear quite simple... 

This must be a concerted process rather than a secondary dissociation of formaldehyde or hydroxymethylene from the previous reactions, because of the high energy barriers of these secondary dissociations. 

J. Troe, Theory of multichannel thermal unimolecular reactions. 2. Application to the thermal dissociation of formaldehyde, /. Phys. Chem. A109 (37) (2(X)5) 8320-8328. 

Figure 8.21 Optimized transition states for the dissociation of formaldehyde into H2 + CO (95 and 96) and acetaldehyde into CH4 + CO (97). Distances (in A) computed at (U)< B97X-D/aug-cc-pVTZ, CCSD(T)/aug-cc-pVTZ ° (italics), and CASPT2/aug-cc-pVDZ (bold).

Ab initio calculations including configuration interactions were reported for the addition of the NH2 radical to ethylene 177>, for dissociation of formaldehyde into radicals 178> and molecular products 179>, and for the electrocyclic transformation between cyclobutene and butadiene 180>. 

A prototypical example of a transition state amenable to single determinant MBPT/CC methods is the unimolecular dissociation of formaldehyde... 

The ab-initio study of process I, the dissociation of formaldehyde into radical products, was simultaneously done by Fink 9 ) (using Nesbet s method of symmetry and equivalence restrictions and a limited Cl) and by Hayes and Morokuma 2) (using a GSMO.CI method and the "point system in the selection of the configurations). Both sets of potential energy curves exhibit the same behavior. One is shoAvn below (Fig. 10). 

Fig. 10. Potential energy curves for the dissociation of formaldehyde into radical products. The first two singlets — the ground state and the an state — are shown (O)- They correlate respectively with the systems HCO and HCO ( A") +H . The first two triplets — the low-lying an state and the high-lying Ai mz state — are also represented (A)...

Dissociation of formaldehyde, CHgO, at comparably low temperatures is obviously determined by a complex decomposition mechanism. Conclusions on the unimolecular dissociation can only be drawn from measurements at high temperatures under shock wave conditions. In this system the primary dissociation leading to formyl radicals is followed by decomposition of CHO and subsequent reactions of H atoms with CH2O and CHO. By analysing the chain mechanism the rate constant of the unimolecular reaction was derived. ... 

For the dissociation of formaldehyde, the true barrier height is believed to be roughly 80.6 kcal/mol. For this system, the LSDA and gradient-corrected DFT calculations provided almost identical barrier heights of 74.0 and 75.0 kcal/mol, respectively. The quoted double- plus polarization HF, singles and doubles Q, and multiple configuration SCF plus Cl calculations gave 100.2, 92.8, and 84.4 kcal/mol, respectively. 

Araujo M, Lasorne B, Magalhaes AL, Worth GA, Bearpark Ml, Robb MA (2009) The molecular dissociation of formaldehyde at medium photoexdtation energies a quantum chemistry and direct quantum dynamics study. J Chem Phys 131 144301... 

Unit 5 Dissociation of formaldehyde (stationary points on PES Woodward-Hoffmann rules)... 

The hydroperojg l radieal, HOO% is an intermediate in some important chemical reactions. In addition to its production by the reactions discussed above, in polluted atmospheres, hydroperoxyl radical is made by the following two reactions, starting with photol5Tic dissociation of formaldehyde to produce a reactive formyl radical ... 

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