Unimolecular Dissociation Reaction of Formaldehyde H2CO - H2 CO

A qualitatively correct description of the dissociation process requires at least four active electrons in the two CH bonds of H2CO. During the dissociation process, two electrons, one from each CH bond, pair up to form the HH bond while the other two form a lone pair on C in CO. 

The basis set used is Dunning s cc-pVDZ [16]. The CASSCF wave function was obtained with CAS(4,4). The geometries of the equilibrium and TS structures were determined with this basis set and active space. The orbitals were then localized in the active orbital space. The orbitals were transformed so as to have maximum overlap with two carbon sp2 orbitals and hydrogen Is orbitals. The sp1 orbitals were used with the fixed hybridization ratio of 2s to 2p orbitals (1 2) and with a fixed angle of 120° relative to the CO axis 

There are 20 linearly independent spin-paired functions corresponding to the dimension of CAS(4,4), which are listed in Table 1. Structures (I) to (VII) are classified as CH bond structures and the structures (VIII) to (X) as HH bond structures. Structure (XI) is classified as neither of the above, since these structures can be regarded both as structures polarized further from one of (II) to (V) and (IX) to (X). 

The CASVB wave functions obtained for the equilibrium and TS structures are given in Fig. 6. 

In the equilibrium structure, the main VB structure is the covalent CH bonds structure (I) as expected. The second most important are those where one of the CH bonds is connected with a covalent bond and the other with an ionic bond made by electron transfer from the hydrogen atom to the carbon atom, (II) and (IV). In contrast, the contribution from the structures that describe electron transfer from the carbon atom to a hydrogen atom is small and negative. The contribution from the HH bond structure (VIII) and ionic structures, (IX) and (X), is very small. The total occupation number of CH bonds is 0.9654, while that of HH bond is -0.0147. This indicates almost no bond formation between two hydrogen atoms in the equilibrium structure. 

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The complete active space valence bond (CASVB) method is an approach for interpreting complete active space self-consistent field (CASSCF) wave functions by means of valence bond resonance structures built on atom-like localized orbitals. The transformation from CASSCF to CASVB wave functions does not change the variational space, and thus it is done without loss of information on the total energy and wave function. In the present article, some applications of the CASVB method to chemical reactions are reviewed following a brief introduction to this method unimolecular dissociation reaction of formaldehyde, H2CO — H2+CO, and hydrogen exchange reactions, H2+X — H+HX (X=F, Cl, Br, and I). 

In this article, we present applications of CASVB to chemical reactions the unimolecular dissociation reaction of formaldehyde, H2CO — H2+CO [5], and a series of hydrogen exchange reactions, H2+X — H+HX (X-F, Cl, Br, and I). The method in this article is based on the occupation numbers of VB structures that are defined by the weights of the spin-paired functions in the CASVB functions, so that we could obtain a quantitative description of the nature of electronic structures and chemical bonds even during reactions. 

Transition state barrier height for the unimolecular dissociation reaction of formaldehyde H2CO H2 CO [35]. This reaction is Woodward-Hoffmann... 

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