Example bimolecular formation of formaldehyde

Let us first consider the bimolecular formation reaction in (10.6) for formaldehyde, which can be written in greater electronic detail as 

From the Lewis stracture diagrams, we recognize that the reaction involves formal transformation of two bonding electron pairs, namely, dissociation of 7hh and tico reactant bonds (formally reducing bond order byai front 1 to 0 and bco from 3 to 2) and formation of two new Tch product bonds (formally increasing each Z ch ch from 0 to 1). How can this chemical magic occur  

The intermediate NLSi region of Fig. 10.5 (separating NTSi and NTS2) is characterized by a confusing succession of strange NLS structures that can all be loosely classified as H + HCO (NLS2) structures, such as 

Let us first try to obtain a reactant-like perspective on the IRC pathway. We can employ the CHOOSE keyHst (Section 5.5) to specify a reactant-Uke bond pattern, and thereby continue to follow the progress of the NBO push-pull delocalizations (10.11 and 10.12) whose values are plotted at the left of 

As suggested by the resonance structure depictions on the left, proton transfer delocalization (10.14) is favored in collinear geometry (Z h co— 180°, which maximizes overlap of the He donor NBO with the t hh acceptor NBO), whereas hydride-addition delocalization (10.15) is favored by bent angular geometry (Z-H CO — 135°, which maximizes overlap of the h donor NBO with one of the four cloverleaf lobes of the ti co acceptor NBO). These qualitative NBO considerations dictate the qualitative features of transition state geometry (Fig. 10.2), with the cationic H atom nearly coaligned with the CO axis and the anionic H atom canted away from this axis to coalign with the adjacent cloverleaf lobe of the n co orbital. 

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