Photochemical Decomposition of Formaldehyde

Gas phase irradiation of formaldehyde raises it to its Si state, which - depending on the frequency of the exciting radiation - decomposes along two competing pathways [20, 21]  

Si is the HOMO— LUMO state with the configuration [...62 ] and consequent state symmetry M2. A vibrational perturbation of irrep 02 is required in order to induce internal conversion to the totally symmetric Sq. Fig. 10.6 depicts the six symmetry coordinates of H2CO, none of which has the proper irrep to promote 1C. As has been demonstrated experimentally [24] the process occurs via coupling of antisymmetric in-plane (62) and out-of-plane (61) vibrations, that together provide a second-order term of the proper 02 symmetry (cf. Section 6.1.2.1). 

Only one non-correspondence between ground-state H2CO and its molecular fragmentation products appears on the left side of Fig. 10.5 Ui 62. The 

In full agreement with the symmetry analysis, the reaction path obtained from refined high-level calculations of the ground-state fragmentation pathway [25, 26, 27], is indeed in-plane, retaining but not full symmetry. 

we recognize that although formaldehyde itself is non-planar in Si, the inversion barrier is very low (1-2 kcal/mol) [28, 29], so formal imposition of planarity cannot introduce any error. The radical pathway, involving rupture of a single CH bond, does not retain C2(z) or a(zx) its highest possible symmetry is Formal symmetry can be retained for the purpose of the analysis by anasymmetrization (Section 7.1.1.1) - a particularly simple procedure in the present instance. 

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