Singlet-triplet transitions formaldehyde

In an electronic transition, an electron from one molecular orbital moves to another orbital, with a concomitant increase or decrease in the energy of the molecule. The lowest-energy electronic transition of formaldehyde promotes a nonbonding (ri) electron to the antibonding pi orbital (it ).1 1 There are two possible transitions, depending on the spin quantum numbers in the excited state (Figure 18-11). The state in which the spins are opposed is called a singlet state. If the spins are parallel, we have a triplet state. 

The lowest-energy excited singlet and triplet states are called S, and Th respectively. In general, Tj has lower energy than S,. In formaldehyde, the transition n —>, it (T1) requires... 

Chemiluminescence.—The triplet potential-energy surface of dioxetan has been found to intersect the singlet surface between dioxetan and the transition state for dioxetan decomposition to formaldehyde. This provides support for the argument that the reason for efficient triplet ketone formation on thermolysis of dioxetans is because intersystem crossing is an integral part of the reaction.293 The decomposition mechanism of 1,2-dioxetans has been the subject of another theoretical investigation.294 ... 

Dewar and Kirschner have reported that the potential-energy surface of triplet dioxetan intersects the surface for the singlet between dioxetan and the transition state for the decomposition of dioxetan to formaldehyde. This finding suggests a reason for the previously known, but rather surprising, thermolysis of dioxetans to produce triplet ketones. 

Table 2. Experimental equilibrium geometries and (0,0) transition energies of the njt singlet and triplet (A 2) states of formaldehyde...

The vibronic structure in the spectrum is consistent with but does not prove that the transition is A" <- Aj. Attempts to assign the bands using selection rules appropriate to other electronic transitions either failed or were inconclusive. In the absence of further information, the exact nature of the electronic transition cannot be determined although it can be assigned as A <- X with reasonable certainty. A careful search, out to 800 nm, was made for the corresponding triplet-singlet transition but no further bands were observed. In sharp contrast to the spectra of formaldehyde and thioform-aldehyde, the triplet-singlet system in thioketene is too weak to be observed by... 

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