Forbidden processes

In coimection with the energy transfer modes, an important question, to which we now turn, is the significance of classical chaos in the long-time energy flow process, in particnlar the relative importance of chaotic classical dynamics, versus classically forbidden processes involving dynamical tuimelling . 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

There is another usefiil viewpoint of concerted reactions that is based on the idea that transition states can be classified as aromatic or antiaromatic, just as is the case for ground-state molecules. A stabilized aromatic transition state will lead to a low activation energy, i.e., an allowed reaction. An antiaromatic transition state will result in a high energy barrier and correspond to a forbidden process. The analysis of concerted reactions by this process consists of examining the array of orbitals that would be present in the transition state and classifying the system as aromatic or antiaromatic. 

The photochemical cycloaddition of a carbonyl compound 1 to an alkene 2 to yield an oxetane 3, is called the Patemo-Buchi reaction - This reaction belongs to the more general class of photochemical [2 + 2]-cycloadditions, and is just as these, according to the Woodward-Hofmann rules, photochemically a symmetry-allowed process, and thermally a symmetry-forbidden process. 

The generalized Woodward-Hoffmann rule suggests that a synchronous addition of disulfonium dications at the double C=C bond of alkenes would be a thermally forbidden process and so would be hardly probable. Simulation of the frontal attack by ethylene on l,4-dithioniabicyclo[2.2.0]hexane 115 gave no optimal structure of an intermediate complex. On the other hand in the lateral approach of the reactants, orbital factors favor attack of the double bond by one of the sulfonium sulfur atoms of the dication. This pattern corresponds to SN2-like substitution at sulfur atom as depicted in Figure 5. Using such a reactant orientation, the structure of intermediate jc-complex was successfully optimized. The distances between the reaction centers in the complex, that is, between the carbon atoms of the ethylene fragment and the nearest sulfur atom of the dication, are 2.74 and 2.96 A, respectively. 

Based on this tentative assignment of Fe(CO)3(C2H4) as having a singlet ground state, addition of ethylene to Fe(CO)3 now becomes a spin-forbidden process, which might seem inconsistent with the observed rapid addition. However, as mentioned above, addition on the triplet surface should be barrierless, and hence rapid. Conversion of the initially formed triplet adduct to the... 

Net addition of CO to 3Fe(CO)3(H2) to form Fe(CO)4(H)2 was observed upon photolysis of Fe(CO)s in sc Ar in the presence of H2 (24). The intermediate species 3Fe(CO)3(H2) is involved as a minor product of the photolysis, and was proposed to arise from addition of H2 to Fe(CO)3 or Fe(C0)3Ar (24). Experimentally, this species was shown to decay in the presence of excess CO with a pseudo-first-order rate constant 0bs — 4.1( + 0.3) x 107s-1. The mechanism for this spin-forbidden process was however unclear from experiment alone, and computation was used to explore the various possibilities (24). 

The first excited state of cyclobutene (o27t ) is correlated with the upper excited state ( /J /2 /3) of butadiene making it a high energy symmetry forbidden process. 

Intersystem crossing is a spin-forbidden process between states of different multiplicity, so the magnitude of the spin-orbit coupling is important in controlling the rate of intersystem crossing. Transitions between... 

The Coulombic mechanism would require that both 3D — 3D and 1A —> 3A were allowed transitions, which clearly they are not as both are spin-forbidden processes. Thus, triplet-triplet energy transfer by the long-range Coulombic mechanism is forbidden. 

Finally, it should be mentioned that rearrangement of the cp-cobaltacyclopentadiene intermediate to the thermodynamically more stable [(T7 -cp)Co(i7 -cyclobutadiene)] complex (which is catalytically inactive) is a thermally forbidden process [Eq.(47)]. 

Unsaturation is also important in the metal-catalysed disrotatory ring opening of XXXII ( hexamethyl-Dewar-benzene , HMDB) to hexamethylbenzene. This formally forbidden process is catalysed by monomeric HMDBRhCl (the reaction being of order 1/2 in [HMDBRhCl]2 and order 1 in substrate) Closely related is the conversion of XXXIII(a) to XXXIV, presumably by way of an unsaturated intermediate XXXIII ( ), since free CO or added alkene ligands inhibit the process . ... 

In addition to energetic considerations, however, there are other factors such as spin conservation that also determine the importance of various sets of products. As discussed in Chapter 3.A, since the ground state of 03 is a singlet, dissociation into either two singlet states (e.g., reaction (5)) or into two triplet states is expected to predominate. However, as discussed shortly, both hot-band absorption by rovibra-tionally excited 03 and by a spin-forbidden process are believed to contribute significantly to the atmospheric photochemistry of 03. 

This spin-forbidden process appears to be the major source of O( D) beyond 325 nm, particularly at low temperatures where the contribution of vibrationally excited 03 is minimized. Thus, in contrast to the decreasing yields of O( D) with temperature in the 306-to 324-nm region, yields in the 325- to 329-nrn region are relatively constant with temperature at lower temperatures, the quantum yield in this region approaches 0.06, which has therefore been assigned by Talukdar et al. (1998) as the upper limit for the production of O( D) in this region by reaction (8). 

Table 4.6 summarizes the wavelength and temperature dependence for O( D) production recommended by Talukdar et al. (1998). Beyond 329 nm, the quantum yield from the spin-forbidden process appears to be 0.05-0.06 (Silvente et al., 1997 Talukdar et al., 1998). The absorption cross sections of 03 become sufficiently small beyond 360 nm that O( D) production beyond this wavelength is not expected to be significant for atmospheric applications. 

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