Spin-allowed Reactions

Recently, Moskaleva et al. have proposed a new mechanism based on electronic structure calculations." Earlier experimental studies by Kasdan et al. determined that methyne (HC) has a doublet ground state and with a doublet-quartet energy splitting (AEdq) of 71.5 + O.SkJ/mol." Moskaleva et al. noted that the initially proposed mechanism (for HCN and N(" S) atom formation) is therefore spin-forbidden, and they also proposed a more favorable and spin-allowed reaction on the doublet surface. This new route on the doublet energy surface proceeds through the formation of an NCN intermediate, with concomitant formation of (doublet) hydrogen atom. 

The forbidden nature of this reaction is supported by the rate constant for this reaction (3.5 0.9 x 107 dm3 mol 1 second ), as measured from the IR kinetics experiment. The rate was more than two orders of magnitude less than the rate of the spin allowed reaction (55) [Eq. (10)]. 

The thermal reaction of matrix-isolated lh with acetylene leads to the formation of the intensely red-colored vinylcarbene 18 as the primary product.62 This carbene, which can also be formulated as a 1,3-diradical, has a triplet ground state and is thus formed in a spin-allowed reaction from triplet carbene lh (Scheme... 

The spin-forbidden reaction between HNO and 02 is quite slow [ 10 M s (147)] compared to the spin-allowed reaction between NO- and 02 [Eq. 9 ... 

The specificity of an oxide catalyst depends on its local electronic structure, both the electron density and the spin density. Making and breaking of bonds affects the spin of the active metal - spin-allowed reactions are kine-tically faster and thus have an advantage over spin-forbidden reactions. 

Thus, the change in oxidation state that would alter the spin is either essential to cater for spin-forbidden transitions or should not occur, to let a spin-allowed reaction to occur rapidly. This principle of specificity was identified in a large array of metal-oxo cluster reactions with small molecules [53, 54] and should be transferable to real catalyst under the concept of local electronic structures at active sites. 

For spin-allowed reactions the DJ values obtained by thermochemical methods adequately represent the energy threshold of reaction. For spin-forbidden reactions this is not so. If the lowest... 

Table 1.6 compares dissociation results of spin-allowed reactions of triatomic molecules. 

The bands of PH(A) are of very low intensity when PH3 is photolyzed in the 159 to 121 nm range but increase very sharply at <121 nm suggesting formation by a spin-allowed reaction [15]. The maximum emission of the spin-allowed primary product PH(b) is weaker than those of PH(A) of PH2(A) by a factor of about 100 no explanation was proposed [21]. 

The threshold for the spin-allowed channel lies between 18190 and 18755 cm but probably close to the upper limit. The energy release by the dissociation from 7vi gives AH°<211 kJ/ mol for the spin-allowed reaction [6]. 

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