Triplet ground state reactivity differences

It has been possible to record the IR and UV spectra of several derivatives of the carbene [75] - tetrachlorocyclopentadienylidene [80], indenylidene [81] and fluorenylidene [82] (Bell and Dunkin, 1985). These carbenes were formed by UV photolysis of the corresponding diazo precursors frozen in inert matrices and have a triplet ground state. The carbenes [80]-[82] react with CO in inert matrices at 30 K, but exhibit a lower reactivity than the carbene [75]. Furthermore, they were stabilized in a pure CO matrix at 12 K, whereas the free carbene [75] could not be detected under these conditions. The different reactivity towards CO between [75] and [80]-[82] may be associated with the different steric shielding of the carbene centres and with the different triplet-singlet gap as well. 

The most common sacrificial electron acceptor in the environment is molecular oxygen, whereas the main sacrificial donors are organic compounds. In consequence, the self-cleaning processes consist in oxidation of organic pollutants by molecular oxygen in its triplet ground state the reactions are driven by energy from solar radiation. In nature, many different photoinitiators or photosensitizers are reactive, but the most common environmental photosensitizers include hiunic substances (HS), whereas the best photoinitiators are transition metal complexes. 

The Six Atom Cluster. For the six atom cluster two different structures are of interest. There is a nearly planar C structure, with one atom above a five-membered ring, and there is the octahedral (Oj ) structure. If all atoms are in the d s state the C structure is preferred and the wavefunction is a closed shell singlet. This does not mean that Ni is non-reactive to since the energy difference to the Oj structure is only 10 Real/mol, and the 0, structure has a triplet ground state with two open shells. Since the exothermicity is most certainly larger than 10 kcal/mol, this structural change is expected to occur during the interaction of with Nig. 

Nitrene (AHf = 357(1) kj mor. S ° = 181.253J mol K ), also called imidogen radical, nitrogen monohydride, and azene has a triplet ground state (X S ). The reactivity of the first excited singlet state (a A) is different... 

Although the bimetallic species PtRhCH2 has a triplet ground state ( A"), the reactive channels on the triplet and the singlet PES profiles have been investigated here. In consideration of different terminals Rh and Pt in the precursors K and K3, the dehydrogenation process mediated by Rh or Pt may take place. Corresponding reactive channels have been explored, and they are denoted as pathway I and pathway II, respectively. 

At this point let us briefly consider the relationship between the carbonyl triplet state and another system capable of hydrogen atom abstraction alkoxy radicals. A comparison of the differences and/or similarities between the reactivity of the carbonyl triplet and that of an alkoxy radical should indicate whether the triplet state behaves as a normal ground state radical or if electronic excitation imparts unique properties leading to reactions not characteristic of ground state radicals. 

Carbenes are electron-deficient two-coordinate carbon compounds that have two nonbonding electrons at one carbon. In the ground state, the two unshared electrons may be either in the same orbital and have antiparallel spins (singlet state S), or in two different orbitals with parallel spins (triplet state T). They can be considered as typical representatives of reactive intermediates and have found a broad range of applications in synthetic chemistry. 

A thermal oxidation of 2,3-dimethyl-2-butene, 16, occurs in NaY when the temperature of the oxygen-loaded zeolite in raised above — 20°C [35], Similar thermally initiated oxidations were not observed for the less electron rich tram-or cix-2-butene. Remarkably, pinacolone was conclusively identified as one of the products of the reaction of 16, This ketone is not a product of the photochemical Frei oxidation (vide supra) and underscores the very different character of these two reactions and the complexity of the oxygen/16 potential energy surface, A rationale for the different behavior could lie in the different electronic states of the reactive oxygen-CT complex in the thermal and photochemical reactions. Irradiation could produce an excited triplet-state CT complex ( [16 O2] ) and/ or ion pair ( [16 02 ] ) with different accessible reaction channels than those available to a vibrationally excited ground-state triplet complex ( [16 "02]) and/... 

Table 7.1 summarizes the ground states for a variety of stmcturally diverse car-bene types and Table 7.2 gives a few specific values for the energy difference between singlets and triplets for some common carbenes. As you can see, in many cases the two reactive intermediates are very close in energy. 

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