Dioxygen singlet state

It will be noted that O2 is always evolved when H2O2 acts as a reducing agent, and sometimes this gives rise to a red chemiluminescence if the dioxygen molecule is produced in a singlet state (p. 605), e.g. ... 

The absorption spectra of the silicon(IV) naphthalocyanines follows the pattern already seen with the analogous zinc(II) (Section 9.22.12.1.4) and aluminum(III) (Section 9.22.13.2) derivatives 354 the red band is shifted about 100 nm further to the red, with intensification. Bis(trihex-ylsiloxy)silicon(IV) naphthalocyanine has Amax(PhH) = 776 nm (e = 650,000 M-1 cm-1) and <1>A 0.35 (oxygen-saturated benzene).389 The solution is fluorescent (main emission at 780 nm, Tf 2.85 ns) the triplet energy (ca. 22 k cal mol-1) is a little less than the energy of the first singlet state of dioxygen, and the process (Equation (9)) is reversible (cf. compounds (40) and (41), Section 9.22.10.2). 

For a more in-depth interpretation of the inertness of dioxygen, the fact that 02 is a triplet state bi-radical, i.e. it has two unpaired electrons in the 2jig orbitals, needs to be considered. It follows that the oxidation of singlet state substrates by the triplet 02 to form singlet products is spin-forbidden and, as a consequence, relatively slow. 

In some cases, the ground state is not a singlet state, e.g. dioxygen, anion and cation radicals of aromatic molecules. 

The excited singlet state of dioxygen (O2). The term singlet oxygen should not be used since it is unclear... 

Exchanged di-, tri- and tetraperoxomolybdates decompose with simultaneous release of two oxygen atoms in one molecule of excited, singlet-state dioxygen ( 02). This O2 diffuses into solution and performs specific oxidations, e.g. olefin hydroperoxidation [10] ... 

Normally the term radical (or free radical ) is confined to molecules or ions with one unpaired electron (called doublet states because the electron has two magnetic quantum numbers +1/2). In the non-metal field the most common paramagnetic species other than radicals are those with two unpaired electrons, called triplet states (magnetic quantum numbers 0, 1). Quite the most important triplet-state molecule is dioxygen and it is a great pity that ESR spectroscopy can, for various reasons, only be used to detect O2 in the gas-phase or certain crystalline solids. Other important triplet-states are sometimes obtained on photo-excitation of ordinary (singlet-state) molecules or ions, and these have reactions in some ways typical of di-radicals (i.e.,... 

Because the ground state of dioxygen is a triplet with two unpaired electrons with parallel spins, its reactivity with organic molecules in the singlet state is severely... 

Ground-State dioxygen has two unpaired electrons ( 02 ), which makes it a biradical with a triplet electronic state (see Table 4). Its radical character is limited because the H O bond is weak (—AGbf, 51 kcalmol" ), and the triplet state precludes direct reaction with singlet-state substrate molecules with saturated a bonding. 

Ground-state dioxygen ( 2) can be activated by photolytic energy transfer to the singlet state ( 2) (eqnation 129) and this may be dnplicated at the snrface of biomembranes by solar radiation. In the laboratory, singlet dioxygen is conveniently prodnced by the stoichiometric oxidation of hydrogen peroxide by hypochlorous acid (eqnation 130). 

Ground-state dioxygen ( 02) can be activated by photolytic energy transfer to the singlet state ( 02),... 

The fluorescence quantum yield of anthracene in degassed acetonitrile is 0.3. It is reduced by 30% upon admission of air. Estimate the lifetime of the singlet state of anthracene in degassed solution and the rate constant of fluorescence. Refer to Section 2.2.1. The concentration of dioxygen in air-saturated acetonitrile is 2x10 3m. [tj = 5.6 ns, kf = 5.4 x 107 s ]... 

Calculate the concentration required to quench 99% of singlet excited naphthalene, given that the lifetime of the naphthalene singlet state is 100 ns in the absence of dioxygen and A. /V-di methyl aniline and that the rate constant of diffusion /cdill is 5 x 109m 1 s [0.2m]... 

Singlet molecular oxygen The oxygen molecule (dioxygen), 02, in an excited singlet state. The ground state of 02 is a triplet. 

Moreover, the products of typical reactions of singlet-state dioxygen with organic substrates (Reactions 5.11 and 5.12, for example) are quite different in character from the reactions of dioxygen with organic substrates catalyzed by oxygenase enzymes (see Section V) ... 

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