Everything about Singlet Oxygen totally explained
Singlet oxygen is the common name used for the two metastable states of molecular oxygen (O
2) with higher energy than the ground state
triplet oxygen . The energy difference between the lowest energy of O
2 in the singlet state and the lowest energy in the triplet state is about 3625 kelvin (T
e (a¹Δ
g <- X³Σ
g-) = 7918.1 cm
-1.)
Molecular
oxygen differs from most molecules in having an open-shell triplet ground state, O
2(X³Σ
g-).
Molecular orbital theory predicts two low-lying excited
singlet states O
2(a¹Δ
g) and O
2(b¹Σ
g+) (for nomenclature see article on
Molecular term symbol). These electronic states differ only in the spin and the occupancy of oxygen's two degenerate
antibonding π
g-orbitals (see
degenerate energy level). The O
2(b¹Σ
g+)-state is very short lived and relaxes quickly to the lowest lying
excited state, O
2(a¹Δ
g). Thus, the O
2(a¹Δ
g)-state is commonly referred to as singlet oxygen.
Physics
The energy difference between ground state and singlet oxygen is 94.2 kJ/mol and corresponds to a transition in the near-
infrared at ~1270 nm. In the isolated molecule, the transition is strictly forbidden by spin, symmetry and parity
selection rules, making it one of nature's most forbidden transitions. In other words, direct excitation of ground state oxygen by light to form singlet oxygen is very improbable. As a consequence, singlet oxygen in the gas phase is extremely long lived (72 minutes). Interaction with solvents, however, reduces the lifetime to microseconds or even nanoseconds.
Direct detection of singlet oxygen is possible through its extremely weak
phosphorescence at 1270 nm, which isn't visible to the eye. However, at high singlet oxygen concentrations, the
fluorescence of the so-called singlet oxygen dimol (simultaneous emission from two singlet oxygen molecules upon collision) can be observed as a red glow at 634 nm .
Chemistry
The chemistry of singlet oxygen is different from that of ground state oxygen. Singlet oxygen can participate in
Diels-Alder reactions and
ene reactions. It can be generated in a photosensitized process by energy transfer from dye molecules such as
rose bengal,
methylene blue or
porphyrins, or by chemical processes such as spontaneous decomposition of
hydrogen trioxide in water or the reaction of
hydrogen peroxide with
hypochlorite . Singlet oxygen reacts with an alkene -C=C-CH- by abstraction of the
allylic proton in an
ene reaction type reaction to the allyl
hydroperoxide HO-O-C-C=C. It can then be reduced to the
allyl alcohol. With some substrates
dioxetanes are formed and cyclic dienes such as
1,3-Cyclohexadiene form [4+2]
cycloaddition adducts. .
Biochemistry
In
photosynthesis, singlet oxygen can be produced from the light-harvesting
chlorophyll molecules. One of the roles of
carotenoids in photosynthetic systems is to prevent damage caused by produced singlet oxygen by either removing excess
light energy from
chlorophyll molecules or quenching the singlet oxygen molecules directly.
In
mammalian
biology, singlet oxygen is a form of
reactive oxygen species, which is linked to oxidation of
LDL cholesterol and resultant
cardiovascular effects.
Polyphenol antioxidants can scavenge and reduce concentrations of reactive oxygen species and may prevent such deleterious oxidative effects .
Singlet oxygen is the active species
photodynamic therapy.
Further Information
Get more info on 'Singlet Oxygen'.
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