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Singlet oxygen
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<h2 id="electronic-structure">Electronic structure</h2> <p>Singlet oxygen refers to one of two singlet electronic excited states. The two singlet states are denoted 1Σ+g and 1Δg (the preceding superscript "1" indicates a singlet state). The singlet states of oxygen are 158 and 95 <a href="/facts/Kilojoule/9ndq9jD2">kilojoules</a> per <a href="/facts/Mole_(unit)/kCMFwCc0">mole</a> higher in energy than the triplet ground state of oxygen. Under most common laboratory conditions, the higher energy 1Σ+g singlet state rapidly converts to the more stable, lower energy 1Δg singlet state.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> This more stable of the two excited states has its two valence electrons spin-paired in one π* orbital while the second π* orbital is empty. This state is referred to by the title term, <i>singlet oxygen</i>, commonly abbreviated 1O2, to distinguish it from the triplet ground state molecule, 3O2.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a><a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p><p><a href="/facts/Molecular_orbital_theory/VJ6Kg8bI">Molecular orbital theory</a> predicts the electronic ground state denoted by the <a href="/facts/Molecular_term_symbol/plQdFUQX">molecular term symbol</a> 3Σ–g, and two low-lying excited <a href="/facts/Singlet_state/CK39GJa4">singlet states</a> with term symbols 1Δg and 1Σ+g. These three electronic states differ only in the spin and the occupancy of oxygen's two <a href="/facts/Antibonding/9lhLNyt3">antibonding</a> πg-orbitals, which are <a href="/facts/Degenerate_energy_level/ulrnMXTR">degenerate</a> (equal in energy). These two orbitals are classified as <a href="/facts/Antibonding/9lhLNyt3">antibonding</a> and are of higher energy. Following <a href="/facts/Hund%27s_rule_of_maximum_multiplicity/SqQFS8vR">Hund's first rule</a>, in the ground state, these electrons are <a href="/facts/Electron_pair/MENP5Gm0">unpaired</a> and have like (same) spin. This open-shell triplet ground state of molecular oxygen differs from most stable diatomic molecules, which have singlet (1Σ+g) ground states.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> </p><p>Two less stable, higher energy <a href="/facts/Excited_state/czcLYNnr">excited states</a> are readily accessible from this ground state, again in accordance with <a href="/facts/Hund%27s_rule_of_maximum_multiplicity/SqQFS8vR">Hund's first rule</a>;<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> the first moves one of the high energy unpaired ground state electrons from one degenerate orbital to the other, where it "flips" and pairs the other, and creates a new state, a singlet state referred to as the 1Δg state (a <a href="/facts/Term_symbol/7v7IFEM4">term symbol</a>, where the preceding superscripted "1" indicates it as a singlet state).<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a><a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> Alternatively, both electrons can remain in their degenerate ground state orbitals, but the spin of one can "flip" so that it is now opposite to the second (i.e., it is still in a separate degenerate orbital, but no longer of like spin); this also creates a new state, a singlet state referred to as the 1Σ+g state.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a><a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> The ground and first two singlet excited states of oxygen can be described by the simple scheme in the figure below.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a><a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> </p> <p>The 1Δg singlet state is 7882.4 cm−1 above the triplet 3Σ−g ground state.,<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a><a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> which in other units corresponds to 94.29 kJ/mol or 0.9773 eV. The 1Σ+g singlet is 13 120.9 cm−1<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a><a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> (157.0 kJ/mol or 1.6268 eV) above the ground state. </p><p>Radiative transitions between the three low-lying electronic states of oxygen are formally forbidden as electric dipole processes.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> The two singlet-triplet transitions are forbidden both because of the spin <a href="/facts/Selection_rule/q0l2TSEq">selection rule</a> ΔS = 0 and because of the <a href="/facts/Parity_(physics)/emSbJ4hD">parity</a> rule that g-g transitions are forbidden.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> The singlet-singlet transition between the two excited states is spin-allowed but parity-forbidden. </p><p>The lower, O2(1Δg) state is commonly referred to as <i>singlet oxygen</i>. The energy difference of 94.3 kJ/mol between ground state and singlet oxygen corresponds to a forbidden singlet-triplet transition in the near-<a href="/facts/Infrared/44XdJNcc">infrared</a> at ~1270 nm.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> As a consequence, singlet oxygen in the gas phase is relatively long lived (54-86 milliseconds),<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> although interaction with solvents reduces the lifetime to microseconds or even nanoseconds.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> In 2021, the lifetime of airborne singlet oxygen at air/solid interfaces was measured to be 550 microseconds.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> </p><p>The higher 1Σ+g state is moderately short lived. In the gas phase, it relaxes primarily to the ground state triplet with a mean lifetime of 11.8 seconds.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> However in solvents such as <a href="/facts/Carbon_disulfide/b04Tadmy">CS2</a> and <a href="/facts/Carbon_tetrachloride/S2ekjT0E">CCl4</a>, it relaxes to the lower singlet 1Δg in milliseconds due to radiationless decay channels.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> </p> <h3>Paramagnetism due to orbital angular momentum</h3> <p>Both singlet oxygen states have no unpaired electrons and therefore no net electron spin. The 1Δg is however <a href="/facts/Paramagnetism/vj1SMbbp">paramagnetic</a> as shown by the observation of an <a href="/facts/Electron_paramagnetic_resonance/TDHV8c9e">electron paramagnetic resonance</a> (EPR) spectrum.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a><a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a><a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> The paramagnetism of the 1Δg state is due to a net <i>orbital</i> (and not spin) electronic angular momentum. In a magnetic field the degeneracy of the M L {\displaystyle M_{L}} levels is split into two levels with z projections of angular momenta +1<a href="/facts/Reduced_Planck_constant/qGHAe3qq"><i>ħ</i></a> and −1<i>ħ</i> around the molecular axis. The magnetic transition between these levels gives rise to the g = 1 {\displaystyle g=1} EPR transition. </p> <h2 id="production">Production</h2> <p>Various methods for the production of singlet oxygen exist. Irradiation of oxygen gas in the presence of an organic dye as a sensitizer, such as <a href="/facts/Rose_bengal/aEwp2Hcy">rose bengal</a>, <a href="/facts/Methylene_blue/NIo9Xaag">methylene blue</a>, or <a href="/facts/Porphyrins/lKNtdXp1">porphyrins</a>—a <a href="/facts/Photochemistry/GsXNBWon">photochemical method</a>—results in its production.<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a><a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> Large steady state concentrations of singlet oxygen are reported from the reaction of triplet excited state <a href="/facts/Pyruvic_acid/qJTgl4J9">pyruvic acid</a> with dissolved oxygen in water.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> Singlet oxygen can also be produced by <a href="/facts/Chemical_synthesis/3KXsVVwC">chemical procedures</a> without irradiation. One chemical method involves the decomposition of triethylsilyl hydrotrioxide generated <i>in situ</i> from <a href="/facts/Triethylsilane/QTL8MaQ3">triethylsilane</a> and ozone.<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a> </p> (C2H5)3SiH + O3 → (C2H5)3SiOOOH → (C2H5)3SiOH + O2(1Δg) <p>Another method uses a reaction of <a href="/facts/Hydrogen_peroxide/vju6HBIc">hydrogen peroxide</a> with <a href="/facts/Sodium_hypochlorite/sp99uHHQ">sodium hypochlorite</a> in aqueous solution:<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a> </p> H2O2 + NaOCl → O2(1Δg) + NaCl + H2O<p> A <a href="/facts/Retro-Diels%E2%80%93Alder_reaction/Pjj4WQLW">retro-Diels Alder reaction</a> of the diphenylanthracene peroxide can also yield singlet oxygen, along with an diphenylanthracene:<a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a> </p> <p>A third method liberates singlet oxygen via phosphite ozonides, which are, in turn, generated <i><a href="/facts/In_situ/YolKlaCC">in situ</a></i> such as triphenyl phosphite ozonide.<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a><a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a> Phosphite ozonides will decompose to give singlet oxygen:<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a> </p> (RO)3P + O3 → (RO)3PO3 (RO)3PO3 → (RO)3PO + O2(1Δg) <p>An advantage of this method is that it is amenable to non-aqueous conditions.<a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a> </p> <h2 id="reactions">Reactions</h2> <p>Because of differences in their electron shells, singlet and triplet oxygen differ in their chemical properties; singlet oxygen is highly reactive.<a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a> The lifetime of singlet oxygen depends on the medium and pressure. In normal organic solvents, the lifetime is only a few microseconds whereas in solvents lacking C-H bonds, the lifetime can be as long as seconds.<a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a><a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a> </p><p>Unlike ground state oxygen, singlet oxygen participates in <a href="/facts/Diels%E2%80%93Alder/zc6dVpKl">Diels–Alder</a> [4+2]- and [2+2]-<a href="/facts/Cycloaddition/Gme6TDFP">cycloaddition</a> reactions and formal concerted <a href="/facts/Ene_reaction/jDxofA4D">ene reactions</a> (<a href="/facts/Schenck_ene_reaction/e2UDszSv">Schenck ene reaction</a>), causing <a href="/facts/Photooxygenation/6VvDq04U">photooxygenation</a>.<a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a> It oxidizes thioethers to sulfoxides. <a href="/facts/Organometallic/bhqFz2P0">Organometallic</a> complexes are often degraded by singlet oxygen.<a class="footnote-ref" id="fnref:45" href="#fn:45"><sup>45</sup></a><a class="footnote-ref" id="fnref:46" href="#fn:46"><sup>46</sup></a> With some substrates <a href="/facts/1,2-dioxetane/bwb3S6TP">1,2-dioxetanes</a> are formed; cyclic dienes such as <a href="/facts/1,3-Cyclohexadiene/53eNrh1p">1,3-cyclohexadiene</a> form [4+2] <a href="/facts/Cycloaddition/Gme6TDFP">cycloaddition</a> adducts.<a class="footnote-ref" id="fnref:47" href="#fn:47"><sup>47</sup></a> </p><p>The [4+2]-cycloaddition between singlet oxygen and furans is widely used in <a href="/facts/Organic_synthesis/aprl6Iq7">organic synthesis</a>.<a class="footnote-ref" id="fnref:48" href="#fn:48"><sup>48</sup></a><a class="footnote-ref" id="fnref:49" href="#fn:49"><sup>49</sup></a> </p><p>In singlet oxygen reactions with alkenic <a href="/facts/Allyl_group/CDxPwyvF">allyl groups</a>, e.g., citronella, shown, by abstraction of the <a href="/facts/Allylic/CDxPwyvF">allylic</a> proton, in an <a href="/facts/Ene_reaction/jDxofA4D">ene-like reaction</a>, yielding the allyl <a href="/facts/Hydroperoxide/w7hdgtVF">hydroperoxide</a>, R–O–OH (R = <a href="/facts/Alkyl/vmEk8X4v">alkyl</a>), which can then be reduced to the corresponding <a href="/facts/Allylic_alcohol/6HW6IS5U">allylic alcohol</a>.<a class="footnote-ref" id="fnref:50" href="#fn:50"><sup>50</sup></a><a class="footnote-ref" id="fnref:51" href="#fn:51"><sup>51</sup></a><a class="footnote-ref" id="fnref:52" href="#fn:52"><sup>52</sup></a><a class="footnote-ref" id="fnref:53" href="#fn:53"><sup>53</sup></a> </p><p>In reactions with water, <a href="/facts/Trioxidane/9S3ARK9m">trioxidane</a>, an unusual molecule with three consecutive linked oxygen atoms, is formed. </p> <h2 id="biochemistry">Biochemistry</h2> <p>In <a href="/facts/Photosynthesis/TMV7w693">photosynthesis</a>, singlet oxygen can be produced from the light-harvesting <a href="/facts/Chlorophyll/1Sb0NyyM">chlorophyll</a> molecules. One of the roles of <a href="/facts/Carotenoid/Jm2eHYwW">carotenoids</a> in photosynthetic systems is to prevent damage caused by produced singlet oxygen by either removing excess <a href="/facts/Light/jeofpMn4">light</a> energy from <a href="/facts/Chlorophyll/1Sb0NyyM">chlorophyll</a> molecules or quenching the singlet oxygen molecules directly. </p><p>In <a href="/facts/Mammal/phSwTzBo">mammalian</a> <a href="/facts/Biology/1M1ee3pB">biology</a>, singlet oxygen is one of the <a href="/facts/Reactive_oxygen_species/GpwSRugH">reactive oxygen species</a>, which is linked to oxidation of <a href="/facts/LDL/R5VLPxAg">LDL</a> <a href="/facts/Cholesterol/TnfPlYc1">cholesterol</a> and resultant <a href="/facts/Cardiovascular/WXwudzzc">cardiovascular</a> effects. <a href="/facts/Polyphenol_antioxidant/ll7HjWVa">Polyphenol antioxidants</a> can scavenge and reduce concentrations of reactive oxygen species and may prevent such deleterious oxidative effects.<a class="footnote-ref" id="fnref:54" href="#fn:54"><sup>54</sup></a> </p><p>Ingestion of pigments capable of producing singlet oxygen with activation by light can produce severe <a href="/facts/Photosensitivity/dohEzKCm">photosensitivity</a> of skin (see <a href="/facts/Phototoxicity/9MjkWKEM">phototoxicity</a>, <a href="/facts/Photosensitivity_in_humans/XwkmnUfg">photosensitivity in humans</a>, <a href="/facts/Photodermatitis/xa2TCgo8">photodermatitis</a>, <a href="/facts/Phytophotodermatitis/DdxCVdsq">phytophotodermatitis</a>). This is especially a concern in herbivorous animals (see <a href="/facts/Photosensitivity_in_animals/FuMbhR8K">Photosensitivity in animals</a>). </p><p>Singlet oxygen is the active species in <a href="/facts/Photodynamic_therapy/qHIQzT5K">photodynamic therapy</a>. </p> <h2 id="analytical-and-physical-chemistry">Analytical and physical chemistry</h2> <p>Singlet oxygen luminesces concomitant with its decay to the triplet ground state. This phenomenon was first observed in the thermal degradation of the endo peroxide of <a href="/facts/Rubrene/fw0fSgUy">rubrene</a>.<a class="footnote-ref" id="fnref:55" href="#fn:55"><sup>55</sup></a> </p> <h2 id="further-reading">Further reading</h2> <ul><li><a href="/facts/Robert_S._Mulliken/l17P0TBQ">Mulliken RS</a> (1928). <a href="https://doi.org/10.1038%2F122505a0">"Interpretation of the atmospheric oxygen bands; electronic levels of the oxygen molecule"</a>. <i><a href="/facts/Nature_(journal)/XmlJXkFx">Nature</a></i>. 122 (3075): 505. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1928Natur.122..505M">1928Natur.122..505M</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1038%2F122505a0">10.1038/122505a0</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:4105859">4105859</a>.</li> <li>Chou, Pi-Tai; Wei, Guor-Tzo; Lin, Chih-Hung; Wei, Ching-Yen; Chang, Chie-Hung (1996-01-01). "Direct Spectroscopic Evidence of Photosensitized O2 765 nm (1Σ+g → 3Σ-g) and O2 Dimol 634 and 703 nm ((1Δg)2 → (3Σ-g)2) Vibronic Emission in Solution". <i>Journal of the American Chemical Society</i>. 118 (12): 3031–3032. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1021%2Fja952352p">10.1021/ja952352p</a>. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0002-7863">0002-7863</a>.</li> <li>Földes T, Čermák P, Macko M, Veis P, Macko P (January 2009). "Cavity ring-down spectroscopy of singlet oxygen generated in microwave plasma". <i>Chemical Physics Letters</i>. 467 (4–6): 233–236. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2009CPL...467..233F">2009CPL...467..233F</a>. <a href="/facts/CiteSeerX_(identifier)/SceDmd3c">CiteSeerX</a> <a href="https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.186.9272">10.1.1.186.9272</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1016%2Fj.cplett.2008.11.040">10.1016/j.cplett.2008.11.040</a>.</li> <li>Nosaka Y, Daimon T, Nosaka, AY, Murakami Y (2004). "Singlet oxygen formation in photocatalytic TiO₂ aqueous suspension". <i>Phys. Chem. Chem. Phys</i>. 6 (11): 2917–2918. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2004PCCP....6.2917N">2004PCCP....6.2917N</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1039%2FB405084C">10.1039/B405084C</a>.</ref></li></ul> <ul><li>Bodner, G.M. (2002) <i>Lecture Demonstration Movie Sheets: 8.4 Liquid Oxygen—Paramagnetism and Color,</i> West Lafayette, IN, USA: Purdue University Department of Chemistry, see <a href="http://chemed.chem.purdue.edu/demos/main_pages/14.2.html">Liquid Oxygen---Paramagnetism and Color</a> and <a href="http://chemed.chem.purdue.edu/demos/index.html">Lecture Demonstration Movie Sheets</a>, accessed 11 August 2015; alternatively, see Bodner, G.M.; K. Keyes & T.J. Greenbowe (1995) <i>Purdue University Lecture Demonstration Manual,</i> 2nd Edn, p. TBD, New York, NY, USA: John Wiley and Sons. [Earlier appearing reference on magnetic properties of oxygen states.]</li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://webbook.nist.gov/cgi/cbook.cgi?ID=C7782447&Units=SI&Mask=1000#Diatomic">The NIST webbook on oxygen</a></li> <li><a href="http://www.photobiology.com/educational/len2/singox.html">Photochemistry & Photobiology tutorial on Singlet Oxygen</a></li> <li><a href="http://chemed.chem.purdue.edu/demos/main_pages/5.12.html">Demonstration of the Red Singlet Oxygen Dimol Emission (Purdue University)</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Wayne RP (1969). "Singlet Molecular Oxygen". In Pitts JN, Hammond GS, Noyes WA (eds.). Advances in Photochemistry. Vol. 7. pp. 311–71. doi:10.1002/9780470133378.ch4. ISBN 9780470133378. <a href="9780470133378" target="_blank">9780470133378</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Klán P, Wirz J (2009). Photochemistry of Organic Compounds: From Concepts to Practice (Repr. 2010 ed.). Chichester, West Sussex, U.K.: Wiley. ISBN 978-1405190886. <a href="978-1405190886" target="_blank">978-1405190886</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Atkins P, de Paula J (2006). Atkins' Physical Chemistry (8th ed.). W.H.Freeman. pp. 482–3. ISBN 978-0-7167-8759-4. <a href="978-0-7167-8759-4" target="_blank">978-0-7167-8759-4</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Hill C. "Molecular Term Symbols" (PDF). Archived from the original (PDF) on 5 September 2017. Retrieved 10 October 2016. <a href="https://web.archive.org/web/20170905054529/https://www.christianhill.co.uk/static/teaching/cribs/molecular_term_symbols.pdf" target="_blank">https://web.archive.org/web/20170905054529/https://www.christianhill.co.uk/static/teaching/cribs/molecular_term_symbols.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Klán P, Wirz J (2009). Photochemistry of Organic Compounds: From Concepts to Practice (Repr. 2010 ed.). Chichester, West Sussex, U.K.: Wiley. ISBN 978-1405190886. <a href="978-1405190886" target="_blank">978-1405190886</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Klán P, Wirz J (2009). Photochemistry of Organic Compounds: From Concepts to Practice (Repr. 2010 ed.). Chichester, West Sussex, U.K.: Wiley. ISBN 978-1405190886. <a href="978-1405190886" target="_blank">978-1405190886</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Atkins P, de Paula J (2006). Atkins' Physical Chemistry (8th ed.). W.H.Freeman. pp. 482–3. ISBN 978-0-7167-8759-4. <a href="978-0-7167-8759-4" target="_blank">978-0-7167-8759-4</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Levine IN (1991). Quantum Chemistry (4th ed.). Prentice-Hall. p. 383. ISBN 978-0-205-12770-2. <a href="978-0-205-12770-2" target="_blank">978-0-205-12770-2</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Frimer AA (1985). Singlet Oxygen: Volume I, Physical-Chemical Aspects. Boca Raton, Fla.: CRC Press. pp. 4–7. ISBN 9780849364396. <a href="9780849364396" target="_blank">9780849364396</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Klán P, Wirz J (2009). Photochemistry of Organic Compounds: From Concepts to Practice (Repr. 2010 ed.). Chichester, West Sussex, U.K.: Wiley. ISBN 978-1405190886. <a href="978-1405190886" target="_blank">978-1405190886</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Atkins P, de Paula J (2006). Atkins' Physical Chemistry (8th ed.). W.H.Freeman. pp. 482–3. ISBN 978-0-7167-8759-4. <a href="978-0-7167-8759-4" target="_blank">978-0-7167-8759-4</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Klán P, Wirz J (2009). Photochemistry of Organic Compounds: From Concepts to Practice (Repr. 2010 ed.). Chichester, West Sussex, U.K.: Wiley. ISBN 978-1405190886. <a href="978-1405190886" target="_blank">978-1405190886</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Atkins P, de Paula J (2006). Atkins' Physical Chemistry (8th ed.). W.H.Freeman. pp. 482–3. 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