The Three Forms of Molecular Oxygen

Mlchael Lalngl California State University, Northridge, CA 91330

: Everyone is familiar with the oxygen in the air ( I ) , and most have oreoared a "oure" samole of this important ele- ment. here &e many ways of doing this, and onk of the best is to heat a mixture of 90 parts KCIO2and 10 parts MnOn (by mass) and collect the gas that is evolved in a gas j& by downward displacement of water (2).

I t is certain that pure oxygen gas is very reactive. Not only will a glowing splint burst into flame when dropped into oxygen, but heated iron filings will hurn when dropped into the pure gas. This latter reaction is of course close to that of iron rusting (3), although rusting requires oxygen, water, and an electrolyte to proceed (4).

On being cooled helow -183 O C oxygen condenses into a beautiful pale blue liquid with a remarkable property: it sticks to the poles of a magnet (5)! This property of heing attracted to a magnetic field, called paramagnetism, is found in any molecule or material that contains unpaired elec- trons. So these are a few of the facts about the behavior of elemental oxygen. The question is whether the bonding model for the pair of oxygen atoms in the diatomic molecule fits these facts.

The simple molecular orhital model for diatomic mole- cules works beautifully (6). Figure 1 shows the energy level diagram for the Oz molecule. Pairs of electrons are assigned to the a and the two r honding orbitals (BO), and one electron is assiened to each of the two r* antihondine orbi- tals (AB), the latter twoelectrons having their spins p&allel as reauired bv Hund's rule (7). These two electrons obev the "ma&num multiplicity, minimum energy" rule, andthey are clearly of parallel spin. Thus, the 0 2 molecule has two unpaired electrons and is paramagnetic. This explanation of the paramagnetism of oxygen was the great triumph of the molecular orbital model.

But what prevents us from assigning the two electrons to the same orhital hut with opposite spin (Fig. 2)? And what of keeping the two electrons in different r* orbitals but assign- ing opposite spins to them (Fig. 3)?

The three electron distributions described above are clearly different. The first of these, the ground state, has two unpaired electrons and is paramagnetic, while the other two have no unpaired electrons and are both diamagnetic. In other words, the simple molecular orhital picture of honding in diatomic molecules predicts that three forms of oxygen molecules could exist. The oxygen molecules with the elec- tron configurations illustrated in Figures 2 and 3 have all electrons paired so they will not be attracted by a magnetic field. Therefore, they will differ from the paramagnetic form shown in Fieure 1 and so should he detectable. Furthermore. they shouldbave different chemical properties. Even though the two nuclei are identical in each case. the form with two unpaired electrons will differ in chemical reactivity from the ones in which all electrons are oaired. Also. because the ele~tronconfi~urationsarediftere~t, these three forms must differ in enerrv. Unfortunatelv, we cannot deduce their rela- tive energies-&her than to say that the form with two un- paired electrons will he lowest in energy (Fig. 4), in keeping with Hund's rule (7).

, ' On leave from University of Natal, Durban, South Africa.

Flgve 1. Energy level diagram of the O2 molecule (ground state blplet form. 3Z)drawn to scale, wlth wbltal energies measured by photmlecbon spectroc- copy. Note the two unpaired electrons of parallel spin that occupy the two w' antibonding wbltals (AB). (Energies are In kJ Iml X 100).

Figwe 2. (lefl) Schematle energy level dlagam of me oxygen molecule in the more stable slnglet state, 'A. This form lies 94.7 kJ (7882 cm-', 1269 nm) in energy above the blplet ground state.

Figure 3. (rlgM) Smematloenergy level diagram of the oxygen molecule In the second (and less sable) singlet state '2. This state lies 158 kJ (13120 cm-'. 762 nm) in energy above the triplet ground state.

Figure 4. Schematic diagram showing Gle relative enargles of the highest occupied orbitals of the oxygen molecules in the three states: 32, 'A, '2.

We can also ponder on the 0-0 bond lengths in the three different electronic states: will they be the same or different? In each state there are six honding electrons and two anti- bonding electrons. Clearly the formal bond order in each case will he 2. We are therefore led to predict that the three forms would have similar 0-0 bond lengths, with the bond lengths in the forms with all electrons paired predicted to be somewhat longer than in the paramagnetic form.

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The question to ask now is: does the oxygen molecule actually exist in more than one electronic form? The answer is emphatically yes. All three states have been detected. The commonest form, the ground state with two unpaired elec- trons, is termed a triplet and given the symho132. The other two forms with paired electrons are called singlet states and are termed "excited" because they are higher in energy than is the triplet ground state. They aregiven the symholslAand '2, with 'A lower in energy than '2 (7).

The singlet 'A form ex& not as a theoretical curiosity hut as a useful material, valuahle as a reagent in organic chemis- try (8). There are several detailed reviews that describe its preparation, physical characteristics, and reactions (9). This chemically useful lA form can be prepared either hy reacting hydrogen peroxide with sodium hypochlorite or by excita- tion of ordinarv 3 trinlet oxveen with ultraviolet liaht in the presence of a sensitiz& dyL?oncentrationsofupio 10% in theeas nhasecan he obtained. and the lifetime of the sinnlet 'A form can he between 1 s A d 45 min, depending onthe concentration of the eas and whether the phase is aaseous or . condensed.

Singlet 'A oxygen undergoes two-electron reactions (anal- ogous to those of ethylene), unlike paramagnetic triplet 32 oxygen, which takes part in free radical processes.

The reactions of 'A singlet oxygen are important because they represent very smooth methods of introducing oxygen into an organic compound in a highly stereospecific fashion. There are three common modes of reaction of 'A oxygen: the ene reaction, the Diels-Alder type of cycloaddition, and the addition to an activated douhle hond. All three types have been used in organic synthesis involving oxidation of olefins, especially in the field of natural producta (10). Examples of each type of reaction of 'A oxygen, abbreviated '02, are given below.

The Ene Reaction This reaction involves the migration of a double hond to

form an allylic hydroperoxide, which in turn can he dehy- drated to yield an unsaturated ketone.

Alternatively, the hydroperoxide group can be reduced to OH to form an allylic alcohol.

Dlels-Alder Type of Addltlon Here the lOz adds across two douhle bonds to form a six-

membered ring, which in turn can be decomposed to give two carbonyl groups.

One of the fust examples of this type of addition is the classic synthesis of (f )-ascaridole (2) in near quantitative yield from a-terpene (1).

Addition to an Actlvated Double Bond In this reaction the '0. adds directlv across the double

bond to yield a four-memhered 1.2-dioxetane ring, which in turn can he cleaved to yield a di-aldehyde type of structure.

Note that both C-C and 0-0 bonds are cleaved simulta- neously. An interesting example of this reaction is the syn- thesis of a musk fragrance (3).

Quite apart from its importance in synthetic organic chemistry, singlet oxygen plays an important role in autoxi- dation (i.e., photodegradation of polymers and vulcanized rubber in air). Notice, too, the implications for biology and life forms that are dependent on oxygen transpiration. The same photosensitizing dyes that yield 'A singlet oxygen also sensitize the oxidative destruction of nucleic acids and en- zymesand can produce skin cancer. Inaddition, many of the carcinogenic polycyclic aromatic hydrocarbons are known to be good photosensitizers with the ability to generate singlet oxygen. Furthermore, the reactions of certain oxygenases so resemble that of 'A oxygen that it becomes a prime candi- date as the reactive species in these enzvmatic reactions (11).

One of the uniquesspects of this 'A singlet form of oxygen is that it can be detected visually because it takes.part in reactions that yield "cold light", a process called chemilumi- nescence (12). The best known reaction of this class is the reaction of luminol that gives a beautiful blue glow in the dark (13).

Finally, we come back to the question of the 0-0 bond lengths. They were in f a d measured many years ago by a careful analysis of the ultraviolet absorption spectrum of oxygen gas. The experimentall determined hond lengths are: 32 (ground state) 1.2074 A: 'A (lowest excited state) 1.2155 A, '2 (second excited state) 1.2268 A (14). Three thines are evident: (1) all of the bond lengths are similar (in kee$ng with the foimal bond order hein$i in each case); (2)' the bond lenaths are loneer in the excited states; and (3) the' 'A state h&the short& of the hond lengths of the two excited states. All three observations are in line with what might have been predicted.

We may conclude by saying that a logical application of the simple rules of the molecular orbital bonding theory for diatomic molecules (6) led us to predict the existence of three spin isomers of the oxygen molecule: one triplet form with two unpaired electrons and two singlet forms with all electrons mired. In addition. the exoerimental chemist has shown t h k the reactivity df the singlet 'A form differs moueh from that of the usual t r i ~ l e t 32 form to be useful in - organic synthesis.

Acknowledgment I thank my friend Erneat Bovey whose interest in the topic

led me to write this article.

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Metealfe.H.C.:WiUiams. J.E.:C&ka. J.F. Modem Chemistrv:Holt.Rinehartand

2. la) Lexla, M.; Wal1er.G. ThinkingChemi*Lry.O.lord University: Oxford, 1986:p312. (bl Bueat, R. B. Ekmenfaaf Chsmlatry;AuatralianAeademy of Science: Canberm. 1983; Vol. 1, p 368.

3. Ref. l b . 0 ls9 i. Gf. 20; 311. 5. (a1 Ebbing, D. D. Omem1 Chemistry; Houghton-Mimin: Boaton, 19% p 256. (bl

Broam, T. L.: Le May, H. E. Chemistry, 4th ed.; PrentieHall: Englmwcd Cliffa. NJ, 1988, D 290.

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6. (a) Hohlaw, H. F.; Robinam. W. R. Gemml Chemistry, 8th ed.; Heath: Lerindnn, LO. Waaarrman. H. H::Iws, J. L. Tet&dmn 1981.37,1625. MA, 1988:~~175,176.(b) Mahan,B.M.:Mysrs,R. J. UniuerailyCkmisfry,4thed.; 11. la) Fate , C. S. Am. Chop Res. 1%8,2,104. lb) Matsuura, T. Tetrohedmn l977,33, Benjamin: M d o Park, CA 1981; PP 630-632. 2539. Id Naqui, A,; Chanec.B.; Cadmau, E. Ann. Re". Biochem. 1986,33,137.

7. H e n h a , G. Molecular S p c f m and Molecvlor S t m t u m I. Sprelm of Dinlamic 11. Shakhaahi, B. Z ChemimlDemnsfrofians; Univ. of Wiseomin: Madiaon. 1981: pp Moleeulea, 2nd ed.. Van Nostrand. New York. 1950; pp 335,346. 133-145,136167.

8. Fieser, M.; Fieser, L. F. Reagentsfor Orgonir Synchears: Wiiey: New York, 1986:Vol IS. Aly8a.H. N.;Dutfon,F.B. TeIrdDomomtmlio~: J. Chem.Educ:Eqaton.PA, 1965: 12, p 363. (Seedso sxamples quoted since 1914 in Vola. 611.) p81.

9. (a)Keams,D. R. Chem.Roo. 1971,71, (4),p395. (b) Wueman,H.H.;Mmay.R.W., 14. (a) Gmnwwd, N. N., E m h a w . k Chemistry ofthe Elomnt8; P~RRRR: Oxford. Eds. Singlet Oxygen; Academic: New York, 1979. 1984: p705. lb) Ref7. p 560.

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