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). It 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). Flgve 1. Energy level diagram of the O2 molecule (ground state blplet form. 3Z)drawnto scale, wlth wbltal energies measured by photmlecbon spectroc- On being cooled helow -183 OC oxygen condenses into a copy. Note the two unpaired electrons of parallel spin that occupy the two w' beautiful pale blue liquid with a remarkable property: it antibonding wbltals (AB). (Energies are In kJIml X 100). 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 1shows 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 Figwe 2. (lefl) Schematle energy level dlagam of me oxygen molecule in the "ma&num multiplicity, minimum energy" rule, andthey more stable slnglet state, 'A. This form lies 94.7 kJ (7882 cm-', 1269 nm) in are clearly of parallel spin. Thus, the 02molecule has two energy above the blplet ground state. unpaired electrons and is paramagnetic. This explanation of the paramagnetism of oxygen was the great triumph of the molecular orbital model. Figure 3. (rlgM) Smematloenergy level diagram of the oxygen molecule In the But what prevents us from assigning the two electrons to second (and less sable) singlet state '2.This state lies 158 kJ (13120 cm-'. the same orhital hut with opposite spin (Fig. 2)? And what of 762 nm) in energy above the triplet ground state. 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 1and so should he detectable. Furthermore. Figure 4. Schematic diagram showing Gle relative enargles of the highest they shouldbave different chemical properties. Even though occupied orbitals of the oxygen molecules in the three states: 32,'A, '2. 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 We can also ponder on the 0-0 bond lengths in the three differ in enerrv. Unfortunatelv, we cannot deduce their rela- different electronic states: will they be the same or different? tive energies-&her than to say that the form with two un- In each state there are six honding electrons and two anti- paired electrons will he lowest in energy (Fig. 4), in keeping bonding electrons. Clearly the formal bond order in each with Hund's rule (7). 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 ' On leave from University of Natal, Durban, South Africa. somewhat longer than in the paramagnetic form. Volume 66 Number 6 June 1969 453 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 Note that both C-C and 0-0 bonds are cleaved simulta- two forms with paired electrons are called singlet states and neously. An interesting example of this reaction is the syn- are termed "excited" because they are higher in energy than thesis of a musk fragrance (3). 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 Quite apart from its importance in synthetic organic 'A form can he between 1 s Ad 45 min, depending onthe chemistry, singlet oxygen plays an important role in autoxi- concentration of the eas and whether the phase is aaseous. or dation (i.e., photodegradation of polymers and vulcanized condensed. rubber in air). Notice, too, the implications for biology and Singlet 'A oxygen undergoes two-electron reactions (anal- life forms that are dependent on oxygen transpiration. The ogous to those of ethylene), unlike paramagnetic triplet 32 same photosensitizing dyes that yield 'A singlet oxygen also oxygen, which takes part in free radical processes. sensitize the oxidative destruction of nucleic acids and en- The reactions of 'A singlet oxygen are important because zymesand can produce skin cancer. Inaddition, many of the they represent very smooth methods of introducing oxygen carcinogenic polycyclic aromatic hydrocarbons are known to into an organic compound in a highly stereospecific fashion. be good photosensitizers with the ability to generate singlet There are three common modes of reaction of 'A oxygen: the oxygen. Furthermore, the reactions of certain oxygenases so ene reaction, the Diels-Alder type of cycloaddition, and the resemble that of 'A oxygen that it becomes a prime candi- addition to an activated douhle hond. All three types have date as the reactive species in these enzvmatic reactions (11). been used in organic synthesis involving oxidation of olefins, One of the uniquesspects of this 'A singlet form of oxygen especially in the field of natural producta (10). Examples of is that it can be detected visually because it takes.part in each type of reaction of 'A oxygen, abbreviated '02, are given reactions that yield "cold light", a process called chemilumi- below. nescence (12). The best known reaction of this class is the The Ene Reaction reaction of luminol that gives a beautiful blue glow in the dark (13). This reaction involves the migration of a double hond to Finally, we come back to the question of the 0-0 bond form an allylic hydroperoxide, which in turn can he dehy- lengths. They were in fad measured many years ago by a drated to yield an unsaturated ketone. 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)' Alternatively, the hydroperoxide group can be reduced to the bond lenaths are loneer in the excited states; and (3) the' OH to form an allylic alcohol. 'A state h&the short& of the hond lengths of the two excited states. All three observations are in line with what Dlels-Alder Type of Addltlon might have been predicted. Here the lOz adds across two douhle bonds to form a six- We may conclude by saying that a logical application of membered ring, which in turn can be decomposed to give two the simple rules of the molecular orbital bonding theory for carbonyl groups. 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 thk the reactivity df the singlet 'A form differs moueh from that of the usual tri~let32 form be useful in One of the fust examples of this type of addition is the - to classic synthesis of (f)-ascaridole (2) in near quantitative organic synthesis.