In the Classroom

The Properties of Investigated with Easily Accessible Instrumentation The “One-Photon-Two-Molecule” Mechanism Revisited

Manfred Adelhelm, Natasha Aristov, and Achim Habekost* Department of Chemistry, Padagogische Hochschule Ludwigsburg, Reuteallee 46, D-71634 Ludwigsburg, Germany *[email protected]

Oxygen has spectacular and unusual properties (1). Students between the ground state and first excited state of oxygen, but are generally familiar with this gas as making up about 20% of the only if two ground-state molecules are promoted and relaxed to atmosphere, being required for combustion, participating in many two excited-state molecules. This has been nicely presented as an oxidation reactions, and being a colorless gas. The revelation that it exercise in spectral interpretation for the general chemistry is blue as a liquid and paramagnetic is surprising. The observation laboratory in this Journal (11). A key point to understanding that, under other conditions (as the product of a chemical the absorption spectrum was the observation of the same features reaction), oxygen emits red light, provides further amazement. both for gaseous oxygen at high pressures and for large oxygen- Over the years, several methods of demonstrating the paramag- layer thicknesses, on the order of the thickness of the earth's netic and optical properties of oxygen have been published in this atmosphere.1 Inter-molecular interactions, more likely in con- Journal (2-6). Using these demonstrations is an elegant way of densed or high-pressure phases and more likely to be seen in a gas introducing or reinforcing the concepts of molecular orbital (MO) reservoir as huge as the atmosphere, were proposed, and later theory. Most recently, a demonstration using a small, strong, confirmed, to be the origin of the observed absorption bands neodynium magnet was published (7). The availability of these (12-15). In fact, the earliest spectroscopists considered the “ ” magnets allows the demonstration of magnetic oxygen in most existence of an O4 molecule to be likely (16). Nevertheless, in classrooms. The experiments described here show that the ob- later interpretations, a “one-photon-two-molecule” mechanism servation of the optical properties of oxygen is also easily possible was invoked, where the two molecules are interacting transiently with instruments that are commonly accessible, for example, hand- as a “collision pair” (6, 17, 18). held spectrophotometers and digital cameras. Much work has been done to understand the properties of A review of the literature led us to consider the usual O2 dimers and the likelihood of the formation of a didactical discussion associated with the presentation of the molecule in both the gas and various condensed phases of oxygen. paramagnetism and optical phenomena of oxygen. For example, Interest remains, not just because of the theoretical challenge of often the blue color of and its paramagnetism are isolating and characterizing this interesting molecule, but also for presented in such a way to suggest that these two properties are practical reasons. It had been thought that oxygen polymers intimately connected to one another. In fact, the structure of might serve as highly energetic materials (19). Theoretical liquid oxygen is not simple, given that its open valence shell gives calculations showed, however, no such long-lived O4 species to rise to intermolecular interactions stronger than van der Waals be available, as the barrier to dissociation to two O2 molecules forces (8). The earliest measurements of oxygen's magnetic was predicted to be low, about 20 kJ/mol (20). Even octaoxygen, properties implied that some interactions among oxygen - O8, has been observed in (21). The tetraoxygen cules must be occurring (9).Thatis,part of the liquefied oxygen molecule, (O2)2/O4, continues to be the object of attention, for is paramagnetic and consists of oxygen molecules and clusters of example, as a key metastable component in atmospheric pro- oxygen molecules with parallel aligned spins, but it is not a priori cesses such as the production of , the de-excitation of “ ” given that this is also the blue part. On the contrary, the blue vibrationally hot O2 (22, 23), and in the absorption of solar part of the oxygen is most likely to consist of diamagnetic oxygen radiation (15). It also plays a role in chemiluminescent processes dimers (10). Thus, it is not the paramagnetism per se that is now commonly used in analytical methods (24, 25). In view of consistent with the observation of a blue color, but rather the the results of scattering experiments, sophisticated spectroscopy, deviation of the observed paramagnetism from the theoretically and extensive theoretical calculations, we argue in this article that predicted value, which is usually not shown in classrooms. the one-photon-two-molecule terminology for the absorption While using the paramagnetism of oxygen to demonstrate and the chemiluminescence spectra of oxygen be abandoned. the reliability of MO theory for predicting electronic structure is legitimate, understanding the blue color (due to photon Demonstrations absorptions) of the liquid and the red chemiluminescence goes Paramagnetism beyond the predictions of MO theory for O2 molecules. The strongest absorption and emission bands near 630 and 570 nm Our method of preparing liquid oxygen follows that of (in the red and the yellow-green) correspond to transitions Shakhashiri (4, 26). We collect about 20-50 mL of liquid

40 Journal of Chemical Education Vol. 87 No. 1 January 2010 pubs.acs.org/jchemeduc r 2009 American Chemical Society and Division of Chemical Education, Inc. _ 10.1021/ed800008g_ Published on Web 12/18/2009_ In the Classroom

Table 1. Lines Observed in the Absorption and Emission Spectra of Liquid Oxygen Relative Intensity (indicated by the number of x's)

Transition Handheld Spectroscope Handheld Spectroscope Absorption Transition Wavelength/nm Spectrophotometer (digital camera) (VideoCom) 3Σ - v f 1Δ v 2 g ( =0) 2 g ( = 0) 634 xxx xxx xxx 3Σ - v f 1Δ v 2 g ( =0) 2 g ( = 1) 578 xxxxxx xxxxxx xxxx 3Σ - v f 1Δ v 2 g ( =0) 2 g ( = 2) 534 xx xx x - P þ 2 3Σ (v =0)f 1Δ þ 1 (v = 0) 478 xxx xx xx g g Pg 3Σ - v f 1Δ þ 1 þ v 2 g ( =0) g g ( = 1) 446 x - P þ P þ 2 3Σ (v =0)f 1 þ 1 (v = 0) 380 xx g Pg Pg 3Σ - v f 1 þ þ 1 þ v 2 g ( =0) g g ( = 1) 362 xx Transition Monochromator/ Handheld Spectroscope Emission Transition Wavelength/nm Photomultiplier Tube (digital camera) 3Σ - v r 1Δ v a 2 g ( =0) 2 g ( =0) 634 xxxxx xxxxx 3Σ - v r 1Δ v a 2 g ( =0) 2 g ( =1) 578 xxx a v: level for both oxygen molecules. Recording the Spectrum with a Spectrophotometer A small transparent Dewar, ∼10 cm length and 4 cm diameter, is precooled with liquid nitrogen, filled with liquid oxygen prepared as above, and placed into the beam path of a spectrophotometer (for example, a double-beam UV-vis Per- kin-Elmer 555) in place of the usual cuvette.2 The oxygen remains liquefied for about 30 min, which is sufficient time to take a visible spectrum (Figure 2, panels A and B).

Recording the Spectrum with a HandHeld Spectroscope and Camera Asmallvolume,2-3cm3, of liquefied oxygen is poured into a tall test tube (D50 Duran glass). Alternatively, a trans- parent (unsilvered) Dewar can be used in place of the test tube. The test tube is held up to a strong light source (generally, bright sunlight is sufficient) and observed through a handheld spectro- Figure 1. Electronic configurations of the three lowest electronic states of scope (A. Kruss Optronic HS 1504, available through Leybold oxygen. For a more thorough treatment of the electronic structure of 11 16 27 28 didactic, catalog no. 667-339). Ice buildup is removed as needed oxygen, see refs , , , and . with a paper towel saturated with ethanol. The lens opening of a digital camera is placed directly behind the ocular of the oxygen and direct it in a stream past a 0.25 T permanent magnet. spectroscope to take a photograph of the spectrum (Figure 2A). To remove ice crystals, we pour the oxygen through a cone lined We used a Nikon CoolPix S4 because the diameter of its lens with filter paper that has been pierced in the center with a needle. opening is exactly the same as that of the ocular on the For larger audiences, a video camera and monitor are used to spectroscope, saving us the labor of shielding the camera lens enhance viewing ability. from stray light. As can be seen from Figure 2A, only low resolution is possible at the wide lens openings necessary to see Optical Properties the black absorption lines. This is a powerful demonstration of the complementary It is not difficult to assign the observed absorption and behavior of absorbed and scattered light. Viewing what is emission lines (Table 1) to well-documented transitions (18) perceived by the naked eye as a blue liquid, one sees through a among the lower electronic states of oxygen (Figure 1). The spectroscope red, (yellow-)green, and blue. Students' attention spectra were calibrated against a spectrum of Nd(NO3)3 (not needs to be drawn to the black lines breaking up the continuity of shown here). the spectrum, that is, the light wavelengths that have been absorbed by the liquid oxygen. Their absence from the spectrum Blue Color causes the color to appear blue. This concept is reinforced by The blue color of liquid oxygen can be plainly seen, but can examining the reverse process of emission of the red and yellow- be more precisely investigated with spectrophotometers or, by green photons in the chemiluminescence process discussed below. each student individually, with handheld spectroscopes. Note An elegant alternative is to use a Leybold didactic 337 47 that the spectra can be measured only to 350 nm, since Duran VideoCom CCD camera to record the spectrum (Figure 2C). A “test tube” glass is not transparent to UV. light source irradiates a Dewar containing the liquid oxygen. It is

41 r 2009 American Chemical Society and Division of Chemical Education, Inc. _ pubs.acs.org/jchemeduc _ Vol. 87 No. 1 January 2010 _ Journal of Chemical Education In the Classroom

Figure 2. (A) Liquid oxygen absorption spectrum as seen through a Figure 3. (A) Spectrum of the Mallet reaction recorded with the prism - handheld spectroscope. (B) UV vis spectrum of liquid oxygen recorded monochromator, photomultipler tube, and Cassy interface. (B) Photo- with a double-beam spectrophotometer (Perkin-Elmer 555) showing graph of the Mallet reaction chemiluminescence spectrum taken with a features at 630, 578, 548, 446, 380, and 362 nm. Measurements digital camera through the ocular of a handheld spectroscope. (C) beyond 350 nm are prevented by the Duran glass. (C) The absorption Mercury emission of the laboratory ceiling lighting taken as a calibration spectrum seen through the handheld spectroscope as recorded by the spectrum. Leybold didactic VideoCom. Discussion: Experimental and Theoretical Evidence for an then spectrally analyzed in a (mounted) handheld spectroscope Oxygen Dimer and projected by a focusing lens (f = 150 mm) onto the VideoCom. It is necessary to work in the dark. Other discussions of the history of the search for O4 can be found in refs 19, 30 and 31. The earliest work on the magnetism Chemiluminescence of of liquid oxygen and its absorption spectrum suggested the The reaction between hypochlorite anion and hydrogen existence of O4 molecules (9, 12, 16). Since then, oxygen dimers peroxide in basic conditions will produce excited-state singlet have been made (the references cited represent only some of the oxygen, as first reported by Mallet in 1927 (29). After a brief literature on this subject) under “soft” conditions, as in a gas cell - þ induction period of about 10 s, an intense red emission is at cold temperatures (15, 23, 32 35),inO2 O2 molecular observed. This emission was spectrally resolved in two ways. beam scattering (10), and in molecular beam expansions (36, 37), The spectrum recorded with a prism monochromator and and metastable excited states of (O2)2* have been produced photomultiplier tube (Type 1 P 28A) is shown in Figure 3A. (19, 38). Evidence for oxygen dimers has been observed in the The monochromator is driven by a step motor. The analog signal atmosphere (13). That some O4 species exists that is more than was input to a Cassy interface (Leybold didactic) and then just a molecule pair is implied, for example, by the observation of f þ processed using the Cassy-Lab software. The image photo- the reaction 2O2 O3 O formation at energies lower than the graphed with a digital camera aimed through the ocular of a bond dissociation energy of O2 (36). In fact, rovibrational [ 3Σ - ] f handheld spectroscope pointed at the reaction vessel is shown in spectra have been obtained for the O2( g )v=0 2 [ 1Δ ] Figure 3B. To calibrate the wavelength axis, a mercury vapor O2( g)v=0 2 transition in van der Waals dimers, an indication spectrum (from the laboratory ceiling lighting) is shown for that these states of the (O2)2/O4 species exist for longer than just comparison (Figure 3C). collisional interactions (34).

42 r Journal of Chemical Education _ Vol. 87 No. 1 January 2010 _ pubs.acs.org/jchemeduc _ 2009 American Chemical Society and Division of Chemical Education, Inc. In the Classroom

Only van der Waals complexes with some spin coupling between the O2 monomers can explain the experimental ob- servations made thus far. For the dimer formed under soft conditions (molecular-beam scattering), the relative contribu- - - tion to the O2 O2 binding energy from spin spin interactions between two ground-state monomers has been found to be about 15% of that from the van der Waals interaction (10). Reviewing and summarizing the data of all these sources, the binding energies of the dimer formed from ground-state O2 are - - on the order of 10 20 meV and the O2 O2 separation is about - 3 3.6 Å: those that correlate to excited-state O2 molecules have well depths of about 6-8 meV and separations of 3.2 - 4.0 Å. (For comparison, standard reference tables give the bond energy of a single O-O bond as about 1.5 eV and the bond length as about 1.5 Å.) A metastable state is seen at 4.1 eV above the separated ground-state O2 and found to be made from ground- 1Σ - state O2 and a highly excited u state of O2. This state must have a lifetime on the order of microseconds to survive to detection (38). High-energy repulsive states of (O ) have been 2 2 - inferred in photoelectron detachment studies of O4 that 3Σ - 1Δ þ 3Σ - correlate to 2 g and to g g (39). A dimeric species must be responsible for the chemilumines- Figure 4. (Top panel) Possible bonding combinations of the two π* 1Δ 1 D cence in the Mallet reaction owing to the relaxation of two g orbitals of two oxygen molecules to form a B1u state of O4 in 2h O to two ground-state O . As in the absorption, this transition is symmetry (H geometry). The z-axisisperpendiculartotheplaneofthe 2 2 π allowed only when two molecules are coupled together. Weakly paper. To make the B3g orbital, the * lobes are mixed above and 1Δ below the plane of the paper; for the B2u, they are in-plane. (Bottom bound states of (O2)2 with both O2 in g have been identified spectroscopically and predicted theoretically, as mentioned above. left panel) The limiting geometries of approach of two oxygen molecules: for H and X, the approach axis is shown out of the plane That such a dimer is being produced is reasonable in light of Khan of the paper as indicated by the wide skewed arrow, and for T and L and Kasha's (18) determination that only excited-state oxygen approach, the collision geometry is in the plane. (Bottom right panel) molecules can be produced in the Mallet reaction. The calculated lowest energy geometry for cyclic, puckered, O4 and The possibility of reactive processes concomitant with a further considered, but eliminated, covalently bound O4 species electronic transitions, that is, that the trade partners, (10, 34, 40, 41). has been evaluated. An interesting result obtained in an ion-molecule collision, neutralization, dissociative reionization Although theoretical calculations indicate its existence reaction indicates that, for metastable excited states of (O2)2,no as a possibility (20, 42), a completely covalently bound, cyclic, exchange reaction takes place. The decomposition products of O4 molecule (bottom right of Figure 4) has not been 16 18 16 18 the dissociation of O2 O2 are O2 and O2; no mixed observed. It is expected to be a puckered ring of D2d 16 18 species, O O, was observed (38). For reactions involving the symmetry, with bond lengths similar to that in O2.There ground states of the oxygen monomers, one notices that only the can be at least two reasons for this: First, formation of cyclic π* electrons (using, for simplicity, the notation for the separated tetraoxygen in the gas is endothermic by about 4.27 eV σ oxygen monomers) can delocalize in this simple picture, the p (but it is bound by a low barrier to redissociation to 2O2 and π remain in the original molecules (top part of Figure 4). In molecules, on the order of 0.5 eV) (30). Second, statistical fact, delocalization of the π* could even lead to a strengthening of mechanical calculations of the phase space available to free- the original O-O double bonds in the dimer, since it withdraws pair, metastable-pair, and true dimeric (covalently bound) some antibonding electron density from the original bonds. states of oxygen showed relative fractions of about 51%, 47%, Thus, a reactive process can be ruled as being unlikely. In the and 2%, respectively, near -183 °C, the boiling point of top part of Figure 4, we show a simple “frontier molecular oxygen, and with the covalently bound-state fraction drop- ” π orbital picture of the interaction of the antibonding * orbitals ping off to zero above this temperature (43). Thus, cyclic O4 of two oxygen molecules in an “H” geometry, in which the isnotalikelyproductinthegasorliquidphases.Alternative oxygen molecules are parallel to each other, D2h symmetry group. limiting structures for O4 have been considered and elimi- Other limiting geometries of approach are shown in the bottom nated, for example, a branched D3h shown in the bottom right left of Figure 4. Of these, ab initio calculations have yielded of Figure 4. stable structures only for the H and X approaches. Singlet, triplet, An extensive molecular dynamics study of liquid oxygen (8) 3Σ - þ 3Σ - and quintet states are possible from g g . For these, an showed that the electronic structure of O2 in the liquid is not H geometry is the lowest energy species for the singlet and triplet, different from the gas, the energy levels are mostly broadened but while the quintet is most stable in an X geometry, D2d, where the hardly shifted. Thus, it is reasonable to extrapolate the results for bond axes of the separated monomers are at an angle to each O4 in the liquid to those in the gas phase. For the liquid, it was other (40). The singlet states of the dimer that dissociates to also found that the most stable geometry in O4 is the H structure. 1Δ þ 1Δ - the g g excited states of O2 are equally stable in H or Magnetic saturation was observed at O2 O2 separations less X geometries (34). This reconciles the early postulation of a than 3.1 Å and a persistence of magnetic alignment was observed “floppy” but “somewhat rigid” complex (32). to separations as large as 4.4 Å.

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Conclusion 14. Tiedje, H. F.; DeMille, S.; MacArthur, L.; Brooks, R. L. Can. J. Phys. 2001, 79, 773–781. Considering all the experimental and calculational data 15. Sneep, M.; Ityaksov, D.; Aben, I.; Linnartz, H.; Ubachs, W. collected thus far, we recommend no longer referring to a “one- 2006 – ” J. Quant. Spectrosc. Radiat. Transfer , 98, 405 424. photon-two-molecule mechanism when discussing the absorp- 16. Wulf, O. R. Proc. Natl. Acad. Sci. U.S.A. 1928, 14, 614–617. tion and chemiluminescence spectra of oxygen. Rather, they are 17. Ogryzlo, E. A. J. Chem. Educ. 1965, 42, 647–648. due to spin-coupled and van der Waals-coupled metastable bound 18. Khan, A. U.; Kasha, M. J. Am. Chem. Soc. 1970, 192, 3293–3300. states. The oxygen molecules interact strongly enough with one 19. Bevsek, H. M.; Ahmed, M.; Peterka, D. S.; Sailes, F. C.; Suits, A. G. another so that the spin and symmetry forbidden transitions 1997, – 3Σ - 1Δ Faraday Discuss. 108, 131 138. between g and g of the monomers become allowed in the 1992, – 3Σ - 1Δ 20. Seidl, E. T.; Schaefer, H. F., III. J. Chem. Phys. 96, 1176 coupled dimer between 2( g )and2( g). 1183. Acknowledgment 21. Lundegaard, L. F.; Weck, G.; McMahon, M. I.; Desgreniers, S.; Loubeyre, P. Nature 2006, 443, 201–204. We would like to thank Steffen Rieger for his photographic 22. Miller, R. L.; Suits, A. G.; Houston, P. L.; Toumi, R.; Mack, J. A.; services and Regina Hornstein and Hans Strecker for their Wodtke, A. M. Science 1994, 256, 1831. technical assistance. 23. Naus, H.; Ubachs, W. Appl. Opt. 1999, 38, 3423–3428. 24. Lu, C.; Song, G.; Lin, J.-M. Trends Anal. Chem. 2006, 25, 985–995. Notes 25. Francis, P. S.; Barnett, N. W.; Lewis, S. L.; Lim, K. F. Luminescence 1. 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