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

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The Properties of Oxygen Investigated with Easily Accessible Instrumentation the “One-Photon-Two-Molecule” Mechanism Revisited In the Classroom The Properties of Oxygen 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 tetraoxygen 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 liquid oxygen 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 mole- O8, has been observed in solid oxygen (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 ozone, 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.
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