SYMMETRICAL AND ANTI-SYMMETRICAL HmROGEN* HBRRICK L. JOHNSTON, THEOHIOSTATEUNIVERSITY, COLUMBUS, OAR) A striking feature in the spectrum of molecular hydrogen is the alternation in intensities which occurs in the rotation lines. One observes alternately weak and strong lines. The relative intensities of adjacent weak and strong lines are roughly one to three. Mecke (1)pointed out that such an alternation usually appeared in the spectra of diatomic molecules whose atoms were alike. Heisenberg (2) and also Hund (3) treated the problem from a theoretical standpoint and showed, by the equations of the wave mechanics, that all homopolar molecules should exist in two forms due to differences that should exist in the nature of the binding between the atoms. One form, called symmetrical because associated with what is referred to in the language of wave mechauics as a "symmetrical eigenfunction," exists only in even rotation states. The other form, called anti-symmetrical because associated with an "anti-symmetrical eigenfunction," exists only in odd rotation states. Hydrogen, they thus reasoned from a theoretical standpoint, should exist as a mixture of two varieties of molecules, differing in the manner in which the atoms are coupled. One form, symmetrical hydrogen, is responsible for the weak lines of the hydrogen spectrum, while the other form, anti-symmetrical hydrogen, is responsible for the strong lines. The one-to-three ratio of intensities is a measure of the relative numbers of molecules in these two states. The two forms were held to he transformable into one another and to exist in a state of equilibrium which, on account of differences in their energies, should be dependent on temperature. By making use of the distances between the lines in the spectrum and of the approximate oneto-three ratio of intensities it is possible to make a quite accurate calculation of the ratio in which the two kinds of hydrogen molecule should exist at any temperatnre. At very high temperatures the proportion should he very nearly equal to that at room temperature and approach exactly 75% anti-symmetrical to 25% symmetrical at i n k i t e temperature, in the limit. At the temperature of liquid air the proportions of the two should be nearly equal and at liquid hydrogen temperatnre 99.7% should exist in the symmetrical form and only 0.3% in the anti-symmetrical. So, as the temperature is lowered the anti-symmetrical molecules are converted to symmetrical. With a return to higher temperatures the antisymmetrical form is restored. It was originally supposed that approach to equilibrium between the forms would be rapid at any temperature. But Dennison (4) was the first A n address given before the Physical Science Section of the Tenth Ohio State Educational Conference, held in Columbus, April 3, 4. and 5, 1930; and printed in the Proceedings. 2030
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to present grounds for the belief that this latter supposition was apparently not true. It has long been a problem to reconcile the experimental values for the specific heat of hydrogen gas with the theoretical values which, it seemed, should be subject to rather accurate calculation. It was always assumed, before Dennison, that one was dealing with a gas in thermal equilibrium. Dennison suggested that the transformation between symmetrical and anti-symmetrical states did not occur immediately and hence that the experimental values were obtained with non-equilibrium mixtures of the two varieties. He verified his suggestion by making a calculation of the specific heats to be expected for hydrogen gas in which the proportion of symmetrical to anti-symmetrical remained unchanged from that at room temperature and he obtained values in agreement with the experimental data. Up to this point the evidence in favor of the existence of symmetrical and anti-symmetrical hydrogen was of a theoretical nature. The first direct experimental evidence for the existence of two kinds of hydrogen and their slow rate of transformation into one another was made public in a paper by Giauque and Johnston (5). In this paper the authors presented the results of experiments which showed that hydrogen kept under pressure a t the temperature of liquid air for a period of six months possessed a lower melting point and a lower vapor pressure a t its melting point than did ordinary hydrogen. The melting point of the special hydrogen was 0.04O lower and its vapor pressure at the melting point 0.4 of a millimeter lower than that of ordinary hydrogen. The limits of error were about 0.01' in the temperature measurement and something less than 0.1 of 1 millimeter in the pressure measurement. This was regarded as evidence for the existence of two forms of hydrogen and of their slow rate of transformation into one another. Several days later verification from quite a different experimental angle was produced. McLennan and McLeod (6) at the Uni-lersity of Toronto made the discovery that a ray of mouochromatic light passed through liquid hydrogen emerges as polychromatic light. In addition to the strong spectral line corresponding to the wave-length of the incident beam were two fainter lines on the long wave-length side. Quantitatively, the positions of the new lines corresponded, respectively, to absorption between the number one and number three rotation levels of hydrogen and between the zero and number two levels. This is known as the Raman effect and is well known for other liquids and gases. But the significant thing observed by McLennan and McLeod was the fact that the one-to-three line, which according to theory is associated with antisymmetrical hydrogen molecules, and which was initially the stronger of the two new lines, gradually faded while the zero-to-two line, associated with symmetrical hydrogen, became more intense. This, of course, was properly interpreted as evi-
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dence for the slow transformation of one form of hydrogen to another. A few weeks later Bonhoeffer and Harteck (7), in the &st of several papers which reported results of their very extensive investigations, presented evidence, obtained by a still different experimental method, for the existence of the two forms of hydrogen. Further, they obtained information on the normal rate of the transformation between the two types a t various temperatures; on conditions which influence the rate and on the existence of catalysts which within a few minutes bring about the transformation which normally would require weeks or months to attain equilibrium. Thus they found that. at the temperature of liquid hydrogen, charcoal was an effective catalyst in converting the 75% antisymmetrical mixture existing at room temperature to the 99.7% symmetrical mixture stable at the lower temperature. Also at the temperature of liquid air charcoal behaves as an effective catalyst in producing the roughly 50% mixture of the varieties. The reconversion to the 75% anti-symmetrical mixture at room temperature is best brought about by platinum or palladium black as catalysts. They examined a number of other catalysts and found that other conditions, such as pressure and liquefaction, were effective in producing more rapid approach to equilibrium. Experimentally, they were able to detect the transformation very quickly and very simply by measuring the heat conductivity of the gas for, as they showed conclusively, the symmetrical variety is the better heat conductor of the two forms, at least in the neighborhood of liquid air temperatures. A change in the proportion of the two forms present in the mixture thus produces a difference in the heat conductivity of the gas. With the means of preparing large quantities of practically pure symmetrical hydrogen, they were able to make extensive studies of the properties of symmetrical hydrogen as distinct from the usual mixture rich in the anti-symmetrical variety. Thus they found the melting point of symmetrical hydrogen to be O.OgOlower than that of ordinary hydrogen and the vapor pressure at the melting point 0.9 of a millimeter lower than that of ordinary hydrogen. These figures are in complete agreement with the 0.04' degree and 0.4 millimeter low values obtained by Giauque and Johnston for hydrogen stored a t liquid air temperature, which is a half and half mixture of the two varieties. Bonhoeffer and Harteck adopted a new nomenclature, para- and orthohydrogen, to replace the older nomenclature, symmetrical and antisymmetrical, but it is doubtful if such a substitution is advisable in view of other usages associated with the terms para and ortho. We shall attempt to demonstrate the existence of the two forms of hydrogen by means of the heat conductivity method of Bonhoeffer and
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Harteck. As the conductivity vessel, we have a cylindrical glass bulb with sealed-in electrodes connected by about six inches of very fine platinum wire. At room temperature this wire has a resistance of about 175 ohms and at liquid air temperature a resistance of about 50 ohms with intermediate resistances at intermediate temperatures. The resistance of the wire thus serves as a thermometer to measure its temperature. The bulb is surrounded with liquid air. First we will admit into the bulb ordinary hydrogen, a t a pressure of about four centimeters, from this cylinder of commercial hydrogen. And now we will heat the wire by applying a constant voltage across its terminals. By means of this Wheatstone bridge we measure the resistance of the wire. This we now find to be about 126 ohms. This corresponds to a temperature some hundred degrees above that of liquid air. However, the resistance remains constant due to the balance reached between the heat generated in the wire and the heat conducted away by the hydrogen to the walls of the vessel. Next we will pump out this ordinary hydrogen and refill to the same pressure with hydrogen which has been standing for some time in contact with charcoal in this bulb fiUed with liquid air. You observe the deflection produced on the galvanometer. We now rebalance the bridge and observe that the resistance of the heated wire is now about 123 ohms. This corresponds to a cooler wire and means that the hydrogen now in the vessel is carrying the heat away from the wire more rapidly than did the ordinary hydrogen. We will next pump out this hydrogen also and refill the bulb with hydrogen again from the supply in contact with the cold charcoal but passed through this tube over a small quantity of platinum black. You observe that the galvanometer is deflected back in the original direction. The bridge reading corresponds again to the approximately 126 ohms obtained with the ordinary 75-25% mixture of the gas. The question may arise iu your minds as to whether symmetrical hydrogen may differ chemically from ordinary hydrogen. The answer is "yes," although only to a slight extent. It is to be expected that the voltage of the hydrogen electrode with symmetrical hydrogen should differ measurably from the voltage produced with usual hydrogen. Experimentally this difference escapes detection, however, since the platinum present in the electrode catalyzes the transformation of symmetrical hydrogen to the ordinary variety. It might also be expected that the reduang action of hydrogen rich in the symmetrical variety should be slightly greater than that of ordinary hydrogen. You may inquire as to the physical difference in the two forms of hydrogen. The difference is probably due to differences in the direction of spin of the two hydrogen nuclei which are joined to make the molecule. We know from other sorts of data that in a great many atoms the protons are
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spinning like little tops. It is probable that in symmetrical hydrogen the two hydrogen nuclei are spinning in opposite directions, clockwise and counter-clockwise, we might say; while in anti-symmetrical hydrogen they both spin in the same direction. You wonder if other molecules might also show this mixture of varieties. The answer is "yes." In fact the alternation in intensities of the rotation lines of the spectrum is found in chlorine as in hydrogen, although the relative intensities are somewhat different. It is probable that chlorine exists in four forms-two pairs of varieties: the members of each pair in rapid equilibrium with one another, the members of opposite pairs not coming rapidly to equilibrium. Fluorine probably exists in the same types of varieties as does chlorine. In nitrogen there are probably three varieties of molecules. In these latter cases, however, we cannot expect to obtain the same sort of experimental evidence for the existence of different varieties since calculation shows that in order to get appreciable variation in the ratios of the various varieties from those existing a t room temperature one would have to cool the gases to lower temperatures than for hydrogen. Experimentally, those gases cease to be gases a t temperatures considerably higher than for hydrogen. Other common molecules are known to exist as mixtures of different varieties-varieties as truly distinct as those illustrated in the hydrogen case-due not to the manner in which spinning protons are coupled but to the manner in which spinning electrons are coupled. Approach to equilibrium in these cases is ordinarily very rapid, however. Thus ordinary oxygen gas composed of two like atoms of mass 16 exists as a mixture of three different varieties which are present in different proportions a t room temperature (8). In the case of nitric oxide we have four different varieties as a result of electron spin and it is probable that each of these exists in three different forms due to the presence of spin in the nitrogen nucleus. So that, in all, probably twelve distinct varieties of this common gas are present. In general, it is safe to predict that for a great niany molecules familiar to us in chemistry there must exist a mixture of several such varieties. Literature Cited MECKE,Physik. Z., 25, 597 (1924); Z. Physik, 31, 709 (1925). HEISENBERG, 2. Physik, 41, 239 (1927). H m , Ibid., 42, 93 (1927). DENNISON, PIOC.Roy. SOC. (London), 115, 483 (1927). GIAUQUE and JOHNSTON, J. Am. Chem. Soc., 50, 3221 (1928). MCLENNAN and MCLEOD, Nature (London), 113, 152 (1929). BONHOEETER and HARTECK, SiBb. preuss. Akad. Wiss., 1929; Nnturm'ssenschaften. 17, 182, 321 (1929); Z. physik. Cl~em.,(B) 4 , 113 (1929). Cf. MULLIKEN, Phys Reo., 32, 880 (1928).
