367
NOTES 5
Table I
Compound 4
Cyclohexane Methanol Nitrobenzene Acetonitrile Water
Y D i e l e a t r i c conatant at 25OMeasd Lit.' (IO' aps) (static)
2.02 32.66 34.8 35.9 78.5
2.015 32.63 34.82 36.0 78.54
- 3
x
0
3 ,
sulfate and then distilled, the methanol was rectified in a 4-ft Podbielniak column, and the nitrobenzene was fractionally crystallized.
*I
0 0
a
2
Acknowledgment. It is a pleasure to thank J. P. Eubanks for extensive technical assistance. (6) W. J. McDowell and K. A. Allen, J . Phys. Chenc., 63,747 (1959). (6) J. E.Lind, Jr., and R. M. Fuoss,ibid., 65, 999 (1961).
1
Experimental Determination of the Electronic 0
Absorption Spectrum Ascribable to the 0%(e1/elJ
-
Ion Adsorbed on Porous Glass
1
Figure 3. Correlation of NaCl activity coefficients with dielectric constants of amides containing water; symbols as defined in Figure 1; XI = mole fraction of water; el and €18 = respective dielectric constant of water and of aqueous-organic mixture; DMF, dimethylformamide; the function (~I/EI~) 1 is identical with the function (1 p ) / p , p = e~r/e, used in ref 4.
-
-
in the same general direction as does Lanier's curve for sodium chloride in dimethylformamide (DMF, included in Figure 3), but they are concave upward, instead of downward as were most of Lanier's curves. The curvatures are compatible with smooth extension to the point (0,O)for pure water, which is the limiting member of each series. The results for potassium chloride are similar.
Experimental Section The amides were described previously.2 Capacitances were measured with the equipment (except the cell) described by McDowell and Allen,6 a General Radio Type 716-CS1 bridge plus Type 722-NQ precision capacitor, permitting direct reading of up to 2300 ppf. All measurements were made a t 1 Mc/sec. The was made of cell, following a design used by FUOSS,~ stainless steel and Teflon, plus a thin glass disk to provide a wettable surface under the Teflon. It was calibrated with l,:bdichloroethane, e 10.36 a t 25" (ref 3), and gave satisfactory measurements on known compounds (see 'Fable I). The cyclohexane (spectral grade) was dried in a molecular sieve column; the dichloroethane and acetonitrile were dried with calcium
by Hiroshi Tsubomura, Naoto Yamamoto, Hiroyasu Sato, Kunio Yoshinaga, Hideyuki Ishida, and Kiyoshi Sugishima Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, Japan (Received August 8,1067)
A tube of porous glass' 3-4 cm long and with an outer diameter of 11 mm was placed in a vacuum line. N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) was adsorbed onto it, and the absorption spectrum of the glass was measured with light incident across the tube. The absorption spectrum obtained was essentially that of the TMPD molecule itself. The sample was then irradiated with a high-pressure mercury lamp, The absorption spectrum arising from the irradiation is shown in Figure 1. In the visible region, it has three characteristic bands readily identified with those of TMPD+ and another band with a maximum a t 4200 A (hereafter called the Y band). The intensity of the Y band produced under various conditions is proportional to the intensity of the TMPD+ band. The specimen also gives an esr pattern shown in Figure 2 (curve a), which is regarded to be the sum of the symmetric esr (1) Made and kindly donated by Central Research Laboratories, Mitsubishi Electric Gorp., Amagasaki, Japan. It is prepared by treating alkali borosilicate glass with strong acid, and said to have pores with fairly uniform diameter, 30-100 A. The surface area is measured to be about 271 m*/g. The glass is slightly opaque. Volume 71, Number 1
January 1068
368 I
4000
I
I
"
6000 Wavelength, A.
I
I
1
20 gauss
6000
Figure 1. Curve 1: the electronic absorption spectrum of TMPD adsorbed on porous glass and irradiated by ultraviolet light. Curve b: that of T M P D + (Br-) in an ethanol solution.
spectrum centered at Q = 2.003 for TMPD+ and another spectrum with considerably large anisotropic g factors (curve b). The latter seems to be in accordance ion formed on the with the spectrum assigned to the 02surface of zinc o ~ i d e . ~ t ~ It has been confirmed that the irradiation of porous glass itself causes neither a special electronic spectrum nor an esr spectrum. The intensity of the Y band depends much on the treatment of the porous glass before adsorption. The results in Figure 1 are for the glass activated a t about 500" for several hours under vacuum. With higher temperature and longer time of activation, the intensity of the band decreases. By a similar experiment using the glass treated with hydrogen gas at 500°, both the TMPD + band and the Y band appear only faintly. To the contrary, the introduction of pure, dry oxygen (10 mm pressure) into the vacuum line for about 1 min increases to a large extent the intensities of both absorption bands produced by irradiati~n.~ In the case where an aromatic hydrocarbon (A) as well as TMPD is adsorbed on the porous glass, there is a marked effect on the intensity of the photochemically formed Y band. For the case of anthracene, terphenyl, or naphthalene used as (A), the Y band disappears and, instead, the spectrum corresponding to the A- anion appears. For the case of fluorene, both the Y band and the band of fluorene- appear. For the case of diphenyl, no spectrum of diphenyl- appears but the Y band does appear. These results are in good accord with the sequence of the electron affinities of the aromatic hydrocarbons. The species related to the Y band, therefore, may be roughly regarded to have an electron affinity nearly the same as that of fluorene. Similar photoionization experiments using N,Ndimethyl-p-phenylenediamine instead of TMPD resulted in the appearance of the Y band as well as those for the phenylenediamine cation. The Journal of Physical Chemistry
I
NOTES
HFigure 2. Curve a: the esr spectrum of irradiated TMPD adsorbed on porous glass. Curve b: the spectrum is tentatively divided into two components in such a way that their sum agrees with curve a.
All the above results seem to suggest fairly strongly that the carrier for the Y band is 02-. The photoionization of TMPD in low temperature matrices has been extensively studiedB6I6 I t is concluded that photoionization under near-ultraviolet light is caused by a two-step mechanism via the lowest triplet state. For the present case, therefore, it is reasonable to suppose that electrons are ejected from TMPD by a two-step excitation. Our present results suggest that the electrons thus ejected return quickly to TMPD+ unless an acceptor, O2or an aromatic, traps it. Thompson and Kleinberg' reported many years ago that alkali superoxides formed in liquid ammonia showed a band at about 3800 A, which they assigned to O2-. We prepared potassium superoxide and measured the absorption spectrum of its solution in liquid ammonia. No absorption peak has been found, but an absorption arising monotonically to 3000 A has been obtained. (2) For example, R. J. Kokes, Proc. 3rd Intern. Congr. Catalysis, Amsterdam, 1084, 1, 484 (1965); J. H. Lunsford and J. P. Jayne, J . Chem. Phys., 44, 1487 (1966). (3) According to McLachlan, et al., the OS- ion in an aqueous solution shows an ultraviolet absorption spectrum at about the same region of wavelength as the Y band, but its esr spectrum is quite different from that obtained in the present paper: A. D. McLachlan, M. C. R. Symons, and M. G. Townsend, J . Chem. SOC.,952 (1959). (4) Under humid conditions, TMPD+ is instantaneously formed by the introduction of oxygen. Under strictly dry conditions, no TMPD + is formed with the introduction of oxygen until the specimen is irradiated. (6) N. Yamamoto, Y. Nakato, and H. Tsubomura, Bull. Chem. SOC. Japan, 40, 451 (1967), and papers cited therein. (6) K. D . Cadogan and A. C. Albrecht, J. Chem. Phys., 43, 2560 (1965); G . E. Johnson, W. M. McClain, and A. C. Albrecht, ibid., 43, 2911 (1965); W. M. McClain and A. C. Albrecht, ibid., 43, 466 (1965).
(7) J. K. Thompson and J. Kleinberg, J . Am. Chem. Soc., 7 3 , 1243 (1951). We are indebted to Professor Kleinberg for detailed information on his work published a long time ago.
NOTES Rolfe, et aL,* recently reported a broad absorption spectrum at 2500 A, together with a fluorescence spectrum which they assign to 02-in the crystal of alkali halides fused in an oxygen atmosphere. Also, Czapski and Dorfmang assigned the absorption band at about 2400 A obtained by the pulse radiolysis of the H2O-02 system to 02-. These two results seem to offer reliable data for the spectrum of the 0,- ion. Our spectrum, though at a different wavelength, may also be valid, because the position of the spectrum may be susceptible to the environment. Another alternative is that our Y band might be the first band of Oz-, which is much weaker than the second band, which may be the spectra found by the above authors. (In our experiment, no absorption band below 3500 A can be measured.)
369 ~~
Table I: Absorptions for Hz and Dz on Rhodium (after Evacuation) HI'
4372 3384 3200 3154 2995 2930 2879 2766 2734 2711 2193 2187 2163 2131 2118 2114 2058
(8) J. Rolfe, F. R. Lipsett, and W. J. King, Phya. Rev., 123, 447 (1961). (9) G . Czapski and L. M. Dorfman, J . Phys. Chem., 68, 1169 (1964).
Interpretation of Infrared Spectra of (1
1584 1564 1556 1538 1533 1528 1518 1506 1447 1409
See ref 1.
Chemisorbed Hydrogen and Deuterium
by W. Hayden Smith, H. C. Eckstrom, and F. B"r Department o j Chemistry, University of Kentucky, Lexington, Kentucky (Received August $9, 1967)
Results obtained by one of these writers (F. B.) for D, on rhodium and previously published but uninterpreted data, for Hz on rhodium' may be correlated using a recently proposed group theoretical method2 and the observed isotopic shift to verify the origin of the bands. As noted by Pickering and Eckstrom,' numerous absorptions are observed in the Rh-H, system in the presence of a few torr of gas. Upon evacuation of the system only the absorptions listed in Table I persist. These may be considered as due to chemisorbed hydrogen and deuterium in a tightly held form. A decrease in reflectivity in presence of the gas phase partially remains upon evacuation as further evidence of chemisorbed hydrogen (or deuterium), Several investigator^^-^ have studied the chemisorption of hydrogen on atomically clean single crystal substrates. In all cases, the evidence has indicated an atomic rather than a molecular mode of absorption for hydrogen on these substrates. The infrared results of Becker and Gobeli4 for atomic hydrogen chemisorbed on single crystal silicon surfaces would indicate that infrared absorption arising from an Rh-H stretch should be expected in the 2000-2200-~m-~region for the RhHz system. In the case of D,, the isotope effect should lower the Rh-D stretches to a frequency of approximately v(Rh-H)/&. The observation of a group
of bands for the Rh-H2 system at 2114-2193 cm-I, and a displaced group at 1506-1584 cm-l for the Rh-D2 system provides a convincing assignment of these bands as due to chemisorbed hydrogen. The observation of a number of bands in each region is ascribed to the polycrystalline nature of the Rh mirrors used in these experiments. Such mirrors exhibit a random crystallographic orientation and relatively different adsorption sites on each type of face exposed. The 2,+ vibrational motions of gas phase D,h molecules can create no varying dipole at low pressures and hence are infrared inactive. In the case of the evacuated rhodium-hydrogen system,' an absorption is found at 4372 cm-l. This band lies near the Zg+ stretching motion of gas phase hydrogen at 4160 cm-l. Because these bands are at the limits of observability,' intensity considerations make the observation of overtones unlikely, unless they are in position to allow Fermi resonance or some other intensity enhancement to occur. For this reason, we do not assign 4372 cm-' as a possible overtone of the 2100-cm-' bands. Before proposing an assignment of the 4372-cm-l band, we first must discuss the symmetry properties of Hz chemisorbed upon a crystalline substrate, and show that the molecular stretching mode is thereby formally (1) H. L. Pickering and H. C. Eckstrom, J . Phys. Chem., 6 3 , 512 (1959). (2) W. H Smith and H. C. Eckstrom, J . Chem. Phys., 46, 3657 (1967). (3) J. A. Becker, Solid State Phya., 7 , 379 (1958). (4) G. E. Becker and G . W. Gobeli, J . Chem. Phys., 38,2942 (1963). (5) P. J. Estrup and J. Anderson, ibid., 45,2254 (1966). Volume 78, Number 1 January 1068
