NOTES Rolfe, et aL,* recently reported a broad absorption spectrum a t 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 a t 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 a t 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. I n 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 a t 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 a t low pressures and hence are infrared inactive. I n 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 a t 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
NOTES
370 allowed in the infrared spectrum. It is clear that the mode of adsorption should be a linear one Rh. * *H-H in order that a nearly gas phase hydrogen stretching frequency will persist. A bridged mode of adsorption like H-H
/
\\
Rh
Rh
would clearly allow more H-H, Rh-H vibrational coupling interactions than a linear mode of adsorption with the consequence of lowering the frequency of the H-H stretch. As an example, consider hydrogen chemisorbed in a linear fashion upon a rhodium (110) crystallographic face. Using the group theoretical procedures,2 the following correlation mapping (Table 11) is derived for the vibrational motions, assuming a gas phase R h . .H-H molecule, and yields an infrared active A' species for the molecular stretching mode of Hz at the adsorption site. A similar result holds for other infrared crystallographic faces ; Le., the mode becomes active.
Table I1 : Hydrogen Chemisorbed Linearly upon a Rhodium (110) Crystallographic Facea
e Rx, Ry, X , Y
vi, YS, YZ,
-Correlation Molecular group
Site grow
C,V
c 0
Factor group C0(p1ml)
A' A', A"
A', A"
E+ II
mapping-
A'
be reversibly interchangeable. Further, a linear mode of adsorption upon crystal faces with a very different atom density or arrangement would be expected to exhibit large frequency shifts due to the change in intermolecular interactions. The possibility that such shifts are all less than the resolution limit of 40 cm-I certainly cannot be ruled out at this point. Nevertheless, we conclude that the 4372-cm-I band is due to hydrogen, molecularly chemisorbed in a linear fashion on the rhodium film upon certain undetermined, but similar crystallographic faces. It is significant that we are here proposing a molecular adsorption mode for H2, whereas the previously mentioned investigations3-6 detected only an atomic mode of adsorption. This will lead us to an alternate, and preferable, explanation for the bands found in the 2000 cm-' region in the present experiments. A review of the experimental condition^^-^ reveals that these studies on single crystal substrates were carried out in ultrahigh vacua on atomically clean surfaces. The infrared studies being discussed here did not meet these stringent conditions, being conducted at pressures of torr. It is well known that a t such pressures an almost immediate oxygen coverage is formed from the residual gas. This would have been sufficient to remove and/or cover the sites where hydrogen might be dissociated. We are left with the observed result that only molecular hydrogen may be adsorbed in such cases. This conclusion implies strongly that the 2000-cm-' bands are only accidentally coincident with the Si-H stretch observed by Becker4 since dissociation of hydrogen is now precluded under the experimental conditions of Bar and Eckstrom and Pickering and Eckstrom.' Again referring to a correlation mapping (Table 111)
All species infrared and Raman active. R, species nonexistent for linear absorption. Table I11 : Bridged Hydrogen on a (110) Rhodium Crystallographic Face"
The experimental conditions precluded a determination of true line widths. This was due to the low resolution necessitated by the reduced transmission resulting from the large number of reflections needed to observe these extremely weak adsorptions. Nevertheless, the single line at 4372 cm-I indicates either that the H-H stretching frequency is not strongly dependent upon the crystallographic orientation of the adsorbing surface or that linear adsorption is only occurring on certain crystallographic faces which do not differ too greatly in the density of atoms exposed. The latter choice is preferable since the observations, to be discussed below, lead to the conclusion that a t least two modes of adsorption occur under these experimental conditions. It has been demonstrated that a given crystallographic surface will adsorb molecules in different structures depending upon the extant experimental conditions.6 When the conditions change, the structures are found to The Journal of Phyeical Chemistry
Molecular group C2"
Correlation mapping Site Factor group group Ca c2v (PW)
R,
Ai A2
A A
Y, Rx
Bi Ba
B B
I
x, RY
7
Ai, Az Ai, Az Bi, Bz BI, Bi
All modes are infrared and Raman active, excext A2 which is Raman active only. (1
for a rhodium (110) surface with hydrogen chemisorbed in a bridged mode H-H
/
Rh
\\
Rh
NOTES
37 1
we find that both the Rh-H, and H-H stretches are shown to be infrared active species. The general result again is that whatever geometry is assumed, the nondegenerate stretching modes are infrared active, and consist of a single line per type of adsorbing face, unless split by the correlation fielda2 The infrared spectra listed in Table I contain a number of lines in the 3000cm-l region which have not been assigned. A possible assignment presents itself if these bands are considered to be the perturbed H-H stretches associated with a ,H-H Rh/ \Rh bridged structure. The 2000-~m-~ bands have essentially the same interpretation given at first, that of being Rh-H stretches, but now, due to the bridged structure are capable of vibrational coupling with the higher frequency H-H motions of the same symmetry species. The corresponding motions of the metal-hydrogen bond in these two extremes may be considered analogous ones in which the H-H interaction of neighboring protons on the surface is changed from a molecular bond to an atom-atom interaction term.
Quantum Yield of Photonitrosation of
Cyclohexane under a Flash Lamp by Kuya Fukuzawa and Hajime Miyama Basic Research Laboratories, Toyo Rayon Co., Ltd., Kamakura, Japan (Received August 8.2, 1967)
Baumgartner, et al.,’ reported the quantum yield of photonitrosation of cyclohexane as 1.48, while Shimokawa, et a1.,2 reported a rather contradictory value of 0.7 for the same reaction. To explain these data the former believes that the reaction proceeds via the following chain mechanism NOCl r i CsHio-CH2
Rh
/H-H\
‘\Rh+Rh
I
I
Rh
Again, since the metal-H frequency is so similar in both cases, it is clear that special sites must be responsible for dissociation, which then diffuse mobile hydrogen atoms onto the observed sites. In the case of the atomic mode of adsorption, the H . . . H motions will be included among the lattice modes predicted in the correlation mappings, Tables I1 and 111, and should be observable optically. In the 3000- and 2000-cm-l regions the large number of bands observed can be attributed to multiple crystallographic sites available on the polycrystalline films, each exhibiting as would be expected a different perturbation of the fundamentals discussed above. The discussion herein applies a group theoretical model to the discussion of spectra for H2 and D2 on polycrystalline rhodium substrates, and predicts the existence of certain fundamentals lying outside the region available to the investigators. A theoretical discussion of experimental problems in these regions has been given to indicate how these data may be obtaineda8 Acknowledgment. This work was supported in part by U. S. Atomic Energy Commission Contract No. AT(40-1)-2948.
(6) H. C. Eckstrom and W. H. Smith, J . Opt. SOC.Am., 57, 1132 (1967).
r+ C1. +C5Hio-CH* + HCl r+ NOCl --+ C5Hlo-CHN0 + C1, 1
1
C;Hlo-CH.
The latter, however, assumes the scheme NOCl
H . . .. H I I
+ hv + N O + C1.
7
+ hv +NO + C1.
1
+ C1. +C6Hlo-CH. + HC1 r-----l r----C5Hlo-CH. + NO +C5Hlo-CHNO 7
1
C5Hlo-CH2
Thus the quantum yield must necessarily be smaller than or at best equal to unity. There seems to be no compromise between these two observations and further investigations still to be awaited. We therefore performed this photoreaction under an intense flash lamp. Our conclusion is that the quantum yield is about 0.65 0.20 even under intense light.
Experimental Section The reaction vessel used in this work was of a triply walled, coaxial type with a flash lamp mounted at the center. The inner chamber was filled with a filter solution, and the outer chamber with a solution of cyclohexane. A photomultiplier was attached to the outer surface to serve as a monitor. The flash light was separated into peak wavelengths of 3600, 4100, and 5050 A by means of water solutions of cupric and cobaltous sulfate, cupric nitrate, and cupric and calcium chloride, respectively. A 0.006 M solution of potassium ferrioxalate3 was filled in the reaction chamber to measure the absolute number of incident light quanta at 3600 A, which together with the monitoring photomultiplier enabled us (1) P. Baumgartner, A. Desohamps, and C. Roux-Guerraz, Compt. Rend., 259, 4021 (1964). (2) Y. Shimokawa, et al., Kowo Kauaku Zasshi, 68, 937 (1965). (3) C. A. Parker, Proc. Roy. SOC.(London), A220, 104 (1953). Volume 78, Number 1
January 1968
