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were prepareld from conductivity water and analytical grade sulfuric acid. Either purified oxygen or nitrogen, or a known mixture of both, was bubbled through the test electrode compartment. At the beginning of the experiment, tiny oxides present on the test electrode were reduced by palssing a cathodic pulse across the electrode-solution interface. After this, the solution was quickly changed to prevent traces of hydrogen (evolved during the cathodic pulse) from interfering with the oxygen adsorption process. The fresh solution was equilibrated with oxygen a t a known partial pressure. The electrode then assumes a steady (rest) potential and has a definite coverage of oxygen. The amount of adsorbed oxygen on the electrode was analyzed by cathodic pulse reduction. At constant current density of 50 ma. cm.-2, the reduction was sufficiently rapid to render negligible any additional adsorption of oxygen from solution during the measurements.2 'The number of coulombs used in the reduction was read off from the potential-time transient displayed on 1,he oscilloscope. From capacity measurement data, obtained from initial slope of the transients, the roughness factors were determined for the electrodes. Column a of Table I gives the value of quantity of change, &, required to reduce adsorbed oxygen. These values are obtained from the slopes of the corresponding isotherms plotted as p ~ ~ ~ 'us.~ p/ ~$ ~~ ' "unless ~, stated otherwise, where p is the partial pressure of oxygen. A4ssuming,for simplicity, closest packing of atoms in the surface, a layer of adsorbed oxygen atoms with one oxygen atom attached to each surface metal atom corresponds to a coverage of about 500 $2. cm.-2 (column b). Column c of Table I shows the relative coverage, 8, for the metals studied. It appears that the maximum coverage on these metals is proportional to the number of unpaired d-electrons per atom, which they possess (column d), to the limit of a full monolayer (e.g., Ru). These results strongly suggest that the unpaired d-electrons directly participate in the bonding of oxygen atoms adsorbed on these noble metals in the oxygen-saturated solutions DATAON Metal
THE
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each of which contributes on the average all of its 0.55 unpaired d-electrons, and the maximum relative corerage 8 = 0.27 results. On rhodium, as well as on iridium, with about 1.7 unpaired d-electrons per atom, according to the present model, a relative coverage of about 0.85, or 450 pC. cm.-2, is expected (cj. Table I). Ruthenium, with more than two unpaired d-electrons per atom, shows (in the solutions saturated under one atmosphere pressure of oxygen) a coverage which is close to a monolayer. Gold, with no unpaired delectrons available, indicates little adsorption. Further work, particularly on alloys, is in progress and may have important bearings on the study of bonding of adsorbed oxygen onto transition metals and, in general, on the mechanism of heterogeneous catalysis on these electro-catalysts. Acknowledgment.-This work is supported by United States Army Electronics Research and Development Laboratory, Fort Monmouth, New Jersey, under Contract No. DA 36-039-SC-88921. THE ELECTROCHEMISTRY LABORATORY iVf. L. B. RAO OF I'ESPI'SYLVAXIA A. DAMJANOVIC THEUNIVERSITY PHILADELPHIA 4, PENNSYLVANIA J. O'M. BOCXRXS RECEIVED SEPTEMBER 12, 1963
NUCLEAR MAGKETIC RESOKAKCE SPECTROSCOPY. LONG-RANGE PHOSPHORUS-31-HYDROGEN-1 SPIK-SPIN COUPLING
sir:
The contributions to spin-spin coupling involving nuclei of elements other than first row members of t,he periodic t'able (heavy nuclei) have recently received considerable attention.1--3 Although reports of longrange spin-spin coupling between similar nuclei (H-H and F-F) separated by four or more bonds have appeared in t,he literature, relatively few cases have been detected where the nuclei have differed and cases involving heavy nuclei are limited. Roberts4J and Cross6 have observed coupling between protons and TABLEI flourine through four, five, and six bonds and pointed OXYGEN ADSORPTIOX o s DIFFERENTNOBLEXETALS out the importance of the stereochemical relationship (e) id) (a) (b) of the nuclei. h long-range P31-H1 coupling (four Observed Calculated Fraction N o . of unpaired bonds) reported for diet,hyl phenylphosphate and clioxygen of surface oxygen d-electrons per coverage of coveragea atom covered by ethyl phenylphosphonite' is questionable because of the (pC. cin. a. monolayer by oxygen, presence of a magnetically asymmetric center which (pC. om. - 2 ) (0) results in a complex ABC3 Fine splitting 1106 510 0.22 0.55 has been calculated and observed for the methyl reso1106 0,226 500 0.55-0.6 nance of the etho.xy groups in the diethylacetals of 500 135 0.27 0.55-0.6 530 0.90 480 1.7" acetaldehyde and acetophenone which contain similar 525 0.84 440 1.T ABC3 systems'O. l1 and no long-range coupling possi-2)
Pd Pt Pt Rh Ir RU
5006 530 0.95 2.2d Au
