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J. A. E. KAIL,J. A. SAUER,AND A. E. WOODWARD
atoms which are bonded only to the surface, and carbon or sulfur atoms, or both. On the basis of these data, proposal of surface fragments is sheer speculation, although formation of chemisorbed carbon monosulfide would release a hydrogen to form a CH2group. Discussion Although adsorption of sulfur compounds by coordination of the sulfur atom with the surface may occur, our data certainly indicate that the adsorption of sulfur compounds on clean silicasupported nickel is a much more varied and complicated process than this. Our data indicate that HzS can have at most only weak interaction a t room temperature with the surface. On our silica-supported nickel, it certainly does not form a strong coordination bond. Common to all of the other sulfur compounds studied is the fact that a major portion of the chemisorbed molecules were dissociatively adsorbed in the sense that all underwent carbon-hydrogen bond rupture. This fact, together with the behavior of HtS, suggests that metal-carbon bonds may be primarily responsible for the tightly held chemisorbed species. This suggestion can only be regarded as tentative on the basis of these data.
Vol. 66
In the past, it generally has been a s ~ u m e d ~ J - ' ~ that poisoning by sulfur compounds is great, due to the sulfur atom forming a strong chemisorption bond. While this may be true for some systems, our data suggest that for our system carbon-metal bonds rather than sulfur-metal bonds are responsible for the primary strong chemisorption. At higher temperatures than those employed here, the poisoning process might be envisioned as starting first with adsorption of the sulfur compound by carbon-metal bond formation and then dissociation to sulfide the catalyst surface. The sulfiding of the surface by decomposition after the primary chemisorption would be responsible for the long term poisoning of the surface. This view is consistent with the observation that while H2Sdid not poison the surface at room temperature, when the temperature was raised high enough to decompose the H2S, the surface was poisoned. Acknowledgment.-This work was supported in part by a grant from the National Science Foundation for an Undergraduate Research Participation Program. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
PROTON MAGNETIC RESONANCE OF SOME SYNTHETIC POLYPEPTIDE S1 BY J. A. E. KAIL,J. A. SAUER, AND A. E. WOODWARD Department of Physics, The Pennsylvania State University, University Park, Pennsylvania Received December 89, 1961
Nuclear magnetic resonance spectra have been obtained for five synthetic poly-( a-amino acids) in the 77-470"K.temperature range. It is found that the temperature dependence of the n.m.r. second moment depends markedly on the side chain substituent, and that the processes occurring in the accessible temperature range can be accounted for wholly in terms of side chain motion. At 77°K. poly-(phenyl-L-alanine) and poly-( 7-benzyl-L-glutamate) are in the rigid lattice while for poly-( L-alanine) and poly-( L-leucine) some methyl group rotation is taking place. For the latter two polymers complete methyl rotation is taking place a t about 120°K. and above. The gradual second moment decrease for poly-(L-leucine) and poly-(phenyl-Lalanine) in the 120-400'K. region is attributed to an increasing mean amplitude of isobutyl and benzyl group oscillation, respectively, vith increasing temperature, a state of complete rotation being reached a t -400°K. For poly( ?-benzyl-L-glutamate) the n.m.r. line narrows rapidly in the 280-370°K. region to values
