AN h " V S T I G A T I 0 N
OF hlOLECULAR AIOTION IN U R E A d 4
ADDUCTS
3757
A Nuclear Magnetic Resonance Investigation of Molecular Motion in
Urea-d,-n-Hydrocarbon and Urea-d,-Fatty Acid Adducts
by Kimiko Umemoto and Steven S. Danyluk' Department of Chemistry, University of Toronto, Toronto, Canada
(Received March 10, 1967)
The wide-line proton magnetic resonance spectra have been measured for urea-d4 adducts of even-numbered straight-chain hydrocarbons from C12H26 to C18H38 and for u r e a 4 adducts of stearic and palmitic acids from 80 to 300°K. For the n-hydrocarbon adducts, marked decreases are noted for the line widths and second moments over relatively narrow temperature ranges a t low temperatures. The transition temperatures for these changes increase in a regular manner with increasing chain length of the n-hydrocarbon. For the fatty acid adducts, three distinct regions are noted in both the line-width and second-moment curves. I n the temperature range 80-200°K the parameters decrease a t a moderate rate; in the range 200-240°K the changes are much more marked, and, finally, above 240°K the changes are again gradual. The line-width and second-moment changes for the adducts have been attributed to the onset of specific motional processes of the enclathrated molecules. From a comparison of observed and calculated second moment values it is concluded that the methyl groups of n-C12H26,n-C14H30,stearic acid, and palmitic acid are rotating freely, based upon the nmr time scale, about the threefold axis a t 95°K. Under the same conditions the motion of a significant fraction of the methyl groups in the n-C16H34 and n-Cl~H38adducts is more restricted. It is also concluded that the large second-moment transitions observed at low temperatures for the n-hydrocarbon adducts, and a t higher temperatures for the fatty acid adducts, result in each case from the onset of rotational motion of the long-chain derivatives about their longitudinal axes. The much higher transition temperatures of the fatty acid adducts have been attributed to the hindering effect of intermolecular hydrogen-bonding interactions.
Introduction The structural stabilities and motional properties of urea adducts of n-hydrocarbon derivatives have been studied by a variety of technique^.^-'^ From wideline proton magnetic resonance measurements of nC13H28 and n-C14H34-urea-d4adducts Gilson and ItlcDow7ellg concluded that the hydrocarbon molecules were able to rotate more or less freely about their longitudinal axis at temperatures well below the decomposition points of the adducts. The onset of rotational motion is further confirmed by recent calorimetric which show the presence of an anomalously high heat absorption for urea adducts of straight-chain hydrocarbons at temperatures roughly corresponding to the regions in which second-moment transitions are observed in the nmr spectra. In contrast, no anoma-
lous heat absorption was noted in the heat capacity curves for adducts of isomeric branched-chain hydrocarbons.'2 Both the nmr and heat capacity measure(1) Address all correspondence to this author a t Argonne National Laboratory, Argonne, Ill. (2) ,(a) R.J. Meakins, Trans. Faraday SOC.,51, 953 (1955); (b) A. A. \ . Stuart, Rec. TTUV.Chim., 75, 906 (1956). (3) G. Caroti and B. Casu, Riv. Combust., 12,451 (1956). (4) G. B.Barlow and P. J. Corish, J . Chem. SOC.,1706 (1959). (5) R.Mecke and W. Kutzelnigg, 2.Anal. Chem., 170, 114 (1959). (6) P. H.H.Fischer and C. A. McDowell, Can. J . Chem., 38, 167 (1960). (7) G. Geiseler and P. Richter, Chsm. Ber., 93,2511 (1960). (8) R. A. Durie and R. J. Harrisson, Spectrochim. Acta, 18, 1505 (1962). (9) D.F. R. Gilson and C. A. McDowell, Mol. Phys., 4,125 (1961). (10) H.G. hlcAdie, Can. J . Chem., 40, 2195 (1962).
Volume 7 1 , Number 19 November 1967
3758
ments also indicate that the onset of rotational motion of the straight-chain hydrocarbon molecules occurs a t much lower temperatures in the adducts than in the corresponding solid hydrocarbons. This suggests that the intermolecular forces between hydrocarbon molecules and the urea lattice and between hydrocarbon molecules in different channels are much weaker than in t,he hydrocarbon alone. Although fewer comparable data are available for urea adducts of straight-chain hydrocarbon derivatives with polar groups such as OH or COOH, it might be expected that the rotational motion of these molecules would be hindered because of intermolecular hydrogen bonding with urea molecules and with hydrogen-bonding groups on adjacent molecules in the urea channel. An indication that this is the case is provided by recent epr mea~urements’~ of urea-decanoic acid and urea-sebacic acid adducts which show a much smaller rotational amplitude for the p protons of the acid radicals than for corresponding protons of ester radicals. In view of the continuing interest in the theoretical and practical aspects of urea adducts,14 additional information pertaining to the motional properties of the enclosed molecules would be desirable. In this connection we have measured the proton resonance spectra for urea-dc adducts of the even-numbered straightchain hydrocarbons from n-ClzHz6t o n-C18H38and for the adducts of stearic and palmitic acids, over a wide range of temperature. The line-width changes and second-moment curves for these adducts are discussed in terms of the various modes of motion possible.
KIMIKOUMEMOTO AND STEVEN S. DANYLUK
derivative signal, corrected for a small contribution (
