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Carbon-13 Chemical Shifts of the Carbonyl Group. IV. Dilution Curves for Acetic Acid in Representative Solvents1-’ Gary E. Maciel and Daniel D. Traficante
Contribution f r o m the Department of Chemistry, University of California, Davis, California, and The Frank J. Seiler Laboratory, O f i c e of Aerospace Research, U. S. Air Force Academy, Colorado. Received August 30, 1965 Abstract: C13chemical shifts of the carbonyl group of acetic acid were determined in the solvents water, acetone, chloroform, and cyclohexane over the range of volume fractions of acetic acid from 1.0 to about Relatively large dilution effectson the C13shifts were observed with acetone as solvent, a smaller effect with chloroform, and only small effects in water or cyclohexane as solvents. A curious hump was found in the water dilution curve. These results are understandable in terms of the influences of making and/or breaking of hydrogen bonds between the carboxyl group and various species.
T
he structural identity of the species present when carboxylic acids are present in solutions or in their pure gaseous, liquid, or crystalline states has been a subject of interest for many year^.^!^ Various physical methods 4including infrared,6- 12,19,2R Raman, 4 , 5 , l9 and proton magnetic r e ~ o n a n c e ~ ~ - ~spectroscopy ~~~3-~5 have been applied to the problem of distinguishing between the monomeric, dimeric, and other species. In the light of the considerable contribution made by previous proton n.m.r. investigations to the understanding of these systems, we have applied C13 magnetic resonance techniques, which seemed ideally suited to such a study on the basis of the reported effects of hydrogen bonding on carbonyl CI3 ~hifts~~z9-32 and on (1) Supported in part by the National Science Foundation under Grant GP 3776. (2) Part 111: G. E. Maciel and J. J. Natterstad, J . Chem. Phys., 42, 2752(1965). (3) Reproduction in whole or in part is permitted for any purpose by the U. S. Government. (4) G. C. Pimentel and A. L. McClellan, “The Hydrogen Bond,” W. H. Freeman and Co., San Francisco, Calif., 1960, Chapters 3 and 4. (5) G. A. Allen and M. A. Caldin, Quart. Rev. (London), 7, 255 (1952). (6) L. J. Bellamy, R. F. Lake, and R. J. Pace, Spectrochim. Acta, 19, 443 (1963). (7) J. D. S . Goulder, ibid., 15, 657 (1959). (8) S. Bratoz, D . Hadzi, and N. Sheppard, ibid., 8, 249 (1956). (9) D. Hadzi and M. Pintar, ibid., 12, 162 (1958). (10) R. E. Lunden, F. E. Harris, and L. I
