摘要:
NUMBER 19, 1966 673 The Chemical Shift of the Hydroxide Ion By G. M. SHELDRICK ( Univei&ty Chemical Laboratory, Cambridge) IN an n.m.r. study of aqueous solutions of acids and bases, Gutowsky and Saikal showed that if the proton chemical shift was assumed to be the weighed mean of the H,O and H30+, OH- shifts respectively, then the 7-values of H,O+ and OH- were both about -5 p.p.m. Although this value for H30+ is consistent with later experimental estimates in other s0lvents,29~ no other experi- mental estimate of the chemical shift of OH- seems to have been reported. Mushel"' has obtained theoretical estimates in good agreement with the above values using an electrostatic model. Here we present evidence that this value for the chemical shift of OH- is in error by 13 p.p.m.The proton magnetic resonance spectra of solutions of tetramethylammonium hydroxide in dry ammonia at room temperature were recorded a t 40 Mc./sec. using cyclopentane as internal standard. We shall assume that T = 8-491 & 0.002 p.p.m. for cyclopentane, which we find for a dilute solution in CC1, containing Me,Si, in good agreement with the literature value (8.49 4 0.02 p.p.m.) .5 The tetramethylammonium ion was identified as a well resolved 1 : 1 : 1 triplet, J = 0.61 & 0.02 c./sec., a t r = 6.58 & 0-01 p.p.m., exactly the values we find for a solution of tetra- methylammonium chloride in dry ammonia. The spectra of the solutions of tetramethylammonium hydroxide, but not those of tetramethylammonium chloride, showed a sharp line at T = 7.88 -& 0.01 p.p.m., of approximately 1/12 the total intensity of the tetramethylammonium signal.Thus we assign this to OH-. In confirmation of this assignment saturated solutions of tetramethyl- ammonium and lithium hydroxide gave weak signals a t r = 7-890 4 0.005 and 7.701 f 0-003674 CHEMICAL COMMUNICATIONS p.p.m. respectively in 1,2-dimethoxyethane con- taining a little Me,Si as internal standard. Neither signal was found in a blank. The 1,2-dimethoxy- ethane was dried over potassium anthracene, the ammonia over potassium, and vacuum-line tech- niques were used throughout. A small amount of sodamide was added to the solutions in ammonia to remove the last traces of water; this caused the ammonia triple-line spectrum to collapse to a broad single line in the expected manner.6 There seems to be a simple qualitative explana- tion for the aqueous solution results.Hydrogen- bonding invariably gives rise to a large low-field shift for the proton involved. Whereas it is unlikely that the OH- proton H-bonds to any- thing, in aqueous solution about three water molecules will be strongly H-bonded to the oxygen atom of the OH- ion. A low-field shift of about 4 p.p.m. per H-bond would result in a weighed mean proton chemical shift consistent with the observed values, since the value assigned here to OH- is about 3 p.p.m. to high-field of water. This model also rationalises the small solvent dependence of the OH- chemical shift found above; although ion pairing probably pre- dominates for lithium hydroxide in 1,2-dimethoxy- ethane, the chemical shift assigned to OH- in this system differs by only -0.2 p.p.m. from the tetramethylammonium hydroxide value. However it is still possible to argue qualitatively that the strong electrostatic field should reduce the shielding of the OH- proton relative to H,O, since r = 9.7 p.p.m. for water in the gas phase,' to high-field of the value assigned here to OH-. (Received, August 30th, 1966; Corn. 642.) H. S. Gutowsky and A. Saika, J . Chem. Phys., 1953, 21, 1688. C. MacLean and E. L. Mackor, J . Chem. Phys., 1961, 34, 2207. R. Radeglia, 2. phys. Chem. (Leipzig), 1966, 231, 339. J. I. Musher, J . Chem. Phys., 1961, 35, 1989. T. J. Swift, S. B. Marks, and W. G. Sayre, J . Chem. Phys., 1966, 44, 2797. W. G. Schneider, H. J. Bernstein, and J. A. Pople, J . Chem. Phys., 1958, 28, 601. ii K. B. Wiberg and B. J. Nist, J . Amer. Chem. SOL, 1961, 83, 1226.
ISSN:0009-241X
DOI:10.1039/C19660000673
出版商:RSC
年代:1966
数据来源: RSC