(5) W. Dew. Horrocks, Jr., J. P. Sipe 111, and D. Sudnick in "NMR Shift Reagents", R. E. Sievers, Ed., Academic Press, New York, N.Y., 1973,p
53. (6)W. Dew. Horrocks and J. P. Sipe 111, J. Am. Chem. SOC., 93, 6800 I1 971 ). (7)2. W.' Wolkowski, C. Beaute, and R. Jantzen, Chem. Commun.. 619 (1972). (8)K. Ajisaka and M. Kainosho, J. Am. Chem. SOC.,97, 330 (1975). (9)C.M. Dobson, R. J. P. Willlams and A. V. Xavier, J. Chem. SOC.,Dalton Trans., 2662 (1973). (10) J. Reuben, J. Magn. Reson., 11, 103 (1973). (11) R. M. Golding and M. P. Halton, Aust. J. Chem., 25, 2577 (1972). (12)W. B. Lewis, J. A. Jackson, J. F. Lemons, and H. Taube, J. Cbem. Phys., 36, 694 (1962). (13)J. Reuben and D. Fiat, J. Cbem. Phys., 51, 4909 (1969). (14 J F Desreux and C. N. Reilley, submitted. (151 M. R. Wilicott, R. E. Lenkinski, and R. E. Davis, J. Am. Chem. SOC.,94, 1742 (1972);R. E. Davis and M. R. Willcott, ibid., 94, 1744 (1972). (16)P. G. Hoei, "Introduction to Mathematical Statistics", John Wiley, New York, 1971,p 164. (17) Reference 1, p 505.
(18) B. F. G. Johnson, J. Lewis, P. McArdle and J. R. Norton, J. Chem. SOC., Chem. Commun., 535 (1972). (19)0.A. Gansow, P. A. Loeffler, R . E. Davis, M-R. WiIIcott, and R . E. Lenkinski, J. Am. Chem. Soc., 95, 3389,3390 (1973). (20)M. Hirayama, E. Edagawa, and Y. Hanyu, J. Chem. SOC.,Chem. Commun.. 1343 (1972). (21)J. Albertsson, Acta. Chem. Scand., 24, 1213 (1970);J. Albertsson. bid., 26, 985 (1972);J. Albertsson, ibid., 26, 1005 (1972);J. Albertsson, ibid., 26, 1023 (1972). (22)H. Donato, Jr., and R. P. Martin, J. Am. Chem. Soc., 94, 4129 (1972).
RECEIVEDfor review March 10, 1975. Accepted July 23, 1975. The authors wish to acknowledge research support from the National Institutes of Health, the National Science Foundation, U.S. Army Research Office-Durham, and the F.N.R.S. of Belgium. J.F.D. is Charge de recherche an F.N.R.S.-Belgium.
Chemical Shifts and Protonation Shifts in Carbon- 13 Nuclear Magnetic Resonance Studies of Aqueous Amines Joseph E. Sarneski, Henry L. Surprenant, Frederlck K. Molen, and Charles N. Reilley Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, N.C. 275 14
Experimental 13C NMR chemlcal shlfts are glven for a group of aliphatic monoamines, amlno alcohols and amino ethers, cyclohexylamlnes, and dlamlnes; 13C spectra were obtalned in aqueous solutlon for the free base and protonated form of all molecules. A method for the prediction of the 13C chemlcal shlfts of the aqueous free monoamlnes has been developed as an extenslon of the parameters of Llndeman and Adams; application to the other amino systems is discussed. The sign and magnitude of the monoamine 13C protonation shifts reveal a marked dependence on molecular structure. The effects of Important structural features on observed 13C protonation shlfts have been incorporated into a h e a r parameterlzatlon scheme. Comparlson of experlmental 13C protonation shifts and calculated (CNDO) charge density changes falled to yleld useful results.
Carbon-13 NMR spectrometry provides a useful tool for investigating structural features of complex molecules in solution. Because of the wide shift range of 13C NMR (200 ppm vs. 10 ppm for 'H NMR) and the singlet peaks obtained under usual proton noise decoupling, I3C NMR often allows the observation of distinct reasonances for all non-equivalent carbons in complex molecules. The assignment of these resonances is of paramount importance and is greatly aided by available additivity relationships for 13C shifts of hydrocarbons and for the effect of various substituent groups ( 1 , 2 ) . An often useful method for making 13C resonance assignments in molecules possessing basic sites [NHz ( 3 ) , COO( 4 ) , PhO- ( 5 ) ,RS-] is to monitor the chemical shifts as a function of pH. The change in chemical shift upon protonation (6 protonated form - 6 free base) may be termed the "protonation shift" and exhibits characteristic trends (magnitude and direction) for 'H (6, 7) and I3C nuclei (3-5) in the vicinity of the basic center. Advantage of this assignment method has been made in 13C studies of amino acids ( 8 ) , peptides (9, 1 0 ) and antibiotics ( I I ) , all of which possess amino functionalities. 2116
Although the amino group protonation has received attention for spectral assignment purposes, a systematic study of the effects of molecular structure on 13C protonation shifts has not been reported. Brown (3) characterized the 13C shifts of linear aliphatic amines from which can be derived rudimentary protonation parameters for CY, p, y, and 6 carbon nuclei. More recently Morishima and coworkers (12) have described the protonation shifts for various substituted piperidines which show some deviation from the behavior described by Brown ( 3 ) .The purpose of this work is to explore the variation in protonation shifts of 13C nuclei in aliphatic amines with changes in substitution at the amine center and adjacent carbon atoms. A parameterization scheme is developed which will allow prediction of I3C protonation shifts for aliphatic amine compounds. A similar treatment is presented for the calculation of l3C shifts of amines in aqueous solution.
EXPERIMENTAL Materials. Amines were obtained commercially whenever possible. Certain tertiary amines possessing one or two N-methyl substituents were synthesized from the appropriate secondary or primary amines using a standard method (13). N-Methyl-tert-butylamine was prepared by the procedure of Dannley et al. (14). For study, amines (and their hydrochlorides) were dissolved in Dz0 to give -30-40% solutions where possible; however, in many highly substituted amines, solubility was limited to less than 2%. Measurements. FT I3C NMR spectra were obtained a t 25.2 MHz on a Varian XL-100-15 spectrometer a t 40 f 2 "C probe temperature. Samples were generally run in 10-mm tubes with proton noise decoupling. Solvent deuterium signal furnished an internal lock. Normally a 2500-Hz spectral width was examined using 8K data points. Using 30-40% solutions, 100 transients provided good spectra; but for sparingly soluble amines as many as 10,000 transients were required to gain adequate S/N in the frequency spectrum. Large pulse angles (60-70°) were employed except for tertbutyl compounds (here only 10-15') where the nonprotonated carbon saturates easily. Spectra were obtained with internal dioxane [and tetramethylammonium (TMA) chloride] as reference but are cited referenced to external TMS. The chemical shift of dilute aqueous dioxane and TMA measured in a 10-mm tube fitted with a concentric sealed capillary of neat TMS was found to be
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
+ 67.73 ppm Gint, TMA + 56.54 ppm
G e x t . ~=~aint. s Dioxane 6ext.TMS
=I
CH) as calculated with Lindeman and Adams’ constants (16);
For individual solutions, spectral reproducibility was better than 10.1 ppm. Free (or protonated) amine data were obtained in solutions whose pD measured >2.5 units above (or below) the pK of a given species. pD was measured using a combination electrode with an Orion Model 601 digital pH meter. pD adjustments were made with concentrated HCl or a concentrated solution of KOH in DzO.
RESULTS The 13C spectra of 48 aqueous aliphatic monoamines (free base and protonated forms) were investigated and data for the 179 unique 13C nuclei in these molecules are summarized in Table I. The labels a , 6, y, and 6 describe carbon nuclei one, two, three, and four bonds removed from the amine nitrogen, N-C,-C,&-C6. Data are presented as aqueous free amine 13C chemical shifts and values for the characteristic changes in 13C shift on amine protonation, i.e. “protonation shifts”, A, are given in parentheses. Satisfactory assignment of the resonances in most of these compounds could be accomplished by the use of (a) well documented effects on 13C shifts of the electronegative amine center (1, 2), (b) the systematic effects on 13C shifts engendered at a and p carbons by increasing substitution on a hydrocarbon chain (1, 2 ) , (c) in some cases, the relative peak intensities, and (d) in protonated amines, cu-carbons often exhibit splitting of
