methanolic extract was filtered and mixed with an excess of a saturated solution of NaBPh, in MeOH. After that mixture had aged for several hours at -20 "C, the BPh4-salt of the quaternary ion product was isolated by filtration and recrystallized from MeCOEt or MeCOEt plus hexane.The nitrogen in the product in the reactant amine) was converted to NH, by Kjeldahl digestion and then to N, by reaction with NaOBr." The Kjeldahl digestion procedure was modifiedI8 to achieve quantitative conversion of pyridinium nitrogen, and the quantitative result of each digestion was verified by titration of the NH3 produced. The IsN abundances in the N2 were measured with a Micromass 602E isotope ratio mass (17) Shearer, G. B.: Kohl, D. H.: Commoner, B. Soil Sci. 1974, 218,Fish, U. B.; Collier, P. R. Anal. Chem. 1958, 30, 151-152. 308-316.spectrometer. The abundances, 6, and 6,, respectively, in N, generated from amine reactant and quaternary ammonium product were measured relative to N 2 generated from a standard sample of ammonium sulfate. [6 = 1000(R/RStd -l), where R and Rstd are the observed 1sN'4N/14N2 ratios in sample and standard.] The true value of the ratio of lsNI4N/l4N2 ratios from reactant and product, Ro/R,, was thus given by (lo00 + S,)/(lOOO + 6, ) . Most measurements of 6, were by direct Kjeldahl digestion of the amine, but digestions of product from complete reaction of amine with excess Me1 or MeOTf was shown to give the same values for 6, within the experimental reproducibility. Acknowledgment.The I4N/l5N isotope effect on K, has been measured for the conjugate acids of five tertiary amines: 4-methylpyridine (K,l4/K,l5 = 1.0226 f 0.0012), pyridine (1.021 1 i 0.0008), 3-acetylpyridine (1.0189 f 0.0008), quinuclidine (1.0135 f 0.0007), and N,N-dimethyl-4-methylaniline (1.0125 f 0.0008). These values are smaller than would be expected from the valence force fields for amines and their conjugate acids; this observed diminution of the isotope effect is attributed to hydrogen bonding by solvent water to the nitrogen of the unprotonated amine.
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