Hydrogen Isotope Fractionation Factors for Benzylamine and Benzylammonium Ion. Comparison of Fractionation Factors for Neutral and Positively-Charged Nitrogen-Hydrogen Bonds

Abstract

Deuterium-protium fractionation factors were determined for benzylamine and benzylammonium ion by the traditional 1H N M R method and for the benzylammonium ion by a newly devised 13C N M R method. The results, 4phCH2NL2 = 0.958 f 0.070 and 4phCH2NL,+ = 1.081 f 0.019, when combined with the solvent isotope effect on the ionization of benzylammonium ion, also determined here, K H / K D = 3.36 f 0.13, give @ P~C H~N L , += 0.80 f 0.13 as the medium effect for transfer of this ion from H20 to D20. These results show that introduction of a positive charge into an N-L bond does not decrease its fractionation factor as is the case for 0-L bonds, where ~O L + /~O L = 0.7. This difference is attributed to the tetrahedral structure of ammonium ions and the consequent stiffness of their bondbending vibrations. Elsevier:Amsterdam, 1976; Chapter 1.(5) The special symbol "8' is generally used for the fractionation factor of the hydronium ion.(6) (a) Kresge, A. J.; Allred, A. L.

Hydrogen isotope fractionation factors for N,N-dimethylbenzylammonium ion and some related species: an unusually strong preference for deuterium over protium

“…[α-13 C]Methyltrimethylammonium iodide was prepared previously. 4 The identities of these substances were confirmed by their 1 H NMR spectra. All other materials were best available commercial grades.…”

“…than that for the neutral N᎐L bonds of benzylamine, 2, φ = 0.96. 4 Examination of vibrational frequencies of NL 4 + and NL 3 as surrogates for PhCH 2 NL 3 + and PhCH 2 NL 2 showed that the expected bond weakening accompanying positive charge introduction was indeed reflected in lowered stretching vibra- † The special symbol 'l ' is generally used for the fractionation factor of the hydronium ion. ‡ The symbol 'L' denotes either protium or deuterium, i.e.…”

Escherichia coli Uracil DNA Glycosylase: NMR Characterization of the Short Hydrogen Bond from His187 to Uracil O2

“…One factor that may increase or decrease φ, but is unrelated to bond stretching frequency differences in the solvent and solute sites, is a change in bending frequencies accompanying H-bond formation in the solute (52). For instance, fractionation studies on N,N-substituted benzylammonium ions by Kresge and co-workers have shown that anomalously high φ values may result from compensatory increases in bond bending vibration frequencies upon H bond formation (53,54). Thus, a strong hydrogen bond that undergoes a decrease in stretching frequency as compared to solvent may have a larger than expected φ value because of a compensatory increase in a bending vibration frequencies.…”

Fast and efficient peptide bond formation using bis‐[α,α‐bis(trifluoromethyl)‐benzyloxy]diphenylsulfur. Part I

“…The first is the difference in basicity of these amines. Benzylamine has a p K a of 9.36 (43), whereas that of H‐ l ‐Phe‐OEt is 2 units lower (7.23,44). This difference in p K a makes the environment of the Boc‐ l ‐Ser(Bzl)‐OH/benzylamine‐mediated condensation more basic than that of the Boc‐ l ‐Ser(Bzl)‐OH/H‐ l ‐Phe‐OEt (Table 2, entry 3), with the consequence that racemization increased to 1.8%.…”