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

Abstract

Deuterium fractionation factors were determined by the ¹H and ¹³C NMR methods in aqueous solution for PhCH₂NLMe₂⁺ (ϕ = 1.47 ± 0.05), PhCH₂OL (ϕ = 1.04 ± 0.06), PhCO₂L (ϕ = 1.04 ± 0.08), and CH₃CO₂L (ϕ = 0.99 ± 0.02). The medium effect for transferring PhCH₂NMe₂ from H₂O to D₂O, ϕ = 1.025 ± 0.003, was also determined by partitioning this substance between water and immiscible organic solvents, and a UV spectroscopic method was used to measure the solvent isotope effect on the acid ionization of PhCH₂NLMe₂⁺, (Q_a)_H/(Q_a)_D = 4.88 ± 0.16. This solvent isotope effect agrees well with the value predicted using the relevant fractionation factors, (Q_a)_H/(Q_a)_D = 4.38 ± 0.28. The unusually large value of ϕ for PhCH₂NLMe₂⁺ is attributed to stiffened bending vibrations of its N–L bond imposed by the tetrahedral structure of the ion and the bulk of its methyl groups.