Position-specific secondary deuterium isotope effects on basicity of pyridine.
Secondary isotope effects (IEs) on basicities of various deuterated pyridine isotopologues and of 2,6-lutidine-2,6-(CD(3))(2) have been accurately measured in aqueous solution by an NMR titration method applicable to a mixture. Deuteration at any position of pyridine increases the basicity, but the IE per deuterium is largest for substitution at the 3-position and smallest for the 2-position, which is closest to the site of N-protonation, smaller even than that for 2-CD(3) substitution. Computations at the B3LYP/cc-pVTZ level overestimate the magnitude of the measured IEs but largely reproduce the variability with isotopic position. Because the calculated IEs are based on changes in vibrational frequencies on N-protonation, the correspondence between calculated and experimental IEs implies that they arise from zero-point energies, rather than from inductive effects.
Perrin
Charles L
CL
Department of Chemistry & Biochemistry, University of California-San Diego, La Jolla, California 92093-0358, USA. cperrin@ucsd.edu
Karri
Phaneendrasai
P
D003198:Computer Simulation; D003903:Deuterium / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D011725:Pyridines / Q000737:chemistry*; D012996:Solutions
D011725:Pyridines; D012996:Solutions; D003903:Deuterium