The Hydrogen-Bonded 2-Pyridone Dimer Model System. 1. Combined NMR and FT-IR Spectroscopy Study

Abstract

2-Pyridone (PD), converting to 2-hydroxypyridine (HP) through a lactam−lactim isomerization mechanism, can form three different cyclic dimers by hydrogen bond formation: (PD)2, (PD−HP), and (HP)2. We investigate the complexation chemistry of pyridone in dichloromethane-d2 using a combined NMR and Fourier transform infrared (FT-IR) approach. Temperature-dependent 1H NMR spectra indicate that at low temperatures (2 dimer, in exchange with PD monomers. At higher temperatures a proton exchange mechanism sets in, leading to a collapse of the doublet of 15N labeled 2-pyridone. Linear FT-IR spectra indicate the existence of several pyridone species, where, however, a straightforward interpretation is hampered by extensive spectral overlap of many vibrational transitions in both the fingerprint and the NH/OH stretching regions. Two-dimensional IR correlation spectroscopy applied on concentration-dependent and temperature-dependent data sets reveals the existence of the (PD)2 cyclic dimer, of PD−CD2Cl2 solute−solvent complexes, and of PD−PD chainlike dimers. Regarding the difference in effective time scales of the NMR and FT-IR experiments, milliseconds vs (sub)picoseconds, the cyclic dimers (PD−HP) and (HP)2, and the chainlike conformations HP−PD, may function as intermediates in reaction pathways through which the protons exchange between PD units in cyclic (PD)2