The hydrogen-bonded 2-pyridone dimer model system. 1. Combined NMR and FT-IR spectroscopy study.
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-d(2) using a combined NMR and Fourier transform infrared (FT-IR) approach. Temperature-dependent (1)H NMR spectra indicate that at low temperatures (<200 K) pyridone in solution predominantly exists as a cyclic (PD)(2) dimer, in exchange with PD monomers. At higher temperatures a proton exchange mechanism sets in, leading to a collapse of the doublet of (15)N 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-CD(2)Cl(2) 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).
Szyc
Łukasz
Ł
Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Strasse 2A, D-12489 Berlin, Germany.
Guo
Jing
J
Yang
Ming
M
Dreyer
Jens
J
Tolstoy
Peter M
PM
Nibbering
Erik T J
ET
Czarnik-Matusewicz
Bogusława
B
Elsaesser
Thomas
T
Limbach
Hans-Heinrich
HH
D019281:Dimerization; D006860:Hydrogen Bonding; D007536:Isomerism; D007769:Lactams / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D011728:Pyridones / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry; D012997:Solvents / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared
D007769:Lactams; D011728:Pyridones; D012996:Solutions; D012997:Solvents; C017679:2-hydroxypyridine