Knowledge Bank

Fluorescence-dip infrared spectroscopy (FDIRS) and IR-UV hole-burning spectroscopy have been used to probe the structures and dynamics of hydrogen bonding in the $S0$ and $S1$ states of 2-pyridone (2PYR), its dimer $(2PYR)2$, and its water-containing clusters $2PYR-(H2O)n$ (with $n=1$ and 2). This double resonance scheme combines the sensitivity of laser-induced fluorescence with the structure-resolving power of IR spectroscopy. 2-pyridone is the keto form of 2-hydroxypyridine (2HOP) and contains the same H-bonding sites that are found in the pyrimidine base, uracil (U). The pyridone dimer provides a first glimpse of an ``improper'' base pairing intcraction in modeling the U-U dimer. The 2-pyridone dimer is known to be a cyclic, doubly H-bonded structure with $C2h$ symmetry. The infrared spectrum of this dimer has a single NH stretch which carries all the oscillator strength in this region. It produces an intense absorption with a maximum just below $2700cm-1$, and a long tail stretching nearly $200cm-1$ beyond this maximum. Distinct, sharp sub-structure with irregular spacing of about $5-10cm-1$ and widths of $2-5cm-1$ underlies this envelope. The implications of this sub-structure for the strong coupling present in the pyridone dimer will be discussed.