The infrared spectroscopy of hydrogen-bonded bridges: 2-pyridone-(water)n and 2-hydroxypyridine-(water)n clusters, n=1,2

Abstract

The water-containing clusters of the two tautomers 2-hydroxypyridine (2HP) and 2-pyridone (2PYR) are studied in the hydride stretch region of the infrared using the techniques of resonant ion-dip infrared spectroscopy (RIDIRS) and fluorescence-dip infrared spectroscopy (FDIRS). The results on 2PYR-(water)n build on previous high-resolution ultraviolet spectroscopy [Held and Pratt, J. Am. Chem. Soc. 115, 9708 (1993)] on the n=1,2 clusters and the infrared depletion spectra of Matsuda et al. [J. Chem. Phys. 110, 8397 (1999)] on the n=1 cluster. The 2PYR-W2 FDIR spectrum reflects the consequences of extending and strengthening the H-bonded bridge between N–H and C=O sites in 2PYR. The spectrum shows evidence of strong coupling along the bridge, both in the form of the hydride stretch normal modes and in the breadth of the observed infrared transitions. RIDIR spectra of the 2HP-Wn clusters are compared with those of 2PYR-Wn in order to assess the spectroscopic consequences of forming the analogous water bridges in the lactim tautomer. Density functional theory calculations are compared with the RIDIR spectra to deduce that the 2HP-Wn clusters are indeed water-containing bridge structures closely analogous to their 2PYR counterparts. The IR spectra of the 2HP-Wn clusters bear a striking resemblance to those of 2PYR-Wn. Potential reasons for the unusual breadth of the bridge XH stretches are discussed.