Knowledge Bank

The structures and dynamics of 2-hydroxypyridine-$(water)n$ clusters (with n=1 and 2) have been investigated using resonant two-photon ionization time-of-flight mass spectroscopy (R2PI-TOFMS), IR-UV hole-burning, and resonant ion-dip infrared spectroscopy (RIDIRS). The double resonance technique of RIDIRS provides both wavelength and mass selectivity in recording the hydride stretch infrared spectrum (2200 to $3800cm-1$) of a chosen cluster. The RIDIR spectrum of 2HOP monomer contains a narrow transition at $3598cm1$, about $55cm-1$ lower in wavenumber than those of the corresponding hydroxy groups in phenol and cis/trans 2-naphthol. This suggests the presence of a weak intramolecular H-bond. The spectra of $2HOP-(H2O)1-2$ contain sharp free OH stretch transitions near $3715cm-1$ and H-bonded OH stretch transitions in the $3100-3500cm-1$ region, which are remarkably broad (approaching $100cm-1$ in $2HOP-(H2O)2)$ and show distinct sub-structure indicative of strong coupling to background states. A comparison with the results of density functional theory calculations indicates that the water molecules in $2HOP-(H2O)1$ and $2HOP-(H2O)2$ form bridges between the OH and N groups of 2HOP, accepting a H-bond from the OH group and donating a H-bond to the nitrogen lone pair. Possible reasons for the unusual breadth of these transitions will be put forward.