Strained hydrogen bonding in imidazole trimer: a combined infrared, Raman, and theory study.

In this work, a careful analysis of anharmonic couplings in NH and some CH stretch modes of imidazole is carried out. This includes IR and Raman spectra of the isolated molecule and aggregates up to the trimer, together with two different theoretical approaches to the calculation of anharmonic shifts and absolute band positions. The imidazole dimer is vibrationally characterized for the first time in vacuum isolation under supersonic jet conditions, showing substantial shifts from previous helium droplet experiments and evidence for Fermi resonance for the hydrogen-bonded NH stretch.

Helium nanodroplet study of the hydrogen-bonded OH vibrations in HCl-H₂O clusters.

Mixed (HCl)N(H2O)M clusters have been assembled in He droplets from the constituting molecules. Spectra of the clusters were obtained in the range of hydrogen-bonded OH vibrations (3100-3700 cm(-1)) by infrared laser depletion spectroscopy. The observed bands were assigned to cyclic hydrogen-bonded aggregates containing up to two HCl and three H2O molecules.

The OH stretching spectrum of warm water clusters.

The O-H stretching fundamental spectra of water clusters are well known from cold jet expansions, but their detection in the atmosphere remains elusive. Mild supersonic jet expansions of neat water vapor are used to detect trimers and tetramers with intermediate thermal excitation. Spectra close to the evaporative limit are obtained by interfering neighboring expansion fans in a multi slit nozzle experiment.

Infrared absorption imaging of 2D supersonic jet expansions: Free expansion, cluster formation, and shock wave patterns.

N2O/He gas mixtures are expanded through a 10 × 0.5 mm(2) slit nozzle and imaged by direct absorption vibrational spectroscopy, employing a HgCdTe focal plane array detector after interferometric modulation. N2O cluster formation in the free supersonic expansion is visualized.

Communication: Where does the first water molecule go in imidazole?

Supersonic jet FTIR spectroscopy supplemented by (18)O substitution shows unambiguously that water prefers to act as an O-H···N hydrogen bond donor towards imidazole, instead of acting as a N-H···O acceptor. Previous matrix isolation, helium droplet, and aromatic substitution experiments had remained ambiguous, as are standard quantum chemical calculations. The finding is supported by a study of the analogous methanol complexes and by higher level quantum chemical calculations.