Thin film infrared spectroscopy on planar silver halide: a new technology for water and other liquids in the mesoscopic domain

Abstract

The essentially unexplored phenomenon of electrostriction on polar solids is now exploited to restrict the motion of polar liquids in infrared spectroscopic experiments. We have found a wide variety of water oligomers for which theoretical properties had previously been calculated utilizing a recently developed cell for thin film infrared spectroscopy. Short acquisition times (to detect transient species) and low signal energies (less disruption of transient species) on planar silver halide (many reflections, limited penetration depth (therefore, small sample size), restraint of molecular motion by electrostriction (thus making possible capture of transient spectra)) were utilized to yield heretofore unknown spectroscopic properties of water. The results included identification of five oligomers (cyclic hexamers (chair, boat), cyclic pentamer and bicyclic books (1, 2)). The new technique of lifetime tagging was used to track these species, leading to the selection of marker peaks for specific oligomers. Each oligomer possessed groups of peaks in the symmetric OH stretching region, for which a new type of model, isomotomers, supported its identification. Intramolecular H-bonding in books 1 and 2 provided extra support for the assignments. Other polar liquids (N-methylformamide, dichloromethane) are briefly discussed. The tetrahedral character of water appears to result from an average over many linear oligomers.