Surface Electrostatic Immobilization of Thin Layers of Water on Silver Halide. Experimental and Calculated Infrared Spectrum of Cyclic Trimer of Water and a Ponderal Isotope Effect

Abstract

Evaporation of water on a planar AgX surface leads to a strongly bound monolayer for which IR spectra display the marker peaks for modest numbers of oligomers. From 700-1800 spectra for each isotopomer, H(2)O(16) and H(2)O(18), a pair was selected with moderate intensity at 1616 cm(-1) (a peak reported for the cyclic trimer of water) from the monolayer portion of the experiment. Every selected spectrum had lesser peaks for other oligomers. The sum of a spectroscopic pair reveals the vibrational spectra of the cyclic trimers of H(2)O(16) and H(2)O(18). All fundamentals in the mid-IR are seen including the bending, OH stretching, and intramolecular H-bonding regions, the last never previously recognized. The relative prevalence of cyclic trimer can be attributed to the "low" water concentration on the surface. In addition, a ponderal effect leads to higher concentrations of cyclic trimer in the H(2)O(18) spectra than in the H(2)O(16) spectra and allows observation of combination bands in the H(2)O(18) spectra, representing a new type of isotope effect. The spectroscopic results for the two water isotopomers are much more extensive than those obtained through matrix isolation. Remarkably complete spectra of the cyclic trimer are obtained for the first time, especially for H(2)O(18). DFT calculations with the cyclic trimer on a simplified model for the AgCl surface yield spectra consistent with the experimental spectrum. The technique can be extended to other oligomers of water and many other OH compounds.

N‐Methyl‐trimethylacetamide in Thin Films Displays Infrared Spectra of π‐Helices, with Visible Static and Dynamic Growth Phases, and then a β‐Sheet

“…The first point was to search for the 3 10 -helix, using expanded TFIS (XTFIS), in which part of the Figure 2 is expanded by approximately six times, as shown in the early spectra in The simplest (minimal) peptide model is HCONHCH 3 . An increase in the p-helix content with increased substitution in the acyl portion suggested the examination of N-methyl-trimethylacetamide) (NMT).…”

“…It is important to note that the quality of the spectra arises from the immobilization of polar molecules on polar surfaces, such as the AgX element used for the attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic measurements. [2][3][4] …”

N‐Alkylacylamides in Thin Films Display Infrared Spectra of 310‐, α‐, and π‐Helices with Visible Static and Dynamic Growth Phases

“…[6] TFIS has been used to make a spectroscopic identification of the oligomers of water [cyclic trimer, cyclic pentamer, cyclic hexamers (boat and chair), and bicyclic hexamers]. [7,8] TFIS utilizes a short acquisition time (revealing transient structures), limited IR energy (low signal, Sg, avoiding disruption of transient species), [5,6] and multiple reflections on planar AgX as the substrate for Fourier transform IR spectroscopy-attenuated total internal reflectance (FTIR-ATR), with limited penetration depth and molecular motion restricted by electrostatic immobilization. [8] .…”

“…[7,8] TFIS utilizes a short acquisition time (revealing transient structures), limited IR energy (low signal, Sg, avoiding disruption of transient species), [5,6] and multiple reflections on planar AgX as the substrate for Fourier transform IR spectroscopy-attenuated total internal reflectance (FTIR-ATR), with limited penetration depth and molecular motion restricted by electrostatic immobilization. [8] . Continuous purging with a slow stream of dry N 2 removes any extraneous water and is an important feature of the original design, [2] as shown in Figure S1 in the Supporting Information.…”

“…TFIS has been used to make a spectroscopic identification of the oligomers of water [cyclic trimer, cyclic pentamer, cyclic hexamers (boat and chair), and bicyclic hexamers] 7. 8…”