Thin‐Film Infrared Spectroscopy of Acetonitrile

Abstract

Acetonitrile and [D3]acetonitrile in the vicinal region of a planar AgX fiber contain linear dipole-dipole linked oligomers as shown by 1) comparison of infrared band intensity ratios in the gaseous and condensed phases and 2) remarkable plots of absorbance (C--N stretch) versus time during evaporation from an AgX planar fiber element. The plots (CH3CN 2252 cm(-1), CD3CN 2262 cm(-1)) reveal the presence of octamers, hexamers, tetramers, and dimers along with some heptamer, trimer, and monomer structures. A novel isotope effect arises from the somewhat smaller size of the CD3CN resulting in an increase in the CN band intensity. The organized oligomers may be termed pseudocrystals and are the main components responsible for absorption intensity in the infrared spectrum of acetonitrile, on the AgX planar fiber or in an IR cell.

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

“…These additional facts may be important in understanding the mechanisms of the conversions. Although a fortunate choice of the experimental conditions may be important for the observed behavior, we have also observed steps (as in the evolution of the helices) in the evaporation of acetonitrile [5] and other solvents like dichloromethane (unpublished results) in the regular measurement of spectra on AgX.…”

“…[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] .…”

“…The 1670-1674 cm À1 peaks have decreased, whereas a maximum has appeared at 1655 cm À1 and the absorption of the p-helix has disappeared, all over a period of 20.9 s. The darker lines arise from the overlapping of several spectra ("steps" for the static phase of helix growth, see Section 2.6) (steps) is about 0.02 per step, in agreement with our estimates for single-layer deposition for acetonitrile . [5] 2.1.4. The p-Helix of NMA Another species was formed after the loss of the a-helix, and, as pointed out in the Introduction, the most reasonable assignment was the p-helix.…”

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

“…[1] We acquired a series of NMT spectra at Sg 1000, and plotted the absorbances at a number of wavelengths versus time, as shown in Figure 1. For the first time, in many plots of this type, there were sharp changes in the absorbances at two of the wavelengths, an increase at 1637 cm À1 (p-helix) and a decrease at 1653 cm À1 (first p -helix) with a ratio (increase/decrease) of 2.2:1.…”

“…[1] An important result was that some of the infrared (IR) spectroscopic properties of a p-helix were found. The substantial increase in the p-helix content with increased substitution in the acyl portion suggested that an amide with a larger acyl group should be examined.…”