Complex Structures in Thin Films Detected By Infrared Spectroscopy

Abstract

Thin films are ubiquitous and important in chemistry, materials science, biology and engineering, but are difficult to study on the molecular level. Infrared spectroscopy is useful because of the rich detail in molecular information and the speed of acquisition of spectra. We have developed an attenuated total internal reflectance (ATR) technique for samples in thin films on a planar AgX (X=Cl, Br) fiber that reveals unsuspected interactions. We initially discovered surface enhanced infrared absorption (SEIRA) for trypsin and p‐nitrobenzoic acid and more recently, complex structures in thin films of alkanols (some known) and acetonitrile (previously unknown). The structures are H‐bonded oligomers for alkanols and dipole‐dipole oligomers for acetonitrile. Almost all molecules associate in liquid and solid form (gas phase clusters) as a result of dipole‐dipole, hydrogen‐bonding, polarization, and dispersion interactions. We have generalized previous indications: there is an infrared signature of increased absorption intensity or changes in spectrum for many associations. We now report that the peptide model N‐methylformamide associates rapidly (2 sec) to an α‐helical oligomer (λmax 1652 cm−1) which then is transformed (10 sec) into the weakly absorbing cyclic hexamer. Peaks for monomer, dimer and trimers are observed in the first 0.5 sec. The implications for peptide IR will be discussed.