Exploring the boundaries of cryospectroscopy: a new approach towards separating monomer and complex spectra
26 February 2016
UAntwerp, Campus Drie Eiken, Promotiezaal Q0.02 - Universiteitsplein 1 - 2610 Antwerpen-Wilrijk
Liene De Beuckeleer
PhD defence Liene De Beuckeleer - Faculty of Science, Department of Chemistry
Weak intermolecular interactions such as hydrogen or halogen bonds perturb the vibrational modes of molecules. These perturbations can be studied by dissolving the molecules in liquid noble gases and measuring the solutions with infrared spectroscopy. Many studies have been carried out investigating the wavenumber shifts and integrated intensities of bands due to interactions between different molecules. But studies on interactions between identical molecules are scares. This is because the bands due to self-association of molecules only shift slightly from the monomer wavenumber and therefore completely overlap with the monomer bands. This overlap makes a trustworthy interpretation of the experimental spectra impossible.
To overcome this limitation new numerical methods have been developed to separate monomer and complex contributions of overlapping spectra. After recording spectra of different concentrations at constant temperature, the separated monomer and different complex spectra are obtained by fitting a polynomial to the measured absorbances at each wavenumber versus the absorbances at a reference wavenumber. This approach enabled us to thoroughly analyse the self-associating behaviour of HCl, ND3, pyrrole and acetone. Next the methodology was adjusted to separate overlapping spectra of mixtures of molecules. The new method was applied on spectra of CF3X with dimethyl ether and CF3X with acetone (X=Br or I).
For all studied systems the new methodology showed added value as it makes it possible to observe and identify the features due oligomers in liquid noble gasses, which was impossible with earlier procedures. Although acetone has been extensively used in previous infrared studies in liquid noble gases, its dimer bands have always escaped detection and were therefore never observed before. The use of the new methodology now results in a separated monomer and dimer contribution which proves that polynomial regression pushes the boundaries of cryospectroscopy as it extracts more detailed information from the recorded data than before. The boundaries were further explored by the analysis of spectra recorded for a mixture of acetone and the halogen bond donor CF3I. To conclude, it can be stated that least-squares fitting of polynomials to data recorded at constant temperature can be seen as a valuable tool for studying the formation of complexes held together by weak intermolecular interactions.