RESUMO
Ion-pair interactions between pyridinium cations and various carboxylate anions are explored using noisy light based coherent anti-Stokes Raman scattering (I(2)CARS). Binary mixtures of pyridine and various carboxylic acids (including halo-acetic acids, straight-chain carboxylic acids, and pivalic acid) are prepared. A Brønsted type acid-base reaction occurs in these mixtures to create pyridinium and carboxylate ions. Both pyridine, itself, and pyridinium have strong I(2)CARS signals originating from their ring breathing modes. The vibrational frequency of the ring breathing mode for pyridine is blue-shifted by hydrogen bonding, and that same mode for pyridinium is red-shifted by ion-pair interaction. Frequency shift data for the ring breathing mode of pyridine and pyridinium are presented. These data are discussed in terms of a simplistic model for the electronic behavior of these compounds.
RESUMO
The effects of hydrogen bonding on the ring stretching modes (both ring breathing and triangle) of pyridine are experimentally investigated using noisy light based coherent Raman scattering spectroscopy. Three systems, pyridine/formamide, pyridine/water, and pyridine/acetic acid, provide varying degrees of strength for the diluent-pyridine hydrogen bond complex. Formamide forms a relatively weaker hydrogen bond, while acetic acid essentially fully transfers a proton to pyridine. Both dilution studies and temperature studies are performed on the three systems. Together, these provide a broad context in which a very simple model for the electronic behavior of pyridine is formulated. This model is based on a molecular orbital picture and electrostatic arguments, and it well explains the observed experimental results. Additionally, a new mechanism for the line broadening of the ring breathing mode for the pyridine-water hydrogen bonded complex is proposed.
