Ion-Pair Interaction in Pyridinium Carboxylate Solutions

Abstract

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.

The origin of the conductivity maximum vs. mixing ratio in pyridine/acetic acid and water/acetic acid

“…These caveats notwithstanding, what we observed from simulation ( clusters of Py(HAc) n (agreeing with Limbach 30 ), clustered anions Ac(HAc) n − (agreeing with Limbach 30 ), and pyridinium ions that were ion-paired but never seen to be free [agreeing with Fig. 10(a) from the work of Ulness 32,33 ]. Even the agreement with Ulness on the 1:1 mixture is remarkable: they find a nonzero but small ratio of free pyridine to bound pyridinium acetate pairs, while we found 1 free pyridine and 7 pyridine/acetic acid pairs.…”

“…32 However, Holbrey and co-workers 34 performed simulations to try to fit to neutron scattering data of several pyridine/CH 3 COOH mixtures, finding a distribution of (HAc) n chains, but very few pyridinium ions of any sort (free or ion-paired), in disagreement with the work of Ulness. 32,33 The idea that acidity might depend on the chain length could potentially be exploited in a quantitative attempt to reproduce the conductivity curve, but the uncertain existence of free pyridinium ions is a concern that AIMD simulations could help resolve.…”

The dehydration of N-acetylglucosamine (GlcNAc) to enantiopure dihydroxyethyl acetamidofuran (Di-HAF)

“…However, as the pK a of the acid decreases, the coordination between the pyridinium ion and the conjugated base becomes weaker. 27 The increased concentration of free pyridinium could be responsible for acceleration of the reaction.…”

Study of the Halogen Bonding between Pyridine and Perfluoroalkyl Iodide in Solution Phase Using the Combination of FTIR and19F NMR

“…As it is experimentally challenging to directly measure the strength of the halogen bond in a particular system, most research in the field has been focused on the geometry and structural impact. Like hydrogen bond [41,42], halogen bond often leads to observable changes in the vibrational spectra of the participating molecules. Focusing on the halogen bond impact on the perfluoroalkyl iodide, Messina et al [43] led the study of the systems between 𝛼, 𝜔-diiodoperfluoroalkanes, and other electron donors using FTIR and Raman spectroscopy.…”