Intramolecular Polarization Contributions to the pKa's of Carboxylic Acids Through the Chain Length Dependence of Vibrational Tag-Shifts in Cryogenically Cooled Pyridinium-(CH2)n-COOH (n = 1-7) Cations.

Abstract

The pK_a's of acids are known to depend on the ionic strength of an electrolyte solution, an effect that qualitatively results from electrostatic interactions of the acid and conjugate base with proximal ions. Here, we explore an intramolecular variation on this theme in which a carboxylic acid group is tethered to a pyridinium cationic charge center with increasingly long alkyl linkages in the series Py⁺-(CH₂)_n-COOH, with n = 1-7. The effective acidities of the carboxylic acid group in the isolated cations are determined by recording the red-shifts in the frequencies of the acid OH stretches upon attachment to D₂ and N₂ molecules, measured by using cryogenic ion spectroscopy. The short chains indeed lead to substantial increases in acidity, with the effect falling off in the range of n = 4-5. The response of the CO stretch on the acid head group is surprisingly strong and, in contrast to the expected red shift of the OH, displays a blue shift as the chain length is reduced. Electronic structure calculations recover these trends and indicate that the proximal cationic charge center acts to draw electron density away from the acid C═O bond. This acts to blue-shift the CO stretch while weakening the C-C bond that attaches the head group to the chain.