The common effect of hydrophobic and electrophilic types of hydration dependent on protonation in the sec-butylamine/HCl/H2O system studied by FTIR spectroscopy

Hydration of sec-butylamine in an aqueous solution was studied by FTIR spectroscopy. HDO diluted in H2O was used as a probe of water environment. Quantitative difference spectra method of data analysis was performed to remove the contribution of bulk water in the solution spectra. As HCl is introduced to the system, the main component band of affected HDO spectrum, ascribed to hydrophobicaly hydrated water, undergoes a gradual shift from ca. 2511 cm−1 for the free amine, to about 2520 cm−1 for the protonated ammonium cation. The process of amine N atom protonation is evidenced by systematic buildup of the ca. 2250 cm−1 band, attributed to N–D oscillators, with decreasing pH. In the case of free amine and neutralized solutions at high pH, a sharp, low intensity band at ca. 2450 cm−1 is also visible and attributed to the HDO molecules forming hydrogen bond directly to the amineʹs lone electron pair. The hydrophobic effect of alkyl side chain is influenced by the polarizing power exerted on water molecules by the electrophilic amine group. This polarizing effect is cooperatively transmitted to the interactions inside the clathrate-like water ‘cage’ around the alkyl group, effectively strengthening it.