Hydrogen Bonding and the Inductive Effect in Crystalline and Solution Phases of Hexylamine:LiCF₃SO₃ and Dipropylamine:LiCF₃SO₃: Application to Branched Poly(ethylenimine)

Raman and infrared spectroscopy were used to study the nature of hydrogen bonding and the cation inductive effect in solutions of LiCF3SO3 dissolved in hexylamine, a primary amine, and dipropylamine, a secondary amine. Comparison of pure hexylamine and hexylamine dissolved in CCl4 established that the Raman band maximum of the symmetric stretching mode, νs(NH2), in pure hexylamine originates in molecules undergoing no hydrogen bonding interactions. The addition of LiCF3SO3 to hexylamine or dipropylamine shifts the frequencies of the solvent NH stretching modes by two effects: the breaking of hydrogen bonds and the cation inductive effect. Comparison of the infrared and Raman spectra allows (to some degree) the separation of these two effects. During these studies, crystalline compounds of hexylamine:LiCF3SO3 and dipropylamine:LiCF3SO3 were discovered, and their structures were solved by single-crystal X-ray diffraction techniques. Vibrational spectra of these crystals and detailed structural knowledge of the cation−solvent interactions complement analogous spectroscopic studies of the solution phases. The cation−polymer and hydrogen bonding interactions of branched poly(ethylenimine) (BPEI) complexed with LiCF3SO3 were modeled by the solutions of hexylamine and dipropylamine containing dissolved LiCF3SO3. Specifically, lithium ion interactions with the primary and secondary amine groups in BPEI were modeled by the solution studies with hexylamine and dipropylamine, respectively. The analysis of the hexylamine system was particularly useful because the primary amine group of BPEI dominates the NH stretching region of the spectrum.