Proton-Bridge Motions in Amine Conjugate Acid Ions Having Intramolecular Hydrogen Bonds to Hydroxyl and Amine Groups

Vibrational spectra of two gaseous cations having NH···O intramolecular ionic hydrogen bonds and of nine protonated di- and polyamines having NH···N internal proton bridges, recorded using IR Multiple Photon Dissociation (IRMPD) of mass-selected ions, are reported. The band positions of hydroxyl stretching frequencies do not shift when a protonated amine becomes hydrogen bonded to oxygen. In three protonated diamines, lower frequency bands (550–650 cm–1) disappear upon isotopic substitution, as well as several bands in the 1100–1350 cm–1 region. By treating the internal proton bridge as a linear triatomic, theory assigns the lowest frequency bands to N–H···N asymmetric stretches. A 2-dimensional model, based on quantization on a surface fit to points calculated using a double hybrid functional B2-P3LYP/cc-pVTZ//B3LYP/6-31G**, predicts their positions accurately. In at least one case, the conjugate acid of 1,5-cis-bis­(dimethylamino)­cyclooctane, a N–H···N bend shows up in the domain predicted by DFT normal mode calculations, but in most other cases the observed bands have frequencies 20–25% lower than expected for bending vibrations. Protonated Me2NCH2­CMe2­CH2CH2­CH2­NMe2 shows three well-resolved bands at 620, 1200, and 1320 cm–1, of which the lowest can be assigned to the asymmetric stretch. Other ions observed include doubly protonated 1,2,4,5-(Me2NCH2)4-benzene and 1,2,4-(Me2NCH2)3-benzene-5-CH2OH. Apart from the aforementioned rigid ion derived from the alicyclic diamine, the other ions enjoy greater conformational mobility, and coupling to low-frequency C–C bond torsions may account for the shift of vibrations with N–H···N character to lower frequencies. Low-barrier hydrogen bonding (LBHB) accounts for the fact that N–H···N asymmetric stretching vibrations of near linear proton bridges occur at frequencies below 650 cm–1.