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 center dot center dot center dot O intramolecular ionic hydrogen bonds and of nine protonated di- and polyamines having NH center dot center dot center dot 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 center dot center dot center dot 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 center dot center dot center dot 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 diaminc, 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 center dot center dot center dot N character to lower frequencies. Low-barrier hydrogen bonding (LBHB) accounts for the fact that N-H center dot center dot center dot N asymmetric stretching vibrations of near linear proton bridges occur at frequencies below 650 cm(-1).