title: Proton-bridge motions in amine conjugate acid ions having intramolecular hydrogen bonds to hydroxyl and amine groups.
abstract: 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 Me(2)NCH(2)CMe(2)CH(2)CH(2)CH(2)NMe(2) 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-(Me(2)NCH(2))(4)-benzene and 1,2,4-(Me(2)NCH(2))(3)-benzene-5-CH(2)OH. 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).
authors: Department of Chemistry, University of California, Riverside, California 92521-0403, USA.
authors: Hou U
authors: HU
authors: Ung
authors: Aaron R
authors: AR
authors: Moehlig
authors: Sevana
authors: S
authors: Khodagholian
authors: Giel
authors: G
authors: Berden
authors: Jos
authors: J
authors: Oomens
authors: Thomas Hellman
authors: TH
authors: Morton
mesh_terms: D000143:Acids / Q000737:chemistry*; D000588:Amines / Q000737:chemistry*; D006860:Hydrogen Bonding; D006878:Hydroxides / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D015394:Molecular Structure; D011522:Protons; D011789:Quantum Theory
chemical_list: D000143:Acids; D000588:Amines; D006878:Hydroxides; D007477:Ions; D011522:Protons; C031356:hydroxide ion