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).

Preferred protonation site of a series of sulfa drugs in the gas phase revealed by IR spectroscopy

“…Scaling factors for N-H stretches may depend strongly on the type of NH moiety and the degree of H-bonding [71] and the use of a uniform scaling factor in this frequency range often gives disappointing results. Moreover, for the charged (protonated) systems studied here, deviations likely become worse [72], because intramolecular interactions are stronger and the degree of proton sharing between two Lewis basic sites increases [73,74]. Furthermore, note that scaling factors reported in the literature have almost exclusively been derived for test sets of neutral molecules.…”

Complexes of Crown Ether Macrocycles with Methyl Guanidinium: Insights into the Capture of Charge in Peptides

“…In the 18‐crown‐6 complex, two partly merged moderately broad bands are observed at 3150–3450 cm −1 , and a narrower and weaker band component is found at 3530 cm −1 . It should be noted that broadened bands in proton bonded systems often arise from proton delocalization effects . While such scenario would be appealing, the a priori expectation for the present systems is that the highly basic guanidine moiety retains the proton with a limited sharing with the crown ether.…”

Structure–property relationships in protic ionic liquids: a study of solvent–solvent and solvent–solute interactions

“…It is likely that a low barrier hydrogen bond exists between the two nitrogen atoms, resulting in rapid proton exchange between these two centers on the (formally singly-charged) [HTMEDA] + cation. 51 The presence of this low barrier hydrogen bond will result in a lower proton affinity of the monoprotonated [HTMEDA] + cation compared to the unprotonated amine. 24 27,52 .…”