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Low Barrier Hydrogen Bonds in Acyclic Tertiary Diamines


The potential energy surface for a hydron between two basic nitrogens has two wells corresponding to the hydron localized on one nitrogen or the other. An energy barrier for hydron transit corresponds to its being situated midway between the two nitrogens. The term "low barrier hydrogen bond" (LBHB) is used to describe systems where the zero point energy is comparable to the barrier height. This work focuses on acyclic tertiary diamines which, when a single hydron is introduced, form a strong intramolecular hydrogen bond.

Infrared Multiple Photon Dissociation (IRMPD) is a type of action spectroscopy, which allows the study of isolated ions in the gas phase. IRMPD spectra of the monoprotonated diamines synthesized here show low-frequency vibrations that disappear when bridging H+ is replaced by D+. Anharmonic theoretical calculations, which treat the NH+--N system as a linear triatomic with a double well potential, make predictions for LBHBs in line with the experimental results.

DFT calculations, used to construct 1- and 2-dimensional potential energy surfaces, show low hydron transit barriers and predict the first vibrational transitions associated with the asymmetric NHN and symmetric NN stretches. The asymmetric stretch is calculated to appear around 540 cm-1. A broad band in the IRMPD of protonated (CH3)2NCH2CH2C(CH3)2CH2CH2N(CH3)2 (540-660 cm-1) disappears upon deuteration. Conjugate acid ions of the unsymmetrically substituted isomer, (CH3)2NCH2C(CH3)2CH2CH2CH2N(CH3)2, exhibit a much narrower band in the same domain (620 cm-1), which also disappears upon substitution of bridging H+ with D+. Deuterium substitution confirms peak assignments: calculations of the D+-bridged species predict the asymmetric stretch to shift to around 300 cm-1, outside the experimentally accessible domain.

NHN bends of the intramolecular proton bridges appear at much lower frequencies than calculated by full-dimensional harmonic or anharmonic calculations. The proton-bridged ions display bands around 1330 cm-1, which disappear for the D+-bridged species. The isotopic shift of these bands to lower frequency is presumably obscured by other bands in this region.

Along with protonated tetramethylputrescine, these cations represent the first documented cases of LBHBs where the bridging proton has been specifically replaced with a bridging deuterium, allowing for assignment of the asymmetric stretch.

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