Low barrier hydrogen bond in protonated proton sponge. X-ray diffraction, infrared, and theoretical ab initio and density functional theory studies

Abstract

A new proton sponge, 2,7-dibromo-1,8-bis(dimethylamino)naphthalene (Br2DMAN), and its protonated (HBr) or deuterated (DBr) forms were studied by using x-ray diffraction and infrared spectroscopy, as well as ab initio and density functional theory calculations. In the crystalline lattice of the Br2DMAN.HBr salt, symmetrical (NHN)+ hydrogen bonding of length 2.547(3) Å is observed. No deuterium geometrical isotope effect was detected. The structure refinement suggests disordering of protons between two equivalent positions at the nitrogen atoms. These findings are in agreement with infrared spectra, which are characterized by an intense band centered at ∼560 cm−1 assigned to the ν(NHN) protonic transition. Deuteration leads to a shift of this band to ∼340 cm−1, so that the isotopic ratio νH/νD (ISR) of 1.65 is characteristic of an unusual potential for the proton motion. The results of MP2 calculations are in fairly good agreement with the experimental data. The theoretical N…N hydrogen bond length is 2.575 Å, while the distance between the minima equals to ca. 0.5 Å with a barrier height of 0.70 kcal/mol. The calculated difference between the 0→1 proton (deuteron) levels is 509 (284) cm−1, giving an ISR value of 1.76.

Symmetrization of Cationic Hydrogen Bridges of Protonated Sponges Induced by Solvent and Counteranion Interactions as Revealed by NMR Spectroscopy

“…Finally, we note that 2,7-symmetrically substituted proton sponges, in particular 8 H + [40] and 9 H + , [41] are known to exhibit unusual hydrogen-bond geometries and vibrations. However, it is difficult at present to establish a connection with the NMR spectroscopic parameters found in this study.…”

“…[39] Prior to this, protonated sponges of the DMANH + type were studied mainly by a combination of various experimental and theoretical methods and had focused on the question of whether or not the proton moves in a single or double well. [40][41][42][43] In solution, such information was obtained by studying the effects of isotopic substitution on NMR spectroscopic chemical shifts. [11,44] Other NMR spectroscopic studies of protonated 1,8-bis(dimethylamino)naphthalenes focused on the determination of the equilibrium constants of tautomerism by analyzing the temperature dependence of the coupling constants JA C H T U N G T R E N N U N G (N,H) and JA C H T U N G T R E N N U N G (H,N).…”

Hydrogen Motion in Proton Sponge Cations: A Theoretical Study

“…On the other hand, the second-order Møller-Plesset perturbation theory (MP2) [26] is widely used for calculations on hydrogen-bonded systems. [15][16][17] MP2 was shown to give reliable results for NH 3 ···HX (X = halogen) complexes, [27][28][29][30][31] and for the phenol-ammonia complex. [32] Going beyond MP2 is not feasible for real proton sponge molecules owing to the large number of geometry optimizations required to construct the PES.…”

“…[6] In principle, the barrier is tunable by the appropriate choice of substituents at the organic core. Thus, putting bulky substituents in the 2,7-positions of 1,8-bis(dimethylamino)naphthalene usually results in a contracted N···N distance, [14][15][16][17] and, therefore, in a lower barrier. For example, for 2,7-dibromo-1,8-bis(dimethylamino)naphthalene and for 2,7-dimethoxy-1,8-bis(dimethylamino)naphthalene, the barrier calculated at the MP2/6-31G** level is about 0.7 [15] and 1.32 kcal mol À1 , respectively.…”

Effect of quantum nuclear motion on hydrogen bonding

“…Most of chemistry can be understood in terms of semi-classical motion of nuclei on potential energy surfaces. In contrast, the quantum dynamics of protons involved in hydrogen bonds plays an important role in liquid water [1][2][3], ice [4,5], transport of protons and hydroxide ions in water [6], surface melting of ice [7], the bond orientation of water and isotopic fractionation at the liquid-vapour interface [8], isotopic fractionation in water condensation [9], proton transport in water-filled carbon nanotubes [10], hydrogen chloride hydrates [11], proton sponges [12,13], water-hydroxyl overlayers on metal surfaces [14], and in some proton transfer reactions in enzymes [15]. Experimentally, the magnitude of these nuclear quantum effects are reflected in isotope effects, where hydrogen is replaced with deuterium.…”