[NHN]⁺ Hydrogen Bonding in Protonated 1,8-Bis(dimethylamino)-2,7-dimethoxynaphthalene. X-ray Diffraction, Infrared, and Theoretical ab Initio and DFT Studies

Abstract

Structural (X-ray diffraction), infrared spectroscopic, and theoretical MP2 and DFT studies on the HBr and DBr adducts of 1,8-bis(dimethylamino)2,7-dimethoxynaphthalene ((CH3O)2.DMAN) were performed. This particular proton sponge has been chosen for its strong basicity and display of the buttressing effect influencing the hydrogen bond dynamics and properties. The studies revealed a symmetric, planar DMAN.H+ cation with a short (NHN)+ hydrogen bond of 2.567(3) A. The X-ray diffraction results suggest that the proton is in the central position in the bridge, while the calculations show two potential energy minima with the zero point energy level close to the top of the barrier. The infrared spectra display an (NHN)+ band at 488 cm(-1) and an (NDN)+ band at 235 cm(-1), respectively. It gives the isotopic ratio of 2.08, the highest value reported to date. Such a result suggests a peculiar shape of the potential for the proton motion, characterized by an extremely high positive anharmonicity. The calculations reproduce this particular potential, yielding an ISR value displaying a very good agreement with the experimental one. The anharmonic frequencies, however, show the discrepancy between the observed and calculated transitions.

Hydrogen Motion in Proton Sponge Cations: A Theoretical Study

“…[15][16][17] This term means an anharmonic potential that is steeper than the harmonic one. The vibrational energy levels are then divergent.…”

“…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. [16] The hydrogen motion potential is highly anharmonic. In general, anharmonicity increases with increasing hydrogenbond strength as the potential barrier decreases.…”

“…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.…”

The Formation of High‐Order Polybromides in a Room‐Temperature Ionic Liquid: From Monoanions ([Br5]− to [Br11]−) to the Isolation of [PC16H36]2[Br24] as Determined by van der Waals Bonding Radii

“…The shortest CÀH···Br distances are 2.818 (H2B···Br (8)) and 2.909 (H15D···Br(9)), which lie in the range of a bromine-hydrogen-bridge bond (2.40-2.90 ). [22] Longer bromine-hydrogen interactions of 2.936 (H16B···Br(11)), 3.012 (H5B···Br(2)), 3.034 (H1A···Br(4)) and 3.036 (H13A···Br(12)) are also observed. These hydrogen interactions are found on all but two of the apices of the square pyramidal structures, affording significant stabilisation, and thus, templating, of the polybromide structure.…”

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

“…[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).…”