Hydrogen bonds, proton sponges

A.V. Degtyarev et al., 2,7-Disubstituted proton sponges as borderline systems for investigating barrier-free intramolecular hydrogen bonds. Protonated 2,7-bis(trimethylsilyl)-and 2,7-di (hydroxymethyl)-l,8-bis(dimethylamino)naphthalenes. Tetrahedron 64, 6209-6224 (2008)... 

The proton sponges are exceptionally strong bases (see Section 4, p. 321 for a discussion) and one of the reasons for this is the strong hydrogen bonds with double-minimum potentials (A[8( H) — 8( H)] 0.5 present in the... 

Table 12 Structural details of the hydrogen bond in protonated proton sponges.

Sometimes a strongly hydrogen-bonded system, such as the fluori-nated proton sponge compound shown in Scheme 2.21, can have an impact on reactivity.25... 

Abstract The problem of the low-barrier hydrogen bond in protonated naphthalene proton sponges is reviewed. Experimental data related to the infra-red and NMR spectra are presented, and the isotope effects are discussed. An unusual potential for the proton motion that leads to a reverse anharmonicity was shown The potential energy curve becomes much steeper than in the case of the harmonic potential. The isotopic ratio, i.e., vH/VD (v-stretching vibration frequency), reaches values above 2. The MP2 calculations reproduce the potential energy curve and the vibrational H/D levels quite well. A critical review of contemporary theoretical approaches to the barrier height for the proton transfer in the simplest homoconjugated ions is also presented. 

H.A. Staab, T. Saupe, Proton sponges and the geometry of hydrogen bonds aromatic nitrogen bases with exceptional basicities. Angew. Chem. Int. Ed. Engl. 27, 865-1008 (1988)... 

A.J. Bienko et al., Low barrier hydrogen bond in protonated proton sponge. X-ray diffraction, infrared, and theoretical ab initio density functional theory studies. J. Chem. Phys. 119, 4313 4319 (2003)... 

M. Pietrzak et al., 13C detected scalar nitrogen-nitrogen couplings across the intramolecular symmetric NHN hydrogen bond of proton sponge. J. Am. Chem. Soc. 123,4338 1339 (2001)... 

M. Pietrzak et al., Counteranion and solvent effects on scalar couplings and hydrogen bond structures of protonated sponges, to be published... 

Proton Sponge-HF (Figure 3.24) is a particularly useful system which is an effective fluoride-ion donor for appropriate fluorinations [38,155] it is likely that the proton in this system is much less involved in H-bonding to fluoride than is the proton in hydrogen fluoride. 

Proton sponges derived from N-heteroaromatic bases, geometry of hydrogen bonds 99AG(E)865. 

Such compounds are very basic (p/T[, = 1.9 cf. NHj, p T, = 4.74 l-dimethyl-aminonapthalene, p/w, = 4.57). and have been nicknamed proton sponges from their avidity for hydrogen ions." - The strong, symmetric N—H—N hydrogen bond (see Chapter 8) stabilizes the conjugate acid. Note, however, that a second proton cannot be added without incurring the original steric problem Diprotonation is only half complete in 86% sulfuric acid ... 

Theoretical calculations of an [H3N H- NH3]+ system have shown that when the N - N distance is 2.75 A, the potential curve has two minima with a barrier of 10.9 klrnol with a decrease in the distance to ca 2.50 A, the barrier disappears. As seen in Table 5, the minimal N N distance for proton sponge cations is 2.54-2.55 A, i.e. none of the compounds has a symmetrical barrier-free H-bond. Indeed, in 80% of cases, the X-ray studies on salts of proton sponge 1 have revealed an asymmetric hydrogen bridge with the distances N—H and N - H in the ranges 0.84-1.27 A and 1.42-1.86 A, respectively. It is believed that such strong variations are due to the influence of the anion, either by an electric field effect or by structural changes in the crystal lattice. 

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