Naphthalene hydrogen bonding Proton

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)... 

A considerable amount of the strain in l,8-bis(dimethylamino)naphtha-lene is relieved by protonation and the N—H N bond length (260 pm) in the protonated amine shows that the molecule is able to adopt a conformation [55] with an intramolecular hydrogen bond (Truter and Vickery, 1972). The infra-red spectrum of protonated l,8-bis(dimethylamino)naphthalene and the chemical shift (5 19.5) of the acidic proton in the nmr spectrum confirm the presence of an intramolecular hydrogen bond (Alder et al., 1968). The magnitude of the isotope effect on the chemical shift (Altman et al., 1978) and the appearance of two Nls peaks in the photoelectron spectrum... 

The strength of the hydrogen bonds in the protonated amines can be increased by the use of larger and additional groups to provide further steric strain. For example, the difference in strain between l,8-bis(dimethylamino)-naphthalene [33] and 1,8-bis(diethylamino)naphthalene leads to a piC,-value for the ethyl derivative enhanced by 0.5 units (Hibbert, 1974), and a similar difference is also observed in the fluorene series for [41] and [42]. Further strain can be introduced into the naphthalene series by the use of 2- and 7-methoxyl groups as substituents. This is illustrated by the pA -values of 1,8-bis(dimethylamino)naphthalene [piC, = 12.1 (Hibbert, 1974)] and l,8-bis(di-methylamino)-2,7-dimethoxynaphthalene [p, = 16.1 (Hibbert and Simpson, 1987a)] and by the piC,-values of [35] pK = 7.49 (Hibbert and Hunte,... 

In protonated l,8-bis(dimethylamino)-naphthalene [9a] 2hJ(N j,N g) = + 8.7 Hz, which is mediated by the proton since in the unprotonated compound [9b] the corresponding J(N 1,N 8) coupling was not detected. In [9a] the (N-H-N)+ interaction corresponds to a classical low-barrier hydrogen bond with a symmetric configuration. 

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. 

A.F. Pozharskii, V.A. Ozeryanskii, Peri-Naphthylene-diamines 23 investigation of the intramolecular hydrogen bond in protonated 1, 8-bis(dimethylamino) naphthalenes by H NMR spectroscopy. Russian Chem. Bull. 47, 66-73 (1998)... 

Protonation becomes a rapid reaction in protic solvents and in the presence of acids, as demonstrated for, e.g., -butyl acrylate in aqueous solution [207], methyl acrylate in EtOH [208], cinnamates in the presence of phenol in DMF [209], and benzaldehyde in ethanolic buffer solution [210]. Rate constants for protonation of aromatic radical anions (anthracene [211], naphthalene, 2-methoxynaphthalene, 2,3-dimethoxynaphthalene) by a number of proton donors including phenols, acetic acid, and benzoic acids in aprotic DMF were found to vary from 5.0 X 10 M- s-> (for anthracene, in the presence of p-chlorophenol) to 6.2 x lO s (for anthracene, in the presence of pentachlorophenol) [212]. For dimedone, PhOH, or PhC02H the rate of protonation depends on the hydrogen-bond basicity of the solvent and increases in the order DMSO < DMF MeCN [213],... 

The intramolecular hydrogen bond in protonated 59 and 60 strongly reduces the rate of the proton transfer with respect to the normal proton transfer from an OH and NH2+ group. In the series of 1,8-disubstituted naphthalenes, the strength of the hydrogen bond is as reported in Scheme 8114. 

A strong hydrogen bond of sodium 4,5-dihydroxynaphthalene-2,7-disulphonate (39) has been observed by NMR at a low temperature (5(OH) 17.72 ppm) . This value has been related to the O- O distance in relation to other measurements, in order to use OH chemical shifts to obtain oxygen-oxygen distances. One possible drawback of using values from the mono-ionized 1,8-dihydroxy naphthalene directly is that the chemical shift of the OH proton is likely to have a sizeable ring current contribution. Phenols are also involved in catalytic triads, e.g. in ketosteroid isomerase , leading to OH chemical shifts of 18.2 ppm. 

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