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Pietrzak M, Wehling JP, Kong S, Tolstoy PM, Shenderovich IG, Lopez C, Claramunt RM, Elguero J, Denisov GS, Limbach HH: Symmetrization of cationic hydrogen bridges of protonated sponges induced by solvent and counteranion interactions as revealed by NMR spectroscopy. Chemistry. 2010 Feb 1;16(5):1679-90.
The properties of the intramolecular hydrogen bonds of doubly (15) N-labeled protonated sponges of the 1,8-bis (dimethylamino) naphthalene (DMANH (+)) type have been studied as a function of the solvent, counteranion, and temperature using low-temperature NMR spectroscopy. Information about the hydrogen-bond symmetries was obtained by the analysis of the chemical shifts delta (H) and delta (N) and the scalar coupling constants J (N,N), J (N,H), J (H,N) of the (15) NH (15) N hydrogen bonds. Whereas the individual couplings J (N,H) and J (H,N) were averaged by a fast intramolecular proton tautomerism between two forms, it is shown that the sum |J (N,H)+J (H,N)| generally represents a measure of the hydrogen-bond strength in a similar way to delta (H) and J (N,N). The NMR spectroscopic parameters of DMANH (+) and of 4-nitro-DMANH (+) are independent of the anion in the case of CD (3) CN, which indicates ion-pair dissociation in this solvent. By contrast, studies using CD (2) Cl (2), [D (8)] toluene as well as the freon mixture CDF (3)/CDF (2) Cl, which is liquid down to 100 K, revealed an influence of temperature and of the counteranions. Whereas a small counteranion such as trifluoroacetate perturbed the hydrogen bond, the large noncoordinating anion tetrakis [3,5-bis (trifluoromethyl) phenyl] borate B [{C (6) H (3)(CF (3))(2)}(4)](-) (BARF (-)), which exhibits a delocalized charge, made the hydrogen bond more symmetric. Lowering the temperature led to a similar symmetrization, an effect that is discussed in terms of solvent ordering at low temperature and differential solvent order/disorder at high temperatures. By contrast, toluene molecules that are ordered around the cation led to typical high-field shifts of the hydrogen-bonded proton as well as of those bound to carbon, an effect that is absent in the case of neutral NHN chelates. |
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