Benzoic acid proton shifts

A ruthenium porphyrin hydride complex was lirst prepared by protonation of the dianion, [Ru(TTP) in THF using benzoic acid or water as the proton source. The diamagnetic complex, formulated as the anionic Ru(If) hydride Ru(TTP)(H )(THF)l , showed by H NMR spectroscopy that the two faces of the porphyrin were not equivalent, and the hydride resonance appeared dramatically shifted upheld to —57.04 ppm. The hydride ligand in the osmium analogue resonates at —66.06 ppm. Reaction of [Ru(TTP)(H)(THF)j with excess benzoic-acid led to loss of the hydride ligand and formation of Ru(TTP)(THF)2. 

Fig. 9 H/D isotope effects on the hydrogen bond geometries H/D isotope effects, A5 = 8Hd - SHh. for the encapsulated benzoic acid (BA) derivatives (d12 mesitylene, open circles) and non-encapsulated (CDF3/ CDF2C1 filled circles) dimers of carboxylic acids as a function of the bridging proton chemical shift 8Hh...

There are many other highly specialized cases of proton-shift tautomerism, including an internal Michael reaction (see 15-24) in which 2-(2,2-dicyano-l-methylethenyl)benzoic acid (161) exists largely in the open chain form rather an its tautomer (162) in the sohd state, but in solution there is an increasing amount of 162 as the solvent becomes more polar. ... 

In ethanol, the 31,850 cm maximum (A) is reduced to a shoulder near 33,000 cm. Whereas in heptane a normal "Weller shift" of fluorescence indicating Intramolecular proton transfer is observed, in alcohols we note a fluorescence band at higher frequency with a maximum at 23,800 cm. The latter is due to the anion (D) and corresponds to the absorption maximum of D at 29,400 cm The intensity of this emission increases with increasing pH and remains constant above pH = 9 (in ethanol). The anilide of 2-methoxy benzoic acid shows a ultraviolet fluorescence that was too weak to be recorded (21). Low quantum efficiency may, therefore, be the reason that the fluorescence of C has not been detected. The only evidence of the presence of C is the close similarity of the absorption spectra of SAN and the methoxy derivative in ethanol (21). 

The protonation of diazoalkanes (7.14) is also a versatile method for forming alkanediazonium ions and ion pairs. A classical example is Huisgen and Riichardt s investigation of 1-diazopropane (1956). In benzene, the reaction of 1-diazopropane with benzoic acid yields almost exclusively propyl benzoate, whereas in water with perchloric acid a propanol mixture containing 28% propan-2-ol was found. The 1,2-H shift leading to propan-2-ol seems to be minimized by the formation of the diazonium-benzoate ion pair. 

At the methylidene group carbon atom of 3 a relatively high electron density NMRCCDCls) 5 = 8S.5 ppm) is located which could be caused by the mesomeric electron-releasing effect of the silicon-substituted enamine nitrogen atom. Therefore this methylidene group carbon atom is able to pick up a proton from some acids. Even weak acids react with 3 if a stable Si-0 bond can be formed with the corresponding anion. 1,4-Addition of methanesulfonic acid, picric acid, benzoic acid and hydroquinone to 3 (solution in THE) led to hexacoordinate salen-silicon complexes by precipitation of compounds 4, 5, 6 and 7 respectively (Scheme 1). The Si chemical shifts of the solids obtained (CP/MAS-NMR) are presented in Table 2. 

Protonated para-benzoic acids exhibit a primary band 230 nm and a secondary weaker band 270 nm [37-39]. The primary band is red-shifted by substitutions in the aromatic ring whether electron donating or withdrawing [38]. Electron withdrawing substituents alter ,ax of the secondary band only if they are JtJt -chromophores e.g. -NO2 and -NHCOCH3 [38], two substituents which we excluded from the correlation for that reason. 

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