Formation of hydrogen-bonded chains between a strong base with guanidine-like character and phenols with various p Ka values — an FT-IR study

Abstract

The complexes formed by phenols with 1,3,4,6,7,8-hexahydro-l-methyl-2 H-pyrimido[1,2- a]pyrimidine (mTBD), an N-base with guanidine-like character, were studied as a function of the p K a of the phenols by FT-IR spectroscopy. The following phenols were used: 4-cyanophenol (4-CNPh), pentachlorophenol (PCP) and 2,6-dichloro-4-nitrophenol (DNPh). In the case of chloroform solutions of 1: 1 mixtures of the phenols with MTBD the corresponding complexes are formed completely. With increasing acidity of the phenols the hydrogen bonds become increasingly asymmetrical. The O … N ⇋ −O … H +N hydrogen bond in the 4-CNPh-MTBD complex shows large proton polarizability. In the other cases only the polar structure is realized. With increasing phenol MTBD ratio, the formation of chains with two phenol molecules is observed. With decreasing p K a of the phenols the fluctuation is limited to the phenol-phenolate bond and finally, the phenol-protonated MTBD bond begins to dissociate. In acetonitrile solutions, N +H …O − hydrogen bonds are observed in the case of the 1:1 mixture of 4-CNPh with MTBD. A weak continuum indicates the presence of homoconjugated phenol-phenolate bonds with large proton polarizability. In the case of 2:1 mixtures only protonated MTBD and homoconjugated phenol-phenolate bonds are observed, independent of the p K a of the phenols. The results are discussed with regard to the proton pathway in bacteriorhodopsin.