FORMATION OF HYDROGEN-BONDED CHAINS BETWEEN A STRONG BASE WITH GUANIDINE-LIKE CHARACTER AND PHENOLS WITH VARIOUS PK(A) VALUES - AN FT-IR STUDY

The complexes formed by phenols with 1,3,4,6,7,8-hexahydro-1-methyl-2H -pyrimido[ 1,2-a]pyrimidine (mTBD), an N-base with guanidine-like char acter, were studied as a function of the pK(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 th e case of chloroform solutions of 1:1 mixtures of the phenols with MTB D the corresponding complexes are formed completely. With increasing a cidity of the phenols the hydrogen bonds become increasingly asymmetri cal. The OH ... N reversible arrow(-)O ... H+N hydrogen bond in the 4- CNPh-MTBD complex shows large proton polarizability. In the other case s only the polar structure is realized. With increasing phenol MTBD ra tio, the formation of chains with two phenol molecules is observed. Wi th decreasing pK(a) of the phenols the fluctuation is limited to the p henol-phenolate bond and finally, the phenol-protonated MTBD bond begi ns 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 we ak continuum indicates the presence of homoconjugated phenol-phenolate bonds with large proton polarizability. In the case of 2:1 mixtures o nly protonated MTBD and homoconjugated phenol-phenolate bonds are obse rved, independent of the pK(a) of the phenols. The results are discuss ed with regard to the proton pathway in bacteriorhodopsin.