Spectroscopic and PM5 semiempirical studies of the proton accepting properties of 1,8-bis(tetramethylguanidino) naphthalene

The strongly basic 1,8-bis(tetramethylguanidino)naphthalene (TMGN) has been mono- and di-protonated by pentachlorophenol (PCP) and HClO4 acid. Formation of hydrogen bonds in the 1:1 and 1:2 mixtures in acetonitrile has been studied by FT-IR, H-1 NMR; C-13 NMR spectroscopy as well as in the gas phase by ESI MS and PM5 semiempirical methods. The spectroscopic data show that in the 1:1 mixtures of TMGN with PCP or perchloric acid the base becomes protonated forming an intramolecular [N-H center dot center dot center dot N](+) hydrogen bond. This hydrogen bond is strongly asymmetrical and is at most of medium strength. In the 1:2 mixtures, the intramolecular [N-H center dot center dot center dot N](+) hydrogen bond is conserved irrespective of the kind of the acid and, in the case of the TMGN complex with PCP, additionally an intermolecular [O center dot center dot center dot H center dot center dot center dot O](-) hydrogen bond is formed whereas in the TMGN complex with HClO4 another N atom of the guanidine groups is protonated. The ESI NIS measurements indicate the formation of protonated TMGN and deprotonated carboxylic acids and phenols as separated ions instead of a complex in the gas phase. The ClO4- anion forms, however, complexes with the protonated TMGN molecules. We show that the anion has a strong influence on the fragmentation pathway. The structures of TMGN and its selected protonated species are visualized using the semiempirical PM5 method. The calculated hydrogen bond parameters are in good agreement with the X-ray data. (C) 2007 Elsevier B.V. All rights reserved.