PHOTOLYTIC GENERATION OF NITRENIUM IONS: KINETIC STUDIES AND POLYMERIZATION REACTIONS

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2005-11-29

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Abstract

Nitrenium ions are highly reactive transient species that contain a positively charged and dicoordinate nitrogen atom. The nitrogen atom contains only six electrons in its valence shell and thus the nitrogen is electron deficient and bears a positive charge. Nitrenium ions are of interest due to their suspected role in carcinogenesis since amines are known to form covalent bonds to DNA. The synthesis and photolysis of 1-(N-methyl-N-(1-naphthyl)amino)-2,4,6-trimethylpyridinium tetrafluoroborate, by laser flash photolysis, allowed for the direct observation of N-methyl-1-naphthylnitrenium ion as well as measurements of N-methyl-1-naphthylnitrenium ion's lifetime and trapping rate constants. It was determined that N-methyl-1-naphthylnitrenium ion has an absorption maximum centered around 500 nm and a lifetime of 835 ns. The trapping rate constants
with simple nucleophiles, such as chloride, alcohols, and amines, were determined to be on the order of 108 - 109 M-1s-1. These trapping rate constants were compared to other arylnitrenium ion systems to determine what factors contribute to a chemical's inherent carcinogenicity. The synthesis and photolysis of 1-(N-methyl-N-(2-naphthyl)amino)-2,4,6-trimethylpyridinium tetrafluoroborate was also performed. Although no transient intermediate was observed, products from photolysis are consistent with arylnitrenium ion products.

Nitrenium ions are also of interest due to their possible role in the polymerization of aniline to form polyaniline (PANI). PANI is of interest because of it is an electronically conducting polymer with many commercial aspects and the mechanism of formation has been under dispute for decades. Although it is generally agreed that the initial dimerization step is due to radical cation coupling, the mechanism for aniline polymerization is argued as proceeding through either a radical cation mechanism or via a nitrenium ion mechanism. Synthesis of a photochemical precursors of an aniline dimer 4-(N-anilino)phenyl azide produced what is believed to be a 4-(N-anilino)phenylnitrenium ion which has an absorption maximum centered around 490 nm. Spectroscopic analysis by MALDI-TOF-MS and X-ray photoelectron spectroscopy (XPS), shows that PANI is a photoproduct. The extrapolated data and results from similar systems, supports the hypothesis that polymerization involves a nitrenium ion intermediate.

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