| Summary: | Nitrenium ions are common reactive intermediates with high activities towards some biological nucleophiles. In this paper, we employed femtosecond transient absorption (fs-TA) and nanosecond transient absorption (ns-TA) as well as nanosecond time-resolved resonance Raman (ns-TR<sup>3</sup>) spectroscopy and density function theory (DFT) calculations to study the spectroscopic properties of the <i>N</i>(4,4′−dibromodiphenylamino)−2,4,6−trimethylpyridinium BF<sub>4</sub><sup>−</sup> salt (<b>1</b>) in an acidic aqueous solution. Efficient cleavage of the N−N bond (4 ps) to form the <i>N,N</i>−di(4−bromophenyl)nitrenium ion (<b>DN</b>) was also observed in the acidic aqueous solution. As a result, the dication intermediate <b>4</b> appears more likely to be produced after abstracting a proton for the nitrenium ion <b>DN</b> in the acid solution first, followed by an electron abstraction to form the radical cation intermediate <b>3</b>. These new and more extensive time-resolved spectroscopic data will be useful to help to develop an improved understanding of the identity, nature, and properties of nitrenium ions involved in reactions under acidic aqueous conditions.
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