Photolytic Studies of Aryl and Heteroaryl Nitrenium Ions: Laser Flash Photolysis Studies

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The objective of the thesis was to understand the chemical and kinetic behavior of arylnitrenium ions and a heteroaromatic nitrenium ion through photolytic studies. N-(4,4'-dichlorodiphenyl) nitrenium ion and N-(4,4'-dibromodiphenyl) nitrenium ion are the halogenated counterparts of diphenylnitrenium ions and are generated photochemically from their respective N-(4,4'-dihalogenated diphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate salts. The halogenated diarylnitrenium ions are ground state singlets that live for more than 1 x 105 ns in acetonitrile. In the absence of nucleophiles, these ions decay to form a dimerized hydrazine. These ions react with nucleophiles such as water and alcohol at a rate constant of 104 - 105M-1s-1 and at diffusion limit with chlorides. With arenes, these ions react via electron transfer mechanism and nucleophilic addition process. The rate constants for the electron transfer mechanisms are between 105 - 109 M-1 s-1 and depend on the Eox of the arenes. Arenes with Eox above 1.78 V showed no reactivity towards the ion.
Unlike other diarylnitrenium ions, the halogenated diarylnitrenium ions react with hydrogen atom donors via a hydrogen atom transfer mechanism. The triplet behavior of these ions is attributed to singlet-triplet intersystem crossing facilitated by the lower singlet-triplet energy gap. Therefore, it has been concluded that substituting halogens in diphenylnitrenium ion lowers the singlet-triplet energy gap and increases the lifetime of these ions.
(N-Methyl-N-4-biphenylyl) nitrenium ion generated by photolysis was reacted with amino acids and proteins to determine their reactivity with these ions. Eight amino acids were observed to react with the ion at a rate constant of 107 - 109 M-1 s-1. The rate constants depend on the nucleophilicity of the side chains of the amino acids. In addition, this ion also reacts rapidly with proteins with a rate constant of 108 M-1 s-1, comparable to their reactions with ss-DNA. Investigations on generating the quinoline N-oxide nitrenium ions showed that the transient species from the photolysis of 4-azidoquinoline N-oxide (4-AzQO) shows more characteristics of a nitrenium ion. The formation of 4-aminoquinoline N-oxide upon photolysis of 4-AzQO in acetonitrile with 10% sulphuric acid and the electron transfer reactions observed with arenes, indicate that the transient species generated could be a heteroaromatic nitrenium ion. However, more experiments are needed to confirm the assignment.