Investigations on Nitrogen, Sulfur and Oxygen Based Heterocyclic Radicals: A Computational and Matrix Isolation Infrared Spectroscopic Studies

Abstract

The formation of the reactive open-shell ring systems simultaneously containing C, N, S and O presents considerable interest in astrochemistry and astrobiology due to their crucial role in primordial evolution. Equally, heterocyclic radicals have recently gained a lot of importance due to the advancements in photo- and electroredox methodologies and their generation and utility in organic transformations. Despite the widespread interest, systematic studies on heterocyclic radicals are rare; particularly, gaining insights into controlling their stability and reactivity will be enormously helpful. In the first chapter, we focused on the thiazole radical and biradical by considering 2- iodothiazole (IT) and 2,5-diiodothiazole (DIT) as precursors. Their photochemistry under matrix isolation conditions in solid argon at 4 K revealed various photoproducts. Through extensive photochemical experiments at different wavelengths of light and kinetic mode measurements, major pathways and intriguing photoproducts, including the target radical, were identified. The second chapter focuses on a benzene-fused thiazole to restrict the multiple ring-opening reaction channels in the thiazole photochemistry. The study revealed a reversible photoinduced ring- opening of 2-iodobenzothiazole (IBT), forming a 2-isocyanophenylthiyl radical under 254 nm light and reverting under 365 nm light or annealing at 30 K, facilitated by iodine. The mechanistic investigations were unravelled with the help of the photochemistry of benzothiazole (BT) under similar conditions and computations. These findings underscore the significance of light-induced C-S bond scission in sulfur-based heterocycles, which is relevant to photovoltaic and organic electronic applications. In the following two chapters, the dehydro-radical isomers of pyrazole and imidazole were investigated. Computations revealed that among pyrazole radicals, the N-centereddehydropyrazole P-a, a π-radical, is thermodynamically more stable than the C-centered σ- radicals. Photochemistry of 3-iodopyrazole (3-IP) and 4-iodopyrazole (4-IP) under cryogenic conditions showed various ring-opening pathways, with 4-IP favouring C5-N1 bond cleavage and 3-IP showing a preference for N1-N2 bond cleavage, leading to distinct photoproducts. However, the attempts to characterize the target pyrazole radicals were unsuccessful. Conversely, the creation of the 2-dehydroimidazole radical I-b from precursor 2-iodoimidazole (2-II) at 254 nm and the subsequent ring-opening products, were successfully demonstrated. Similarly, photochemical studies of 4-iodoimidazole (4-II) revealed the formation of the 4-dehydroimidazole radical I-c. Interestingly, the photochemistry of 5-iodoimidazole (5-II), the tautomeric pair of 4- II was also observed. In the last chapter, 42 mono-radical isomers of adenine-uracil base pairs (AU bps) were computationally studied, revealing significant modifications in H-bonding upon radical formation. The significance of spin delocalization through radical-lone pair interaction is one of the highlights of it. Spin density and electron density analyses confirmed strong interactions between adenine and uracil, affecting H-bond stability and spin delocalization between the base pairs, observed in a few radical isomers. Experimental studies on 5-iodouracil (5IU) under cryogenic conditions revealed that UV irradiation led to partial bleaching and ring-opening, producing various fragmentation products like ethynylcarbamoyl isocyanate and small fragmented products such as HNCO. Overall, the thesis exemplifies the successful generation of certain heterocyclic radicals and their photoproducts through ring opening and fragmentation pathways, which have significant implications in biochemistry and astrochemistry.