Ambar Banerjee

Unearthing the Mechanism of Prebiotic Nitrile Bond Reduction in Hydrogen Cyanide through a Curious Association of Two Molecular Radical Anions

HCN is clearly associated with the prebiotic chemical evolution of life. It has been known for decades that the radiolysis of HCN solutions produces sugars, amino acids and nucleobases. Remarkably, recent experimental studies have shown that the photolytic reduction of aqueous HCN by a photoredox reagent [Cu(CN)3](2-) specifically yields sugars, which are the essential building blocks of RNA. Although a mechanistic understanding of such reductions with solvated electrons is poor, the general consensus is that they involve neutral free radicals. We show herein through the use of electronic structure studies and molecular simulations that the reduction of the nitrile bond of HCN is initiated through the formation of a molecular dipole-bound anion from the photoredox reagent. Our theoretical studies show how HCN binds to the photoexcited reagent and then extracts an electron from the reagent and is ultimately detached as a dipole-bound anion. The dipole-bound anionic form of [HCN](-) can easily convert into a solvated valence-bound form of [HCN](-). After the formation of solvated [HCN](-), an extraordinary chemical event ensues through a counter-intuitive coupling of two valence-bound anions to form a solvated molecular dianionic intermediate, [HCN]2(2-). Finally, a proton-coupled electron transfer occurs within the dianionic entity to complete the reduction. This mechanistic scenario is applicable to the reduction of other prebiotic nitrile species and avoids neutral radical-based pathways, thereby preventing the proliferation of reactive species and preserving chemical selectivity. Furthermore, we show how such similar nitrile reduction pathways operate to yield the sugar precursors.

A Serendipitous Rendezvous with a Four-Center Two-Electron Bonded Intermediate in the Aerial Oxidation of Hydrazine

Oxidation by dioxygen has a rich repertoire of mechanistic intricacies. Herein, we report a hitherto unknown paradigm of dioxygen activation reaction which propagates through a four center two electron (4c-2e) bound species. Using static DFT and ab initio quantum chemical techniques we have unraveled the oxidation pathway for hydrazine and its methylated analogues by dioxygen which involves formation of this unconventional 4c-2e bonded species en route to the oxidation products. Inconvertible evidence in favor of such an unprecedented dioxygen activation route is provided by capturing the events of formation of the 4c-2e species in aqueous phase for hydrazine and its congeners and the experimentally observed products from the respective 4c-2e species, like H2O2 and N2H2 , diazene in the case of hydrazine using Car-Parrinello molecular dynamics simulations.

Deciphering the cryptic role of a catalytic electron in a photochemical bond dissociation using excited state aromaticity markers

The breaking of bonds by catalytic electrons has gained prominence very recently but has been limited to cases where electrons from external sources have been used. Here, we show that upon photoexcitation, an electron of intramolecular origin is transferred from one part of a molecule to another followed by bond cleavage and then returns to its original moiety on completion of its catalytic function. By a proper assessment of the dramatic changes in aromaticity in excited-state intermediates along the photoreaction coordinate captured by the magnetically induced current density (MICD) technique, we show that in 5-phenyltetrazole, an excited electron, which migrates from the phenyl ring to the tetrazole ring, induces bond cleavage catalytically. Using the MICD technique, we establish for the first time a link between the phenomenon of excited-state electron/charge transfer among aromatic rings and the intricate interplay of aromatic, antiaromatic and non-aromatic states.

Unraveling the Microscopic Origin of Triplet Lasing from Organic Solids

We present a heuristic mechanism for the origin of the unusual triplet lasing from (E)-3-(((4-nitrophenyl)imino)methyl)-2H-thiochroman-4-olate·BF2.We demonstrate that whereas the moderate lifetime (1.03 μs) of the first triplet state (T1) prohibits triplet-triplet annihilation, the relatively faster S1 → T1 intersystem crossing and the 104 times smaller reverse intersystem crossing effectively help achieve population inversion in the T1 state. Furthermore, the triplet lasing wavelength (675 nm) for the tetramer does not overlap with the triplet-triplet absorptions wavelength, indicating that the spin-forbidden emission cross section is very large. Additionally, the almost complete absence of a vibrational progression in the vibronic phosphorescence spectrum of the monomer plays an important role in ensuring efficient triplet-state lasing from this organic material. Our results show that controlling the triplet-state lifetimes combined with lowering of the triplet-triplet absorption in the emission region and small vibronic coupling will be the key steps when designing novel organic triplet-lasing materials.

Theoretical Investigations on the Mechanistic Aspects of O2 Activation by a Biomimetic Dinitrosyl Iron Complex

Though dinitrosyl-iron complexes (DNICs) are largely believed to act as NO carriers, several experiments on model DNICs have suggested that they can also act as nitrating agents in presence of dioxygen. Oxygen activation by DNICs has been implicated as a possible route for protein tyrosine nitration (PTN), which leads to neurodegenerative disorders. Herein using static and dynamic theoretical techniques we unravel a previously unknown dual state mechanistic paradigm for dioxygen activation of a biomimetic nitrating DNIC complex leading to phenolic nitration. Our computations reveal that the model DNIC, the ground electronic state of which is singlet, has a low-lying triplet state and an inherent singlet-triplet spin-crossover of DNICs can be triggered by fluxional changes in the bite angle of the two NO ligands. The presence of a low-lying triplet state in the DNIC affords an avenue for O2 activation other than a direct O2 activation by O2 -induced spin-crossover of the singlet ground state. These two low-lying channels facilitate the formation of a peroxynitrite species. Nitration of phenolic substrates is facilitated by the release of NO2 . The corresponding minimum energy crossing points (MECP) have been located. Along the reaction path, the changes in the electronic structure scenarios have been studied and interpreted. Our report also sheds light on the plausible mechanistic pathway of PTN by reactive species formed once O2 activation by DNICs have been achieved.

Understanding the Unexpected Product Distribution in the Aerial Oxidation of Carbene-Stabilized Diphosphorus Complex

Oxidation of nonmetallic singlet molecules by oxygen has its own share of intricacies. Herein, by means of DFT and ab initio techniques, mechanistic details of the aerial oxidation of an N-heterocyclic carbene (NHC) stabilized diphosphorus complex are revealed. This particular oxidation process is known to produce an unexpected P-P bond containing diphosphorus tetroxide complex, instead of the more thermodynamically stable oxo-bridged (P-O-P) compound. These findings suggest that the P-P bond containing less stabilized species is a kinetically controlled product (KCP) and obtained due to the presence of lower lying transition states (TSs) in the pathway leading to its formation, relative to the higher lying corresponding minimum-energy crossing points (MECPs) present in the pathway involved in the formation of the oxo-bridged species, which is the thermodynamically controlled product (TCP). Thus, an intriguing variant of the well-known KCP/TCP phenomenon is presented here, in which the KCP is formed not by merely traditionally known lower barrier heights of TSs involved in the formation of KCP, but by faster transmission of a system through a low barrier TS relative to a higher lying MECP. Additionally, the faster kinetics of an irreversible unimolecular O-O dissociation step, which avoids the formation of the TCP is a contributing factor in dictating the fate of the reaction. The insights provided herein may help to understand the oxidation of other P-P-containing species, such as black phosphorene.