Sequestering and metamorphosis of molecular nitrogen at ambient conditions: role of bonding and back-bonding

20 July 2022, Version 2
This content is a preprint and has not undergone peer review at the time of posting.


The optical absorbance behavior of a molecule during the reaction is yet to be explored due to the deficiency in appropriate theoretical and experimental tools. In the present study, the Intrinsic Reaction Coordinate (IRC) trapping methodology is implemented in a new approach to capture the electronic motion during the nitrogen activation reaction by heterocyclic carbenes. This technique acts as a Handycam (Molecular Handycam) to shoot photographs of electrons in the event of a chemical reaction beyond the Born-Oppenheimer approximation. The use of the newly designed molecular Handycam to analyse electron flow at the frontier molecular orbitals during the adduct formation reaction of molecular nitrogen with heterocyclic carbene corroborated that in the carbene −N 2 reaction, carbene operates as a σ −acceptor and π −donor where the bonding and the back bonding processes occur before and after the transition state of the reaction. There is a significant time delay between the bonding and the back bonding process, implying that the bonding back-bonding is not simultaneous for this reaction. These findings ameliorate the DFT-based designing of heterocyclic carbenes to model a congenial catalyst for the sequestering of atmospheric nitrogen and its metamorphosis to ammonia at an ambient and metal-free condition. It is also brought to light that the optical absorption behavior of a molecule may change along the reaction path. Though molecular nitrogen, model carbene and the carbene−N 2 adduct are non-responsive to visible light, an evanescent optical absorbance in the visible region is observed during the chemical reaction.


N-heterocyclic carbene
Nitrogen activation
P-heterocyclic carbene
Molecular Handycam
Evanescent Optical Absorbance

Supplementary materials

Ammonia synthesis from molecular nitrogen at the metal-free condition
Employing an appropriate carbene, molecular nitrogen may be sequestered and converted to ammonia in a suitable reduction condition. Carbene acts as a σ -acceptor and π-donor in this process. The electronic level investigation proclaimed that the bonding and the back bonding processes are not instantaneous. These two processes occur at different stages of the reaction. The bonding takes place before reaching the transition state while the back bonding occurs after the transition state which implies that these two processes are independent of each other.


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