The Role of Intramolecular Hydrogen Bonds vs. Other Weak Interactions on the Conformation of Hyponitrous Acid and Its Mono- and Dithio-Derivatives

Copper(I) Complex Mediated Nitric Oxide Reductive Coupling: Ligand Hydrogen Bonding Derived Proton Transfer Promotes N2O(g) Release

A cuprous chelate bearing a secondary sphere hydrogen bonding functionality, [(PV-tmpa)Cu^I]⁺, transforms ^•NO_(g) to N₂O_(g) in high-yields in methanol. Ligand derived proton transfer facilitates N-O bond cleavage of a putative hyponitrite intermediate releasing N₂O_(g), underscoring the crucial balance between H-bonding capabilities and acidities in (bio)chemical ^•NO_(g) coupling systems.

Probing the Most Stable Isomer of Zirconium Bis(phenoxy-imine) Cation: A Computational Investigation

The possibility of coexistence of multiple isomers for zirconium bis(phenoxy-imine) catalyst has been systematically studied by computational approaches. The energetics among the five different isomers of neutral Zr-catalyst have been assessed quantum mechanically. The results suggest that isomer cis-N/trans-O/cis-Me is the most stable among the five isomers in accordance with the general observations of these kinds of phenoxy-imine catalyst. However, for the polymerization reaction, the active species is known to be the cationic form of the Zr-catalyst. The Zr-cation can exist in three different isomers, viz., cis-N/trans-O (A), cis-N/cis-O (B), and trans-N/cis-O (C), and the presence of flexible ligands makes the modeling considerably challenging to determine the most preferable isomers. For the efficient modeling, altogether 80 different structures for each of the three cationic isomers have been generated by using molecular dynamics simulations, and subsequently, the quantum mechanical optimization of these structures has been performed to obtain the most preferable conformation for each isomer. The existing probability derived from the obtained free energy values suggests that isomer C is comparable with isomer A. Even more, isomer A of the cation can be present in two different conformations, where the orientation of side groups is altered at the imine nitrogen atoms. The transition state calculations also confirm that the Zr-cation can exist as a mixture of three structures, "up-down" and "down-down" orientations of the isomers A along with isomer C's "up-up" orientation. However, by varying the substituents at imine nitrogen atoms, one could modulate multimodal to unimodal polymerization behavior of the Zr-catalysts. We believe that this study should provide a starting point for theoretically exploring the mechanistic pathway of the complicated polymerization reactions.

Nitric oxide coupling to generate N2O promoted by a single-heme system as examined by density functional theory

Bacteria utilize a heme/non-heme enzyme system to detoxify nitric oxide (NO) to N₂O. In order to probe the capacity of a single-heme system to mediate this NO-to-N₂O transformation, various scenarios for addition of electrons, protons, and a second NO molecule to a heme nitrosyl to generate N₂O were explored by density functional theory calculations. We describe, utilizing this single-heme system, several stepwise intermediates along pathways that enable the critical N-N bond formation step yielding the desired Fe-N₂O product. We also report a hitherto unreported directional second protonation that results in either productive N₂O formation with loss of water, or formation of a non-productive hyponitrous acid adduct Fe{HONNOH}.