Probing the origins of activation barriers in nitrous oxide capture reactions by analyzing Lewis acid–base pairs with dimethylxanthene-linked group-13 (P) and group-15 (B) elements

Abstract

Nitrogen oxides, including nitrous oxide (N₂O), are undoubtedly classified as greenhouse gases. In this research, we conducted a theoretical investigation into the N₂O capturing reactions using frustrated Lewis pair (FLP)-assisted molecules based on the intramolecular dimethylxanthene backbone G13/P (G13 = B, Al, Ga, In, and Tl) and B/G15 (G15 = N, P, As, Sb, and Bi). Density functional theory (DFT) along with frontier molecular orbital (FMO) theory, activation strain model (ASM), and energy decomposition analysis–the natural orbitals for the chemical valence (EDA–NOCV) were employed to analyze the molecular reaction barriers and chemical reactivity. From the DFT calculations, it is evident that only B/P-Rea, among the FLP-type molecules utilizing G13/P and B/G15 as Lewis base/Lewis acid sites, exhibits the ability to capture N₂O in a reversible reaction. The EDA examinations suggest that the bonding nature between the G13/G15-FLP and N₂O in the G13/G15-TS structure can be elucidated by the donor–acceptor interaction (singlet–singlet bonding) model, instead of the electron-sharing interaction (triplet–triplet bonding) model. FMO and NOCV analyses reveal that the reaction between N₂O and the G13/G15-based FLP exhibits two distinct bonding properties: one is the forward bonding, the lone pair of G15 → the p–π* orbital of the N-terminus of N₂O, which is a significantly strong FLP-to-N₂O interaction. The other is the back-bonding, the empty orbital of G13 ← the filled p–π orbital of the O atom of N₂O, which is a relatively weak N₂O-to-FLP interaction. In basic terms, Lewis base interacting with N₂O is more pivotal than the interaction of Lewis acid with N₂O. The ASM analytical findings indicate that the deformation energy of the small N₂O molecule significantly influences the reaction barrier of the N₂O capture reaction by the intramolecular dimethylxanthene-linked G13/G15-FLPs. The theoretical evidence for N₂O capture by intramolecular dimethylxanthene-linked G13/G15-based FLPs shows that the relationships between geometrical structures and energetic values are in accordance with the Hammond postulate.