Structural diversity in nitrobenzene trimer anions: messenger-tagged infrared photodissociation spectroscopy and theoretical study.

The nitrobenzene trimer anion, NB3- was generated using a supersonic expansion source in the gas phase. The structure was characterizated using infrared photodissociation (IRPD) spectroscopy of the argon-tagged quantum chemical calculations. The optimized structures of NB3- and NB3(Ar)- were categorized into four types of isomers, including CH···O, π-π, and co-contributing (CH···O + π-π, CH···O + CH···π) interactions. Among the isomers stabilized predominantly by CH···O hydrogen bonds, three structural motifs can be distinguished: Y-type, triangular-type, and planar-type isomers. These isomers exhibit pronounced energy differences, with the π-π stacking and CH···π interactions further contributing to their conformational stabilization. Both simulated spectra of planar-type and Y-type isomers exhibit agreement with the experimental spectrum, suggesting the potential coexistence of planar-type and Y-type isomers. The strongest IRPD bands observed at 3019 and 3033 cm-1 are tentatively attributed to the CH···O interactions of the two conformers. The singly occupied molecular orbital (SOMO) reveals that the negative charge distribution in both isomers (a) and (f) are localized on one nitrobenzene unit. This NB unit can be regarded as a negative charge, and it forms an electrostatic attraction with the positive potential region of the benzene ring on the additional NB units to stabilize the structure. More importantly, multiple low-energy isomers stabilized by distinct non-covalent interactions (NCIs) might coexist under the experimental conditions, suggesting a complex conformational landscape that may extend to larger aggregates and condensed phases. These results highlight the importance of considering conformational heterogeneity when interpreting reaction dynamics and non-covalent interactions in molecular assemblies..