Mass-Selected Infrared Photodissociation Spectroscopic and Theoretical Insights into Nitrobenzene Dimer Anion Tagged by Argon.

The nitrobenzene dimer anion, NB₂^-, tagged with argon in the gas phase was successfully generated by using a collinear tandem time-of-flight mass spectrometer equipped with a supersonic ion source. Precise characterization was achieved through infrared photodissociation (IRPD) spectroscopy combining with theoretical calculation. Nine distinct absorptions were observed. The optimized structures of NB₂(Ar)^- were categorized into three types of isomers, including double-hydrogen-bonded (DHB), T-shaped (TS), and antiparallel π-π stacking (APS) isomers. The vibrational analysis indicated that the calculated spectrum of the APS isomer (e) exhibited better agreement with the experimentally observed spectrum than that of the lowest energy DHB isomer (a). Singly occupied molecular orbitals (SOMOs) and natural population analysis (NPA) confirmed that the negative charge in NB₂(Ar)^- was predominantly localized on one NB unit carrying a charge of -0.932 e, closely resembling a single electron charge. The other NB unit exhibited a minimal negative charge of -0.069 e, and Ar had a charge of 0.001 e. This distribution of charge suggests that NB₂^- can be viewed as a molecule-anion species, which consists of two separate frameworks: one neutral NB and one anionic NB framework. Energy decomposition analysis (EDA) further revealed that the stabilization of the APS isomer resulted from a nearly equal contribution of dispersion and electrostatic forces. However, for the DHB isomer, only the electrostatic interaction emerged as the primary stabilizing force.