Gas-phase nitronium ion affinities

Area of Science:

• Physical Chemistry
• Gas-Phase Ion Chemistry
• Mass Spectrometry

Background:

• Nitronium ion (NO2+) affinities are crucial for understanding chemical reactivity and molecular interactions.
• Previous studies have lacked a comprehensive scale for NO2+ affinities across diverse ligands.
• The kinetic method and ion-transfer equilibria provide avenues for thermodynamic measurements.

Purpose of the Study:

• To establish a relative and absolute gas-phase nitronium ion affinity scale for various ligands.
• To investigate the correlation between nitronium ion affinities and proton affinities.
• To extend the kinetic method to polyatomic ion-bound dimers.

Main Methods:

• Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was employed to measure ion-transfer equilibria.
• The kinetic method, analyzing metastable fragmentation of nitronium ion-bound dimers, was applied.
• The established affinity scale was calibrated using a literature value for water's NO2+ affinity.

Main Results:

• A scale of relative and absolute gas-phase NO2+ affinities was determined for 18 ligands.
• A reasonably linear correlation was observed between NO2+ affinities and proton affinities (PAs) for diverse ligands.
• NO2+ binding energies were found to be significantly lower than PAs and related cation binding energies.

Conclusions:

• The study provides a valuable tool for predicting NO2+ affinities based on PAs, with improved linearity for specific ligand classes.
• The extension of the kinetic method broadens its applicability to polyatomic ions.
• Experimental determination of ammonia's NO2+ affinity enabled the evaluation of nitramide's amino group PA, verifying theoretical predictions.