Protonated nitro group: structure, energy and conjugation.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Organic Chemistry

Background:

• Protonation of nitro compounds is crucial for understanding their reactivity.
• The electronic structure and bonding in protonated species influence molecular properties.
• Comparing protonated nitro groups (NO2H+) with isoelectronic analogues like carboxyl groups (CO2H) provides insights into chemical behavior.

Purpose of the Study:

• To investigate the structure and electronic properties of protonated nitro compounds.
• To compare the behavior of the protonated nitro group (NO2H+) with the isoelectronic carboxyl group (CO2H).
• To elucidate the nature and strength of interactions within the NO2H+ group and with substituents.

Main Methods:

• Ab initio calculations using MP2 and B3LYP functionals.
• High-level basis sets (6-311++G(2d,2p) and 6-311+G(d,p)) were employed.
• Isodesmic reactions were used to quantify stabilization energies.

Main Results:

• Protonated nitro compounds exhibit conformational preferences and equilibria similar to the isoelectronic CO2H group.
• The NO2H+ group shows significantly stronger resonance interactions with donor substituents compared to CO2H.
• Standard resonance formulas partially explain geometry but fail to account for the observed strong interaction energies.

Conclusions:

• The NO2H+ group possesses unique electronic characteristics distinct from CO2H, particularly in its interaction with electron-donating groups.
• Computational methods reveal a powerful resonance stabilization in protonated nitro compounds.
• Further theoretical and experimental studies are warranted to fully understand the bonding and reactivity of these species.