Electronic Free Energy Surface of the Nitrogen Dimer Using First-Principles Finite Temperature Electronic Structure...

Area of Science:

• Computational chemistry
• Quantum mechanics
• Materials science

Background:

• The behavior of diatomic molecules at high temperatures is crucial for understanding chemical reactions and material properties.
• Nitrogen dimer (N2) is a fundamental molecule with a strong triple bond, often used as a benchmark in theoretical studies.

Purpose of the Study:

• To investigate the electronic free energy surface of the nitrogen dimer at various temperatures.
• To understand the competing effects of internal energy and entropy on the nitrogen-nitrogen bond strength.
• To predict the temperature range for thermally driven dissociation of the nitrogen dimer.

Main Methods:

• Full configuration interaction (FCI) and density matrix quantum Monte Carlo (DMQMC) methods were employed.
• Calculations were performed within the free-energy Born-Oppenheimer approximation.
• The electronic free energy surface was analyzed at different temperatures and bond lengths.

Main Results:

• A temperature regime was identified where internal energy leads to nitrogen-nitrogen bond strengthening.
• Entropy contributions were found to be essential for observing bond weakening at elevated temperatures.
• Thermally driven dissociation of the nitrogen dimer is predicted between 22,000 K and 63,200 K, influenced by symmetries and basis set.

Conclusions:

• The study reveals a counterintuitive bond strengthening at moderate temperatures before dissociation.
• The inclusion of spatial and spin symmetries significantly lowers the predicted dissociation temperature.
• Analysis of the density matrix structure provides insight into the temperature-dependent bonding behavior.