On the HCN - HNC Energy Difference.

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Chemical Thermodynamics

Background:

• The isomerization energy of hydrogen cyanide (HCN) to hydrogen isocyanide (HNC) is a critical parameter in understanding interstellar chemistry and molecular stability.
• Previous high-level calculations yielded conflicting values, necessitating a re-evaluation.

Purpose of the Study:

• To accurately determine the 0 K isomerization energy of HCN → HNC.
• To resolve discrepancies between existing computational results.
• To provide reliable thermochemical data for HCN, HNC, and their related species.

Main Methods:

• Utilized state-of-the-art electronic structure methods, specifically the HEAT-456QP level of theory.
• Employed the Active Thermochemical Tables (ATcT) approach for data analysis and validation.
• Cross-validated results through independent computational steps.

Main Results:

• The directly computed energy difference at the HEAT-456QP level is 5236 ± 50 cm⁻¹.
• The ATcT approach corroborated the HEAT-456QP result and identified issues with a prior multireference calculation.
• A recommended isomerization energy of 5212 ± 30 cm⁻¹ at 0 K was established.
• Standard enthalpies of formation for HCN, HNC, and their ions at 0 and 298 K were presented.

Conclusions:

• The refined isomerization energy provides a more accurate value for the HCN → HNC transformation.
• The study highlights the importance of combining high-level electronic structure calculations with robust data analysis methods like ATcT.
• The presented thermochemical data will aid future studies in astrochemistry and chemical kinetics.