Anchoring the Absolute Proton Affinity Scale.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Chemical Physics

Background:

• Proton affinities (PA) are fundamental chemical properties.
• Accurate PA values are crucial for establishing molecular scales.
• Previous calculations had limitations in accuracy and scope.

Purpose of the Study:

• To accurately determine the first-principles proton affinities of ammonia and carbon monoxide.
• To establish reliable high and low anchors for the molecular proton affinity scale.
• To provide highly accurate reference values superseding previous recommendations.

Main Methods:

• Employed the focal-point analysis (FPA) approach for converged calculations.
• Utilized all-electron coupled-cluster (CC) methods up to pentuple excitations.
• Incorporated advanced basis sets (aug-cc-pCVXZ and aug-cc-pVXZ) and relativistic corrections.

Main Results:

• Calculated proton affinities for ammonia (NH3) and carbon monoxide (CO) at 298.15 K and 0.0 K.
• Achieved high accuracy with uncertainties of ±0.3 kJ mol⁻¹ for NH3 and ±0.2 kJ mol⁻¹ for CO.
• The determined values are ΔpaH°(NH3) = 852.6(846.4) kJ mol⁻¹ and ΔpaH°(CO) = 592.4(586.5) kJ mol⁻¹.

Conclusions:

• The study provides the most accurate proton affinities for NH3 and CO to date.
• These results firmly anchor the molecular proton affinity scale.
• The findings offer a reliable benchmark for future computational and experimental studies.