Accurate benchmark calculations on the gas-phase basicities of small molecules

Area of Science:

• Computational chemistry
• Quantum chemistry
• Thermochemistry

Background:

• Gas-phase basicity is a fundamental molecular property.
• Accurate theoretical prediction requires high-level computational methods.
• Experimental data provides benchmarks for theoretical models.

Purpose of the Study:

• To perform accurate benchmark calculations of gas-phase basicities for small molecules.
• To compare theoretical results with experimental data.
• To assess the accuracy of various ab initio methods.

Main Methods:

• Optimized geometries and thermochemical analyses using MP2/aug-cc-pVTZ.
• Gas-phase basicity calculations employing MP2 and CCSD(T) methods.
• Extrapolation of single-point energies to the complete basis set (CBS) limit.

Main Results:

• Ranked ab initio methods by accuracy: CCSD(T)_CBS > CCSD(T)/aug-cc-pVDZ > (MP2/aug-cc-pVQZ ≈ MP2_CBS) > HF/aug-cc-pVQZ.
• Achieved a root-mean-squared-error of 1.0 kcal mol⁻¹ with CCSD(T)_CBS//MP2/aug-cc-pVTZ.
• Identified the importance of electron correlation energy for accurate predictions.

Conclusions:

• Accurate theoretical prediction of gas-phase basicities relies heavily on electron correlation energy.
• Conformational effects can be significant for complex molecules.
• CCSD(T)_CBS//MP2/aug-cc-pVTZ provides a highly accurate approach for gas-phase basicity calculations.