π Interactions Studied with Electronic Structure Methods: The Ethyne Methyl Isocyanide Complex and Thioanisole

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Previous studies reported discrepancies between B3LYP and MP2 methods for certain molecular systems.
• Accurate computational methods are crucial for understanding non-covalent interactions and molecular conformations.

Purpose of the Study:

• To investigate the performance of various electronic structure methods, including density functionals and DFT-D, for molecular systems with π hydrogen bonds and conformational flexibility.
• To assess the accuracy of these methods by comparing them to high-level coupled cluster with singles, doubles, and perturbative triples (CCSD(T))/complete basis set (CBS) calculations.

Main Methods:

• Employed a range of electronic structure methods: meta and double hybrid density functionals, DFT-D, and coupled cluster theory.
• Calculated interaction energies for the ethyne methyl isocyanide complex and potential energy curves for thioanisole conformations.
• Assessed method performance against estimated CCSD(T)/CBS reference values.

Main Results:

• For the ethyne methyl isocyanide complex, B3LYP underestimated interaction energy, likely due to missing dispersion. B3LYP-D, M05-2X, and mPW2-PLYP-D showed good agreement with CCSD(T) estimates.
• Thioanisole's conformational analysis revealed strong dependence on method and basis set. MP2, B3LYP, M05-2X, mPW2-PLYP, and mPW2-PLYP-D provided reasonable agreement with CCSD(T)/CBS, while B3LYP-D and M06-L overestimated rotational barriers.

Conclusions:

• Dispersion corrections are essential for accurately describing π hydrogen bonds and conformational preferences in the studied systems.
• Methods incorporating dispersion, such as B3LYP-D and M05-2X, offer improved accuracy compared to standard B3LYP and MP2 for these systems.
• The choice of electronic structure method and basis set significantly impacts the reliability of calculated molecular properties.