The Fluoride Ion Affinity Revisited: Do We Need the Anchor-Point Approach?

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate computation of gas-phase fluoride ion affinities (FIAs) is crucial for understanding Lewis acid-base interactions.
• Traditional indirect anchor-point methods have been favored over direct FIA computations in Density Functional Theory (DFT) calculations.
• These indirect methods aim to circumvent the challenges associated with modeling the free fluoride ion.

Purpose of the Study:

• To quantitatively evaluate various DFT functionals and composite methods for calculating FIAs.
• To compare the accuracy of direct FIA computation against widely used indirect anchor-point methods.
• To challenge the prevailing assumption that indirect methods are always superior for DFT-level FIA calculations.

Main Methods:

• A benchmark dataset of computed gas-phase FIAs for 71 main-group Lewis acids was utilized.
• 52 DFT functionals and 4 composite methods were assessed.
• Direct FIA computations were compared with two indirect anchor-point methods using fluorophosgene and trimethylsilyl cation.

Main Results:

• Modern DFT functionals with low self-interaction errors and appropriate basis sets reduce the advantage of indirect methods.
• Anchor-point methods based on trimethylsilyl cation often worsen results compared to direct computation.
• Error cancellation in anchor-point methods breaks down with advanced computational setups, negating their benefits.

Conclusions:

• Direct computation of FIAs is the preferable method when using modern DFT functionals and suitable basis sets with diffuse functions.
• The widespread preference for indirect anchor-point methods is not universally valid.
• Optimized direct FIA calculations provide more reliable results for Lewis acid characterization.