Proton Affinity Calculations with High Level Methods

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Physical Chemistry

Background:

• Proton affinity is a fundamental chemical property influencing molecular interactions and reactivity.
• Accurate proton affinity data is crucial for various chemical applications, including reaction mechanism elucidation and drug design.
• Existing literature values for some compounds may contain inaccuracies, necessitating re-evaluation.

Purpose of the Study:

• To accurately calculate proton affinities for a range of organic compounds, from small molecules to polycyclic aromatic hydrocarbons.
• To compare the performance of several high-accuracy computational methods in determining proton affinities.
• To identify and correct potential inaccuracies in currently accepted literature proton affinity values.

Main Methods:

• High-accuracy computational chemistry methods were employed, including W1BD, G4, G3B3, CBS-QB3, and M06-2X.
• Calculations covered a spectrum of molecules, from simple reference compounds to methylbenzenes, naphthalene, and anthracene.
• Computed proton affinities were rigorously compared against established reference values.

Main Results:

• Computed proton affinities generally show excellent agreement with accepted reference values, with notable exceptions.
• Literature values for propene and methylbenzenes appear to be overestimated by 6-7 kJ/mol and 4-5 kJ/mol, respectively.
• The G4 and G3B3 methods demonstrated good agreement with the high-level W1BD method.
• The CBS-QB3 method consistently underestimated proton affinities, with errors increasing for larger molecules.
• The M06-2X functional showed significant deviations for small molecules like CO and ketene but high accuracy for methylbenzenes.

Conclusions:

• The study provides highly accurate proton affinity data, refining existing literature values for specific compounds.
• Computational methods exhibit varying degrees of accuracy, with G4 and G3B3 showing robust performance.
• The findings underscore the importance of employing appropriate computational tools for reliable chemical property determination.