Evaluation of MNDO calculated proton affinities

A large set of charged species arising mainly from protonation or deprotonation of hydrocarbons, alcohols, ethers, carboxylic acids, amines, imines, and nitriles has been studied by means of the semiempirical self‐consistent‐field (SCF) molecular orbital (MO) MNDO method. From the calculated heats of formation of such charged species and those of neutral molecules, MNDO‐estimated proton affinities have been obtained and the results compared with experimental gas‐phase proton affinities. If the small size anions and acetylides, for which the method predicts heats of formation too large, are ruled out, the mean absolute error in calculated proton affinities is ca. 7 kcal/mol for hydrocarbons (22 acid‐base pairs) and ca. 8 kcal/mol for oxygen‐containing compounds (25 acid‐base pairs). For nitrogen‐containing molecules it is necessary to discard, in addition, the values corresponding to the protonation of alkylamines and imines in order to achieve a reasonable mean absolute error of 7–8 kcal/mol.

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