AB initio and semi‐empirical investigation of gas‐phase carbon acidity

Acidities of a representative range of carbon acids were calculated by various levels of ab initio theory and by the semi‐empirical methods AM1 and PM3 in order to ascertain what level is necessary in order to obtain consistently reliable results. The semi‐empirical methods give good agreement with experiment in about a third of the cases, but are significantly, and sometimes seriously, in error for the rest. The 3–21G ab initio method consistently underestimates acidities by a substantial amount. The best results were obtained at the MP2/6–31 + G*//6–31 + G* level. Higher level Møller–Plesset corrections worsened the agreement with experiment.