π-Conjugation as a Direct Estimate of Lewis Acidity

A computational approach to directly estimate the relative acidity of a given Lewis acid is presented. This approach is based on the strength of the π-conjugation in trans-crotonaldehyde-Lewis acid complexes, the species used in the well-known Childs' Lewis acidity scale. It is found that the π-conjugative strength values given by the Energy Decomposition Analysis - Natural Orbital for Chemical Valence method strongly correlate not only with the variation of the bond lengths in the system, which are greatly affected by the nature of the Lewis acid, but also with the downfield shifts experienced by the different nuclei of the conjugated system upon binding to the Lewis acid. These strong correlations indicate that the (easy-to-compute) π-conjugation energies can be used as an alternative Lewis acidity scale.