Comparison of the calculated acidity of cubane with that of other strained and unstrained hydrocarbons

Abstract

Gas-phase acidities of some strained and unstrained hydrocarbons were calculated. The resulting deprotonation enthalpies (DPE's) are within 2-3 kcal mol{sup {minus}1} of those observed when extended basis sets are employed, some accounting of the correlation energy is made, and zero-point energy differences are considered. The best calculated DPE for cubane (406.4 {plus minus} 3 kcal mol{sup {minus}1}) is significantly greater than that calculated for benzene (397 kcal mol{sup {minus}1}) and bicyclobutane (395.4 kcal mol{sup {minus}1}), but less than that for cyclopropane (413 kcal mol{sup {minus}1}). This result shows that the enhanced kinetic acidity of cubane is reflected in its thermodynamic acidity as well. It is noted that the quantity {triangledown}{sup 2}{sub {rho}}{sub c}(C-H) predicts cubane to be less acidic than cyclopropane, as would correlations against J{sub 13{sub C-H}}. Changes of angles between bond paths at carbon upon ionization were calculated. It is found that the change for cubane is unusually large for formal sp{sup 3} centers, thus corroborating the unusual hybridization in the cubyl anion as suggested by Luh and Stock.