COMPARISON OF THE CALCULATED ACIDITY OF CUBANE WITH THAT OF OTHER STRAINED AND UNSTRAINED HYDROCARBONS

Gas-phase acidities of some strained and unstrained hydrocarbons were calculated. The resulting deprotonation enthalpies (DPE's) are within 2-3 kcal mol-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. Our best calculated DPE for cubane (406.4 ± 3 kcal mol-1) is significantly greater than that calculated for benzene (397 kcal mol-1) and bicyclobutane (395.4 kcal mol-1), but less than that for cyclopropane (413 kcal mol-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 ∇2pc(C-H) predicts cubane to be less acidic than cyclopropane, as would correlations against J13C-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 sp3 centers, thus corroborating the unusual hybridization in the cubyl anion as suggested by Luh and Stock. © 1990, American Chemical Society. All rights reserved.