Relationship between experimental pKa values in aqueous solution and a gas phase bond length in bicyclo[2.2.2]octane and cubane carboxylic acids
Linear correlations were established between the calculated bond lengths and the pKa or σI values for a series of 4-substituted bicyclo[2.2.2]octane-1-carboxylic acid and 4-cubane-1-carboxylic acid derivatives. The bond lengths have been calculated at a modest computational level, HF/6-31G(d), both in the gas phase and with the continuum solvation model, polarisable continuum model (PCM). In general, the best correlations are obtained when the PCM model is taken into account, especially when neutral and charged molecules are considered together. The best models in each case show square correlation coefficients (R2) larger than 0.9 and indicate that they can be used as predictive tools. These results expand previous results that indicate the possibility of a relationship between gas phase bond length and pKa values in aqueous solution and indicate that such relationships are more general than hitherto expected.