Hydrogen Bond Strength and Acidity. Structural and Energetic Correlations for Acetylides and Alcohols

Ion−molecule complexes of acetylide anions, RCC - (R = tert -butyl, H, phenyl, p -tolyl), and methanol have been studied using Fourier transform ion cyclotron resonance mass spectrometry. The RCC - complexation energies with HOCH 3 were measured relative to CH 3 O - ·H 2 O and CH 3 O - ·HOCH 3 . These complexes, RCC - ·HOCH 3, all have complexation energies of −21 kcal/mol and are, therefore, hydrogen-bonded. The acetylides vary in basicity over an 8 kcal/mol range, but all have the same complexation energy with methanol. These results show that the frequently observed relationship between acid−base energetics and complexation energy is a property of the specific system and not a general one. Thus, the existence, and magnitude, of these correlations cannot be used to infer structural information about the complexes. The stability of hydrogen-bonded complexes appears to be related to the charge distributions of the separated ion and neutral.