Hydrogen-bonded complexes of methanol and acetylides. Structure and energycorrelations

We examine the connection between acid-base chemistry and hydrogen bond sta bility for anionic ion-molecule complexes. Ion-molecule complexes of substi tuted acetylides, RCC- (R = tert-butyl, H, phenyl, p-tolyl), and methanol w ere studied using Fourier transform ion cyclotron resonance mass spectromet ry. These complexes, RCC-. HOCH3, all have complexation energies of -21 kca l/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 me thanol. In contrast, results reported by Bartmess show that the complexatio n energies of aliphatic alcohols with phenyl acetylide vary with the acidit y of the alcohol. These results show that the frequently observed relations hip between acid-base energetics and complexation energy is a property of t he specific system and not a general one. We believe that these results dem onstrate that the existence, and magnitude, of these correlations cannot be used to infer structural information about the complexes.