Solution chemistry of macrocycles. Part 2. Enthalpic and entropic contributions to the proton basicity of cyclic tetra-amine ligands: 1,4,8,11-tetra-azacyclotetradecane, 1,4,8,12-tetra-azacyclopentadecane, and 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane