Formation of phenolate anion-counterion complexes can explain the vibrational properties of the phenolate anion in solution

Structures of enolate-counterion complexes and structures and vibrational s pectra of phenolate anion-counterion complexes have been calculated by mean s of MP2 and density functional methods. Compared to corresponding monomeri c complexes, higher complexes reveal longer C-O bond lengths which causes a downshift of the C-O stretching mode. In the case of phenolate we find C-O stretching frequencies and isotope shifts upon O-18 and d(2) labeling whic h are in good agreement with recent IR data of phenolate generated in solut ion. The C-O stretching frequency, for example, is predicted to be around 1 270 cm(-1) compared to an experimental value of 1273 cm(-1) and the O-18 sh ift of this mode is calculated to be 18 cm(-1) compared to an experimental shift of 17 cm(-1). For a free phenolate anion, our calculations predict a C-O stretching frequency of similar to 1350 cm(-1). The vibrational spectru m of phenolate anions in solution can thus be explained in terms of higher phenolate anion-counterion complexes in agreement with recent NMR experimen ts of Jackman and Smith (Jackman, L. M.; Smith, B. D. J. Am. Chem. Sec. 198 8, 110, 3829).