GAS-PHASE BASICITIES OF ACID ANHYDRIDES

The gas-phase proton affinities (PA's) of acetic anhydride, 1, and sev eral representative cyclic anhydrides (succinic, 2; methylsuccinic, 3; glutaric, 4; and 3-methylglutaric, 5) were measured through the use o f Fourier transform-ion cyclotron resonance and high-pressure chemical ionization techniques: PA(1) = 844 +/- I kJ/mol, PA(2) = 797 +/- 1 kJ /mol, PA(3) = 807 +/- 1 kJ/mol, PA(4) = 816 +/- 3 kJ/mol, PA(5)= 820 /- 3 kJ/mol. The results were analyzed in the light of molecular orbit al ab initio (MP2/6-31G, G2) and density functional theory.(B3LYP/6-3 1G) calculations. The enol forms of acetic ahydride and its protonate d counterparts were predicted to be significantly less stable than the corresponding diketo conformers. The large proton affinity of acetic anhydride takes its origin from the formation of an intramolecular hyd rogen bond in the protonated form. This is supported by the computatio nal results and by the measurement of a sizable entropy loss upon prot onation. In contrast, the protonation of cyclic anhydrides is accompan ied by an acyl bond fission, thus leading to an entropy gain upon prot onation. The protonated structures of cyclic anhydrides are stabilized by an electrostatic attraction between the two opposite parts of the ion. This effect is more pronounced for glutaric derivatives, and this explains the enhancement of the proton affinity observed when the siz e of the ring increases. It is also related to the increase in entropy of protonation and to the observed methyl substitution effect.