Gas phase protonation of α, β and γ-dicarbonyls: Thermochemistry and structures

An experimental and theoretical study of the protonation of representative dicarbonyl compounds M = 2,3-butanedione (biacetyl), 1, 2,4-pentanedione (acetylacetone), 2, 2,5-hexanedione (acetonylacetone), 3, and methyl-acetoacetate, 4 has been carried out. The experimental proton affinities and protonation entropies have been obtained by the extended kinetic method using the orthogonal distance regression (ODR) treatment. Theoretical proton affinities are calculated at the G2MP2 level of theory while protonation entropies were estimated after a detailed treatment of the internal rotations. The data show that protonation of 1 and 2 is associated with negligible protonation entropies while significant negative values are obtained for molecules 3 and 4. Protonation of 2,3-butanedione, 1a, is associated with a tautomerisation inside the proton transfer complex thus leading to protonated 2-hydroxy-butenone, 1bH+. Protonation thermochemistry of 2,4-pentanedione 2 may be simply rationalized by the protonation of its most stable tautomer, the 4-hydroxy-3-pentene-2-one, 2b, to give its most stable protonated form 2bH+ stabilized by a strong intramolecular hydrogen bond. Protonation of 2,5-hexanedione 3a most probably produces a cyclic structure stabilized by a covalent bonding, 3aHc+. The structure of neutral methyl acetoacetate 4 sampled during protonation in mass spectrometry experiments appears to be its diketonic form 4a; its protonation leading to an internally hydrogen bonded stabilized structure 4aH+.