STRUCTURAL-PROPERTIES OF PROTONATED ACYL DERIVATIVES AS STUDIED BY QUANTUM-MECHANICS

Basicities of 16 acyl compounds including selected aldehydes, ketones, esters, amides and ureas in the gas phase and in water were calculate d with the Becke3LYP/6-31G method. Solvent effects were modeled using a polarizable electrostatic continuum representation of the solvent. The properties of the electron densities of protonated molecules were described by localized bond orbitals. Our results suggest that the car bonyl oxygen is the preferred site of protonation for all molecules st udied. Calculated pKa values in water range from -12 for aldehydes to + 1.5 for ureas. They agree well with published experimental data. We found that a high basicity of acyl compounds at the carbonyl oxygen is coupled with a large amount of additional resonance stabilization at the carbonyl group. The protonation of the leaving group of eaters, am ides and ureas is less preferred, but the basicity difference between C=O and -OR or -NR2 decreases from esters to ureas. Calculated pKa val ues for this site range from -18 for esters to + 3 for ureas. These va lues are often not accessible by experiment. The structure of carboxyl ic acid derivatives protonated at the leaving group is determined by p refragmentation of the molecules into an acylium ion or positively cha rged isocyanate and an alcohol or amine. (C) 1998 Elsevier Science B.V .