TY  - JOUR
A1  - Kallies, Bernd
A1  - Mitzner, Rolf
T1  - Models of water-assisted hydrolyses of methyl formate, formamide, and urea from combined DFT-SCRF calculations
N2  - In this work we present theoretical studies of the hydrolytic reaction of methyl formate, formamide and urea with one water molecule. The studied systems contain two additional water molecules which can act as bifunctional acid- base catalysts. These water molecules catalyze proton transfers between the primary reacting species. Our models describe the concerted transfer of two protons in every reaction step. The calculations have been carried out with the Becke3LYP/6-31G* method. Unspecific solvation effects have been included by means of a polarizable continuum model. Substrate reactivity differences as well as preferences for different reaction pathways can be discussed with the aid of these molecular systems. The studied alternative mechanisms include the common addition-elimination mechanism via a tetrahedral intermediate, and a concerted SN-like mechanism without a reaction intermediate. Our results suggest that the proved decreasing substrate reactivity in the order ester, amide, urea is caused by a rising resonance stabilization of the reaction centre, and not by a different positive partial charge of the carbonyl carbon. It is also concluded, that the probability of a concerted addition of a nucleophile and elimination of a leaving group without a tetrahedral intermediate rises in the order ester, amide, urea. The ordering of reactivity is not influenced by this behaviour.
Y1  - 1998
ER  - 
TY  - JOUR
A1  - Kallies, Bernd
A1  - Mitzner, Rolf
T1  - Structural Properties of Protonated Acyl Derivatives as Studied by Quantum Mechanics
N2  - Basicities of 16 acyl compounds including selected aldehydes, ketones, esters, amides and ureas in the gas phase and in water were calculated 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 carbonyl oxygen is the preferred site of protonation for all molecules studied. 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, amides and ureas is less preferred, but the basicity difference between C=O and -OR or -NR2 decreases from esters to ureas. Calculated pKa values for this site range from -18 for esters to + 3 for ureas. These values are often not accessible by experiment. The structure of carboxylic acid derivatives protonated at the leaving group is determined by prefragmentation of the molecules into an acylium ion or positively charged isocyanate and an alcohol or amine. (C) 1998 Elsevier Science B.V.
Y1  - 1998
ER  - 
TY  - JOUR
A1  - Beitz, Toralf
A1  - Bechmann, Wolfgang
A1  - Mitzner, Rolf
T1  - Investigations of reactions of selected Azaarenes with radicals in water, 2. Chlorine and Bromine radicals
Y1  - 1998
ER  - 
TY  - JOUR
A1  - Beitz, Toralf
A1  - Bechmann, Wolfgang
A1  - Mitzner, Rolf
T1  - Investigations of reactions of selected Azaarenes with radicals in water, 1. Hydroxyl and sulfate radicals
Y1  - 1998
ER  -