title: Quantum mechanical calculations of the cephalosporin nucleus.
abstract: A computational study using several ab initio methods, HF, DFT and MP2 at different basis set levels (6-31G*, 6-31G**, 6-311G** and 6-311++G**) has been performed to determine the possible stable conformations attained by the cephalosporin nucleus. The calculations were carried out in three stages by studying the conformational analysis of the 3-cephem nucleus, the 3-cephem-4-carboxylic acid nucleus and the acetylamino group of the 7-acetylamino-3cephem-4-carboxylic acid. In the first two stages, the potential energy surfaces indicated two minima that correspond to the S1-up and C2-up conformations, with the S1-up being more stable. The energy required for the interconversion of the S1-up to the C2-up is around 7 kcal/mol, indicating the feasibility of interconversion between the two conformers. In the third stage, the acetylamino group attained two conformations with respect to the 3-cephem nucleus. All the geometric parameters obtained in this study were found to be in reasonably good agreement with available X-ray diffraction data, even upon using a simple basis set.
authors.affiliation: Department of Chemistry, Faculty of Science, Benha University, Benha 12211, Egypt. i.elkarim@gmail.com
authors.forename: I A
authors.initials: IA
authors.lastname: Gad El-karim
authors.forename: A A
authors.initials: AA
authors.lastname: Aly
authors.forename: M S
authors.initials: MS
authors.lastname: Amine
authors.forename: S
authors.initials: S
authors.lastname: El-Alfy
mesh_terms: D002264:Carboxylic Acids / Q000737:chemistry; D002511:Cephalosporins / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D011789:Quantum Theory; D013843:Thiazines / Q000737:chemistry
chemical_list: D002264:Carboxylic Acids; D002511:Cephalosporins; D013843:Thiazines