Ab initio calculations on peptide-derived oxazoles and thiazoles: improved molecular mechanics parameters for the AMBER force field.

Ab initio calculations at the RHF/6-31G* and MP2/6-31G*//RHF/6-31G* levels of theory are performed for 2-methyl-4-carboxamido-oxazoles and -thiazoles, including rotational profiles for the ring-carboxamide bond, which showed the expected conjugation and hydrogen bonding effects. On the basis of these data, newly optimised stretch, bend and torsional parameters for the AMBER* force field are derived, along with CHELPG-fitted partial atomic charges.