Semi-empirical study of isocyanate geometries,andβ-lactam formation through alkene-isocyanate cyclo-addition reactions

摘要

Semi-empirical and ab initio geometries and energies of ground state isocyanate molecules are compared with experimental values.Semi-empirical metnods isocyanate geometries compare well with experimental values.Without polarization,3-21G,6-31G,and 6-311G ab initio basis sets all give incorrect linear geometries,Polarization functions improve ab initio isocyanate geometries drastically,to agree with experiment.Semi-empirical calculation(MNDO,AMI and PM3) of the β-lactam forming isocyanate cycloaddition reaction,with various substituted alkenes,show MNDO and AMI transition state geometries significantly different from ab initio transition state geometries;PM3 activation energy barriers and transition states are in better agreement with high level ab initio and PM3 is the most accurate semi-empirical method for the hydrogen isocyanate beat-of-formation.