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

Semi-empirical and ab initio geometries and energies of ground state isocya nate molecules are compared with experimental values. Semi-empirical method s isocyanate geometries compare well with experimental values. Without pola rization, 3-21G, 6-31G, and 6-311G ab initio basis sets all give incorrect linear geometries. Polarization functions improve ab initio isocyanate geom etries drastically, to agree with experiment. Semi-empirical calculations ( NMDO, AM1 and PM3) of the beta-lactam forming isocyanate cycloaddition reac tion, with various substituted alkenes, show MNDO and AM I transition state geometries significantly different from ab initio transition state geometr ies; PM3 activation energy barriers and transition states are in better agr eement with high level ab initio and PM3 is the most accurate semi-empirica l method for the hydrogen isocyanate heat-of-formation. (C) 1999 Elsevier S cience B.V. All rights reserved.