Abstract
Semi-empirical and ab initio geometries and energies of ground state isocyanate molecules are compared with experimental values. Semi-empirical methods 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 calculations (MNDO, AM1 and PM3) of the β-lactam forming isocyanate cycloaddition reaction, with various substituted alkenes, show MNDO and AM1 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 heat-of-formation.
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