Nitrogen inversion, molecular structure

As in the case of the MM2 force field, parameterization of MM3 for amines was based mainly on experimental data with occasional references to ab initio calculations, mainly to evaluate relative conformational energies and derive appropriate torsional parameters. As mentioned above, one notable difference between the two force fields is the removal of lp on sp3 nitrogens from MM3. This simplifies the treatment of vibrational spectra and allows for a realistic treatment of nitrogen inversion which could not be handled by MM2. As usual with MM3, parameterization was aimed at reproducing a variety of molecular properties such as structure, steric energy, dipole moments, moments of inertia, heat of formation and vibrational spectra. A complete list of MM3 parameters for amines is provided in Reference 6. 

Barriers to nitrogen inversion are strongly dependent on molecular structure and, as such, they may be considered as probes for understanding structural effects in molecules. 

The non-empirical studies of the nitrogen inversion process provide a general picture of the energetic origin of the inversion barrier. This picture, however, depends on the energy partitioning scheme used. The attractions Ex versus repulsions E scheme may provide an overall picture and a dichotomic A, R classification of the barriers it does generally not allow a detailed description in terms of molecular structural features. 

The overall stereoregularity and melting points of the polymer are, however, lower for the syndiotactic polymers produced with the lO/MAO catalyst system due to the presence of high concentration of site epimerization related stereo-errors (rrmr pentads). A rather frequent or fast site epimerization rate at higher polymerization temperatures is probably facilitated by the flexibility of the molecular structure of 10 (dynamic umbrella-type reversible inversion at the amido-nitrogen center see [148]) and the rapid same site contact ion-pairing. 

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