Molecular Structure and Conformations of 2,2-Di-tert-butyl-1,3-diaza-2-silacyclopentane:  Gas Electron Diffraction and Quantum Chemical Calculations

Abstract

The geometric structure and conformational properties of the saturated five-membered-ring compound 2,2-di-tert-butyl-1,3-diaza-2-silacyclopentane, (t-Bu)(2)Si(NH)(2)(CH(2))(2), was investigated by gas electron diffraction and quantum chemical methods (B3LYP and MP2 with 6-31G basis sets). The compound exists as a mixture of two conformers, both possessing a twist conformation and C(2) symmetry. In the prevailing form (76(6) % at 305 K) the N-H bonds stagger the adjacent CH(2) groups, and in the minor form the N-H bonds eclipse the CH(2) groups. This conformational mixture corresponds to a free energy difference of DeltaG degrees = 0.69(19) kcal/mol. The B3LYP method predicts a preference for the eclipsed conformer. The largest torsion occurs around the C-C bond with tau(NCCN) = 29.2(24) degrees. The degree of puckering in the title compound is considerably smaller than that in silacyclopentane with tau(CCCC) = 49.7(14) degrees. This has been rationalized by larger angle strain in the title compound.

Pseudorotation in pyrrolidine: rotational coherence spectroscopy and ab initio calculations of a large amplitude intramolecular motion

“…1 Two ways of interconversion of the axial (f = 01) and equatorial (f = 1801) conformers of pyrrolidine: pseudorotation (green, o300 cm À1 ) and ring inversion (red, B1500 cm À1 ). 6 resonance 11 and electron diffraction (ED) 11,12 enable the determination of energetic preferences and structures of lower pseudorotational states, however, information about the complete pseudorotational potential usually remains beyond their scope. The gas-phase ED study of the conformational preferences in pyrrolidine in conjunction with ab initio calculations indicated that the axial conformer is energetically the most stable one, 13 an assignment which was revised later by microwave spectroscopy.…”