Ring Current Model and Anisotropic Magnetic Response of Cyclopropane.

Abstract

Three-dimensional models of the quantum mechanical current density, induced in the electron cloud of the cyclopropane molecule by a uniform magnetic field applied either along the C3 or the C2 symmetry axes (indicated by B∥ and B⊥, respectively), have been constructed via extended calculations. These models of near Hartree−Fock quality, previously shown to provide a good agreement between computed and observed values of magnetic tensors, have been used to interpret the magnitude of the diagonal components of susceptibility (χ), nuclear shielding of carbon (σC) and hydrogen (σH), and shielding at the center of mass (σCM). The source of the exceptionally large in-plane component σ⊥CM, dominating the anomalous average σavCM, is shown to be a strong delocalized current flowing around the methylene moieties and the noncyclic CH2−CH2 fragment. The total current strength for a magnetic field applied in the direction of a C2 symmetry axis is 15.7 nA/T, approximately 1.5 times larger than that calculated for B∥. T...