Structural Effects in Solvolytic Reactions. 42. Effect of Increasing Electron Demand on the Carbon-13 Nuclear Magnetic Resonance Shifts for Substituted 9-Methyl-9-anthracenium Cations. Evidence for the Unimportance of Steric Hindrance to Coplanarity in Causing Anomalous Carbon-13 Nuclear Magnetic Resonance Shifts

Abstract

A series of substituted 9-methyl-9-anthracenium cations were prepared in FSO3H/SO2ClF at −78 °C and their 13C NMR shifts were measured at −80 °C. The Δ℘C+ shifts observed for the electron-donating substituents correlate linearly against the aC+ constants. However, the data observed for the electron-withdrawing substituents deviate upward from the straight line defined by the data of electron-donating substituents, similar to the deviation observed for the 1-aryl-1-phenyl-1-ethyl carbocations. The deviation observed in the latter case was previously attributed to steric inhibition of coplanarity. However, comparison of the behavior of this system with that of the 9-methyl-9-anthracenium system casts doubt upon this explanation. © 1982, American Chemical Society. All rights reserved.