Cyclopropa-arenium cations. Part II. Carbon-13 nuclear magnetic resonance spectra of 1-halogenocyclopropabenzenium cations and their 1,1-dihalogeno-precursors

Abstract

The ¹³C n.m.r. spectra of 1-fluorocyclopropabenzenium cation (9), 1-chloro-2, 5-diphenylcyclopropabenzenium cation (12), 1-fluoro-2, 5-diphenylcyclopropabenzenium cation (13), and their 1, 1-dihalogeno-precursors have been measured and the chemical shift differences between the neutral and charged species have been used to estimate the positive charge distributions. These empirical charge density distributions are in qualitative agreement with SCC–EH MO calculations for the parent cyclopropabenzenium ion (8). Only a relatively small amount of charge is delocalised into the phenyl substituents of (12) and (13). For (9) and (13) the significant increase in the values of the ¹³C–¹⁹F coupling constants and the reduced total downfield shifts of their carbon atoms compared with (12), indicate a substantial withdrawal of charge by back bonding from the fluorine atom.