The nuclear spin functions for the A3(X3)3 system have been developed and used to analyse the NMR spectra of (CH3)3C6H3 and (CF3)3C6H3. The π-electron coupled hyperfine splitting constants, obtained from the Ramsey theory with simple Hückel M.O.'s for benzene and without the assumption of an average energy of excitation for the virtual processes which give rise to the coupling, are in agreement with the hyperfine constants observed between the methyl groups and the ring protons in the above compounds.
Juana Acrivos. "Analysis of the A3(X3)3 Nuclear Spin System and -Electron Coupled Hyperfine Splitting Constants in Aromatic Hydrocarbons" Molecular Physics (1961). doi:10.1080/00268976200100011