A SIMPLIFIED PROCEDURE FOR THE ANALYSIS OF COMPLEX NUCLEAR MAGNETIC RESONANCE SPECTRA: II. THE ANALYSIS OF SYMMETRICAL TRIFLUOROBENZENE, THE AA′A″XX′X″ SYSTEM

Abstract

The analysis of the six spin AA′A″XX′X″ system consisting of two sets of three chemically equivalent nuclei with spin [Formula: see text] and four coupling constants J_AA, J_XX, J_AX, and J_AX′ has been developed with the use of group theory and the appropriate Hamiltonian. It is found that there will be differences in the A and X spectra if J_AX ≠ J_AX′ and simultaneously J_AA ≠ J_XX. If J_AA = J_HH the A and X spectra become identical. If J_AX = J_AX′ each spectrum of course becomes trivial.The complex n.m.r. spectrum of the AA′A″XX′X″ can be decomposed into subspectra, and the analysis of the nuclear magnetic resonance spectra of symmetrical trifluorobenzene has been carried out in this manner. The experimental proton and fluorine spectra of this molecule exhibit marked differences. Calculated spectra have been fitted to the experimental spectra and the following values of the coupling constants have been found:[Formula: see text]The sign of J_HH is the same as the sign of J_HF•J_HF and J_HF′ have opposite signs. It is not possible to relate the signs of J_HH and J_FF to the signs of J_HF and J_HF′ in this type of spectrum. However, comparison of the signs of couplings in related molecules suggest that J_HF, J_HH, and J_FF have the same sign.