Electron Coupling of Nuclear Spins. VI. Relative Signs of JgemHH, JgHH, and JtHH in (2.2) Metacyclophane

The high-resolution proton magnetic resonance spectrum of the methylene groups in (2.2) metacyclophane has been observed at 60 and 15 Mc. In this compound, the C–CH2–CH2–C groups are fixed in position with the dihedral angle between alkyl C–C–C bonds slightly less than the symmetrical, staggered 60° and with little distortion from tetrahedral of the other bond angles in the group. Therefore, the HCCH coupling constants should be characteristic of ethanic trans and gauche orientations and the CH2 should be comparable with the geminal coupling in methane. A complete analysis of the A2X2 and A2B2 type spectra, in combination with previous, unambiguous experimental findings that |JgemHH|≅|JtHH|>|JgHH|, leads to the following assignments in (2.2) metacyclophane: JtHH=±12.3,Jg′HH (the coupling of the ``central'' pair of gauche protons) =±3.2,JgemHH=∓12.0,and JgHH=±4.0,all±0.1 cps. The magnitudes of these constants agree well with previous valencebond calculations for CH4 and the ethanic HCCH group. However, the opposite signs found for the large trans and geminal constants disagrees with the theoretical prediction that both are positive. The observed difference in sign can not be attributed to substituent effects, angular distortions, or to motional averaging. It is concluded that one of the two sets of calculations is in error; the implications of this result are discussed.