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A discussion on the relationship between nuclear spin-spin coupling constants and bond lengths (internuclear distances) will be presented in this paper. The discussion will be focussed on the R-3 rule, the limit of its application and its influencing factors. Specifically, the causes for two long-existing puzzles in NMR coupling constants will be discussed: (a) the abnormal values of the coupling constants in ethylene and acetylene and (b) the “methyl-substitution rule”. It will be pointed out that in solution the coupling of intramolecular nuclear spins is affected by many factors and thus will not strictly obey the R-3rule in general. However, in practice, the R-3 rule will still be valid when various influencing factors are carefully taken into consideration.
A discussion on the relationship between nuclear spin-spin coupling constants and bond lengths (internuclear distances) will be presented in this paper. The discussion will be focussed on the R-3 rule, the limit of its application and its influencing factors. the causes for two long-existing puzzles in NMR coupling constants will be discussed: (a) the abnormal values of the coupling constants in ethylene and acetylene and (b) the “methyl-substitution rule”. It will be pointed pointed that in solution the coupling of intramolecular nuclear spins is affected by many factors and thus will not strictly obey the R-3rule in general. However, in practice, the R-3 rule will still be valid when various influencing factors are conditioned taken into consideration.