Determination of the signs of long-range proton spin coupling constants by nuclear magnetic triple resonance

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Abstract
In a nuclear magnetic resonance spectrum of the type AMX, the relative signs of the spin coupling constants J AM and J MX cannot be determined by double resonance experiments if J AX is unobservably small or zero, but the problem can be solved by the simultaneous application of three radio-frequency fields. This technique is described and demonstrated, and it is suggested that it should prove particularly useful for relating the signs of long-range spin coupling constants to couplings of known sign. Applied to the molecule of 3-bromothiophene-2-aldehyde, this experiment establishes that the long-range coupling to the aldehyde proton has the same sign as the vicinal coupling.