In solution, are the hydrogen bonds in monoprotonated N,N,N',N'-tetramethyl-1,8-naphthaienediamines single- or double-well? To answer this question, isotopic perturbation of equilibrium is applied to a mixture of -d(0), -d(3), -d(6), -d(9), and -d(12) isotopologs. The N-methyls of the 2,7-dimethoxy analogue show intrinsic isotope shifts from the geminal CD3 and from only one distant CD3, an unusual stereochemical effect transmitted across the hydrogen bond. The C-13 NMR splittings and intensities at the various ring carbons of both ions are consistent with perturbation isotope shifts, intrinsic shifts, or a combination of both. The perturbation shifts mean that the protons reside in a double-minimum potential and that each ion is a pair of rapidly interconverting tautomers, The significance of this result for the role of low-barrier hydrogen bonds in enzyme-catalyzed reactions is discussed.