When one sees downfield hydrogens one may have what are called short, strong hydrogen bonds or low-barrier hydrogen bonds. One might see that the value of the fractionation factor that is much less than one and/or a significantly nonzero value of δH - δD when comparing the two chemical shifts. The fractionation factor is a measure of whether a given exchangeable group would prefer to have hydrogen or deuterium at that site. Maurice Kreevoy did some of the early work on the organic chemistry of these H-bonds. It has been a controversial topic in enzymology. See for example, "Strong, Low-Barrier Hydrogen Bonds May Be Available to Enzymes" dx.doi.org/10.1021/bi4014566 | Biochemistry 2014, 53, 344−349