A weak (1:5 kcalmol1) and rare C–H...OC hydrogen bond has been
reported in a class of compounds known as a [6]semirubin (a dipyrrinone).64
There is also evidence for a C–H...N/CH...OH bond in the crystal structures of
a,b-unsaturated ketones carrying a terminal pyridine subunit,65 and for R3Nþ–C–
H...OC hydrogen bonding.66
Deuterium also forms hydrogen bonds; in some systems these seem to be stronger
than the corresponding hydrogen bonds; in others, weaker.67