Different hydrogen-bonding modes in two closely related oximes

Two closely related oximes, namely 1-chloroacetyl-3-ethyl-2,6-diphenylpiperidin-4-one oxime, C21H23ClN2O2, (I), and 1-chloroacetyl-2,6-diphenyl-3-(propan-2-yl)piperidin-4-one oxime, C22H25ClN2O2, (II), despite their identical sets of hydrogen-bond donors and acceptors, display basically different hydrogen-bonding patterns in their crystal structures. While the molecules of (I) are organized into typical centrosymmetric dimers, created by oxime-oxime O-H center dot center dot center dot N hydrogen bonds, in the structure of (II) there are infinite chains of molecules connected by O-H center dot center dot center dot O hydrogen bonds, in which the carbonyl O atom from the chloroacetyl group acts as the hydrogen-bond acceptor. Despite the differences in the hydrogen-bond schemes, the -OH groups are always in typical anti positions (C-N-O-H torsion angles of ca 180 degrees). The oxime group in (I) is disordered, with the hydroxy groups occupying two distinct positions and C-C-N-O torsion angles of approximately 0 and 180 degrees for the two alternatives. This disorder, even though the site-occupancy factor of the less occupied position is as low as ca 0.06, is also observed at lower temperatures, which seems to favour the statistical and not the dynamic nature of this phenomenon.