An Eclipsed C(sp3)‐CH3 Bond in a Crystalline Hydrated Tricyclic Orthoamide: Evidence for CH…O hydrogen bonds

Accurate, low‐temperature (81 K) X‐ray analyses have been made for two crystalline modifications of the tricyclic orthoamide 1b: a cubic trihydrate in space group Pa3 (Z = 8), where the molecule has crystallographic threefold rotation symmetry, and an anhydrous monoclinic form in space group P21/c (Z = 8) where two symmetry‐independent molecules have different configurations, one ail‐trans. (as in the cubic trihydrate), the other cis, cis, trans. In the cubic trihydrate, each orthoamide molecule is attached to a triad of H2o molecules by OH…︁N H‐bonds. A remarkable feature of this structure is the nearly eclipsed conformation about the central C‐CH3 bond. In the anhydrous crystal, both types of molecule have the normal staggered orientation of their Me groups. The reversal of the Me orientation in the trihydrate is attributed to CH…︁O H‐bonding, which must be much stronger and more directionally specific than has been previously assumed.