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An Eclipsed C(sp 3 )‐CH 3 Bond in a Crystalline Hydrated Tricyclic Orthoamide: Evidence for CH…︁O hydrogen bonds
Author(s) -
Seiler Paul,
Dunitz Jack D.
Publication year - 1989
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19890720531
Subject(s) - chemistry , monoclinic crystal system , crystallography , hydrogen bond , molecule , anhydrous , eclipsed conformation , crystal structure , group (periodic table) , conformational isomerism , stereochemistry , intermolecular force , organic chemistry
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 P2 1 /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 H 2 o molecules by OH…N H‐bonds. A remarkable feature of this structure is the nearly eclipsed conformation about the central C‐CH 3 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 CH…O H‐bonding, which must be much stronger and more directionally specific than has been previously assumed.