Crystal chemistry of trialkylsilyl-capped (3Z)-4-(methylthio)-3-penten-1-yne: polymorphism, twinning and ambiguity of order-disorder descriptions.

The crystallization behavior of trimethylsilyl-capped (3Z)-4-(methylthio)-3-penten-1-yne (1-TMS) and the triisopropylsilyl and tert-butyldimethylsilyl analogues (1-TIPS) and (1-TBDMS) was investigated. (1-TMS) crystallizes in the Sohncke space group P21 21 21 with Z' = 1. (1-TIPS) exists as two polymorphs, both crystallizing in P\overline 1 with Z' = 2 independent molecules. Polymorph (I) is an order-disorder (OD) twin. Two interpretations in terms of M = 1 or M = 2 kinds of OD layers are possible, with different degrees of idealization. Polymorph (II) is fully ordered (non-twinned). Its structure can be derived from polymorph (I) by inverting the orientation of every second molecule. (1-TBDMS) (P\overline1, Z' = 2) is an OD twin, where the OD description is unambiguously in M = 1 kinds of layers.