Conformational analysis in multisulfur heterocycles. VIII. 3,3:6,6-Bis(tetramethylene)-s-tetrathiane and 3,3:6,6-bis(pentamethylene)-s-tetrathiane. Slow s-tetrathiane chair-to-twist, chair-to-chair, and twist-to-twist interconversions. X-ray crystallographic studies