Thermal isomerization of silicon and germanium sesquichalcogenides, Thex(4)M(4)E(6) (M = Si, Ge; E = S, Se)

The isomerization of the sesquichalcogenides Thex(4)M(4)E(6) (Thex = 1,1,2- trimethylpropyl, 1: M = Si, E = S; 2: M = Si, E = Se; 3: M = Ge, E = S; 4: M = Ge, E = Se) with the "double-decker"-type structures (Thex(2)M(2)E(2))( 2)E-2. (1dd-4dd) to the corresponding compounds with the adamantane-type st ructures Thex(4)M(4)E(6) (1ad-4ad) was studied. The skeletal rearrangement has been observed to be reversible in the case of sesquichalcogenides 1, 2, and 4; for 1 and 4, the isomerization has reached an equilibrium with pref erence for adamantane-type structures lad and 4ad, respectively, while the equilibrium in 2 has lain more to "double-decker"-type 2dd. On the other ha nd, the isomerization of 3dd to 3ad has been observed to be irreversible un der the condition employed. In the temperature ranges 433-453 K (Idd), 373- 393 K (3dd), and 358-383 K (4dd), the equilibrium has followed reversible f irst-order kinetics. The thermodynamic parameters determined for the forwar d isomerization, the "double-decker"-type structures (dd)-->adamantane-type structures (ad), are as follows: for 1dd-->1ad, E-a = 35.9+/-0.1 kcal mol( -1),lnA = 30.8+/-0.2 s(-1), Delta H-double dagger = 35.0+/-0.1 kcal mol(-1) , Delta S-double dagger = -0.044 cal mol(-1) K-1;for 3dd-->3ad, E-a = 35.9/-0.1 kcal mol(-1),lnA = 38.3+/-0.2 s(-1),Delta H-double dagger = 34.6+/-0. 1 kcal mol(-1),Delta S-double dagger = 13.7 cal mol(-1) K-1;for 4dd-->4ad, E-a = 32.3+/0.1 kcal mol(-1),lnA = 35.7+/-0.1 s(-1), Delta H-double dagger = 31.7+/-0.3 kcal mol(-1), Delta S-double dagger = 10.4 cal mol(-1) K-1. Th e X-ray structure of new sesquichalcogenide 2ad is also reported.