2270
NOTES
cordingly, the observed second moment below 230" K can be attributed to something less than completely free rotation of the CH3 groups. A 40" oscillation of the CH3 group about the C-N axis would account for the observed line width. At temperatures higher than 230°K a sharp line-width transition occurs in which the CH3 groups undergo completely free reorientation and a tumbling motion of the (C€13)qN+group occurs. The second moment expected for rotation (tumbling) of the (CH3)4S+ group about its center of gravity (-1 G2) is in agreement with the observed second moment of 1.55 G2. An activation energy can be derived from the line-width transition12 for this general moleciilar This reorientation, giving E , = 5.3 f 0.5 kcal value is in good agreement with that derived for similar motions (/+phase transition) in Si(CH3)4,2aE , = 7.23 i 0.40 kcal mol-'.
~
3.8
-
3.4
-
3.0-
u1
z
-
I--
! 2.62.2
-
1.8
-
1.4-
Table I : Calculated Second Moments. G2 .-Intramolecular-SI, SL, rig.1-a
rig.4-19
Intermolecular 52, rig.
Rigid lattice 22.4 -1.0 -1.0 Methyl group re5.6 -0.7 -0.3 orientation 40' methyl group oscillation IiIolecular tumbling Experimental Second IiIoments, G2 Low temperature,
