PHASE BEHAVIOUR OF THE CHLOROSUBSTITUTED NEOPENTANES STRUCTURE AND DYNAMIC OF (CH3)4-XC(CH2CL)X
C. Buhrmester, H. Fuess
TU Darmstadt, FB Materialwissenschaft, Petersenstr. 23, 64287 Darmstadt, Germany claudia@friedel.st.mw.tu-darmstadt.de
ODIC phases, phase transition, molecular crystal
Neopentane C(CH3)4 is known to show orientational order-disorder phase transitions and to exist in a disordered plastic phase in the temperature region 140K<T<257K . The shape of the molecule is close to spherical and it may therefore be classified as a globular molecular crystal. The mobility of neopentane in the plastic phase has extensively been studied [1,2,3].
The present investigation intends to examine the influence of substitution of methylgroups in neopentane by CH2Cl-groups which decreases the globular shape and should therefore hinder reorientation of the molecule in the plastic phase.
We prepared chlorine substitued neopentanes (CH3)4-xC(CH2Cl)x with x=1,2,3 and found remarkable differences for the resulting compounds.
Neopentylchlorid (x=1) forms four solid phases (see table 1):
Two cubic plastic ones, a disorderd and a fourth ordered phase.
Table 1: Phase transitions of Neopentylchorid
| Phase transition+ | T [K] | H [kJ/mol] | S [J/molK] |
| m --> I | 252.6 | 2.0 | 7.9 |
| I --> II | 206.5 | 0.3 | 1.5 |
| II --> III | 169.2 | 5.6 | 33.0 |
| III -->IV | 100* |
+m: melt; I, II, III, IV: solid phases with
decreasing temperature resp.
*phase transition seen by 1H NMR measurements
The phase sequence of 1,3-dichloro-2,2-dimethylpropane (x=2) is not yet fully elucidated.
1H NMR measurements indicate an ordered phase IV below 110K which then transforms with increasing temperature to a disordered phase III. DSC experiments indicate a further transition at a rate of 1 K/min to phase II at 194K which is not detected at higher heating rates. Heating rates above 1 K/min immediately show a transition to phase I.
For 1,3-dichloro-2-chloromethyl-2-methylpropane (x=3) only the plastic phase I and an ordered phase II is detected.
Similar values as from DSC measurements for the transition temperatures have been determined by temperature dependent observations of the second moment (1H NMR). Further indications on the degree of order may be estimated from the value of the second moment. Calculations of the second moment for different dynamical models based on the theory of Van Vleck are compared to observations.
In addition infrared measurements were collected between roomtemperature and 77K . The combination of the information from 1H NMR and infrared investigations led to a possible reorientation mechanism for the different phases.
Structural investigations based on x-ray and synchrotron data are in progress. The plastic phase exhibits a body centred cubic lattice with lattice constants between a=7.05A (x=1) and a=7.55 A (x=3). The low temperature ordered structures of neopentylchlorid C 2/c and 1,3-dichloro-2-chloromethyl-2-methylpropane P 21/n are different.