Differential thermal analysis and nuclear quadrupole resonance study on polymorphic 1,4-dichlorobutane

Differential thermal analysis (DTA), and the ³⁵Cl nuclear quadrupole resonance (NQR) frequency (v _Q ) and line width (Δv _Q ) were measured as a function of the temperature in solid 1,4-Dichlorobutane (DCB). Two crystalline modifications forms I and II, stables above and below T _c = 210 K respectively were found according to the thermal history of the sample. A comparative analysis of the NQR frequency in both phases suggests that the crystal structure in form II is closer-packed than in form I. The NQR line width shows the existence of a thermal activated process with an activation energy E _a ? 20 kJ mol^?1, probably related with reorientations of the CH₂Cl end groups. Some orientational disorder is expected in the low temperature modification since the line width is twice as much as that in the high temperature phase.