Comparison of the orientational order parameters determined from X-ray diffraction and ¹³C NMR studies of a hockey stick-shaped compound

Abstract

Orientational order parameters (OOP's) have been determined in the Smectic A, Smectic C₁ and Smectic C₂ phases of a hockey stick-shaped compound from X-ray diffraction measurements and compared with NMR measurements. While the OOP values from NMR measurements abruptly drop at the Smectic C₁ to Smectic C₂ transition, those determined from X-ray diffraction measurements decreases sharply at the Smectic A to Smectic C₁ transition. This drop in the OOP values from X-ray measurements has been explained using a model where the molecular director is randomly oriented in a cone about the smectic layer normal. OOP's from X-ray diffraction and NMR measurements have been successfully fitted with McMillan's theory, after necessary correction to the potential, incorporating tilt angles, for three homologues with n = 10, 11 and 12. The temperature dependences of the optical birefringence, dielectric anisotropy and OOP values determined from birefringence measurements of compound 11 are also reported.

Keywords:

• X-ray diffraction
• NMR measurement
• optical birefringence
• dielectric anisotropy
• orientational order parameter

Acknowledgements

Financial support from Department of Science and Technology, New Delhi (Project No: SR/S2/CMP-20/2005) to MKD and financial support from Department of Science and Technology, New Delhi (Project No: SR/S2/CMP-29/2007) to BD are gratefully acknowledged. The authors would like to thank the referees for their valuable comments and suggestions to improve the manuscript.