NMR of oriented CDH2I and symmetry related isotope effects

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Abstract
Measurements of the orientation parameters of a 13C enriched sample of CDH2I in nematic solvent employing 1H, 2H, and 13C NMR are reported. These parameters denoted as SCH, SCD, SHH, and SHD, obey the following relation which assumes mirror plane symmetry: SCD=−2cosα (SHH+2SHD)−2SCH, rather than the following relations which assume C3 rotational symmetry: SCD=SCH=−2cosα SHH=−2cosαSHD, where α equals the HCH or HCD bond angle. Consequently it is concluded that isotope effects on the orientation parameters, such as SCH≠SCD, reported earlier [J. B. Wooten, A. L. Beyerlein, J. Jacobus, G. B. Savitsky, J. Chem. Phys. 65, 2476 (1976)] result from a change in symmetry upon deuteration. The value of α=112.67° that best describes the reported data on CDH2I and CH3I in nematic phase differs from the gas phase microwave value 111.6° suggesting bond angle distortions in the nematic phase. Thus the preferable method of obtaining SCD for quadrupole coupling constant determinations of the CD bond is from the 13CD dipolar coupling and one molecular parameter, the CD bond length, which is not sensitive to molecular distortions. The quadrupole coupling constant measured in this work for CDH2I using SCD obtained from 13CD dipolar couplings is 163.1±3.8 kHz.