NMR of oriented CDH2I and symmetry related isotope effects

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 S_CH, S_CD, S_HH, and S_HD, obey the following relation which assumes mirror plane symmetry: S_CD=−2cosα (S_HH+2S_HD)−2S_CH, rather than the following relations which assume C₃ rotational symmetry: S_CD=S_CH=−2cosα S_HH=−2cosαS_HD, where α equals the HCH or HCD bond angle. Consequently it is concluded that isotope effects on the orientation parameters, such as S_CH≠S_CD, 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 S_CD 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 S_CD obtained from ¹³CD dipolar couplings is 163.1±3.8 kHz.