Effect of phase symmetry on the NMR spectrum of acetonitrile oriented in a uniaxial–biaxial–uniaxial phase

• Dipolar couplings of acetonitrile oriented in a biaxial liquid crystal obtained.
• Measurements carried out through the range of uniaxial – biaxial – uniaxial phases.
• Dipolar couplings show large changes that coincide with the onset of biaxiality
• HCH bond angle obtained and compared with values obtained in other systems.
• Results discussed in terms of vibration–rotation interaction effects.

We report here the measurement of the CH and the HH dipolar couplings of the methyl group of acetonitrile oriented in the biaxial liquid crystal potassium laurate/1-decanol/water system. These parameters show large variations when measured as a function of temperature. The variations follow the symmetry of the phase as the liquid crystal goes through the sequence of uniaxial – biaxial – uniaxial phases and show a close correspondence to the phase changes that occur in the liquid crystalline solvent coinciding with the onset of biaxiality. The HCH bond angle calculated after incorporating vibrational corrections to the dipolar couplings is discussed in terms of contributions in the case of the biaxial liquid crystal arising from vibration–rotation interaction effects.