Symmetry breaking nuclear quadrupole coupling tensor orientation for cesium-133 nuclei located in a mirror plane

Simultaneous multiple data set fits of all transition peaks of ¹³³Cs nuclei enabled us to obtain accurate cesium-133 nuclear magnetic resonance (NMR) parameters and Euler angles between the principal axis systems of the chemical shift (CS) and quadrupole coupling (Q) tensors of ¹³³Cs nuclei in Cs₂CrO₄ . Although in a previous study of Cs₂CrO₄ by Power et al. (W. P. Power, S. Mooibroek, R. E. Wasylishen, T. S. Cameron, J. Phys. Chem. 1994, 98, 1552), one central transition was observed for cesium sites 1 and 2 in the ¹³³Cs NMR spectra and one Euler angle between the CS tensors and Q tensors was obtained as 52° and 7° for cesium sites 1 and 2, respectively, the present single-crystal ¹³³Cs NMR measurements found two Euler angles (10(2)°, 51.9(1)°, 0°) for site 1 and two central transition peaks for site 2. Three principal components of the CS tensor for Cs1 are oriented along the crystallographic a, b, and c axes, whereas none of the principal components of the Q tensor for Cs1 are oriented along the crystal axes. The principal component V₂₂ of the Q tensor for Cs1 is tilted 10° from the b axis in the bc plane, and the other two components are not located in the ac plane. Therefore, we have found that the requirement that "the quadrupole coupling tensor for a nucleus located in a mirror plane has one principal axis perpendicular to the mirror plane" cannot be applied to Cs1. On the other hand, δ₁₁ and V₂₂ for Cs2 are aligned along the b axis, and the other components of the CS and Q tensors deviate at an angle of 1.4(1)° and 10.1(1)°, respectively, from the a and c axes in the ac plane. A distortion-free powder ¹³³Cs NMR spectrum of Cs₂CrO₄ was measured using a solid-state spin echo technique