Molecular reorientation and boron nuclear quadrupole interaction in crystalline o r t h o-carborane

Molecular reorientation in crystalline ortho‐carborane has been studied by ¹H, ¹⁰B, and ¹¹B nuclear magnetic resonance (NMR) at several frequencies, in the temperature interval 155–370 K. The phase transition originally found by Baughman has been determined in our measurements to occur at 274±2 K. o‐carborane reorientation in the high temperature phase has been found by ¹H and ¹¹B T₁ measurements to have an activation energy E_A=5.8±1.3 kJ/mole. Pulse and cw NMR data are consistent with a picture of the high temperature phase in which o‐carborane molecules undergo rapid isotropic reorientation. NMR data for the low temperature phase demonstrate the reorientation must be anisotropic. The boron lines are quadrupole‐split. The width and shape of the ¹¹B central line are functions of NMR frequency and temperature. At low temperature and low frequency the ¹¹B line is broad and asymmetric. Evidence indicates the width of this line is due mainly to lifetime broadening. ¹H and ¹¹B T₁ data in the high temperature phase, and a theory of the frequency dependence of the ¹¹B central line in the low temperature phase, yield an ’’average’’ ¹¹B quadrupolar coupling constant $(\overline{e^2\mathrm{Qq/h}})$ =1.3±0.3 MHz. The ¹H spin–lattice relaxation time in the low temperature phase is an asymmetric function of inverse temperature. The apparent activation energy for temperatures above the T₁ minimum is E_A=33.5±1.8 kJ/mole at NMR frequencies of 10 and 20 MHz. Below the T₁ minimum, E_A=14.0±1.1 kJ/mole at 10 MHz, and E_A=18.9±0.7 kJ/mole at 20 MHz.