17O NMR–NQR study of molecular motion in the liquid, and α and β solid phases of CO

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Abstract
A comprehensive NMR−NQR study of carbon monoxide enriched 24.5% in 17O (I = 5/2) is reported, which provides interesting information on the motions of the CO dipoles in the liquid and in the a and b solid phases. In the liquid phase the 17O NMR spin-lattice relaxation due to the tumbling rotational motions of the molecular axis, combined with the experimental evaluation of the rigid electric field gradient (EFG) at the 17O site has been utilized to achieve a direct and precise measurement of the reorientational correlation time of the dipole and its temperature dependence. In the b phase, the quadrupole perturbed NMR spectra and the spin-lattice relaxation indicate the occurrence of a precessional motion of the dipole around the c axis. An effective EFG is consequently present, whose order of magnitude is about 103 smaller than the static one. The precessional correlation time is derived, which is surprisingly very close to the reorientational correlation time of the liquid phase, suggesting a quasifree rotation character for the precession. 17O NQR signals in the a phase have been detected and resonance frequencies and linewidths have been measured. From the temperature behavior of the NQR frequency, information on the local order parameter for the librational motion of the dipoles is obtained. The linewidth exhibits a peculiar temperature dependence, with a maximum around 37 K and a dramatic divergence for T→29 K. While an orientational static disorder could be expected in view of the residual entropy, it is shown how the linewidth, as well as some pure quadrupole relaxation measurements, seem to indicate the occurrence in the a phase of a dynamical disorder, with sudden head-to-tail reorientations of the dipole, at least for T≳29 K. In the framework of such a dynamical model, a theoretical picture for the motional contribution to the NQR allows one to obtain insights on the features of the orientational disordering processes. The hypothesis of a phase transition of antiferroelectric type below T?37 cannot be ruled out. Finally, some evidence of softening of the libronic frequencies on approaching the a–b phase transition is obtained.