Observation of the Second-Order Quadrupolar Interaction as a Dominating NMR Relaxation Mechanism in Liquids: The Ultraslow Regime of Motion

We report variable-temperature (VT) O-17 NMR spectra of [5-O-17]-D-glucose in an aqueous solution and in glycerol at 14.1 and 21.1 T. The VT O-17 NMR data cover a wide range of motion for which the molecular rotational correlation time (tau(c)) of glucose changes more than 5 orders of magnitude. The observed line width of the O-17 NMR signal for [5-O-17]-D-glucose displays a maximum at omega(0)tau(c) approximate to 1 and a minimum at omega(0)tau(c) approximate to 150, where omega(0) is the angular Larmor frequency of O-17. Under the ultraslow motion condition (i.e., omega(0)tau(c) > 150), the line width of the observed O-17 NMR signal increases drastically with tau(c), suggesting that the second-order quadrupolar interaction becomes the predominant relaxation mechanism. While this relaxation mechanism has long been predicted by theory, the current study reports the first experimental observation of such a phenomenon. The implications of this new relaxation mechanism on the spectral resolution limit in liquid-state NMR spectroscopy for half-integer spins are discussed.