Mar 27 2013 Wednesday 15:30
Deniz Sezer, Sabancı University
Harvesting Molecular Motions to Polarize Nuclear Spins

Abstract:Nuclear Magnetic Resonance (NMR) spectroscopy, mostly known in the context of medical MRI, consists of manipulating the tiny magnets present in atomic nuclei (i.e., nuclear spins) with a strong externally applied magnetic field. In spite of its tremendous versatility, NMR has one main limitation: relatively low sensitivity. The weak coupling between the nuclear spins and the magnetic field leads to a rather small spin polarization. As a result, to achieve a desired signal-to-noise ratio, the volume from which the NMR signal is acquired needs to contain many equivalent spins and averaging over many separate signal acquisitions becomes necessary. Increasing the nuclear spin polarization will increase the resolution of the technique in both space and time making possible novel applications of MRI to microfluidics (flow of liquids in small channels) and the study of chemical reactions. One well-known phenomenon that employs electron spins to substantially increase the nuclear spin polarization is the Overhauser effect [1]. This spin polarization mechanism was first observed in solids [2] but is also effective in liquids [3]. In liquids, the efficiency of the effect depends on the relative motion of the molecules carrying the nuclear and electron spins. Analytical models that treat diffusing spherical molecules with spins at their geometrical centers [4,5] have served well over the years in rationalizing the extent to which nuclear spins can be polarized under given experimental conditions. In this talk I will argue that recent experiments of nuclear spin polarization in liquids probe a regime where the assumptions of the analytical model fail. I will present a computational/analytical approach for calculating the efficiency of polarization transfer from electron to nuclear spins, which combines the traditional analytical model with realistic simulations of the molecular motions [6]. The predictions of this novel approach are in agreement with experiment.
[1] Overhauser A.W., Polarization of nuclei in metals, Physical Review, 92, 411 (1953).
[2] Carver T.R., Slichter C.P., Polarization of nuclear spins in metals, Physical Review, 92, 212 (1953).
[3] Hausser K.H., Stehlik D., Dynamic nuclear polarization in liquids, Advances in Magnetic Resonance, 3, 79 (1968).
[4] Ayant Y., Belorizky E., Alizon J., and Gallice J., Calcul des densités spectrales résultant d'un mouvement aléatoire de translation en relaxation par interaction dipolaire magnétique dans les liquides, Journal de Physique, 36, 991 (1975).
[5] Hwang L.P., Freed J.H., Dynamic effects of pair correlation functions on spin relaxation by translational diffusion in liquids, Journal of Chemical Physics, 63, 4017 (1975).
[6] Sezer D., Computation of DNP coupling factors of a nitroxide radical in toluene: Seamless combination of MD simulations and analytical calculations, Physical Chemistry Chemical Physics, 15(2): 526 (2013).

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