Oxygen-17 NMR spectroscopy: Basic principles and applications (Part I)

摘要

The oxygen atom is probably the most chemically and biologically important element on earth which contains 50% by weight of oxygen. Oxygen forms compounds with all elements except for a few noble gases and metals. The structure and dynamics, therefore, of oxygen-containing compounds is of great significance. Compared to 1H, 13C, 15N, 31P and 19F NMR, however, 17O NMR has received little attention [1–10]. Between the first observation of a 17O nuclear induction signal in 1951 [11] and the first comprehensive review article of all aspects of 17O NMR in 1981 [4] there have been only about 200 publications dealing with 17O NMR. This limited interest is not surprising since of the three naturally occurring oxygen isotopes (16O, 17O and 18O), only 17O possesses a nuclear spin (I = 5/2). Table 1 lists some useful data of the 17O nucleus. It has a moderate electrical quadrupole moment (Qe =2.63 1030e m2), a very small magnetogyric ratio (c = 3.688 107rad. T1 s1), a low natural abundance (0.037%) and an extremely low absolute sensitivity compared to that of 1H (1.1 10-5). The 17O isotope is therefore one of the more difficult nuclei to observe by NMR spectroscopy. It is however of great interest to use a nucleus, such as oxygen, that is located at strategic molecular sites and is directly involved in inter- and intra-molecular interactions. The 17O NMR parameters, i.e., isotropic shielding, principal elements of the 17O shielding and electric field gradient tensors and transverse and longitudinal relaxation times can be considered as excellent means for probing structure, bonding and dynamics of oxygen containing compounds.