An observation of 23Na NMR triple-quantum dynamic shift in solution

Abstract

Mult iplequantum-coherence spectroscopy of spin-: nuclei in solution has become recently a subject of intensive research, particularly in reference to biological systems. It was found, both theoretically and experimentally, that in all cases where the singlequantum-coherence decay is biexponential, higher-order coherences may be observed ( 1, 2). This situation occurs when the nucleus is bound in a slowly tumbling site, i.e., with a reorientation time, rR, of the order of the inverse of the kumor frequency, G’, or larger, or when it exchanges with such a site (3, 4). According to relaxation theory (5) the resonance frequency in such systems is shifted relative to the Larmor frequency. This shift is referred to as a dynamic frequency shift (DFS). The measurement of the DFS for single-quantum (SQ) coherences has been very difficult since the shifts are comparable to the linewidths (6-8). However, it has been found to have a pronounced effect in ESR spectra ( 9). Theory predicts that this shift is smaller than the linewidth for double-quantum (DQ) coherence ( 10) but significantly larger for tr iple-quantum ( TQ ) coherence ( 20, 2 I ) . In the present Communicat ion we report the first experimental observation of a TQ DFS in solution. In order to have a system without complications due to the presence of several species and chemical exchange we have chosen the stable complex of Na+ with 4,7,13,16,21-pentaoxa-l,lO-diazabicyclo[S.8.5] tricosane (Kryptofix 221) ( 12 ) , dissolved in glycerol. In the following we shall denote the complex as Na cryptate. G lycerol was selected as the solvent due to the steep ,temperature dependence of its viscosity, allowing the rotational correlation time to vary over a wide range. The relaxation times of the TQ coherence and the TQ DFS were measured by the pulse sequence (1, 2, 13, 14)