Spectrometer for studying NMR and relaxation in the doubly rotating frame

Abstract

A spectrometer is described that ensures observation of NMR and relaxation in the effective field H2 acting in the doubly rotating frame (DRF). Unlike conventional NMR spectrometers, the presented apparatus allows the investigation of nuclear spin dynamics and relaxation under conditions of the specially transformed effective multispin dipole-dipole interaction Hamiltonian not studied up to now. The setup is a modified version of the previously described spectrometer for direct recording of NMR in the effective field H,1 in the singly rotating frame (RF), all its operating functions being retained. The new version of the apparatus is designed for the study of NMR and, especially, spin relaxation of 'H and 19F nuclei in solids under the magic-angle conditions in both the RF and the DRF. It enables one to obtain NMR spectra and longitudinal and transverse nuclear relaxation kinetics in the DRF field H,2 directly in their final form. In all operating modes, the apparatus is coherent with respect to the NMR frequency in the laboratory frame. In transient modes, it is also coherent with respect to the NMR frequencies in both the RF and the DRF. In all cases the NMR signals are recorded continuously during a single-scan experiment, as a rule not exceeding 1 s. The sensitivity and resolution capability of the spectrometer are superior to those of the starting one. In particular modes, its sensitivity is comparable with that of conventional NMR spectrometers. The apparatus enables one to investigate in detail slow molecular motions in solids with rates -7H,2 -103-105 s-' and = 7H, 1 -1 O-1 fl7 s -'; various motion parameters, including multiparticle correlations as well as a motion mechanism, can be extracted from the experimental data.

Study of slow molecular motions in α-Crystallin by proton magnetic spin-lattice relaxation in the doubly rotating frame

“…The description of the experimental and theoretical details of NMR relaxation in the DRF are presented in the series of papers mentioned above [5][6][7][8][9][10][11][12]. The scheme of the DRF NMR relaxation experiment is shown in Fig.…”

“…In this case, M 1 (0) = Mo and Mppii (0) = 0, and thus one may observe a DRF free induction decay (FID) only, i.e., a transversal relaxation in the DRF. It should be noted that a more complicated procedure of the DRF spin-locking enables one to obtain MII (0) = M0, M 1 (0) = 0 [8,10,12].…”

“…In the case of the DRF spin-lattice relaxation experiment, Mp1 = M 11 . It should be noted that if M 1 (t) * 0, additional terms appear in M7(t) which oscillate with frequencies (.0 + .0 e2 )/21t [5,10,12]. It is important to note that with this technique, the NMR signal is registered during the action of a high-power rf pulse.…”

“…Following ref. 23 (12) Here T * is a relaxation time of the dipolar subsystem that is equal to T^ = T ail + T(2)* + T 1 (13) TdPP lPP…”

“…With this method, it is possible to study molecular motions with correlation times in the submillisecond range. This method was advanced about 15 years ago [5][6][7][8][9][10][11][12], but it was not applied to biopolymer studies yet.…”