High resolution NMR study of T1 magnetic relaxation dispersion. II. Influence of spin-spin couplings on the longitudinal spin relaxation dispersion in multispin systems

Abstract

Effects of scalar spin-spin interactions on the nuclear magnetic relaxation dispersion (NMRD) of coupled multispin systems were analyzed. Taking spin systems of increasing complexity we demonstrated pronounced influence of the intramolecular spin-spin couplings on the NMRD of protons. First, at low magnetic fields where there is strong coupling of spins the apparent relaxation times of the coupled spins become equal. Second, there are new features, which appear at the positions of the nuclear spin level anticrossings. Finally, in coupled spin systems there can be a coherent contribution to the relaxation kinetics present at low magnetic fields. All these peculiarities caused by spin-spin interactions are superimposed on the features in NMRD, which are conditioned by changes of the motional regime. Neglecting the effects of couplings may lead to misinterpretation of the NMRD curves and significant errors in determining the correlation times of molecular motion. Experimental results presented are in good agreement with theoretical calculations.

High resolution NMR study of T1 magnetic relaxation dispersion. III. Influence of spin 1/2 hetero-nuclei on spin relaxation and polarization transfer among strongly coupled protons

“…In this context, it is important to emphasize that the relaxation rates refer to spin states but not to individual spins. In addition, there were sharp features in the NMRD curves found, which are caused by avoided crossings 14,[19][20][21][22] of energy levels of the coupled spin system. Both effects can be observed even when the condition of fast motion (extreme narrowing ωτ c = 1, consequently, J(ω) = const.)…”

“…14,19 The timing scheme of the experiment consists of 5 consecutive stages. At first, the spin system is relaxed to thermal equilibrium during a sufficiently long time, τ R (far exceeding the relaxation times of the spin system), at the magnetic field B R and acquires .…”

“…They may vary the spin eigen-states with the field and cause pronounced changes in the relaxation behavior. Experimental studies of the third factor are limited by the fact that (with very few exceptions [13][14][15][16][17][18] ) the experimental methods are lacking NMR spectral resolution and therefore allow one to investigate only the relaxation of the total magnetization of solvent or solute molecules. Such studies are unable to directly identify the consequence of spin-spin interactions.…”

“…13,14,19 Although the strength of spin-spin interactions is quite small (not larger than 30 Hz for protons) as compared to the large Zeeman interactions with the magnetic field (often reaching hundreds of MHz), these couplings can strongly affect the eigen-states of the spin system and thus cause a field dependence of the R 1 rates. The features observed are caused by the fact that at low magnetic fields the spins reach the regime of strong coupling.…”

“…They are manifest not only for the individual spins but for the coupled spin system as a whole; thus, they become important for the low-resolution NMRD studies as well and must be taken into account for properly analyzing the data. 14,19 Furthermore, when during the preparation period of the experiment non-thermal polarization is formed, efficient coherent polarization transfer among the strongly coupled spins becomes possible. [20][21][22] Effects similar to those observed for proton systems are expected to exist also for protons coupled to spin 1 2 heteronuclei.…”