Transfer of CIDNP among coupled spins at low magnetic field

Abstract

Coherent polarization transfer among groups of dynamically polarized spins is explored and applied to field cycling experiments where spin evolution proceeds at low magnetic field while observation is performed at high field. The case of two nonequivalent spins-1/2 with scalar spin coupling is considered theoretically in detail for the cases of sudden and adiabatic field change. The criterion for efficient polarization transfer is derived theoretically and consistently confirmed experimentally for three photochemical reactions, involving spin systems of increasing complexity that exhibir chemically induced dynamic nuclear polarization: (I) the two polarized protons of the purine base of adenosine monophosphate; (2) four coupled indole protons of tryptophan; and (3) long-range polarization transfer among the aliphatic protons of cycloundecanone. The importance of polarization transfer in other cases with non-equilibrium population of the nuclear spin levels and the possibility of its utilization in field cycling NMR studies are discussed.

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

“…Then not only relaxation is taking place but also the polarization transfer among the strongly coupled spins. 14,[19][20][21] In the present work, we will once again demonstrate this effect showing in particular that when setting a proper magnetic field B int one can observe a polarization transfer between the coupled protons, which is selective with respect to the spin state of fluorine spins.…”

“…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. However, observing such effects is a difficult experimental task.…”

“…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.)…”

“…Another necessary condition for having coherence at τ = 0 is non-adiabatic field switching, because it can transfer state population differences at high field into coherences at low field. 14,[19][20][21] In this case the spin system starts evolving from a coherent state. Then not only relaxation is taking place but also the polarization transfer among the strongly coupled spins.…”

High-resolution study of nuclear magnetic relaxation dispersion of purine nucleotides: Effects of spin–spin coupling

“…As a consequence of strong coupling the eigen-states of the spin system are collective states of spins. Our recent studies [21,22] of re-distribution of non-thermal polarization among scalar coupled spins at variable external magnetic field revealed that indeed intra-molecular spin-spin couplings may affect the relaxation behavior of the coupled spins at low fields. We found that processes of polarization re-distribution proceed very efficiently at low fields where the spins are strongly coupled [23].…”