Long-Lived ¹H Singlet Spin States Originating from Para-Hydrogen in Cs-Symmetric Molecules Stored for Minutes in High Magnetic Fields

Abstract

Nuclear magnetic resonance (NMR) is a very powerful tool in physics, chemistry, and life sciences, although limited by low sensitivity. This problem can be overcome by hyperpolarization techniques dramatically enhancing the NMR signal. However, this approach is restricted to relatively short time scales depending on the nuclear spin-lattice relaxation time T(1) in the range of seconds. This makes long-lived singlet states very useful as a way to extend the hyperpolarization lifetimes. Para-hydrogen induced polarization (PHIP) is particularly suitable, because para-H(2) possesses singlet symmetry. Most PHIP experiments, however, are performed on asymmetric molecules, and the initial singlet state is directly converted to a NMR observable triplet state decaying with T(1), in the order of seconds. We demonstrate that in symmetric molecules, a long-lived singlet state created by PHIP can be stored for several minutes on protons in high magnetic fields. Subsequently, it is converted into observable high nonthermal magnetization by controlled singlet-triplet conversion via level anticrossing.

Symmetry constraints on spin dynamics: Application to hyperpolarized NMR

“…The transformation of total spin magnetization into singlet order, and back again, is an important manipulation in the NMR of long-lived states [13,14,15,16,17,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44] and in parahydrogen-enhanced NMR [18,19,20,21,22,45,46,47,48,49].…”

Exploiting level anti-crossings for efficient and selective transfer of hyperpolarization in coupled nuclear spin systems

“…Previously, pronounced features in the CIDNP field dependence due to LACs were reported 24 and they have also been considered in PHIP. [25][26][27] However, polarization transfer kinetics was never studied in detail and nor was the coherent nature of the transfer efficiently exploited.…”

Singlet NMR methodology in two-spin-1/2 systems

“…These two facts suggest a number of interesting uses of singlet order in NMR and MRI spectroscopy. A number of researchers have been indeed exploiting its long lifetime for the purposeof storage of hyperpolarisation [4,5,6,7,8,9,10,11,12]. Hyperpolarisation storage, combined with on-demand accessibility, suggests the use of singlet order as a repository of hyperpolarisation from where the signal enhancement gained by a given hyperpolarisation technique can be repeatedly accessed over a long time (tens of minutes) [12].…”

“…Since its discovery in the Levitt group [24,25,26] a number of research groups have investigated the field of singlet NMR over the past 12 years with the purpose of defining and characterising singlet order relaxation properties [3,27,28,29,30], and the conditions for its existence [31,32,33,34], and of developing the methodology to access singlet order in different magnetic regimes [24,26,35,36,37,38,39,9,40,11,41,42,43] and to demonstrate the existence of long-lived singlet order in multiple-spin systems [44,45,46,47,48,49] and in heteronuclear systems [50]. Significant studies on the chemical requirements for obtaining long-lived spin order have been also conducted [17,51,52,53,54] with a record lifetime of 70 min for a 13 C-pair [54], 26 min for a 15 N-pair [51], and 10 min for a 1 H-pair [17] demonstrated.…”