Beyond the $T_1$ Limit: Singlet Nuclear Spin States in Low Magnetic Fields

Low-field nuclear spin singlet states may be used to store nuclear spin order in a room temperature liquid for a time much longer than the spin-lattice relaxation time constant $T_1$. The low-field nuclear spin singlets are unaffected by intramolecular dipole-dipole relaxation, which is generally the predominant relaxation mechanism. We demonstrate storage of nuclear spin order for more than 10 times longer than the measured value of $T_1$. This phenomenon may facilitate the development of nuclear spin hyperpolarization methods and may allow the study of motional processes which occur too slowly for existing NMR techniques. This is the first time that the memory of nuclear spins has been extended well beyond the $T_1$ limit in a system lacking intrinsic magnetic equivalence.

Figure 1

(a) Sequence of magnetic fields used in the experiment. The sample is transported from a region of high magnetic field, $B_{\mathrm{high}}$, into a region of low magnetic field, $B_{\mathrm{low}}$, in a time ${\tau }_{\mathrm{transp}}^{(1)}$. After remaining in $B_{\mathrm{low}}$ for a time ${\tau }_{\mathrm{LF}}$, the sample is transported back into $B_{\mathrm{high}}$, in a time ${\tau }_{\mathrm{transp}}^{(2)}$. (b) Pulse sequence performed at the Larmor frequency of the spins in high field. (c) Idealized sequence of spin state populations during the experiment. The four high-field energy levels belong to the states $|\beta \beta ⟩$, $|\alpha \beta ⟩$, $|\alpha \alpha ⟩$, and $|\beta \alpha ⟩$, reading in a clockwise direction starting with the upper state.

Figure 2

(a) Conventional ${}^1\mathrm{H}\mathrm{NMR}$ spectrum of the solution of 2,3-dibromothiophene (inset) in $\mathrm{DMSO}\mathrm{\text{−}}{\mathrm{d}}^6$. (b) Spectrum generated by the sequence in Fig. 1, using the preparation sequence $A_{+}$ and a storage time ${\tau }_{\mathrm{LF}}=100\mathrm{s}$. (c) Spectrum generated by the sequence in Fig. 1, using the preparation sequence $A_{-}$ and a storage time ${\tau }_{\mathrm{LF}}=100\mathrm{s}$. Expanded sections of the spectra from 7.76 to 7.68 ppm are shown for (b) and (c). All spectra were obtained from a single signal acquisition. The spectra in (b) and (c) are expanded vertically by a factor of 2 compared to (a).

Figure 3

Decay of the diagnostic antiphase signals as a function of time ${\tau }_{\mathrm{LF}}$. The top and bottom halves indicate the integrated amplitudes of the two components of the antiphase multiplet centred at 7.72 ppm. The symbols represent measurements at different storage fields $B_{\mathrm{low}}$. The vertical scale is the integrated amplitude of the spectral peaks as a percentage of their amplitude in an ordinary one-pulse NMR experiment. The solid line fits to decaying exponential functions with time constant $T_{\mathrm{LFS}}=104\mathrm{s}$. The singlet decay is independent of the storage field in this regime.