NMR and magnetization measurements in Li2VOSiO4 and Li2VOGeO4 are reported. The analysis of the susceptibility shows that both compounds are two-dimensional S = 1/2 Heisenberg antiferromagnets on a square lattice with a sizable frustration induced by the competition between the superexchange couplings J1 along the sides of the square and J2 along the diagonal. Li2VOSiO4 undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for J2/J1>0.5. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.

The efficiency of dissolution dynamic nuclear polarization can be boosted by Hartmann-Hahn cross polarization at temperatures near 1.2 K. This enables high throughput of hyperpolarized solutions with substantial gains in buildup times and polarization levels. During dissolution and transport, the (13)C nuclear spin polarization P((13)C) merely decreases from 45 to 40%.

a b s t r a c tIn most applications of dissolution-DNP, the polarization of nuclei with low gyromagnetic ratios such as 13 C is enhanced directly by irradiating the ESR transitions of radicals with narrow ESR lines such as Trityl at low temperatures T = 1.2 K in polarizing fields B 0 6 5 T. In a field B 0 = 6.7 T at T = 1.2 K, DNP with TEMPO leads to a rapid build-up of proton polarization P( C) in excess of 70% within 20 min. After rapid dissolution to room temperature, this is 122 000 times larger than the Boltzmann polarization at 300 K and 6.7 T.Ó 2012 Elsevier B.V. All rights reserved.For a variety of reasons, dynamic nuclear polarization (DNP)[1], when it is used to boost the polarization P(S) of nuclei S with low gyromagnetic ratios c S prior to rapid heating to room temperature, is usually performed in fairly low magnetic fields, most frequently B 0 = 3.35 T. Furthermore, DNP is usually carried out at temperatures in the vicinity of T = 1.2 K. Under such conditions, the electron spin polarization is close to unity (P e = 95%). By irradiating the EPR transitions with microwaves, a significant fraction of this polarization can be transferred to the polarization P S of nuclear spins S such as 13 C, which is defined asExperimental reports on low temperature dissolution DNP at fields above 3.35 T, i.e., at B 0 = 4.6 T using Trityl and at B 0 = 5 T using the widely available free radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) [3,4] show substantial improvements in polarization levels achieved by direct DNP, i.e., P( 13 C) = 35% at 4.6 T and P( 13 C) = 15% at 5 T, albeit at the price of prohibitively long build-up times: s DNP ( 13 C) > 3000 s with Trityl at 4.6 T and s DNP ( 13 C) > 1000 s with TEMPO at 5 T. On the other hand, the build-up times are usually much shorter for protons than for carbon-13 when TEMPO is used as polarizing agent [5][6][7][8]. We have shown recently [9] that the combination of 1 H DNP using TEMPO with cross-polarization (CP) to transfer the enhanced magnetization from 1 H to 13 C allows one to achieve dramatic improvements in both polarization levels and build-up rates.[9] At 3.35 T, polarization levels as high as Increasing the magnetic field B 0 beyond 5 T cannot significantly enhance P e since it is already close to unity at 3.35 T. It is therefore not obvious that an increase of B 0 can yield any improvement in the nuclear spin polarization P S . However, a closer inspection of the mechanism known as 'thermal mixing' (TM) as described by spin temperature theory[1] reveals that one should expect an improvement in DNP efficiency at higher fields. In this work, it is shown that at B 0 = 6.7 T and T = 1.2 K, using frozen glassy solutions containing TEMPO as polarizing agent, a polarization P( 13 C) = 36%can be obtained directly, albeit with a slow build-up s DNP ( 13 C) = 2000 s. With Trityl, it might be possible to achieve higher polarization levels P( 13 C) at B 0 = 6.7 T, but the build-up times are likely to be much longer. With TEMPO at B 0 = 6.7 T and T = 1.2 K, the proton ...

NMR, muon spin rotation (SR), magnetization, and specific-heat measurements in Li 2 VOSiO 4 powders and single crystals are reported. Specific-heat and magnetization measurements evidence that Li 2 VOSiO 4 is a frustrated two-dimensional Sϭ1/2 Heisenberg antiferromagnet on a square lattice with a superexchange coupling J 1 , along the sides of the square, almost equal to J 2 , the one along the diagonal (J 2 /J 1 ϭ1.1Ϯ0.1 with J 2 ϩJ 1 ϭ8.2Ϯ1 K). At T c Ӎ2.8 K a phase transition to a low-temperature collinear order is observed. T c and the sublattice magnetization, derived from NMR and SR, were found practically independent on the magnetic field intensity up to 9 T. The critical exponent of the sublattice magnetization was estimated ␤Ӎ0.235, nearly coincident with the one predicted for a two-dimensional XY system on a finite size. The different magnetic properties found above and below T c are associated with the modifications in the spin Hamiltonian arising from a structural distortion occurring just above T c .

Hyperpolarization of substrates for magnetic resonance spectroscopy (MRS) and imaging (MRI) by dissolution dynamic nuclear polarization (D-DNP) usually involves saturating the ESR transitions of polarizing agents (PAs; e.g., persistent radicals embedded in frozen glassy matrices). This approach has shown enormous potential to achieve greatly enhanced nuclear spin polarization, but the presence of PAs and/or glassing agents in the sample after dissolution can raise concerns for in vivo MRI applications, such as perturbing molecular interactions, and may induce the erosion of hyperpolarization in spectroscopy and MRI. We show that D-DNP can be performed efficiently with hybrid polarizing solids (HYPSOs) with 2,2,6,6-tetramethyl-piperidine-1-oxyl radicals incorporated in a mesostructured silica material and homogeneously distributed along its pore channels. The powder is wetted with a solution containing molecules of interest (for example, metabolites for MRS or MRI) to fill the pore channels (incipient wetness impregnation), and DNP is performed at low temperatures in a very efficient manner. This approach allows high polarization without the need for glass-forming agents and is applicable to a broad range of substrates, including peptides and metabolites. During dissolution, HYPSO is physically retained by simple filtration in the cryostat of the DNP polarizer, and a pure hyperpolarized solution is collected within a few seconds. The resulting solution contains the pure substrate, is free from any paramagnetic or other pollutants, and is ready for in vivo infusion.D-DNP | NMR signal enhancement | molecular imaging | mesostructured hybrid silica | porous materials D issolution dynamic nuclear polarization (D-DNP) (1, 2) usually requires freezing molecules of interest, such as metabolites, together with persistent free radicals often called polarizing agents (PA) in a glassy matrix at very low temperatures (1 < T < 4 K), so that their nuclear spin polarization can be enhanced by up to four to five orders of magnitude. Such enhancements are achieved by saturating the ESR transitions of the PAs. D-DNP is generally performed in moderate magnetic fields (B 0 = 3.35 or in this study, 6.7 T) and followed by rapid dissolution of the frozen sample with a burst of superheated water to give highly polarized solutions. Applications include detection of intermediates in chemical reactions (3-5), protein folding in real time (6), and detection of cancer by monitoring abnormal rates of metabolic reactions in humans (7). PAs with narrow EPR lines, such as trityl radicals, are usually used for the direct polarization of 13 C nuclei (2). In practice, polarizations P( 13 C) of 20% or higher can be obtained after dissolution. We have recently shown that DNP of 13 C can be significantly accelerated by combining increased magnetic fields with polarization of 1 H rather than 13 C [using nitroxide radicals, such as 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO), with broader ESR lines than trityl radicals] followed by Hartmann-Hahn C. In ...