Photochemically Induced Dynamic Nuclear Polarization of Heteronuclear Singlet Order

Abstract

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is a method to hyperpolarize nuclear spins using light. In most cases, CIDNP experiments are performed at high magnetic field and the sample is irradiated by light inside a nuclear magnetic resonance (NMR) spectrometer. Here we demonstrate photo-CIDNP hyperpolarization generated in the Earth's magnetic field and under zero-to ultralow-field (ZULF) conditions. Irradiating a sample for several seconds with inexpensive light-emitting diodes produces a strong hyperpolarization of 1 H and 13 C nuclear spins enhancing the NMR signals by several hundred times. The hyperpolarized spin states at the Earth's field and in ZULF are different. In the latter case the state corresponds to singlet order between scalar-coupled 1 H-13 C nuclear spins. This state has longer lifetime than the state hyperpolarized at Earth's field. The method is simple and cost-efficient and should be applicable to many molecular systems known to exhibit photo-CIDNP, including amino acids and nucleotides. Table of Contents Graphic Main textNuclear magnetic resonance (NMR) is a widespread method to study composition of substances, molecular structure, molecular dynamics, and to perform noninvasive

Magnetometer-detected Nuclear Magnetic Resonance of Photochemically Hyperpolarized Molecules

“…The presented mechanism is effective in the field range of approximately 1 mT to 10 T. However, photo-CIDNP for this system can also be performed directly at much lower fields down to zero field. Strong hyperpolarization of heteronuclear singlet order was previously observed for 13 C-containing BQ 38 . This order is readily detectable in ZULF NMR, which is nevertheless challenging due to low natural abundance of 13 C and the presence of splitting in the ZULF NMR J-spectrum of 13 C-BQ.…”

“…A model system of tetraphenylporphyrin (TPP) and 1,4-benzoquinone (BQ) known to exhibit photo-CIDNP [33][34][35][36][37][38] was studied (see Figure 2a). Stock solutions were made up comprising 4.3 mg BQ in 2.0 mL chloroform, and 2.5 mg TPP in 2.0 mL chloroform.…”

Spin Hyperpolarization in Modern Magnetic Resonance

“…By acquiring photo-CIDNP NMR and EPR data, one can learn valuable information on the structure, reaction mechanism, g -factors and hyperfine coupling constants of radical intermediates pertaining to reactions of interest . In addition, under appropriate zero-to-low field conditions, photo-CIDNP is capable of generating long-lived nuclear spin states with relaxation times of the order of tens of seconds. − These long-lived spin states pave the way to investigations focusing on slow processes. , Further, theoretical arguments predict that the ratio between regular T 1n and the relaxation time of the corresponding long-lived nuclear spin state depends on molecular geometry . This property was exploited to determine torsion angles of a variety of small-molecule isotopologues …”

Multinuclear 1D & 2D NMR with 19F-Photo-CIDNP hyperpolarization in a microfluidic chip with broadband microcoil

“…39 After decades with only a few specialized groups holding up and exploiting the technique, 40,41 in recent years a rapidly growing community of users is arising. 42,43,44,45,46 Arguably the biggest bottleneck for more wide-spread use of photo-CIDNP lies in the rapid degradation of the required sensitizer molecules, and in the, related, practical limitations for optimizing the experimental parameters such as relative concentrations of the sensitizer and target components, and light power, among others. With the earlier mentioned cheap and home-built ESCARGOT PDMS-based NMR devices, we have introduced photo-CIDNP hyperpolarization on microliter-sized samples, with sub-picomole mass sensitivity.…”