Silicon becomes colored: Stable biradicals were prepared from an N‐heterocyclic carbene stabilized SiCl2 and a cyclic alkyl(amino)carbene, and characterized as two polymorphs. The deep‐blue crystals of one polymorph are stable upon exposure to air for about a week, while the solution in THF decomposes rapidly when exposed to air. In a side reaction, the different carbene species react with each other under CH activation and CC bond formation in the presence of the biradical.

Dynamic nuclear polarization is emerging as a potential tool to increase the sensitivity of NMR aiming at the detection of macromolecules in liquid solution. One possibility for such an experimental design is to perform the polarization step between electrons and nuclei at low magnetic fields and then transfer the sample to a higher field for NMR detection. In this case, an independent optimization of the polarizer and detection set ups is required. In the present paper we describe the optimization of a polarizer set up at 15 MHz (1)H NMR/9.7 GHz EPR frequencies based on commercial hardware. The sample consists of the nitroxide radical TEMPONE-D,(15)N in water, for which the dimensions were systematically decreased to fit the homogeneous B(1) region of a dielectric ENDOR resonator. With an available B(1) microwave field up to 13 G we observe a maximum DNP enhancement of -170 at room temperature by irradiating on either one of the EPR lines. The DNP enhancement was saturated at all polarizer concentrations. Pulsed ELDOR experiments revealed that the saturation level of the two hyperfine lines is such that the DNP enhancements are well consistent with the coupling factors derived from NMRD data. By raising the polarizing field and frequencies 10-fold, i.e. to 140 MHz (1)H/94 GHz EPR, we reach an enhancement of -43 at microwave field strengths (B(1) approximately 5 G). The results are discussed in view of an application for a DNP spectrometer.

Nuclear magnetic resonance (NMR) techniques play an essential role in natural science and medicine. In spite of the tremendous utility associated with the small energies detected, the most severe limitation is the low signal‐to‐noise ratio. Dynamic nuclear polarization (DNP), a technique based on transfer of polarization from electron to nuclear spins, has emerged as a tool to enhance sensitivity of NMR. However, the approach in liquids still faces several challenges. Herein we report the observation of room‐temperature, liquid DNP 13C signal enhancements in organic small molecules as high as 600 at 9.4 Tesla and 800 at 1.2 Tesla. A mechanistic investigation of the 13C‐DNP field dependence shows that DNP efficiency is raised by proper choice of the polarizing agent (paramagnetic center) and by halogen atoms as mediators of scalar hyperfine interaction. Observation of sizable DNP of 13CH2 and 13CH3 groups in organic molecules at 9.4 T opens perspective for a broader application of this method.

Contents: 1. Materials and methods 2. General procedure for the oxidation of amines to imines 3. Setup for photocatalytic reactions 4. Optimization 5. Mechanistic experiments 6. Theoretical calculations 7. Characterization of products 8. References 9. NMR spectra Materials and methodsCommercial reagents were used without purification and reactions were run under CO2 atmosphere with exclusion of moisture from reagents using standard techniques for manipulating air-sensitive compounds. In case of dry DBN used for reactions, commercial DBN was dried over activated molecular sieves (3 Å) in a flame-dried Schlenk tube and degassed (several vacuum/argon cycles) prior to use. 1 H NMR spectra (300, 400 and 500 MHz) and 13 C NMR spectra (75.58, 100.62 and 125.71 MHz) were recorded using Bruker spectrometers AVANCE III 300, AVANCE III HD 400, AVANCE III 400, AVANCE III HD 500 and Varian spectrometers Mercury VX 300, VNMRS 300 and Inova 500 with CDCl3 and DMSO-d6 as solvent. NMR spectra were calibrated using the solvent residual signals (CDCl3: δ 1 H = 7.26, δ 13 C = 77.16; DMSO-d6: δ 1 H = 2.50, δ 13 C = 39.52; D2O: δ 1 H = 4.79). ESI mass spectra were recorded on Bruker Daltonic spectrometers maXis (ESI-QTOF-MS) and micrOTOF (ESI-TOF-MS). GC-MS mass spectra were recorded on Thermo Finnigan spectrometers TRACE (Varian GC Capillary Column; wcot fused silica coated CP-SIL 8CB for amines; 30 m x 0.25 mm x 0.25 µm) and DSQ (Varian FactorFour Capillary Column; VF-5ms 30 m x 0.25 mm x 0.25 µm). Gas chromatography was performed on an Agilent Technologies chromatograph 7890A GC System (Supelcowax 10 Fused Silica Capillary Column; 30 m x 0.32 mm x 0.25 µm). GC calibrations were carried out with authentic samples and ndodecane as an internal standard. Gas-phase GC measurements were conducted by a Shimadzu GC-2014 equipped with a TCD detector and a ShinCarbon ST 80/100 Silco column.Absorption-emission spectra were recorded on a Jasco FP-8500 Spectrofluorometer and UV/Vis spectra were recorded on a Jasco V-770 Spectrophotometer. General procedure for the dehydrogenation of amines to iminesA 10 mL two-necked flask containing a stirring bar was charged with 0.134 mmol substrate.After purging the flask three times with vacuum and two times with nitrogen the CO2 atmosphere was incorporated through a CO2-filled balloon. Afterwards dry DMSO (2.5 mL) and DBN (1.2 eq.; 0.16 mL of a 1 M solution in dry DMSO) were added. The resulting mixture was stirred for 48 h at irradiation of visible blue light (the progress can be monitored via GC-MS or TLC). Then, the resulting mixture underwent an aqueous workup (using distilled water; or brine in case of slurry phase separation) and was extracted three times with ethyl acetate.The combined organic layers were dried over anhydrous Na2SO4, filtered and concentrated in vacuo. Products were purified via silica gel chromatography with ethyl acetate and n-hexane and 1 V% triethylamine as solvents (typically 20:80 ethyl acetate:n-hexane).

Distance measurements in RNAs by pulse EPR with TEMPO-labeled nucleotides allow for model free conversion of distances into base-pair separation.

Double electron-electron resonance (DEER) at W-band (95 GHz) was applied to measure the distance between a pair of nitroxide and Gd(3+) chelate spin labels, about 6 nm apart, in a homodimer of the protein ERp29. While high-field DEER measurements on systems with such mixed labels can be highly attractive in terms of sensitivity and the potential to access long distances, a major difficulty arises from the large frequency spacing (about 700 MHz) between the narrow, intense signal of the Gd(3+) central transition and the nitroxide signal. This is particularly problematic when using standard single-mode cavities. Here we show that a novel dual-mode cavity that matches this large frequency separation dramatically increases the sensitivity of DEER measurements, allowing evolution times as long as 12 μs in a protein. This opens the possibility of accessing distances of 8 nm and longer. In addition, orientation selection can be resolved and analyzed, thus providing additional structural information. In the case of W-band DEER on a Gd(3+)-nitroxide pair, only two angles and their distributions have to be determined, which is a much simpler problem to solve than the five angles and their distributions associated with two nitroxide spin labels.

Dynamic nuclear polarization (DNP) permits increasing the NMR signal of nuclei by pumping the electronic spin transitions of paramagnetic centers nearby. This method is emerging as a powerful tool to increase the inherent sensitivity of NMR in structural biology aiming at detection of macromolecules. In aqueous solution, additional technical issues associated with the penetration of microwaves in water and heating effects aggravate the performance of the experiment. To examine the feasibility of low-field (9.7 GHz/0.35 T) DNP in high resolution NMR, we have constructed the prototype of a two-field shuttle DNP spectrometer that polarizes nuclei at 9.7 GHz/0.35 T and detects the NMR spectrum at 14 T. We report our first (1)H and (13)C DNP enhancements with this spectrometer. Effective enhancements up to 15 were observed for small molecules at (1)H 600 MHz/14 T as compared to the Boltzmann signal. The results provide a proof of principle for the feasibility of a shuttle DNP experiment and open up perspectives for the application potential of this method in solution NMR.

Pulsed electron-electron double resonance (PELDOR, also known as DEER) has become a method of choice to measure distances in biomolecules. In this work we show how the performance of the method can be improved at high EPR frequencies (94 GHz) using variable dual frequency irradiation in a dual mode cavity in order to obtain enhanced resolution toward orientation selection. Dipolar evolution traces of a representative RNA duplex and an α-helical peptide were analysed in terms of possible bi-radical structures by considering the inherent ambiguity of symmetry-related solutions.