Investigating Unusual Homonuclear Intermolecular “Through-Space” J Couplings in Organochalcogen Systems

Through-space H-F coupling in a series of 4-substituted-1H-1,2,3-triazoles

In the 1H-NMR spectra of a series of N-1 substituted 4-substituted-1H-1,2,3-triazoles that have been prepared, the lone heterocyclic ring hydrogen (H-5) appears as a singlet in all cases except those compounds that contain a 2-fluorophenyl moiety at Position 4. In those cases, H-5 is a doublet with J ~3.7 Hz. Based on computational chemistry results and geometric considerations, we attribute this splitting to through-space H-F coupling.

Experimental Evidence for Non-Fermi-Contact J Coupling Across Chalcogen Bonds in Ionic Salt Cocrystal Polymorphs

A pair of novel polymorphic ionic cocrystals of 3,4-dicyanotelluradiazole and tetraphenylphosphonium bromide are synthesized and are characterized by single-crystal XRD. Strong and directional non-covalent chalcogen bonds (ChB) between Te and Br are analyzed via solid-state NMR to reveal large and anisotropic J(125Te,79/81Br) coupling tensors, providing unequivocal evidence for non-Fermi contact contributions across ChBs. Along with large 79/81Br quadrupolar couplings for the Br- anions, these data provide new tools to characterize chalcogen bonds and to differentiate between ChB polymorphs.

Solid-State Photo-NMR Study on Light-Induced Nitrosyl Linkage Isomers Uncovers Their Structural, Electronic, and Diamagnetic Nature

A light-induced linkage NO isomer (MS1) in trans-[Ru(¹⁵NO)(py)₄¹⁹F](ClO₄)₂ is detected and measured for the first time by solid-state MAS NMR. Chemical shift tensors of ¹⁵N and ¹⁹F, along with ⁿJ(¹⁵N-¹⁹F) spin-spin couplings and T₁ relaxation times of MS1, are compared with the ground state (GS) at temperatures T < 250 K. Isotropic chemical shifts (¹⁵N and ¹⁹F) are well resolved for two crystallographically independent cations (A and B) [Ru(¹⁵NO)(py)₄¹⁹F]²⁺, allowing to define separately both populations of MS1 isomers and thermal decay rates for two structural sites. The relaxation times T₁ of ¹⁹F in the case of GS (30/38.6 s for sites A/B) and MS1 (11.6/11.8 s for sites A/B) indicate that both isomers are diamagnetic, which is the first experimental evidence of diamagnetic properties of MS1 in ruthenium nitrosyl. After light irradiation (λ = 420 nm), the NO ligand rotates by nearly 180° from F-Ru-N-O to F-Ru-O-N, whereby the isotropic chemical shifts of δ_iso(¹⁵N) increase and those of δ_iso(¹⁹F) decrease. The ⁿJ(¹⁵N-¹⁹F) couplings increase from ²J(¹⁵N-Ru-¹⁹F)_GS = 71 Hz to ³J(¹⁵N-O-Ru-¹⁹F)_MS1 = 105 Hz. These results are interpreted on the basis of DFT-CASTEP calculations including Bader-, Mulliken-, and Hirshfeld-charge density distributions of both states.

NMR Crystallography Enhanced by Quantum Chemical Calculations and Liquid State NMR Spectroscopy for the Investigation of Se-NHC Adducts*

N-heterocyclic carbene ligands (NHC) are widely utilized in catalysis and material science. They are characterized by their steric and electronic properties. Steric properties are usually quantified on the basis of their static structure, which can be determined by X-ray diffraction. The electronic properties are estimated in the liquid state; for example, via the ⁷⁷ Se liquid state NMR of Se-NHC adducts. We demonstrate that ⁷⁷ Se NMR crystallography can contribute to the characterization of the structural and electronic properties of NHC in solid and liquid states. Selected Se-NHC adducts are investigated via ⁷⁷ Se solid state NMR and X-ray crystallography, supported by quantum chemical calculations. This investigation reveals a correlation between the molecular structure of adducts and NMR parameters, including not only isotropic chemical shifts but also the other chemical shift tensor components. Afterwards, the liquid state ⁷⁷ Se NMR data is presented and interpreted in terms of the quantum chemistry modelling. The discrepancy between the structural and electronic properties, and in particular the π-accepting abilities of adducts in the solid and liquid states is discussed. Finally, the ¹³ C isotropic chemical shift from the liquid state NMR and the ¹³ C tensor components are also discussed, and compared with their ⁷⁷ Se counterparts. ⁷⁷ Se NMR crystallography can deliver valuable information about NHC ligands, and together with liquid state ⁷⁷ Se NMR can provide an in-depth outlook on the properties of NHC ligands.

Weak Intermolecular CH···N Hydrogen Bonding: Determination of 13CH-15N Hydrogen-Bond Mediated J Couplings by Solid-State NMR Spectroscopy and First-Principles Calculations

Weak hydrogen bonds are increasingly hypothesized to play key roles in a wide range of chemistry from catalysis to gelation to polymer structure. Here, ¹⁵N/¹³C spin-echo magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) experiments are applied to "view" intermolecular CH···N hydrogen bonding in two selectively labeled organic compounds, 4-[¹⁵N] cyano-4'-[¹³C₂] ethynylbiphenyl (1) and [¹⁵N₃,¹³C₆]-2,4,6-triethynyl-1,3,5-triazine (2). The synthesis of 2-¹⁵N₃,¹³C₆ is reported here for the first time via a multistep procedure, where the key element is the reaction of [¹⁵N₃]-2,4,6-trichloro-1,3,5-triazine (5) with [¹³C₂]-[(trimethylsilyl)ethynyl]zinc chloride (8) to afford its immediate precursor [¹⁵N₃,¹³C₆]-2,4,6-tris[(trimethylsilyl)ethynyl]-1,3,5-triazine (9). Experimentally determined hydrogen-bond-mediated ^2hJ_CN couplings (4.7 ± 0.4 Hz (1) and 4.1 ± 0.3 Hz (2)) are compared with density functional theory (DFT) gauge-including projector augmented wave (GIPAW) calculations, whereby species-independent coupling values ^2hK_CN (29.0 × 10¹⁹ kg m^-2 s^-2 A^-2 (1) and 27.9 × 10¹⁹ kg m^-2 s^-2 A^-2 (2)) quantitatively demonstrate the J couplings for these "weak" CH···N hydrogen bonds to be of a similar magnitude to those for conventionally observed NH···O hydrogen-bonding interactions in uracil (^2hK_NO: 28.1 and 36.8 × 10¹⁹ kg m^-2 s^-2 A^-2). Moreover, the GIPAW calculations show a clear correlation between increasing ^2hJ_CN (and ^3hJ_CN) coupling and reducing C(H)···N and H···N hydrogen-bonding distances, with the Fermi contact term accounting for at least 98% of the isotropic ^2hJ_CN coupling.

Weak Pnictogen Bond with Bismuth: Experimental Evidence Based on Bi-P Through-Space Coupling

To study pnictogen bonding involving bismuth, flexible accordion-like molecular complexes of the composition [P(C6 H4 -o-CH2 SCH3 )3 BiX3 ], (X=Cl, Br, I) have been synthesised and characterised. The strength of the weak and mainly electrostatic interaction between the Bi and P centres strongly depends on the character of the halogen substituent on bismuth, which is confirmed by single-crystal X-ray diffraction analyses, DFT and ab initio computations. Significantly, 209 Bi-31 P through-space coupling (J=2560 Hz) is observed in solid-state 31 P NMR spectra, which is so far unprecedented in the literature, delivering direct information on the magnitude of this pnictogen interaction.

Exploring Through-Space Spin-Spin Couplings for Quantum Information Processing: Facing the Challenge of Coherence Time and Control Quantum States

Nuclear magnetic resonance (NMR) is a powerful tool for studying quantum information processing (QIP). Recently quantum technologies have been proposed to overcome the challenges in large-scale NMR QIP. Furthermore, computational chemistry can promote its improvement. Nuclear spins-¹/₂ are natural qubits and have been used in most NMR quantum computation experiments. However, molecules that enable many qubits NMR QIP implementations should meet some requirements regarding their spectroscopic properties. Exceptionally large through-space (TS) P-P spin-spin coupling constants (SSCC or J) observed in 1,8-diphosphanaphthalenes (PPN) and in naphtho[1,8- cd]-1,2-dithiole phenylphosphines (NTP) were proposed and investigated to provide more accurate control within large-scale NMR QIP. Spectroscopic properties of PPN and NTP derivatives were explored by theoretical strategies using locally dense basis sets (LDBS). ³¹P chemical shifts (δ) calculated at the B3LYP/aug-cc-pVTZ-J level and TS P-P SSCCs at the PBE1PBE/pcJ-2 (LDBS-1) level are very close to the experimental data for the PPN molecule. Differently, for the NTP dimer, PBE1PBE/pcJ-2 (LDBS-2) predicts more accurate ³¹P δ, whereas PBE1PBE/Def2-TZVP (LDBS-1) forecasts more accurate TS P-P SSCCs. From our results, PPN_o-F, PPN_o-ethyl, and PPN_o-NH₂ were the best candidates for NMR QIP, in which the large TS SSCCS could face the need of long-time quantum gates implementations. Therefore, it could overcome natural limitations concerning the development of large-scale NMR.

Perspective: Current advances in solid-state NMR spectroscopy

In contrast to the rapid and revolutionary impact of solution-state Nuclear Magnetic Resonance (NMR) on modern chemistry, the field of solid-state NMR has matured more slowly. This reflects the major technical challenges of much reduced spectral resolution and sensitivity in solid-state as compared to solution-state spectra, as well as the relative complexity of the solid state. In this perspective, we outline the technique developments that have pushed resolution to intrinsic limits and the approaches, including ongoing major developments in the field of Dynamic Nuclear Polarisation, that have enhanced spectral sensitivity. The information on local structure and dynamics that can be obtained using these gains in sensitivity and resolution is illustrated with a diverse range of examples from large biomolecules to energy materials and pharmaceuticals and from both ordered and highly disordered materials. We discuss how parallel developments in quantum chemical calculation, particularly density functional theory, have enabled experimental data to be translated directly into information on local structure and dynamics, giving rise to the developing field of "NMR crystallography."

Polymorphism, Weak Interactions and Phase Transitions in Chalcogen-Phosphorus Heterocycles

A series of P-E-containing heterocycles (E=chalcogen) with aromatic backbones were synthesised and characterised by single-crystal and powder XRD, microanalysis and mass spectrometry. Solution- and solid-state 31 P and 77 Se NMR spectroscopy revealed significant differences between the NMR parameters in solution and in the solid state, related to conformational changes in the molecules. Many compounds were shown to exhibit a number of different polymorphic structures (identified by single-crystal XRD), although the bulk material studied by solid-state NMR spectroscopy often contained just one major polymorph. For the unoxidised heterocycles, the presence of weak intermolecular J couplings was also investigated by DFT calculations.