Ab initio characterization of through-space indirect nuclear spin–spin coupling tensors for fluorine-X (X=F, C, H) spin pairs

Exploring long-range fluorine-carbon J-coupling for conformational analysis of deoxyfluorinated disaccharides: A combined computational and NMR study

In this study, we investigated the potential of long-range fluorine-carbon J-coupling for determining the structures of deoxyfluorinated disaccharides. Three disaccharides, previously synthesized as potential galectin inhibitors, exhibited through-space fluorine-carbon J-couplings. In our independent conformational analysis of these disaccharide derivatives, we employed a combination of density functional theory (DFT) calculations and nuclear magnetic resonance (NMR) experiments. By comparing the calculated nuclear shieldings with the experimental carbon chemical shifts, we were able to identify the most probable conformers for each compound. A model comprising fluoromethane and methane molecules was used to study the relationship between molecular arrangements and intermolecular through-space J-coupling. Our study demonstrates the important effect of internuclear distance and molecular orientation on the magnitude of fluorine-carbon coupling. The experimental values for the fluorine-carbon through-space couplings (TSCs) of the disaccharides corresponded with values calculated for the most probable conformers identified by the conformational analysis. These results unlock the broader application of fluorine-carbon TSCs as powerful tools for conformational analysis of flexible molecules, offering valuable insights for future structural investigations.

Through-space scalar spin-spin coupling: from rigid intramolecular cases to short-lived van der Waals complexes

We will discuss, with the help of few selected examples, how the concept of through-space scalar spin-spin coupling between non covalently bonded nuclei has evolved in recent years. We will first present systems where 'no covalent bond' actually means that the two atoms are separated by a large number of bonds; then we will see cases where it is referred to true van der Waals dimers, but with the two atoms somehow constrained in their positions; we will finish with the most recent examples of liquids and even gaseous mixtures with full translational degrees of freedom in a regime of intermolecular/interatomic fast exchange.

C-F Bonds as "Frozen" Nucleophiles: Unconsummated SN2 Reactions

In this note, we present a series of rigid molecules that show close enforced interactions between Ar-F moieties and -CH₂X groups in a "tetrel bond" configuration similar to a nascent S_N2 attack. We explore the spectroscopic, crystallographic, and chemical reactivity consequences of these unusual interactions, including significant through-space spin-spin couplings, short C-F···CH₂X distances, and differential S_N1 and S_N2 reaction pathways. We also reveal experimental evidence of carbon-based tetrel bonds influencing chemical reactivity in solution. Finally, density functional theory (DFT) calculations are employed throughout this study to confirm and illuminate our experimental data.

Evidence of C-F···H-C Attractive Interaction: Enforced Coplanarity of a Tetrafluorophenylene-Ethynylene-Linked Porphyrin Dimer

The formation of C-F···H-C "hydrogen bonds" has been a controversial subject because, in principle, fluorine is hardly an acceptor for less acidic protons contrasting to the C-F···H-O and C-F···H-N hydrogen bonds. Nevertheless, the interaction is emerging as a powerful implement for confining the torsional rotation in the design of fully coplanar π-conjugated polymers. Heretofore, no evidence of the C-F···H-C interaction has been observed in solutions. We herein disclose comprehensive evidence that the C-F···H-C interaction produces an attractive force. A ¹⁹F-¹H heteronuclear Overhauser effect experiment elucidated the close proximity of the F and H atoms in the doubly edge-facing C-F···H-C interactions of a meso-tetrafluorophenylene-ethynylene-conjugated porphyrin dimer (1). Extensive electronic and photophysical property investigations confirmed that all the aromatic units were torsionally restricted by the C-F···H-C interactions. Moreover, the enforced coplanarity invoked a markedly high π-staking propensity. Thus, we have firmly established the formation of a C-F···H-C interaction that produces a hydrogen-bond-like attractive force in solution.

An Internuclear J-Coupling of 3He Induced by Molecular Confinement

The solution-state ¹³C NMR spectrum of the endofullerene ³He@C₆₀ displays a doublet structure due to a J-coupling of magnitude 77.5 ± 0.2 mHz at 340 K between the ³He nucleus and a ¹³C nucleus of the enclosing carbon surface. The J-coupling increases in magnitude with increasing temperature. Quantum chemistry calculations successfully predict the approximate magnitude of the coupling. This observation shows that the mutual proximity of molecular or atomic species is sufficient to induce a finite scalar nuclear spin-spin coupling, providing that translational motion is restricted by confinement. The phenomenon may have applications to the study of surface interactions and to mechanically bound species.

Dihydrogen contacts observed by through-space indirect NMR coupling

"Through-space" indirect spin-spin couplings between hydrogen atoms formally separated by up to 18 covalent bonds have been detected by NMR experiments in model helical molecules. It is demonstrated that this coupling can provide crucial structural information on the molecular conformation in solution. The coupling pathways have been visualised and analysed by computational methods. The conformational dependence of the coupling is explained in terms of orbital interactions.

A Systematic Comparison of Second-Order Polarization Propagator Approximation (SOPPA) and Equation-of-Motion Coupled Cluster Singles and Doubles (EOM-CCSD) Spin-Spin Coupling Constants for Selected Singly Bonded Molecules, and the Hydrides NH3, H2O, and HF and Their Protonated and Deprotonated Ions and Hydrogen-Bonded Complexes

Second-order polarization propagator approximation (SOPPA) and equation-of-motion coupled cluster singles and doubles (EOM-CCSD) methods have been employed for the calculation of one-bond spin-spin coupling constants in series of small molecules and ions, and of one- and two-bond coupling constants across X-H···Y hydrogen bonds. For isolated molecules, one-bond SOPPA coupling constants (1)J(X-Y) involving (13)C, (15)N, (17)O, and (19)F have larger absolute values than corresponding EOM-CCSD coupling constants, with the EOM-CCSD values being in significantly better agreement with available experimental data. The difference between SOPPA and EOM-CCSD tends to increase as the number of nonbonding electrons on the coupled atoms increases, and the SOPPA values for O-F coupling are significantly in error. Similarly, the absolute values of SOPPA one-bond coupling constants (1)J(X-H) for the hydrides NH3, H2O, and FH and their protonated and deprotonated ions are greater than EOM-CCSD values, with the largest differences occurring for F-H coupling. One- and two-bond coupling constants (1)J(X-H), (1h)J(H-Y), and (2h)J(X-Y) across X-H···Y hydrogen bonds in neutral, protonated, and deprotonated complexes formed from the hydrides are similar at SOPPA and EOM-CCSD, with the largest differences again found for (1)J(F-H) in complexes with F-H as the proton donor, and (2h)J(F-F) for (FHF)(-). The signs of (1)J(X-H), (1h)J(H-Y), and (2h)J(X-Y) are the same at both levels of theory, as is their variation across the proton-transfer coordinate in F-H···NH3. SOPPA would appear to provide a reliable and more cost-effective alternative approach for computing coupling constants across hydrogen bonds, although couplings involving F may be problematic.

Relativistic and electron-correlation effects on the nuclear magnetic resonance shieldings of molecules containing tin and lead atoms

The reference values for NMR magnetic shieldings, σ(ref), are of the highest importance when theoretical analysis of chemical shifts are envisaged. The fact that the nonrelativistically valid relationship among spin-rotation constants and magnetic shieldings is not any longer valid for heavy atoms requires that the search for σ(ref) for such atoms needs new strategies to follow. We present here results of σ(ref) that were obtained by applying our own simple procedure which mixes accurate experimental chemical shifts (δ) and theoretical magnetic shieldings (σ). We calculated σ(Sn) and σ(Pb) in a family of heavy-halogen-containing molecules. We found out that σ(ref)[Sn;Sn(CH3)4] in gas phase should be close to 3864.11 ± 20.05 ppm (0.5%). For Pb atom, σ(ref)[Pb;Pb(CH3)4] should be close to 14475.1 ± 500.7 ppm. Such theoretical values correspond to calculations with the relativistic polarization propagator method, RelPPA, at the RPA level of approach. They are closer to experimental values as compared to those obtained applying few different functionals such as PBE0, B3LYP, BLYP, BP86, KT2, and KT3 of the density functional theory, DFT. We studied tin and lead shieldings of the XY(4-n)Z(n) (X = Sn, Pb; Y, Z = H, F, Cl, Br, I) and PbH(4-n)I(n) (n = 0, 1, 2, 3, 4) family of compounds with four-component functionals as implemented in the DIRAC code. For these systems results of calculations with RelPPA-RPA are more reliable than DFT ones. We argue about why those DFT functionals must be modified in order to obtain more accurate results of NMR magnetic shieldings within the relativistic regime: first, there is a dependence among both electron-correlation and relativistic effects that should be introduced in some way in the functionals; and second, the DIRAC code uses standard nonrelativistic functionals and the functionals B3LYP and PBE0 were parametrized only with data taken from light elements. It can explain why they are not able to properly introduce relativistic effects on nuclear magnetic shieldings. We finally show that in the analysis of magnetic shieldings for the family of compounds mentioned above, one must consider the newest and so-called heavy-atom effect on vicinal heavy atoms, HAVHA. Such effects are among the most important relativistic effects in these kind of compounds.

Synthesis, structure and dynamics of NHC-based palladium macrocycles

A series of macrocyclic CNC pincer pro-ligands based on bis(imidazolium)lutidine salts with octa-, deca- and dodecamethylene spacers have been prepared and their coordination chemistry investigated. Using a Ag2O based transmetallation strategy, cationic palladium(II) chloride complexes [PdCl{CNC-(CH2)n}][BAr(F)4] (n = 8, 10, 12; Ar(F) = 3,5-C6H3(CF3)2) were prepared and fully characterised in solution, by NMR spectroscopy and ESI-MS, and in the solid-state, by X-ray crystallography. The smaller macrocyclic complexes (n = 8 and 10) exhibit dynamic behaviour in solution, involving ring flipping of the alkyl spacer across the Pd-Cl bond, which was interrogated by variable temperature NMR spectroscopy. In the solid-state, distorted coordination geometries are observed with the spacer skewed to one side of the Pd-Cl bond. In contrast, a static C2 symmetric structure is observed for the dodecamethylene based macrocycle. For comparison, palladium(II) fluoride analogues [PdF{CNC-(CH2)n}][BAr(F)4] (n = 8, 10, 12) were also prepared and their solution and solid-state structures contrasted with those of the chlorides. Notably, these complexes exhibit very low frequency (19)F chemical shifts (ca. -400 ppm) and the presence of C-H···F interactions ((2h)J(FC) coupling observed by (13)C NMR spectroscopy). The dynamic behaviour of the fluoride complexes is largely consistent with the smaller ancillary ligand; [PdF{CNC-(CH2)8}][BAr(F)4] exceptionally shows C(2v) time averaged symmetry in solution at room temperature (CD2Cl2, 500 MHz) as a consequence of dual fluxional processes of the pincer backbone and alkyl spacer.

A theoretical structural analysis of the factors that affect (1)J(NH), (1h)J(NH) and (2h)J(NN) in N-H...N hydrogen-bonded complexes

Calculations of (1)J(NH), (1h)J(NH) and (2h)J(NN) spin-spin coupling constants of 27 complexes presenting N-H...N hydrogen bonds have allowed to analyze these through hydrogen-bond coupling as a function of the hybridization of both nitrogen atoms and the charge (+1, 0, - 1) of the complex. The main conclusions are that the hybridization of N atom of the hydrogen bond donor is much more important than that of the hydrogen bond acceptor. Positive and negative charges (cationic and anionic complexes) exert opposite effects while the effect of the transition states 'proton-in-the-middle' is considerable.

A review with comprehensive data on experimental indirect scalar NMR spin-spin coupling constants across hydrogen bonds

Scalar NMR spin-spin coupling constants across hydrogen bonds are fundamental in structural studies and as test grounds for theoretical calculations. Since they are scattered among many articles of different kinds, it seems useful to collect them in the most comprehensive way.

Characterization of indirect 31P-31P spin-spin coupling and phosphorus chemical shift tensors in pentaphenylphosphinophosphonium tetrachlorogallate, [Ph3P-PPh2][GaCl4]

Phosphorus chemical shift and 31P,31P spin-spin coupling tensors have been characterized for pentaphenylphosphinophosphonium tetrachlorogallate, [Ph3P-PPh2][GaCl4], using solid-state 31P NMR spectroscopy. Spectra obtained with magic-angle spinning yield the isotropic value of the indirect spin-spin coupling, |1J(31P,31P)iso|, 323 ± 2 Hz, while 2D spin-echo and rotational resonance experiments provide the effective dipolar coupling constant, Reff, 1.70 ± 0.02 kHz, and demonstrate that Jiso is negative. Within experimental error, the effective dipolar coupling constant and Jiso are unchanged at –120°C. The anisotropy in 1J(31P,31P), ΔJ, has been estimated by comparison of Reff and the value of the dipolar coupling constant, RDD, calculated from the P—P bond length as determined by X-ray diffraction. It is concluded that |ΔJ| is small, with an upper limit of 300 Hz. Calculations of 1J(31P,31P) for model systems H3P-PH+2 and (CH3)3P-P(CH3)+2 using density functional theory as well as multiconfigurational self-consistent field theory (H3P-PH+2) support this conclusion. The experimental spin-spin coupling parameters were used to analyze the 31P NMR spectrum of a stationary powder sample and provide information about the phosphorus chemical shift tensors. The principal components of the phosphorus chemical shift tensor for the phosphorus nucleus bonded to three phenyl groups are δ11 = 36 ppm, δ22 = 23 ppm, and δ33 = –14 ppm with an experimental error of ±2 ppm for each component. The components are oriented such that δ33 is approximately perpendicular to the P—P bond while δ11 forms an angle of 31° with the P—P bond. For the phosphorus nucleus bonded to two phenyl groups, the principal components of the phosphorus chemical shift tensor are δ11 = 23 ppm, δ22 = –8 ppm, and δ33 = –68 ppm with experimental errors of ±2 ppm. In this case, δ33 is also approximately perpendicular to the P—P bond; however, δ22 is close to the P—P bond for this phosphorus nucleus, forming an angle of 13°. The dihedral angle between the δ33 components of the two phosphorus chemical shift tensors is 25°. Results from ab initio calculations are in good agreement with experiment and suggest orientations of the phosphorus chemical shift tensors in the molecular frame of reference.Key words: Nuclear magnetic resonance spectroscopy, phosphorus chemical shift tensors, 31P-31P J-coupling tensors, density functional theory, multiconfigurational self-consistent field theory, phosphinophosphonium salts.