Indirect Nuclear 15N–15N Scalar Coupling through a Hydrogen Bond: Dependence on Structural Parameters Studied by Quantum Chemistry Tools

Solvent effects on acid-base complexes. What is more important: A macroscopic reaction field or solute-solvent interactions?

Can the geometry of an acid-base complex in solution be reproduced in calculations using an implicit accounting for the solvent effect in the form of a macroscopic reaction field? The answer is, "Yes, it can." Is this field equal to the real electric field experienced by the complex in solution? The answer is, "No, it is not." How can the geometry be correct under wrong conditions? This question is answered using density functional theory modeling of geometric and NMR parameters of pyridine⋯HF⋯(HCF₃)_n adducts in the absence and presence of an external electric field. This adduct under field approach shows that the N⋯H distance is a function of the H-F distance whatever method is used to change the geometry of the latter. An explicit account for solute-solvent interactions is required to get a realistic value of the solvent reaction field. Besides that, this approach reveals how certain NMR parameters depend on the solvent reaction field, the solute-solvent interactions, and the geometry of the N⋯H-F hydrogen bond. For some of them, the obtained dependences are far from self-evident.

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.

Simplified calculation approaches designed to reproduce the geometry of hydrogen bonds in molecular complexes in aprotic solvents

The impact of the environment onto the geometry of hydrogen bonds can be critically important for the properties of the questioned molecular system. The paper reports on the design of calculation approaches capable to simulate the effect of aprotic polar solvents on the geometric and NMR parameters of intermolecular hydrogen bonds. A hydrogen fluoride and pyridine complex has been used as the main model system because the experimental estimates of these parameters are available for it. Specifically, F-H, F⋯N, and H-N distances, the values of ¹⁵N NMR shift, and spin-spin coupling constants ¹J(¹⁹F¹H), ^1hJ(¹H¹⁵N), and ^2hJ(¹⁹F¹⁵N) have been analyzed. Calculation approaches based on the gas-phase and the Polarizable Continuum Model (PCM) approximations and their combinations with geometric constraints and additional noncovalent interactions have been probed. The main result of this work is that the effect of an aprotic polar solvent on the geometry of a proton-donor⋯H⋯proton-acceptor complex cannot be reproduced under the PCM approximation if no correction for solvent-solute interactions is made. These interactions can be implicitly accounted for using a simple computational protocol.

A Study of Through-Space and Through-Bond JPP Coupling in a Rigid Nonsymmetrical Bis(phosphine) and Its Metal Complexes

A series of representative late d-block metal complexes bearing a rigid bis(phosphine) ligand, iPr₂P-Ace-PPh₂ (L, Ace = acenaphthene-5,6-diyl), was prepared and fully characterized by various techniques, including multinuclear NMR and single-crystal X-ray diffraction. The heteroleptic nature of the peri-substituted ligand L allows for the direct observation of the J_PP couplings in the ³¹P{¹H} NMR spectra. Magnitudes of J_PP are correlated with the identity and geometry of the metal and the distortions of the ligand L. The forced overlap of the phosphine lone pairs due to the constraints imposed by the rigid acenaphthene skeleton in L results in a large ⁴ J_PP of 180 Hz. Sequestration of the lone pairs, either via oxidation of the phosphine or via metal chelation, results in distinct changes in the magnitude of J_PP. For tetrahedral d¹⁰ complexes ([LMCl₂], M = Zn, Cd, Hg), the J_PP is comparable to or larger than (193-309 Hz) that in free ligand L, although the P···P separation in these complexes is increased by ca. 0.4 Å (compare to free ligand L). The magnitude of J_PP diminishes to 26-117 Hz in square planar d⁸ complexes ([LMX₂], M = Ni, Pd, Pt; X = Cl, Br) and the octahedral Mo⁰ complex ([LMo(CO)₄], 33 Hz). Coupling deformation density calculations indicate the through-space interaction dominates in free L, while in metal complexes the main coupling pathway is via the metal atom.

Modulating intramolecular P⋯N pnictogen interactions

The strength of P⋯N intramolecular pnictogen interactions can be modulated, enhanced or diminished upon substitution of different electron withdrawing or donor groups.

Intramolecular pnicogen interactions in phosphorus and arsenic analogues of proton sponges

A computational study of the intramolecular pnicogen bond in 1,8-bis-substituted naphthalene derivatives (ZXH and ZX₂ with Z = P, As and X = H, F, Cl, and Br), structurally related to proton sponges, has been carried out.