Two-Bond Spin−Spin Coupling across a Hydrogen Bond: X−Y Coupling in the Presence and Absence of the Proton

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-H···O Hydrogen Bonding. The Prototypical Methane-Formaldehyde System: A Critical Assessment

Distinguishing the functionality of C-H···O hydrogen bonds (HBs) remains challenging, because their properties are difficult to quantify reliably. Herein, we present a study of the model methane-formaldehyde complex (MFC). Six stationary points on the MFC potential energy surface (PES) were obtained at the CCSD(T)/ANO2 level. The CCSDT(Q)/CBS interaction energies of the conformers range from only -1.12 kcal mol^-1 to -0.33 kcal mol^-1, denoting a very flat PES. Notably, only the lowest energy stationary point (MFC1) corresponds to a genuine minimum, whereas all other stationary points-including the previously studied ideal case of a_e(C-H···O) = 180°-exhibit some degree of freedom that leads to MFC1. Despite the flat PES, we clearly see that the HB properties of MFC1 align with those of the prototypical water dimer O-H···O HB. Each HB property generally becomes less prominent in the higher-energy conformers. Only the MFC1 conformer prominently exhibits (1) elongated C-H donor bonds, (2) attractive C-H···O═C interactions, (3) n(O) → σ*(C-H) hyperconjugation, (4) critical points in the electron density from Bader's method and from the noncovalent interactions method, (5) positively charged donor hydrogen, and (6) downfield NMR chemical shifts and nonzero ²J(C_M-H_M···O_F) coupling constants. Based on this research, some issues merit further study. The flat PES hinders reliable determinations of the HB-induced shifts of the C-H stretches; a similarly difficult challenge is observed for the experiment. The role of charge transfer in HBs remains an intriguing open question, although our BLW and NBO computations suggest that it is relevant to the C-H···O HB geometries. These issues notwithstanding, the prominence of the HB properties in MFC1 serves as clear evidence that the MFC is predominantly bound by a C-H···O HB.

Characterizing complexes with F-Li...N, H-Li...N, and CH3Li...N lithium bonds: structures, binding energies, and spin-spin coupling constants

Ab initio calculations have been carried out to determine the structures, binding energies, and spin-spin coupling constants of complexes stabilized by X-Li...N bonds with F-Li, H-Li, and CH3Li as the Lewis acids. Complexes of these acids with the nitrogen bases N2, HCN, 1,3,5-triazine, pyrazine, 1,2,3-triazine, pyridine, and NH3 have linear X-Li...N bonds. Methylamine forms a nonlinear lithium bond only when F-Li is the lithium donor. Two bases, HN=CH2 and aziridine, form nonlinear X-Li...N bonds with each acid. Except for complexes with N2, which have small binding energies of about 5 kcal/mol, the binding energies of lithium-bonded complexes are appreciable, varying between 15 and 23 kcal/mol. The one-bond coupling constant 1J(F-Li) may increase or decrease upon complexation, but 1J(H-Li) and 1J(C-Li) decrease significantly. These coupling constants have their smallest values in complexes with nonlinear X-Li...N bonds. No correlations appear to exist between 1J(X-Li) and the X-Li distance and 1liJ(Li-N) and the Li-N distance. Values of the two-bond coupling constants 2liJ(X-N) are extremely small. Comparisons of 2liJ(F-N) with 2hJ(F-N) for coupling across a hydrogen bond and 2xJ(F-N) for coupling across a halogen bond suggest that the extremely small values of 2liJ(X-N) are not due to long X-N distances but to the low valence electron density on Li in lithium-bonded complexes.

The Properties of Weak and Strong Dihydrogen-Bonded DH⋅⋅⋅HA Complexes

The properties of six dihydrogen-bonded (DHB) dimers with the BeH2 molecule as a proton acceptor were calculated by MP2, CCSD(T) and B3LYP methods. The structural, energetic and spectroscopic parameters are presented and analyzed in terms of their possible correlation with the interaction energy and the intermolecular H...H separation. The symmetry-adapted perturbation theory (SAPT) calculations were performed to gain more insight into the nature of the H...H interactions. The studied complexes are divided into three groups based on the calculated intermolecular distances and the interaction energies which range from approximately -1 to -42 kJ mol(-1). The analysis of the interaction energy components indicates that, in contrast to conventional hydrogen bonds, the induction energy is the most important term in the BeH2NH4+ complex. On the other hand, there is no sharp boundary between the DHB complexes classified as hydrogen bonded and van der Waals systems. The complexation-induced changes in vibrational frequencies and in proton shielding constants show a relationship with the interaction energy. The values of the 2hJXH and 3hJBeX coupling constants correlate well with the interaction energy and with the intermolecular distance.

CLOPPA-IPPP Analysis of Electronic Mechanisms of Intermolecular 1hJ(A,H) and 2hJ(A,D) Spin−Spin Coupling Constants in Systems with D−H···A Hydrogen Bonds

The electronic origin of intermolecular (2h)J(A,D) and (1h)J(A,H) couplings is discussed by means of the CLOPPA-IPPP approach in several model complexes with D-H...A hydrogen bonds. It is found that the origin of these couplings is mainly due to the interaction between the acceptor sigma lone pair and vacant molecular orbitals localized in the D-H...A moiety, regardless of the donor and acceptor nuclei. The problem of the larger absolute value of (2h)J(A,D) compared to (1h)J(A,H) is also addressed.

Understanding the Conformational Dependence of Spin-Spin Coupling Constants: Through-Bond and Through-SpaceJ(31P,31P) Coupling in Tetraphosphane-1,4-diides [M(L)x]2[P4R4]

The characteristic dependence of J(31P,31P) spin-spin coupling constants of alkali metal tetraphosphane-1,4-diides on structure and composition has been analyzed by density functional methods. The computations confirm that the structure of the contact ion pairs is conserved in solution. Calculations on model systems M2P4H4, on naked P4H4(2-) anions, and on models including point charges, show that the role of the cations is mainly structural and to a smaller extent electrostatic. Three of the four J(P,P) coupling constants depend characteristically on the conformation of the anion, which in turn is determined by the substituents R and by cation-anion interactions. Several couplings exhibit a large through-space component and are thus strongly dependent on the relative orientation of nonbonding electron pairs on the phosphorus atoms involved. This is shown by visualization of coupling pathways using the recently introduced coupling energy density (CED), in combination with the electron localization function (ELF).