Two-bond Coupling Constants Research Articles

Krasilnikolides A and B and Detalosylkrasilnikolide A, Cytotoxic 20-Membered Macrolides from the Genus Krasilnikovia: Assignment of Anomeric Configuration by J-Based Configuration Analysis.

A chemical investigation of strain RD003821, belonging to the underexplored actinomycetes genus Krasilnikovia, led to the discovery of three novel polyketides: two 20-membered glycomacrolides, krasilnikolides A (1) and B (2), and an aglycone of 1, detalosylkrasilnikolide A (3). A major challenge in the structure elucidation of 1 was to determine the anomeric configuration of the α-l-6-deoxytalose (6dTal) unit, which was achieved by J-based configuration analysis (JBCA) that incorporated anomeric carbon- and proton-specific two-bond 13C-1H spin-spin coupling constants as diagnostic parameters. The updated criteria for the conformation/configuration assignment facilitated discrimination of three out of four stereochemical variants at the anomeric and the adjacent C2 positions, which expanded the scope of the JBCA method to determination of the anomeric configuration of aldohexopyranoses. Compounds 1 and 2 are the first macrolides decorated by 6dTal. Compounds 1-3 exhibited cytotoxicity against P388 murine leukemia cells with IC50 values of 14, 8.4, and 3.9 μM, respectively. In addition, 1-3 were antibacterial against the Gram-positive bacterium Kocuria rhizophila with MIC values of 25, 50, and 100 μg/mL. 1 was inhibitory against Staphylococcus aureus with an MIC of 50 μg/mL.

Complexes of O=C=S with Nitrogen Bases: Chalcogen Bonds, Tetrel Bonds, and Other Secondary Interactions.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate chalcogen-bond formation through the σ-hole at S and tetrel-bond formation through the π-hole at C in complexes of OCS with a series of nitrogen bases. The binding energies of chalcogen- and tetrel-bonded complexes with the sp-hybridized bases correlate exponentially with the N-S and N-C distances, respectively. The presence of secondary interactions between an N-H or C-H group of an sp2 -hybridized base and OCS in chalcogen-bonded complexes decreases the correlation between binding energies and the N-S distance. These secondary interactions are stronger in the tetrel-bonded complexes with the sp2 bases, particularly in the isomers of OCS:imidazole and OCS : N2 H2 , where they may be described as distorted N-H⋅⋅⋅O or N-H⋅⋅⋅S hydrogen bonds. Charge-transfer interactions are consistent with the nature of the primary and secondary interactions in these complexes. The in-plane OCS bending frequencies are blue-shift in the chalcogen-bonded complexes, and red-shifted in the tetrel-bonded complexes. EOM-CCSD spin-spin coupling constants 1c J(N4-S) across chalcogen bonds have absolute values less than 9.0 Hz, while the two-bond coupling constants 2c J(N4-C) do not exceed 4.0 Hz. These are greater in absolute value that the one-bond coupling constants 1t J(N4-C) across tetrel bonds that are less than 0.5 Hz at much shorter N-C distances.

2JHH-resolved HSQC: Exclusive determination of geminal proton-proton coupling constants

The measurement of two-bond proton-proton coupling constants (2JHH) in prochiral CH2 groups from the F2 dimension of 2D spectra is not easy due to the usual presence of complex multiplet J patterns, line broadening effects and strong coupling artifacts. These drawbacks are particularly pronounced and frequent in AB spin systems, as those normally exhibited by the pair of diastereotopic CH2 protons. Here, a novel 2JHH-resolved HSQC experiment for the exclusive and accurate determination of the magnitude of 2JHH from the doublet displayed along the highly-resolved indirect F1 dimension is described. A pragmatic 2JHH NMR profile affords a fast overview of the full range of existing 2JHH values. In addition, a 2JHH/δ(13C)-scaled version proves to be an efficient solution when severe signal overlapping complicate a rigorous analysis. The performance of the method is compared with other current techniques and illustrated by the determination of challenging residual dipolar 2DHH coupling constants of small molecules dissolved in weakly orienting media.

How Microsolvation Numbers at Li Control Aggregation Modes, sp(2)-Stereoinversion, and NMR Coupling Constants (2)JH,H of H2C═C in α-(2,6-Dimethylphenyl)vinyllithium.

The title compound 4 is a trisolvated monomer 4&3THF in THF solution and dimerizes endothermically to form (4&THF)2 with a strongly positive (!) dimerization entropy in toluene as the solvent. In the absence of electron-pair donor ligands, 4 aggregates (>dimer) in hydrocarbon solutions. These results followed from the (13)C-α splitting patterns and the magnitudes of the one-bond (13)C,(6)Li NMR coupling constants in combination with lithiation NMR shifts as secondary NMR criteria. The rate constants of cis/trans sp(2)-stereoinversion could be measured on the (1)H NMR time scale in THF, in which solvent the preinversion lifetime is 0.24 s at 25 °C. This inversion proceeds according to the pseudomonomolecular, ionic mechanism with the typical, strongly negative pseudoactivation entropy. In a different mechanism, the lifetimes are much longer at 25 °C for the dimer (4&t-BuOMe)2 in toluene (ca. 2.5 min) and for donor-free, aggregated 4 in hexane solution (roughly 1 min). The olefinic interproton two-bond coupling constants (2)JH,H of the H2C═CLi part are proposed as an indicator of microsolvation at Li, because they were found to increase linearly with the "explicit" solvation of α-arylvinyllithiums by 0, 1, 2, and 3 electron-pair donor ligands.

Exploring (NH2F)2, H2FP:NFH2, and (PH2F)2Potential Surfaces: Hydrogen Bonds or Pnicogen Bonds?

An ab initio MP2/aug'-cc-pVTZ study has been carried out to identify local minima on the (NH(2)F)(2), H(2)FP:NFH(2), and (PH(2)F)(2) potential surfaces, to characterize the types of interactions which stabilize the complexes found at these minima, and to evaluate their binding energies. With one exception, (NH(2)F)(2) complexes are stabilized by N-H···N or N-H···F hydrogen bonds. Only one complex, that with the smallest binding energy, has a pnicogen N···N bond. In contrast, (PH(2)F)(2) complexes are stabilized by P···P or P···F pnicogen bonds or by an antiparallel alignment of the dipole moment vectors of the two monomers, but not by hydrogen bonds. The most stable complex has an F-P···P-F alignment which approaches linearity. Both hydrogen-bonded and pnicogen-bonded complexes exist on the H(2)FP:NFH(2) surface, with the most stable being the pnicogen-bonded complex with F-P···N-F approaching a linear arrangement. Charge transfer transitions from a lone pair on a P, N, or F atom in one molecule to an antibonding σ* orbital of the other stabilize these complexes. These transitions are most important for complexes with pnicogen bonds. Although net charge transfer occurs in complexes in which the two monomers are inequivalent, charges on N and P do not correlate with N and P absolute chemical shieldings. Rather, these shieldings also reflect charge distributions and overall bonding patterns. EOM-CCSD two-bond spin-spin coupling constants (2h)J(X-Y) across X-H···Y hydrogen bonds tend to be small, due in part to the nonlinearity of many of the hydrogen bonds. (1p)J values across a particular kind of pnicogen bond are relatively large and vary significantly but do not correlate with corresponding distances.

An ab initio study of cooperative effects in ternary complexes X:CNH:Z with X, Z=CNH, FH, ClH, FCl, and HLi: structures, binding energies, and spin–spin coupling constants across intermolecular bonds

A systematic ab initio investigation has been carried out to determine the structures, binding energies, and spin-spin coupling constants of ternary complexes X:CNH:Z and corresponding binary complexes X:CNH and CNH:Z, for X, Z = CNH, FH, ClH, FCl, and HLi. The enhanced binding energies of ternary complexes X:CNH:Z for fixed X as a function of Z decrease in the same order as the binding energies of the binary complexes CNH:Z. In contrast, the enhanced binding energies of the ternary complexes for fixed Z as a function of X do not decrease in the same order as the binding energies of the binary complexes X:CNH, a consequence of the increased stabilities of ternary complexes FCl:CNH:Z due to very strong chlorine-shared halogen bonds. For complexes in which the X···CNH interaction is a D-H···C hydrogen bond for D-H the proton-donor group (N-H, F-H, or Cl-H), spin-spin coupling constants (1)J(D-H) and (2h)J(D-C) in ternary complexes X:CNH:Z decrease in absolute value as the binding energies of binary complexes CNH:Z and the enhanced binding energies of the ternary complexes for fixed X as a function of Z also decrease. However, (2X)J(F-C) increases as the enhanced binding energies of the ternary complexes FCl:CNH:Z decrease, a consequence of the nature of the chlorine-shared halogen bond. The one-bond coupling constants (1)J(N-H) for the CNH···Z interaction in ternary complexes vary significantly, depending on the nature of the X···CNH interaction. The largest values of (1)J(N-H) are found for ternary complexes with FCl as X. Two-bond coupling constants (2h)J(N-A) for A the proton-acceptor atom of Z, and (2d)J(N-H) decrease in absolute value in the order of decreasing enhancement energies of ternary complexes X:CNH:Z for fixed Z as a function of X.

Ab Initio Study of Ternary Complexes A···NCH···C with A,C = HCN, HF, HCl, ClF, and LiH: Energetics and Spin−Spin Coupling Constants across Intermolecular Bonds

A systematic ab initio study has been carried out to investigate the structures, binding energies, and spin-spin coupling constants of binary complexes A...NCH and NCH...C and ternary complexes A...NCH...C for A,C = HCN, HF, HCl, FCl, and HLi. These complexes are stabilized by some combination of hydrogen bonds, dihydrogen bonds, halogen bonds, and lithium bonds. The binding energies of the ternary complexes are enhanced relative to those of the corresponding binary complexes, suggesting that A...NCH is a stronger proton-donor acid and NCH...C is a stronger base relative to the isolated monomer HCN. The ternary complexes with the largest enhancement energies are FH...NCH...HLi, HLi...NCH...NCH, and HLi...NCH...HLi, each containing molecules with large dipole moments, and each with a linear head-to-tail arrangement of dipole moment vectors. The equation-of-motion coupled-cluster single and doubles method (EOM-CCSD) has been employed to evaluate spin-spin coupling constants across intermolecular bonds for binary complexes A...NCH and NCH...C and for ternary complexes A...NCH...C. These data are used to investigate how the presence of C influences coupling constants associated with the A...NCH interaction, and how the presence of A influences coupling constants for the NCH...C interaction. Changes in coupling constants in the ternary complexes relative to the binary may be related to the binding energies of corresponding binary complexes and the enhanced binding energies of the ternary complexes. Two-bond coupling constants across X-H...Y hydrogen bonds [(2h)J(X-Y)] are related to changes in intermolecular distances.

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.

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 Molecules with C, N, and O Double and Triple Bonds and Selected F-Substituted Derivatives.

Ab initio EOM-CCSD and SOPPA calculations with the Ahlrichs (qzp,qz2p) basis set have been carried out to evaluate one-, two-, and three-bond spin-spin coupling constants for molecules HmXYHn and HmXYHn for X, Y = (13)C, (15)N, and (17)O, and selected (19)F-substituted derivatives. In the great majority of cases, EOM-CCSD one-bond C-C, C-N, C-O, C-F, N-N, N-O, and N-F coupling constants and three-bond F-F coupling constants are smaller in absolute value than the corresponding SOPPA coupling constants, with the EOM-CCSD values in better agreement with experimental data. SOPPA tends to significantly overestimate the absolute values of large one- and three-bond couplings involving fluorine. The majority of two-bond SOPPA coupling constants are in better agreement with experiment than EOM-CCSD, although differences between EOM-CCSD and experimental values are not dramatic. A statistical analysis of thirty EOM-CCSD and SOPPA coupling constants versus experimental coupling constants demonstrates that better agreement with experiment is found when EOM-CCSD is the computational method.

Spin−Spin Coupling across Intermolecular F−Cl···N Halogen Bonds

Ab initio EOM-CCSD calculations have been performed to determine one- and two-bond spin-spin coupling constants 1J(F-Cl), 1XJ(Cl-N), and 2XJ(F-N) across F-Cl...N halogen bonds in complexes with F-Cl as the Lewis acid and N2, FCN, HCN, (CH3)CN, LiCN, Z-HNNH, H2CNH, NH2F, NH3, cyclic-NH(CH2)2, and NH2(CH3) as Lewis bases. The structures of these complexes were optimized at MP2 with the aug'-cc-pVTZ basis set. The absolute value of 2XJ(F-N) increases in these complexes as the F-N distance decreases, a behavior similar to that of 2hJ(F-N) for complexes stabilized by F-H...N hydrogen bonds. 1XJ(Cl-N) also tends to increase in absolute value with decreasing F-N distance. 1J(F-Cl) is always positive, decreases upon complex formation as the F-Cl distance increases, and appears to be sensitive to the hybridization of the nitrogen base. The relatively large differences in the values of these coupling constants in the various complexes and their variation along the chlorine-transfer coordinate for F-Cl...NH3 suggest that they should be amenable to experimental investigation.

Ab Initio EOM-CCSD Spin−Spin Coupling Constants for Hydrogen-Bonded Formamide Complexes: Bridging Complexes with NH3, (NH3)2, H2O, (H2O)2, FH, and (FH)2

EOM-CCSD spin-spin coupling constants across hydrogen bonds have been computed for complexes in which NH3, H2O, and FH molecules and their hydrogen-bonded dimers form bridging complexes in the amide region of formamide. The formamide one-bond N-H coupling constant [(1)J(N-H)] across N-H...X hydrogen bonds increases in absolute value upon complexation. The signs of the one-bond coupling constants (1h)J(H-X) indicate that these complexes are stabilized by traditional hydrogen bonds. The two-bond coupling constants for hydrogen bonds with N-H as the donor [(2h)J(N-X)] and the carbonyl oxygen as the acceptor [(2h)J(X-O)] increase in absolute value in the formamide/dimer relative to the corresponding formamide/monomer complex as the hydrogen bonds acquire increased proton-shared character. The largest changes in coupling constants are found for complexes of formamide with FH and (FH)2, suggesting that bridging FH monomers and dimers in particular could be useful NMR spectroscopic probes of amide hydrogen bonding.

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.

HC⋮P and H3C−C⋮P as Proton Acceptors in Protonated Complexes Containing Two Phosphorus Bases: Structures, Binding Energies, and Spin−Spin Coupling Constants

Ab initio calculations at the MP2/aug'-cc-pVTZ level have been carried out to investigate the structures and binding energies of cationic complexes involving protonated sp, sp2, and sp3 phosphorus bases as proton donor ions and the sp-hybridized phosphorus bases H-C[triple bond]P and H3C-C[triple bond]P as proton acceptors. These proton-bound complexes exhibit a variety of structural motifs, but all are stabilized by interactions that occur through the pi cloud of the acceptor base. The binding energies of these complexes range from 6 to 15 kcal/mol. Corresponding complexes with H3C-C[triple bond]P as the proton acceptor are more stable than those with H-C[triple bond]P as the acceptor, a reflection of the greater basicity of H3C-C[triple bond]P. In most complexes with sp2- or sp3-hybridized P-H donor ions, the P-H bond lengthens and the P-H stretching frequency is red-shifted relative to the corresponding monomers. Complex formation also leads to a lengthening of the C[triple bond]P bond and a red shift of the C[triple bond]P stretching vibration. The two-bond coupling constants 2pihJ(P-P) and 2pihJ(P-C) are significantly smaller than 2hJ(P-P) and 2hJ(P-C) for complexes in which hydrogen bonding occurs through lone pairs of electrons on P or C. This reflects the absence of significant s electron density in the hydrogen-bonding regions of these pi complexes.

Molecular Dynamics Computational Study of the 199Hg−199Hg NMR Spin−Spin Coupling Constants of [Hg−Hg−Hg]2+ in SO2 Solution

The isotropic one-bond and two-bond 199Hg-199Hg nuclear magnetic spin-spin coupling constants (J-couplings) of [Hg-Hg-Hg]2+ were calculated using density functional theory, the zeroth-order regular approximation (ZORA) to treat relativistic effects, and Born-Oppenheimer molecular dynamics (BOMD) including SO2 molecules explicitly for the description of solvent effects. The final BOMD average of 150 kHz for 1J (199Hg-199Hg) agrees well with the experimental spin-spin coupling of 140 kHz measured in liquid SO2, while computations not considering explicit solvation at the quantum-mechanical level yielded one-bond coupling constants between 230 and 260 kHz. The two-bond coupling is similarly strongly affected by solvent effects. An analysis of the BOMD data shows that the effect is mainly due to close contacts between the terminal Hg atoms of [Hg-Hg-Hg]2+ and the solvent's oxygen atoms. The results highlight the importance of solvent effects for the NMR parameter of heavy metals and demonstrate the usefulness of treating such solvent effects with the help of molecular dynamics-based averaging.

Complexes with N−H+−P Hydrogen Bonds: Structures, Binding Energies, and Spin−Spin Coupling Constants

Ab-initio MP2/aug'-cc-pVTZ calculations have been performed to determine the structures and binding energies of proton-bound complexes stabilized by N-H+-P hydrogen bonds and to investigate the nature of the proton-transfer coordinate in these systems. Double minima are found only when the difference between the protonation energies of the N and P bases is less than about 4 kcal/mol. The isomer in which the protonated nitrogen base is the donor lies lower on the potential surface and also has a greater binding energy relative to the corresponding isolated monomers. Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been employed to obtain one- and two-bond spin-spin coupling constants across these hydrogen bonds. Two-bond coupling constants (2h)J(N-P) correlate with N-P distances, irrespective of whether the donor ion is N-H+ or P-H+. One-bond coupling constants (1)J(N-H) and (1h)J(H-P) for complexes stabilized by N-H+...P hydrogen bonds correlate with corresponding distances, but similar correlations are not found for (1)J(P-H) and (1h)J(H-N) for complexes with P-H+...N hydrogen bonds. Negative values of (1h)K(H-N) and (1h)K(H-P) indicate that the hydrogen bonds in these complexes are traditional. Comparisons are made with complexes stabilized by N-H+-N and P-H+-P hydrogen bonds.

Ab Initio Study of Hydrogen Bonding and Proton Transfer in 3:1 FH:NH3 and FH:Collidine Complexes: Structures and One- and Two-Bond Coupling Constants across Hydrogen Bonds

Ab initio EOM-CCSD calculations have been performed on 3:1 FH:NH3 complexes at their own optimized MP2/6-31+G(d,p) geometries and at the optimized geometries in the hydrogen-bonding regions of corresponding 3:1 FH:collidine complexes. The isolated gas-phase equilibrium 3:1 FH:NH3 complex has an open structure with a proton-shared Fa-Ha-N hydrogen bond, while the isolated equilibrium 3:1 FH:collidine complex has a perpendicular structure with an Fa-Ha-N hydrogen bond that is on the ion-pair side of proton-shared. The Fa-N coupling constant ((2h)J(Fa-N)) for the equilibrium 3:1 FH:NH3 complex is large and negative, consistent with a proton-shared Fa-Ha-N hydrogen bond; (2h)JFb-Fa is positive, reflecting a short Fb-Fa distance and partial proton transfer from Fb to Fa across the Fb-Hb-Fa hydrogen bond. In contrast, (2h)JFa-N has a smaller absolute value and (2h)JFb-Fa is greater for the 3:1 FH:NH3 complex at the equilibrium 3:1 FH:collidine geometry, consistent with the structural characteristics of the Fa-Ha-N and Fb-Hb-Fa hydrogen bonds. Coupling constants computed at proton-transferred 3:1 FH:collidine perpendicular geometries are consistent with experimental coupling constants for the 3:1 FH:collidine complex in solution and indicate that the role of the solvent is to promote further proton transfer from Fa to N across the Fa-Ha-N hydrogen bond, and from Fb to Fa across the two equivalent Fb-Hb-Fa hydrogen bonds. The best correlations between experimental and computed coupling constants are found for complexes with perpendicular proton-transferred structures, one having the optimized geometry of a 3:1 FH:collidine complex at an Fa-Ha distance of 1.80 A, and the other at the optimized 3:1 FH:collidine geometry with distances derived from the experimental coupling constants. These calculations provide support for the proposed perpendicular structure of the 3:1 FH:collidine complex as the structure which exists in solution.

Characterizing Hydrogen Bonding and Proton Transfer in 2:1 FH:NH3 and FH:Collidine Complexes through One- and Two-Bond Spin−Spin Coupling Constants across Hydrogen Bonds

Ab initio equation-of-motion coupled cluster singles and doubles calculations have been carried out on a variety of 2:1 FH:NH(3) complexes (F(b)H(b):F(a)H(a):NH(3)) to investigate the effects of structural changes on one- and two-bond spin-spin coupling constants across F(a)-H(a)-N and F(b)-H(b)-F(a) hydrogen bonds and to provide insight into experimentally measured coupling constants for 2:1 FH:collidine (2:1 FH:2,4,6-trimethylpyridine) complexes. Coupling constants have been computed for 2:1 FH:NH(3) equilibrium structures and proton-transferred perpendicular and open structures at 2:1 FH:NH(3), FH:pyridine, and FH:collidine geometries. (2h)J(Fa)(-)(N), (1)J(Fa)(-)(Ha), and (1h)J(Ha)(-)(N) exhibit expected dependencies on distances, angles, and the nature of the nitrogen base. In contrast, one- and two-bond coupling constants associated with the F(b)-H(b)-F(a) hydrogen bond, particularly (2h)J(F)()b(-)(F)()a, vary significantly depending on the F-F distance, the orientation of the hydrogen-bonded pair, and the nature of the complex (HF dimer versus the anion FHF(-)). The structure of the 2:1 FH:collidine complex proposed on the basis of experimentally measured coupling constants is supported by the computed coupling constants. This study of the structures of open proton-transferred 2:1 FH:NH(3), FH:pyridine, and FH:collidine complexes and the coupling constants computed for 2:1 FH:NH(3) complexes at these geometries provides insight into the role of the solvent in enhancing proton transfer across both N-H(a)-F(a) and F(b)-H(b)-F(a) hydrogen bonds.

Do coupling constants and chemical shifts provide evidence for the existence of resonance-assisted hydrogen bonds?

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been performed to determine two-bond 17O–17O and 15N–15N spin–spin coupling constants (2h J O–O and 2h J N–N) for ten neutral structures which may exhibit intramolecular O–H–O or N–H–N hydrogen bonds. MP2 chemical shifts of hydrogen-bonded protons have also been evaluated. The molecules include malonaldehyde and its diaza counterpart, and the corresponding saturated analogues. The aim of this study is to investigate whether the magnetic properties of the two-bond spin–spin coupling constants and the chemical shifts of the hydrogen-bonded protons provide evidence for the existence of resonance-assisted hydrogen bonds (RAHBs). The two-bond coupling constant for the equilibrium structures is greater for the hydrogen-bonded unsaturated molecule than for the saturated molecule, a result of the shorter O–O distance and stronger hydrogen bond in the former. However, when the O–O or N–N distances are forced to be the same in corresponding saturated and unsaturated structures, the coupling constants are similar. There is a significant increase in coupling constants when the intramolecular hydrogen bond (IMHB) changes from asymmetric to symmetric, due to much shorter O–O or N–N distances. Symmetrization effects also significantly affect the value of the proton chemical shift. For hydrogen-bonded conformers the trends in chemical shifts and coupling constants are similar. A detailed analysis of the NMR properties of oxygen-containing systems leads to the conclusion that neither the coupling constants nor the proton chemical shifts provide any evidence for the existence of RAHBs. Furthermore, the strength of the O–H···O IMHBs, investigated through the use of appropriate homodesmotic reactions, indicates that the enhanced stability of the IMHB in the unsaturated compound is associated with the σ skeleton of the molecule that allows the oxygen atoms to be in closer proximity than in the saturated analogue.

Spin Coupling through the N→Si Dative Bond in Penta- and Hexacoordinate Hydrido- and Fluorosilicon Complexes; Coupling through a Rapidly Dissociating−Recombining N→Si Bond

Penta- and hexacoordinate silicon complexes with spin ½ nuclei ( 1 H and 1 9 F) directly attached to silicon have been prepared and used to study NMR spin-spin interactions across the N→Si dative bond. In both hydrido and fluoro complexes, two-bond coupling constants were found to drop sharply as the geminal bond angle deviated from 90°. Three-bond coupling constants showed a Karplus-type dependence upon the corresponding dihedral angles for both 1 H and 1 9 F complexes. Four-bond coupling constants 4 J( 1 9 F-Si-N-C- 1 H) across the dative bond were observed. The correlations can be used as tools for the interpolation of bond and dihedral angles from NMR data, in cases where crystal structures are unavailable. They also demonstrate that the N→Si dative bond in hypercoordinate silicon complexes behaves essentially in the manner expected from normal covalent bonds. Rapid dissociation and recombination of the N-Si dative bond was established in several pentacoordinate complexes; however, one-, two-, and three-bond coupling constants could nevertheless be measured across this rapidly dissociating bond.

A theoretical study of the large Hg-Hg spin-spin coupling constants in Hg(2)(2+), Hg(3)(2+), and Hg(2)(2+)-crown ether complexes.

Nuclear spin-spin coupling constants (1)J(Hg-Hg) in the systems Hg(2)(2+) and Hg(3)(2+) represent the largest coupling constants so far observed in NMR experiments. We have performed a computational study on these ions, on Hg(2)(2+) complexes with 18-crown-6 and 15-crown-5, and on Hg(3)(2+) with solvent molecules and counterions. The results obtained with our recently developed program for the density functional computation of heavy nucleus spin-spin coupling constants are in good agreement with experiments. The data reveal that the bare ions Hg(2)(2+) and Hg(3)(2+) would afford much larger coupling constants than those experimentally observed, with an upper limit of approximately 0.9 MHz for Hg(2)(2+). This limit is much larger than that previously estimated by Hückel theory. It is demonstrated that in solution or due to complexation the experimentally determined values are much smaller than the free ion's coupling constants. With the help of intuitive MO arguments, it is illustrated how the environment strongly reduces the coupling constants in Hg(2)(2+) and Hg(3)(2+). The two-bond coupling constant (2)J(Hg-Hg) in Hg(3)(2+) is also examined.