Ernesto Díez

Vicinal proton-proton coupling constants: IV. Effect of individual substituents with second row α-atoms

An equation for the prediction of the vicinal coupling constants 3 J HH in substituted HCCH fragments is formulated as a truncated Fourier series in the torsion angle φ between the coupling hydrogens with coefficients expanded as a Taylor series in a substituent parameter λ. The different terms in this series have definite meanings: (1) the independent terms k n give the angular dependence of 3 J HH in ethane, (2) the linear terms k niλ i represent the effects upon 3 J HH of the individual substituents, and (3) the cross terms k nij λ i λ j account for the effects of interactions between pairs of substituents. This interpretation is based on a model for the effect of the substituents that explains the Fourier coefficients as a sum of contributions from ethane, from individual substituents and from interactions between pairs of substituents. The model reduces to the equation when several simplifying assumptions are made. Otherwise, some refinements can be introduced in the equation. The invariances of the ...

Relationships between torsion angles and ring-puckering coordinates: Part III. Application to heterocyclic puckered five-membered rings

Abstract The conformational analysis of five-membered rings is greatly facilitated by the concept of pseudorotation. The well-known pseudorotation equation for torsion angles (C. Altona and M. Sundaralingam J. Am. Chem. Soc., 94 (1972) 8205) yields a good correspondence between calculated and observed torsion angles for equilateral or nearly equilateral rings. In this communication the pseudorotation equation is extended by additional terms in order to yield an improved correspondence between calculated and observed torsion angles, especially for heterocyclic puckered rings (e.g. tetrahydrothiophene, 1,2-oxathiolane). The additional parameters, valid for a given class of five-membered rings, can be obtained either from a dataset of X-ray structures or from model structures optimized by ab initio or (semi)empirical calculations. Endocyclic bond angles in five-membered rings are correlated with the phase angle of pseudorotation P and with the puckering amplitude φ m . Coefficients for bond angle regressions with pseudorotation parameters are given for furanose and proline rings and for ring D in steroids. The geometry of a five-membered ring can be calculated with the computer program PSEU for any desired value of P and φ m . The nine independent coordinates are calculated from five torsion angles (obtained from a given P and φ m by means of the extended pseudorotation equation) and five bond distances by means of a least-squares procedure. Coefficients for functions relating the pseudorotation parameters to exocyclic bond angles and exocyclic torsion angles in furanose fragments are also established. Cartesian coordinates for model β- D -ribo- and β- D -deoxyribo furanoside fragments encompassing the full pseudorotation pathway are presented as supplementary material.

Transmission mechanisms of NMR long-range J(13C,19F) scalar couplings in 1-F,4-X-cubanes: a DFT and experimental study

In order to get insight into transmission mechanisms of the title spin–spin coupling constants, nJ(C, F1) couplings (n = 4, 5) were calculated at the DFT-B3LYP-6-311-G**/EPR-III level, and couplings were measured in 9 members of the series 1-F,4-X-cubanes, where the α atom of the X group is a carbon atom. Hyperconjugative interactions were evaluated within the NBO approach, using the same level of theory as that employed for calculating spin–spin coupling constants. The unusual 4J(C4,F1) spin–spin coupling constants known in 1-F,4-X-cubanes are rationalized in this work as originating mainly due to two factors, namely, (i) the strong σ-hyperconjugative interactions involving as donors the cage C–C bonds; (ii) the particular arrangement of cage bonds not involving either the C1 or the C4 carbon atoms, C i and C j . For linear X substituents, the direction F1–C1··· C4–X is a three-fold symmetry axis and there are six C i –C j equivalent cage bonds, which are involved in (C i –C  j )→(C1–F1)* and (C i –C  j )→(C4–X)* hyperconjugative interactions. This means that coupling pathways involving such interactions are amplified six times, rendering the substrate very efficient for transmitting ‘trans-cage’ coupling constants. The large halogen substituent effects observed for 4J(C4,F1) spin–spin coupling constants in 1-F,4-X-cubanes (X = halogen atom) are rationalized in terms of interactions between σ-hyperconjugative (involving C–C bonds) and negative hyperconjugative interactions involving the halogen lone-pairs.

Application of 13C nmr spectroscopy to the elucidation of the electrochemical oxidation mechanism of the l-ascorbic and d-araboascorbic acids

Abstract The previously proposed electrochemical oxidation mechanism of the l -ascorbic and d -araboascorbic acids in basic medium must be rejected because of the variation in the carbon chemical shifts and coupling constants with the pH, which shows that the supposed rupture of the furanose rings at pH ∼ 9 does not occur. Electrolysis in basic medium yields the same products as those obtained when the pH of the products of the electrolysis in acid medium are raised to pH ∼ 11. This suggests that the oxidation mechanisms in acid and basic media are similar. The assigned carbon chemical shifts of the oxidation products in acid medium 3 (or 4 ) and in basic medium 5 (or 6 ) and 7 (or 8 ) are reported.

Approximating correlation effects in multiconfigurational self-consistent field calculations of spin-spin coupling constants

The multiconfigurational self-consistent field (MCSCF) method in their approximations restricted and complete active spaces (RAS and CAS) provides a theoretically accurate description of the coupling constants of a wide range of molecules. To obtain accurate results, however, very large basis sets and large configuration spaces must be used. Nuclear magnetic resonance coupling constants for the equilibrium geometry have been calculated for a series of small molecules using approximated correlation contributions. The four contributions to the coupling constants (Fermi contact, spin dipolar, orbital paramagnetic, and orbital diamagnetic) have been calculated at the CAS and RAS MCSCF and second-order polarization propagator approximation levels using a large basis set. An additive model that considers the effect on the coupling constants from excitation of more than two electrons and from core-electron correlation is used to estimate the coupling constants. Compared with the experimental couplings, the best calculated values, which correspond to the MCSCF results, present a mean absolute error of 3.6 Hz and a maximum absolute deviation of 13.4 Hz. A detailed analysis of the different contributions and of the effects of the additive contributions on the coupling constants is carried out.

Prediction of vicinal proton–proton coupling constants 3 J HH from density functional theory calculations

Vicinal coupling constants 3 J HH have been calculated at the optimized geometries for a series of selected molecules with the aim of developing a practical procedure for predicting this kind of coupling. Calculations of couplings and optimizations of molecular geometries have been carried out at the DFT/B3LYP level using a moderate sized basis set. When the Fermi contact contributions to 3 J HH calculated for 25 mono- and 23 1,1-di-substituted ethanes are multiplied by a factor of 0.904, the corresponding predicted couplings J pre are in good agreement with the experimental J exp couplings, with standard deviation σ of 0.10 Hz. When such a comparison is carried out for the remaining sets of molecules the σ deviation increases to 0.26 Hz for a dataset of 21 couplings from 11 monosubstituted cyclohexanes, to 0.19 Hz for a dataset of 40 couplings from 6 norbornane type molecules and to 0.25 Hz for a dataset of 54 couplings from 14 three-membered rings. For the complete dataset of 163 couplings the σ deviation amounts to 0.20 Hz. This figure is further reduced to 0.17 Hz by adding to the J pre coupling a small correction given by the term −0.15cosϕ, depending on the dihedral angle ϕ between the coupled protons. A larger σ deviation of 0.31 Hz was reported for the best empirically parameterized extended Karplus equation. DFT J pre values could be further improved by more accurate calculations for the pertinent substituted ethane constituents of the molecule in question by applying a substituent effect model.

Vicinal proton–proton coupling constants: MCSCF ab initio calculations of ethane

Abstract Ab initio Hartree–Fock self consistent field and multiconfigurational self consistent field calculations have been carried out to study the dihedral angle dependence of the vicinal proton–proton coupling constant in an ethane molecule. The four contributions to 3 J HH , Fermi contact, spin dipolar, orbital paramagnetic and orbital diamagnetic, have been computed with five different basis sets, three of them rebuilt specifically to calculate coupling constants. The importance of the noncontact contributions is small and the correlation effect on these is practically null. The calculated Karplus equation reproduces the experimental equation for 3 J HH with a maximum deviation of 1.0 Hz when the proton–proton torsional angle is 180°. Our best average 〈 3 J HH 〉 coupling constant calculated using the experimental geometry (7.77 Hz) is in good agreement with the experimental value (8.02 Hz).

Vicinal proton—proton coupling constants. Basis set dependence in SCF ab initio calculations

Abstract An SCF ab initio study of the angular dependence and substituent effects upon the vicinal coupling constants has been carried out for the molecules CH 3 CH 3 , CH 2 FCH 3 and CHF 2 CH 3 . The four contributions to 3 J HH ( J FC , J SD , J OD and J OP ) have been computed using the STO-3G, 6–31G, 6–31G * and 6–31G ** basis sets. The major contributions arise from the FC term. The magnitude of the SD contributions is very small and near independent of the size of the basis set. The magnitude of the orbital contributions OR (=OD+OP) decreases as the size of the basis set increases. The FC term slightly overestimates both the individual and the interaction substituent effects for basis sets larger than the STO-3G one. For this basis such effects are underestimated.

NMR J (C,C) scalar coupling analysis of the effects of substituents on the keto-enol tautomeric equilibrium in 2-OH- n -X-pyridines. An experimental and DFT study

In order to study the effect of substituents on the preferential keto/enol forms of six mono-substituted 2-OH-pyridines, the energies corresponding to their DFT-optimized structures for both tautomeric forms were compared. To obtain an idea of how solvent dielectric effects affect such a tautomeric equilibrium, geometry optimizations were performed considering both an isolated molecule and an infinitely diluted dimethylsulfoxide solution (ε = 46.7). It was found that, for all these compounds, the dielectric solvent effect tends to increase the presence of the keto form. NMR 1 J(13C,13C) and 3 J(13C,13C) isotropic couplings were measured for the same mono-substituted 2-OH-pyridines as well as for the corresponding mono-substituted pyridines. These isotropic coupling constants were also calculated within the DFT framework, where all four isotropic contributions, Fermi contact, spin dipolar, paramagnetic spin–orbit and diamagnetic spin–orbit, were taken into account. For the mono-substituted-2-OH-pyridines studied in this work, coupling constants were calculated using both the optimized keto and enol forms. The possible use of these couplings as probes to detect the keto and enol forms is discussed.

Vicinal carbon-proton coupling constants. Angular dependence and fluorine substituent effects

Abstract Data sets of vicinal carbon-proton coupling constants 3JCH have been calculated for propane, 1-fluoropropane and 2-fluoropropane by means of SCF ab initio methods using various standard Gaussian-type basis sets and a double zeta basis set [ 4s2p1d 2s1p ] with additional tight s functions. The major contribution to 3JCH arises from the Fermi contact term. The remaining contributions, spin dipolar, orbital paramagnetic and orbital diamagnetic, are very small. A satisfactory agreement with experimental data is obtained after multiplying the best calculated values by a factor close to 0.8. The dependence of the calculated 3JCH couplings upon the torsion angles φ( 13 C α C β C γ H) , between the coupled nuclei, and φα(X13CαCβCγ), between a substituent X at Cα and the Cγ carbon, as well as the effect of a fluorine substituent bonded to the carbons Cα, Cβ, or Cγ, have been analysed using equations formulated from a substituent effect model. The two-dimensional angular dependence on φ and φα for the 3JCH coupling of propane is described by an equation including 11 terms with a r.m.s. deviation σ of 0.13 Hz. Likewise, the effect upon 3JCH of a fluorine attached to Cα depends on both angles φ and φα. This effect is described by an equation of five terms with a σ of 0.31 Hz. On the other hand, the effects upon 3JCH of a fluorine bonded to Cβ or Cγ depend mainly on the angle φ and are described by equations including, respectively, four and seven terms. The corresponding σ deviations for these equations are 0.13 and 0.28 Hz. The experimental effects upon 3JCH of a substituent OH are satisfactorily reproduced by these equations when the different electronegativity of oxygen and fluorine is taken into account.