Angel L. Esteban

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.

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.

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.

Spin–spin coupling constants in ethylene: equilibrium values

Abstract The nuclear magnetic resonance spin–spin coupling constants for the ethylene have been calculated at the multiconfigurational self-consistent field (MCSCF) method using the restricted (RAS) and complete (CAS) active space approximations. An analysis of the calculated values and of the effects of different factors (active spaces, valence and core–electron correlation) is presented. The effect of increase the number of active orbitals and that of triple and quadruple excitations is important and opposite to the effect of core–electron correlation. The best calculated values for 1 J CC (68.83 Hz), 1 J CH (151.56), 2 J CH (−1.56 Hz), 2 J HH (1.07 Hz) 3 J HH (cis) (11.47 Hz), and 3 J HH (trans) (17.78 Hz) are in good agreement with the best calculated and experimental values.

Conformational effects on 13C NMR parameters in alkyl formates

The 13C NMR spectra of a series of alkyl formates which show the coexistence of s‐cis and s‐trans rotamers at room temperature were measured at 125 MHz. 13C chemical shifts and 1J(CH) couplings are compared for both types of rotamers. Their differences are rationalized in terms of different intramolecular interactions. The 17O NMR spectra of these compounds could be observed only for the most abundant rotamer, which in all cases was identified as the s‐cis rotamer. In the three members of this series with the shortest alkyl chains, a 2J(17O,1H) coupling constant of ca. 40 Hz was observed. In ethyl formate, 13C magnetic shielding constants were calculated using the LORG approach with ab initio optimized geometries in both types of rotamers for the carbon atom β to the dicoordinated oxygen atom. A comparison between the calculated and experimental values yields support for the rationalizations quoted above.

Intramolecular electric field effect on a 1J(C,H) NMR spin–spin coupling constant. an experimental and theoretical study

An experimental study of the effects of intramolecular electric fields on 1J(C,H) coupling constants involving a formyl proton in a series of 5‐X‐salicylaldehydes is reported. The particularly large electric field component along the CcHf bond, calculated for X=NO2 , leads to a measured substituent effect of 7.36 Hz. For other X substituents a linear correlation was found between the calculated substituent electric field component along the Cc—Hf bond and the substituent effect on 1J(Cc ,Hf). To determine if 5‐X substituents affect the intramolecular CO· · ·HO hydrogen bond, NBO analyses were carried out on 5‐NO2 − and 5‐H‐salicylaldehyde, and they were compared with those corresponding to benzaldehyde and phenol. The effects of interactions other than the electrostatic one on 1J(Cc ,Hf) couplings were ruled out. The experimental results are in agreement with theoretical predictions published previously. Copyright

Carbonyl 17O Chemical Shift in the Proximity of a Methyl Group in Amides: an Experimental and Theoretical Study

The effect of a cis‐N‐methyl group on the carbonyl 17O chemical shift, cis‐MSCS, was investigated both from theoretical and experimental points of view in ten amide derivatives. Experimentally, it was observed that the cis‐MSCS in N‐methylformamide (2) corresponds to a shielding effect of 12.0 ppm with respect to formamide (1). LORG calculations at both the 6–31G* and 6–311G** levels reproduced fairly well this trend, i.e. 10.2 and 11.4 ppm, respectively, provided that as the N‐methyl group conformation was such that a C—H bond eclipsed the C—N bond (2a). This is the preferential conformation at the 6–31G*/MP2 level. For other methyl group conformations the LORG calculations did not reproduce that experimental trend. For instance, for an N‐methyl C—H bond eclipsing the N—H bond (2b), deshielding cis‐MSCSs of 3.7 ppm (6–31G*) and 3.6 ppm (6–311G**) were predicted. Analyses of LORG bond–bond contributions suggested that the interaction that defines 2a as the preferential conformation is an attractive interaction between the in‐plane N‐methyl C—H bond and the carbonyl oxygen lone pairs. Experimental trends observed for the 17O chemical shifts measured in the remaining compounds can be rationalized on the same grounds.

Dependence of the dipolar couplings of tetrahydrofuran on the pseudorotation parameters

The dipolar couplings Dij previously measured for tetrahydrofuran (THF) in nematic Phase IV have been analysed in order to obtain information about the puckering amplitude and pseudorotation potential of this molecule. The effective ordering matrix (EOM) method and the ELS theoretical model have been applied by using a hindered pseudorotation potential of type V 2 cos 2φ + V 4 cos 4φ. The optimized value for the equilibrium puckering amplitude of 0·39 A is close to the value of 0·38 A determined by electron diffraction. The actual pseudorotation potential for THF cannot be determined because the surfaces σ(V 2, V 4) for the rms deviations between observed and calculated Dij values show flat valleys instead of local minima. Notwithstanding, the results obtained by means of the ELS model, which allows for the dependence of orientational ordering on molecular shape, are more consistent with the spectroscopic result of almost free pseudorotation for the THF than those provided by the EOM method.