Jesús San Fabián

Influence of Density Functionals and Basis Sets on One-Bond Carbon-Carbon NMR Spin-Spin Coupling Constants.

The basis set and the functional dependence of one-bond carbon-carbon NMR spin-spin coupling constants (SSCC) have been analyzed using density functional theory. Four basis sets (6-311G**, TZVP, EPR-III, and aug-cc-pVTZ-J) and four functionals (PBE, PW91, B3LYP, and B3P86) are tested by comparison with 70 experimental values corresponding to 49 molecules that represent multiple types of hybridization of the carbon atoms. The two hybrid functionals B3P86 and B3LYP combined either EPR-III or aug-cc-pVTZ-J basis sets lead to the best accuracy of calculated SSCC. However, a simple linear regression allows for the obtaining of scaled coupling constants that fit much better with the experimental data and where the differences between the different basis sets and/or functional results are significantly reduced. For large molecules the TZVP basis set can be an appropriate election presenting a good compromise between quality of results and computational cost.

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.

On the unusual 2J C 2H f coupling dependence on syn/anti CHO conformation in 5‐X‐furan‐2‐carboxaldehydes

A remarkable difference for 2J  C 2H f coupling constant in syn and anti conformers of 5‐X‐furan‐2‐carboxaldehydes (X = CH3, Ph, NO2, Br) and a rationalization of this difference are reported. On the basis of the current knowledge of the Fermi‐contact term transmission, a rather unusual dual‐coupling pathway in the syn conformer is presented. The additional coupling pathway resembles somewhat that of the JHH in homoallylic couplings, which are transmitted by hyperconjugative interactions involving the πCC electronic system. The homoallylic coupling pathway can be labeled as σ*CH ← πCC → σ*CH. In the present case, this additional coupling pathway, using an analogous notation, can be labeled as σ*  C 2C C ← LP1(O1)··· LP2(OC) →σ*  C CH f (σ*  C 2C C ) where O1 and OC stand for the ring and carbonyl O atoms, respectively. This additional coupling pathway is not activated in the anti conformers since both oxygen lone pairs do not overlap. Copyright

Improvements in DFT Calculations of Spin–Spin Coupling Constants

Different types of spin-spin coupling constants (SSCCs) for several representative small molecules are evaluated and analyzed using a combination of 10 exchange functionals with 12 correlation functionals. For comparison, calculations performed using MCSCF, SOPPA, other common DFT methods, and also experimental data are considered. A detailed study of the percentage of Hartree-Fock exchange energy in SSCCs and in its four contributions is carried out. From the above analysis, a combined functional formed with local Slater (34%), Hartree-Fock exchange (66%), and P86 correlation functional (S66P86) is proposed in this paper. The accuracy of the values obtained with this hybrid functional (mean absolute deviation of 4.5 Hz) is similar to that of the SOPPA method (mean absolute deviation of 4.6 Hz).

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.

Vibrational contributions to vicinal proton-proton coupling constants3JHH

Vibrational contributions to the couplings of six mono- and five 1,1-di-substituted ethanes, three mono-substituted cyclohexanes, three norbornane-type molecules, and 11 three-membered rings have been calculated at the DFT/B3LYP level for the Fermi contact term using a moderate sized basis set. When, for a data set of 70 couplings, the sums of the values for the equilibrium configurations J e and the respective vibrational contributions at 300 K are multiplied by a factor of 0.8485, the corresponding predicted couplings are in good agreement with the experimental couplings with a standard deviation σ of 0.18 Hz. The same σ results when values are obtained by multiplying the J e values by 0.9016. However, the vibrational contributions must be taken into account, together with the J e values, in order to achieve a procedure for a reliable and accurate prediction of couplings since, globally, contributions amount to about 7% of the J e values and the correlation coefficient between and J e is only 0.68 with a σ deviation of 0.20. The first and diagonal second derivatives of J with respect to each normal coordinate Qk , required to estimate the vibrational contributions, have been obtained from six Jk values computed for molecular geometries positively and negatively displaced from the equilibrium geometry along the normal coordinate Qk and using for δ the values 0.01, 0.05 and 0.10. The computational precision of the results obtained when using one, two and three δ values is analysed.

Natural bond orbital/natural J-coupling study of vicinal couplings.

AbstractNBO-NJC decomposition of vicinal 3JHH spin-spin coupling constants into Lewis, delocalization, and repolarization contributions are presented. A deep study allows to assign the main contributions to specific orbitals or electron delocalizations between two orbitals. 3JHH torsional dependence and the substituent effect are analyzed according to the main orbital contributions for ethane and fluoroethane molecules using different basis sets. The torsional dependence for the energies corresponding to electron delocalization is also studied. FigureNBO-NJC decomposition of vicinal 3JHH spin-spin coupling constants into Lewis, delocalization, and repolarization contributions together with those attributed to specific orbitals or electron delocalizations between two orbitals are presented. Torsional dependence and the substituent effect are analyzed according to the main orbital contributions for ethane and fluoroethane molecules.

Intramolecular surfaces for vicinal proton–proton coupling constants 3JHH

Equations for the intramolecular surfaces of the 3JHH coupling constants in ethane, ethylene, and acetylene are formulated, and the corresponding coefficients are estimated from calculations at the DFT/B3LYP level. The chosen variables are changes in bond lengths, in the torsion angle φ between the coupled protons Ha and Hb, in bond angles, and in dihedral angles. The 3JHH surface of ethane is formulated as an extended Karplus equation with the coefficients of a truncated Fourier series on the torsion angle φ expanded as second-order Taylor series in the chosen variables taking into account the invariance of 3JHH under reflections and rotations of nuclear coordinates. Partial vibrational contributions from linear and square terms corresponding to changes in the geometry of the Ha − Ca − Cb − Hb fragment are important while those from cross terms are small with a few exceptions. The 3JHH surface of ethane is useful to predict contributions to 3JHH from changes in local geometry of derivatives but vibrational contributions are predicted less satisfactorily. The predicted values at the B3LYP/BS2 level of the 3JHH couplings (vibrational contributions at 300 K) from equilibrium geometries are 9.79 (−0.17) for acetylene, and 17.08 (1.93) and 10.73(0.93) for the trans and cis couplings of ethylene.