Maria P. Galache

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.

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

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.

The r α structure of oxetane as studied by NMR spectroscopy in a nematic medium

The proton and carbon-13 NMR spectra of oxetane partially orientated in a nematic solvent was analysed, and the dipolar coupling constants Dij thus determined were used to obtain information about the r α structure of the molecule. It is concluded that the oxetane molecule oscillates between two equivalent bent conformers characterized by a puckering amplitude of 0.128(5) A. The optimized values for the remaining geometric parameters are: r CHα = 1·094(1) A, r CHβ = 1·082(1) A, HCαH = 109·9(1)°, HCβH = 110·2(1)°, and -4·8(5)° for the bending coordinate. The puckering potential function for oxetane cannot be determined because the surface σ(V 2, V 2) for the RMS deviations between observed and calculated Dij values shows a valley of minimum deviation instead of local minima.

Methylene group deformational coordinates in four-membered rings

Abstract The dependence of methylene group deformational coordinates upon the puckering amplitude q has been analyzed for a set of ab initio geometries of the four-membered ring (CH 2 ) 3 X. For small values of the puckering amplitude, the rocking and twisting deformational coordinates are proportional to q , whereas for q > 0.3 A, the inclusion of a cubic term in q is necessary in order to describe the dependence of those deformational coordinates with the puckering amplitude. The wagging deformational coordinate shows a linear correlation with the square of q , although the effect is so weak that, for small variations in q , the wagging coordinate can be considered constant.

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.

Proton and carbon-13 NMR study of partially oriented 1,4-dioxane Determination of the r α structure and of bond contributions to the orientation

The proton and carbon-13 NMR spectra of 1,4-dioxane partially oriented in a nematic solvent were analysed, and the measured dipolar coupling constants, corrected for harmonic vibrations and for correlated deformation, were used to obtain information about the r α structure of the molecule. It is concluded that the 1,4-dioxane molecule oscillates between two equivalent chair conformers, which can be described by a puckering amplitude of 0·55 A. This study also indicates that each methylene group is bent away from the bisector of the ring angle towards the heteroatom. Finally, the molecular orientation of this molecule in N-(p-ethoxybenzylidene)-p′-n-butylaniline can be described by three bond interaction parameters: Xcc , X v, X CH (or X CO), corresponding to the CC bonds, the virtual C2C6 and C3C5 bonds and the CH (or CO) bonds.

The structure and ring puckering vibrations of thietane from its dipolar couplings

The r α structure of thietane has been obtained from its previously measured dipolar couplings. The molecular geometry can be described by a model of hindered pseudorotation characterized by a double minimum potential function. The optimized values for the geometric parameters are: r CH = 1·077 A, HCαH = 109·7°, HCβH = 108·8° and -6·8° for the bending coordinate. The puckering amplitude q corresponding to the potential minimum is 0·24A, but the height of the inversion barrier (δV) cannot be determined because the surface σ(q, δV) for the root mean square (RMS) deviations between observed and calculated dipolar couplings shows a valley of minimum deviation instead of local minima.

The r α structure of cyclobutane from its dipolar couplings

The molecular structure of cyclobutane has been analysed from its previously measured dipolar couplings. Assuming that the cyclobutane molecule oscillates between two equivalent bent conformers, the C-H bond length decreases from an anomalous value of 1·144 A to 1·097 A when harmonic vibration corrections are applied to observed dipolar couplings. The optimized values for the remaining geometric parameters are: 0·26 A for the puckering amplitude, 109.8° for the HCH bond angle and 3.5° for the methylene rocking coordinate. The inversion barrier (ΔV) value cannot be determined because the RMS deviation between observed and calculated direct couplings remains practically unaltered for ΔV values greater than 4 kJ mol-1.

NMR study of tetrahydrofuran oriented in a nematic phase

Abstract The H-1 NMR spectrum of tetrahydrofuran (THF) dissolved in the nematic phase IV (Merck) has been analysed by used of a computer program which incorporates symmetry. A satisfactory fit for the spectrum could be obtained under the assumption that the spin Hamiltonian has effective C(2v)symmetry. Comparison of the empirical values of the direct dipolar couplings with the predictions of different theoretical models shows that agreement is better for free pseudorotation than for interconversion between two equivalent conformation.