J. Guilleme

Basis sets and active space in multiconfigurational self-consistent field calculations of nuclear magnetic resonance spin–spin coupling constants

The dependence of spin–spin nuclear magnetic resonance (NMR) coupling constants on the basis set and electron correlation has been investigated in methane using Hartree–Fock and multiconfigurational self-consistent field wave functions (HF-SCF and MCSCF). The effect of the size, contraction, and tight s functions of the basis sets is analyzed. Some suggestions about the contraction scheme are indicated. MCSCF wave functions with different numbers of active orbitals and different numbers of excited electrons were used. An approximation to determine spin–spin coupling constants at a high level of electron correlation from three calculations with a smaller level of correlation and reduced computational cost is investigated. The best calculated 1JCH and 2JHH couplings are 120.63 and −13.23 Hz, respectively, which are 0.24 and 1.24 Hz smaller than those experimentally obtained for the equilibrium geometry. The remaining error in these coupling constants can be attributed mainly to correlation and not to basis ...

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 ...

MCSCF calculations of NMR spin–spin coupling constant of the HF molecule

The dependence of spin–spin NMR coupling constants on the basis set and electron correlation has been investigated for the hydrogen fluoride using Hartree–Fock (HF-SCF) and multiconfigurational self-consistent field (MCSCF) wave functions. The effect of the size, contraction, and tight s-type, augmented and polarization functions in the basis sets is analyzed. MCSCF wave functions with different number of active orbitals and excited electrons were used within the frozen-core approximation and with all-electron calculations. The correlation effect associated with the core electrons is not negligible. An approximation to determine spin–spin coupling constants at high level of electron correlation and reduced computational cost is applied satisfactorily. The best calculated and estimated 1JFH couplings are 544.20 and 536.63 Hz, respectively, with all electron correlation. Both values agree with the experimental one within the error bars (525±20 Hz).

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.

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.

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.

Vicinal fluorine-proton coupling constants. Ab initio calculations of angular dependence and substituent effects

Abstract A data set of vicinal fluorine-proton coupling constants has been calculated by means of the SCF ab initio and semiempirical INDO/FPT methods. The angular dependence, the effect of individual substituents, and the effect of interaction between two substituents upon the 3 J FH couplings have been studied for the molecules CH 2 FCH 3 , CHF 2 CH 3 , CH 2 FCH 2 F, CF 3 CH 3 , and CHF 2 CH 2 F. The four contributions to 3 J FH ( J FC , J SD , J OD and J OP ) have been computed using the standard basis sets 6-31G, 6-31G ∗ , 6-31G ∗∗ and 6-311G ∗∗ and a double zeta basis set [4s2p1d/2s1p] with additional tight s functions on the H and F. The agreement with the experimental data is better for the last basis set but the trends of the angular dependence and substituent effects are also reproduced by the remaining basis sets. The major contribution arises from the FC term and the remaining contributions are much smaller being the OP the most important. The individual effect of an electronegative substituent depends on the carbon to which is bonded, being more important when the substituent is bonded to the carbon with the coupled hydrogen. The effect of interaction between two substituents seems to be not negligible, reaching values up to 6 Hz. The most important calculated interaction effects are the geminal δC 012 FF , δC 034 FF and δC 134 FF as well as the vicinal δC 213 FF and δC 214 FF .

Molecular reorientation from NMR relaxation times in planar molecules

The spectral density J(0) for the autocorrelation function referred to an arbitrarily chosen body‐fixed coordinate system has been rederived for a planar asymmetric rotor undergoing rotational diffusive motions. The angular dependence of J(0) was found to be more complicated than in the case of coincidence of the body‐fixed frame with the principal axes of rotation. Therefore, the orientation of the principal axes of the rotational diffusion tensor can be determined from four different J(0) or 1/T1 values. This result disagrees with previous conclusions based upon the assumption that the same angular dependence should be expected irrespective of the relative orientation of the diffusion tensor axes. Likewise, the values of the three diffusion coefficients can be in principle determined from more than two (or three) different J(0) values for molecules with (or without) a binary symmetry axis. In practice, however, only two relations between them can usually be obtained due to the uncertainty in T1 measurem...

Methyl carbon chemical shifts of methylbenzoic acids and methylbenzoate anions

The aryl carbon chemical shifts of methylbenzoic acids in CDCl3 and methylbenzoate anions in aqueous NaOH solutions have been assigned. The shifts have been interpreted by means of a model based on the additivity of substituents which is similar to that previously developed for the analysis of the methyl chemical shifts. Corrective parameters included to take into account the interactions of adjacent substituents are found to be related to both the twist angle, θ, of the carboxyl or carboxylate substituents and the conformation of the methyl groups. The derived values of the twist angle are in closer agreement with x‐ray data than those estimated from UV spectra or from Cl‐PPP computations.