M. C. Ruiz de Azúa

Relativistic effects on the nuclear magnetic shielding tensor

A new approach for calculating relativistic corrections to the nuclear magnetic shieldings is presented. Starting from a full relativistic second order perturbation theory expression a two-component formalism is constructed by transforming matrix elements using the elimination of small component scheme and separating out the contributions from the no-virtual pair and the virtual pair part of the second order corrections to the energy. In this way we avoid a strong simplification used previously in the literature. We arrive at final expressions for the relativistic corrections which are equivalent to those of Fukui et al. [J. Chem Phys. 105, 3175 (1996)] and at some other additional terms correcting both the paramagnetic and the diamagnetic part of the nuclear magnetic shielding. Results for some relativistic corrections to the shieldings of the heavy and light nuclei in HX and CH3X (X=Br,I) at both random phase and second order polarization propagator approach levels are given.

Relativistic effects on nuclear magnetic shielding constants in HX and CH3X(X=Br, I) based on the linear response within the elimination of small component approach

Numerical calculations of relativistic effects on nuclear magnetic shielding constants sigma corresponding to all one-body operators obtained within a formalism developed in previous work were carried out. In this formalism, the elimination of small component scheme is applied to evaluate all quantities entering a four-component RSPT(2) expression of magnetic molecular properties. HX and CH3X (X=Br,I) were taken as model compounds. Calculations were carried out at the Hartree-Fock level for first-order quantities, and at the random-phase approximation (RPA) level for second- and third-order ones. It was found that values of sigma(X) are largely affected by several relativistic corrections not previously considered in the bibliography. sigma Values of the H nucleus are in close agreement with four-component RPA ones. Overall relativistic effects on the shift of sigma(X) from HX to CH3X are smaller than the nonrelativistic shifts.

Viewpoint 8 — polarization propagator analysis of spin-spin coupling constants

Abstract The analysis of electronic mechanisms that define indirect NMR spin-spin coupling constants can be efficiently carried out by combining the polarization propagator formalism with the use of localized molecular orbitals. This approach is applied to study several trends of one-bond couplings along the Periodic Table when one of the coupled nuclei bears a lone pair. This analysis is preceded by a revision of the fundamentals of the method in which its approximations, shortcomings and capabilities are commented on. A brief overview of results already published in the literature is also given. The relationship between the classical approach of Pople and Santry and the polarization propagator calculation of the Fermi contact term is discussed.

Theoretical and experimental study of the orientational nitrogen lone-pair effect on 1J(13C13C) couplings in acetoxime

Abstract The difference between 1 J cc(Z) and 1 J cc (E) for acetoxime is experimentally and theoretically studied as a nitrogen lone-pair orientational effect. Experimentally, it is observed that this difference almost disappears upon full protonation. Theoretically, the most recent version of the IPPP program is used, finding that the N lone pair contributes to the couplings a positive component for 1 J cc (Z) and a negative one for 1 J cc (E), which are found to be the main cause of this difference.

Quantum chemical analysis of the orientational lone-pair effect on spin-spin coupling constants

Abstract The quantum-chemical analysis of factors defining the stereospecific behaviour of nuclear spin-spin coupling constants is shown to be an important aid in obtaining experimental information on molecular structures from high-resolution NMR spectroscopy. The analyses presented in this paper are based on the contributions from localized molecular orbitais within the polarization propagator approach (CLOPPA) method and examples are given where factors defining heteroatom lone-pair orientational effects on spin-spin coupling constants are discussed.

Analysis of multipath transmission of spin-spin coupling constants in 1-x-bicycloalkanes: Part II. Additivity of coupling pathways

Abstract The multipath additivity of couplings involving the bridgehead carbon atoms in 1-X-bicycloalkanes (X = CH3, CH2OH, CO2H, F) was analysed using the IPPP method. Couplings are decomposed into a through-space component and different through-bond components transmitted by the pathways that join the coupled nuclei. The sum of these components is compared with the total calculated J coupling. The J(C2-C(X)), J(C1-C2) and J(C1-H2) couplings are discussed, where the notation X-Cl · C2-H2 is used to define the bridgehead carbon atoms. Good additivity is found for J(C2-C(X)), but this is not the case for J(C1-C2). The IPPP results for a given C-C pathway are in qualitative agreement with the empirical additivity rules proposed by Klessinger et al. for these couplings. Good multipath additivity was also found for J(C1-H2).

AB INITIO IPPP-CLOPPA APPROACH TO PERFORM BOND CONTRIBUTION ANALYSIS OF NMR COUPLING CONSTANTS: 1J(NH) IN NH3 AS A FUNCTION OF PYRAMIDALITY

Abstract A method designed to analyse transmission mechanisms of NMR indirect spin–spin coupling constants based on the use of localized molecular orbitals and inner-projections of the polarization propagator at the RPA level has been implemented within an ab initio approach. Former semi-empirical versions of this approach proved to be a valuable tool to obtain insight on transmission mechanisms of NMR parameters. The new features and difficulties found in the implementation of this method at an ab initio level are discussed. In particular, a localization procedure is proposed to deal with vacant molecular orbitals. In order to convey an idea about the potentialities of this approach, in this work a bond contribution analysis of the 1 J(NH) coupling in NH 3 as a function of the pyramidality at the N atom is carried out. The influence of the N lone-pair, the different N–H bonds and the N inner-shell electrons in the coupling transmission are discussed, for pyramidal and planar configurations. Trends that are expected to be of general validity are found. Results obtained are in agreement with previous theoretical studies on 1 J couplings as well as with empirical trends deduced from experimentally measured values of 1 J(NH) couplings in different compounds.

Analysis of the diamagnetic spin-orbital contribution to indirect nuclear spin-spin coupling constants

Abstract A general numerical method for the calculation of the diamagnetic spin-orbital (DSO) contribution to the nuclear spin-spin coupling is presented. Cylindrical coordinates are used to carry out the integration of the perturbative Hamiltonian in order to overcome the singularities which are present at the sites of the coupled nuclei. Advantages and shortcomings of the present method with respect to others already published are discussed. The decomposition of the DSO coupling in terms of localized orbitais describing bonds or lone pairs is also proposed in order to predict in simple terms which bonds or lone pairs yield important contributions to the coupling. This method is thus particularly useful in the analysis of the “through-space” transmission of the DSO interaction.

Natural bond orbitals analysis of C–H⋯O interactions in NCH/H2O and NCH/OCH2, and their effect on nuclear magnetic shielding constants☆

Abstract C–H⋯O interactions and their effect on nuclear magnetic shielding constants for differently coordinated oxygen atoms in NCH/H 2 O, I , and NCH/O CH 2 , II , complexes are analyzed for different intermolecular distances. The effects of electrostatic and charge transfer interactions on the C–H bond lengths and on the nuclear magnetic shielding constants of the nuclei directly involved in the C–H⋯O hydrogen bond, are discussed. Charge transfer effects are analyzed employing Natural Bond Orbitals. The intermolecular n→(C–H)* charge transfer interaction causes a slight lengthening of the corresponding C–H bond. The intermolecular charge transfer interaction in II yields an inhibition of the intramolecular n(p)→(C–H)* charge transfer in O CH 2 . A conspicuous difference between the electrostatic and total effects on the 17 O magnetic shielding constant upon formation of the complex is found in I . Experimental data from the literature, supporting the obtained results, are discussed.

Analysis of multipath transmission of spin-spin coupling constants in 1-X-bicycloalkanes: Part I. The through-space transmission via the bridgehead carbon atoms

Abstract The through-space transmission of couplings involving bridgehead carbon atoms in substituted 1-X-bicycloalkanes (X = H, F, CH 3 ) was analysed theoretically with a modified version of the IPPP-INDO method. The through-space components are further decomposed into a sum of individual “coupling pathways”, each of which involves two occupied and two vacant localized orbitals. The relative importance of these orbitals indicates which are the most important bonds and antibonding orbitals defining the transmission mechanisms. In this way, it is shown that the main trends of the through-space component of j (C1-H) (where H is the proton bonded to the remaining bridgehead carbon atom C H ) and J (C1-C H ) are directly related to the amplitude of the C H -H bonding orbital on C1. The decrease in the through-space values when going from bicyclopentane to bicyclooctane is thus explained by the decrease in this amplitude as the C1-C H distance increases. The dependence of J (C1-H) on the substituent X can also be explained by the effect of X on the C H -H bond. The through-space IPPP results are found to be quite different from those obtained using the NNBI approach, showing that both methods are based on different grounds.