Rubén H. Contreras

Advances in Theoretical and Physical Aspects of Spin–Spin Coupling Constants

Publisher Summary This chapter discusses advances in the theoretical and physical aspects of spin–spin coupling constants. From both theoretical and experimental points of view, the analysis of high-resolution NMR parameters is an important problem, and its significance for the understanding of molecular electronic structure can hardly be stressed enough. When both approaches are taken together, an excellent example of the Born–Opperheimer approximation is obtained because experimentalists measure transitions among nuclear states that are modified by the interactions among magnetic nuclei, external magnetic field, and electrons, while theoreticians study the way the electronic wave function is modified owing to those interactions. The empirical Hamiltonian describes the way nuclear spin energy levels are modified both by the static magnetic field provided by a spectrometer and by the interactions between magnetic nuclei and electrons belonging to a given molecule.

Recent Advances in Theoretical Calculations of Indirect Spin–Spin Coupling Constants

Recent trends in theoretical calculations of indirect spin–spin coupling constants covering the period of May 2003–July 2006 are outlined with the emphasis on the currently used wavefunction- and DFT-based methods. Computational results dealing with the different spin–spin coupling constants in several representative inorganic, organic, and biomolecular species obtained within the ab initio and DFT frameworks are compiled and critically discussed to elucidate some illustrative structural trends and their practical applications.

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.

Proximity effects on nuclear spin–spin coupling constants. Part 2.—The electric field effect on 1J(CH) couplings

The electric field effect on 1J(CH) for C(sp3)—H and C(sp)—H bonds is studied by means of the IPPP-CLOPPA method at an ab initio level. Opposite trends for the dominant Fermi contact, FC, term in both types of bonds are found. It is shown that these opposite trends originate in the π-electronic system associated with the carbon atom in C(sp)—H bonds. In fact, while the σ-transmitted component of the FC term behaves like the FC term in a C(sp3)—H bond, the π-transmitted component shows an opposite electric field effect. Results are compared with the proximity effects that an atom bearing lone pairs exerts on these couplings, which were studied in Part 1 of this series of papers. The influence of an electric field on the bond lengths of these two types of bonds is also analysed.

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.

1hJFH coupling in 2-fluorophenol revisited: Is intramolecular hydrogen bond responsible for this long-range coupling?

The present study shows that a hydrogen bond between the OH group and the fluorine atom is not involved in the 1hJFH spin–spin coupling transmission either for 4‐bromo‐2‐fluorophenol or 2‐fluorophenol. In fact, according to a quantum theory of atoms in molecules analysis, no bond critical point is found between O‐H and F moieties. The nature of the transmission mechanism of the Fermi contact term of the 1hJFH spin–spin coupling is studied by analyzing canonical molecular orbitals (see J. Phys. Chem. A 2010, 114, 1044), and it is observed that virtual orbitals play only a quite minor role in its transmission. This is typical of a Fermi contact term transmitted mainly through exchange interactions owing to the overlap of proximate electronic clouds; therefore, it is suggested to identify them as nTSJFH coupling where n stands for the number of formal bonds separating the coupling nuclei. In the cases studied in this work is n = 4. Results presented in this work could provide an interesting rationalization for different experimental signs known in the current literature for proximate JFH couplings. Copyright

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.

The through-space transmission of 13C19F coupling constants via an intermediate bond: A polarization propagator analysis

Abstract Inner projections of the polarization propagator were used to study the indirect through-space transmission of the Fermi contact term of CF couplings via a CX (X = F, H) intermediate bond. When X = H a large capability for transmitting this component is observed. But for X = F a notable inhibition of this transmission mechanism is observed. The role of the fluorine lone pairs in this behaviour is discussed.

The use of partially restricted molecular orbitals to investigate transmission mechanisms of spin-spin coupling constants. II. the σ and π contributions to the Fermi contact, orbital, and spin-dipolar terms of SCPT-INDO CC coupling constants

Abstract Partially restricted molecular orbital calculations are carried out within the SCPT-INDO method in order to separate σ and π-transmitted components of the Fermi contact, spin-dipolar, and orbital terms of CC coupling constants in several unsaturated hydrocarbons. Very well-defined trends are found for both components of all three interactions. In particular, those found for the π component of the Fermi contact interaction, are in good agreement with those determined empirically, at least in a qualitative sense. The possibility that the SCPT-INDO method can cope better with π-transmitted rather than σ-transmitted components is discussed. Substituent effects for one-bond CC couplings in monosubstituted benzenes are discussed.

The use of partially restricted molecular orbitals to investigate transmission mechanisms of spin-spin coupling constants. I. The? and? contributions within the FPT INDO method

Partially restricted INDO MO Calculations have been carried out to separate the π-electron contributions to spin-spin coupling constants in ethylene, butadiene, benzene and toluene. Results reproduce very well known trends such as the pathway invariance, the alternation in sign and the methyl group replacement rule.