Table of Contents

Abstract: n/a

Abstract: The present review summarizes the information available on the ab initio calculations of spin-spin nuclear coupling constants through hydrogen bonds or in van der Waals complexes. It also reports the sources of experimental data on n^hJ_XY scalar couplings.

Abstract: The known solvent dependence of ¹J(C_c,H_f) and ²J(C₁,H_f) couplings in acetaldehyde is studied from a theoretical viewpoint based on the density functional theory approach where the dielectric solvent effect is taken into account with the polarizable continuum model. The four terms of scalar couplings, Fermi contact, paramagnetic spin orbital, diamagnetic spin orbital and spin dipolar, are calculated but the solvent effect analysis is restricted to the first term since for both couplings it is by far the dominant contribution. Experimental trends of Δ¹J(C_c,H_f) and Δ²J(C₁,H_f) Vs ε (the solvent dielectric constant) are correctly reproduced although they are somewhat underestimated. Specific interactions between solute and solvent molecules are studied for dimethylsulfoxide, DMSO, solutions considering two different one-to-one molecular complexes between acetaldehyde and DMSO. They are determined by interactions of type C=O---H---C and S=O---H---C, and the effects of such interactions on ¹J(C_c,H_f) and ²J(C₁,H_f) couplings are analyzed. Even though only in a semiquantitative way, it is shown that the effect of such interactions on the solvent effects, of Δ¹J(C_c,H_f) and Δ²J(C₁,H_f), tend to improve the agreement between calculated and experimental values. These results seem to indicate that a continuum dielectric model has not enough flexibility for describing quantitatively solvent effects on spin-spin couplings. Apparently, even for relatively weak hydrogen bonding, the contribution from “direct” interactions is of the same order of magnitude as the “dielectric” effect.

Abstract: Complete analysis of ¹H-NMR spectra of trans-1,2-dichlorocyclopentane and trans-1,2-dibromocyclopentane was performed with use of our total lineshape fitting algorithm VALISA. The resulting high precision spin-spin coupling constants were then applied to the problem of conformational analysis, yielding a continuos potential of pseudorotation for the studied compounds in CDCl₃, CCl₄, and CD₃CN solutions.

Abstract: We present an extension of the Polarizable Continuum Model (PCM) to the calculation of solvent effects on indirect spin–spin coupling constants for Hartree–Fock wave functions and Density Functional Theory. This is achieved by implementing the PCM model for singlet and triplet linear response functions. The new code is used for calculating the solvent effects on the indirect spin–spin coupling constants of benzene. For the ¹J(H¹³C) coupling constants, our calculated solvent shifts are in good agreement with experimental observations when geometry relaxation is taken into account. However, our results do not support the extrapolated gas-phase value for this coupling constant. A new experimentally derived ¹J(H ¹³C) for a vibrating benzene molecule at 300 K is proposed.

Abstract: Recent results of experimental spin-spin coupling constants are reviewed and their relation to ab initio calculations is discussed. It is shown that the NMR measurements of spin-spin coupling are density dependent in the gas phase. The extrapolation to the zerodensity limit is required in order to obtain the J_o coupling constants which are free from intermolecular interactions. Such coupling constants can be used as the experimental standards for any comparison with the results of appropriate calculations. It is also pointed out that the effects of the rotational and vibrational motion of nuclei in a molecule can be estimated completely only by theoretical methods.

Abstract: The spin–spin coupling constants in ethane, methylamine, and methanol have been calculated using density-functional theory (DFT), coupled-cluster singlesand-doubles (CCSD) theory, and multiconfigurational self-consistent field (MCSCF) theory so as to benchmark the performance of DFT against high-level ab initio methods and experimental data. For each molecule, the Karplus curve has been evaluated at the three computational levels. The comparisons with ab initio methods indicate that DFT reproduces the ¹J(CH), ¹J(CC), and ¹J(NH) one-bond couplings well but is less accurate for ¹J(CN), ¹J(OH), and ¹J(CO). While DFT performs well for the geminal couplings ²J(HH) and ²J(CH), it tends to overestimate the vicinal ³J(HH) couplings slightly although it is sufficiently accurate for most purposes.