Experimentally and theoretically (ab initio) determined CC spin−spin coupling tensors and 1H and 13C nuclear shielding tensors are reported for ethane (13C2H6), ethene (13C2H4), and ethyne (13C2H2). The experimental anisotropies of the CC coupling tensors, ΔJ CC, for all these molecules, and also the combination J CC, xx − J CC, yy for ethene, were derived from sets of anisotropic couplings (D exp) analyzed from the 1H and 13C NMR spectra of molecules partially oriented in liquid−crystalline environments. Both harmonic vibrations and structural deformations arising from the correlation of vibrational and reorientational motions were taken into account in the D couplings. The ab initio calculations of all the J tensors were performed using MCSCF linear response theory. The best calculated and experimental ΔJ CC values (along with J CC, xx − J CC, yy for ethene) are found to be in good mutual agreement. Together with earlier work on the n J CC tensors in benzene, this study shows that the indirect contribution, 1/2 J CC aniso, to experimental couplings between differently hybridized carbons is small and can generally be omitted. This means that the use of experimental D CC couplings in the determination of molecular order tensors and/or conformation does not introduce serious errors. The experimental determination of the 1H and 13C shielding tensors was based on the liquid crystal director rotation by 90° in mixtures of thermotropic nematogens with opposite anisotropy of diamagnetic susceptibility. Ab initio SCF and MCSCF calculations utilizing gauge-including atomic orbitals produce results in good agreement with experiments.

This study reports experimentally and theoretically (ab initio) determined indirect CC spin−spin coupling tensors n J CC in benzene. The CC spin−spin coupling constants n J CC between the ortho, meta, and para (n = 1, 2, and 3) positioned carbons were experimentally determined in two ways:  firstly by utilizing the 2H/1H isotope effect on the carbon shieldings in neat monodeuteriobenzene and recording the 13C satellite spectrum in a 1H-decoupled 13C NMR spectrum, and secondly by recording the 13C NMR spectrum of fully 13C-enriched benzene (13C6H6) and carrying out its complete analysis. The anisotropies of the corresponding coupling tensors, Δ n J CC, were resolved experimentally by liquid crystal 1H and 13C NMR using dipolar couplings corrected for both harmonic vibrations and deformations. The results obtained in three thermotropic liquid crystal solvents are in good mutual agreement, indicating the reliability of the determinations. The anisotropy of the ortho, meta, and para CC indirect couplings are ca. +17, −4, and +9 Hz, respectively. Also, the signs of the coupling constants are unambiguously determined. The ab initio calculations were performed using multiconfiguration self-consistent field linear response theory with both single-reference and multireference wave functions. The results confirm the signs of the experimental anisotropies in all cases. The magnitude of the ortho coupling anisotropy is excellently reproduced, but the anisotropies are somewhat overestimated in the two other theoretical coupling tensors. The importance of the different physical contributions to the couplings and anisotropies is discussed.

The tensors corresponding to the second-rank NMR observables, nuclear shielding, quadrupole coupling, and spin−spin coupling of formamide (HCONH2, FA) were determined using several first principles quantum chemical methods. The changes induced on the shielding and quadrupole coupling tensors by intermolecular hydrogen bonding were examined computationally. Liquid crystal NMR experiments were performed for dissolved FA in the SDS and CTAB lyotropic mesophases and their isotropic phases and in the gas phase. We report experimental data on shielding, quadrupole coupling, and spin−spin coupling constants. The convergence of the calculations with the basis set completeness and the treatment of electron correlation were investigated. The calculated and experimental data on the anisotropic properties of the C, N, and O shielding tensors are found to be in good agreement, given the large error limits of the latter caused by the low degree of order of FA in these systems. The medium effects on the observables are found to be readily understood by comparison of structurally relaxed FA monomer and chain trimer calculations. The calculated spin−spin coupling constants are in good agreement with the experimental ones. The anisotropic properties of the corresponding tensors are calculated to be small enough to prevent experimental detection and not to disturb structure determinations by using experimental dipolar couplings. The principal components and the orientation of the principal axis systems of each of the NMR tensors are specified.

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