The geometry dependence of the spin–spin coupling constants in ethane: a theoretical study

Unusual behaviour of the spin-spin coupling constant 1JCH upon formation of CHX hydrogen bond

One-bond coupling constants ¹J_XY are usually used as a measure of the corresponding XY interatomic distances. However, the physical nature of this correlation is not well understood and, in some cases, a counterintuitive behaviour of ¹J_XY upon hydrogen bonded complex formation has been reported. In this work, the behavior of ¹J_CH upon formation and strengthening of complexes with CHX hydrogen bonds and upon a proton transfer process is investigated by means of ¹H NMR spectroscopy and quantum chemical calculations. ¹H NMR spectra of 1,1-dinitroethane solution at room temperature in various solvents (carbon tetrachloride, chloroform, dichloromethane, acetone, dimethylformamide and dimethyl sulfoxide) illustrate the increase of ¹J_CH by several Hz upon an increase of the complex strength. Computational results (MP2/aug-cc-pVDZ) reproduce this observation and allow one to conclude that the increase of ¹J_CH is mainly caused by the change of the carbon hybridization (an increase of s-character), rather than by the change in interatomic distance r_CH. The behavior of ¹J_CH was also examined computationally for a wide range of CHX hydrogen bond energies and geometries. For this purpose, quantum-chemical modeling of the partial proton transfer process for complexes formed by 1,1-dinitroethane and trinitromethane as hydrogen bond donors with acetone, pyridine and fluoride anion as hydrogen bond acceptors was performed. The obtained results have confirmed the above-mentioned idea - for rather weak complexes, the dominant impact on the change of ¹J_CH magnitude is the increase of the s-character of carbon atom hybridization, while for complexes with a significantly transferred proton, the exponential decrease of the Fermi-contact term dominates.

The Relativistic Effects on the Carbon-Carbon Coupling Constants Mediated by a Heavy Atom

The (2)JCC, (3)JCC, and (4)JCC spin-spin coupling constants in the systems with a heavy atom (Cd, In, Sn, Sb, Te, Hg, Tl, Pb, Bi, and Po) in the coupling path have been calculated by means of density functional theory. The main goal was to estimate the relativistic effects on spin-spin coupling constants and to explore the factors which may influence them, including the nature of the heavy atom and carbon hybridization. The methods applied range, in order of reduced complexity, from the Dirac-Kohn-Sham (DKS) method (density functional theory with four-component Dirac-Coulomb Hamiltonian), through DFT with two- and one-component zeroth-order regular approximation (ZORA) Hamiltonians, to scalar effective core potentials (ECPs) with the nonrelativistic Hamiltonian. The use of DKS and ZORA methods leads to very similar results, and small-core ECPs of the MDF and MWB variety reproduce correctly the scalar relativistic effects. Scalar relativistic effects usually are larger than the spin-orbit coupling effects. The latter tend to influence the most the coupling constants of the sp(3)-hybridized carbon atoms and in compounds of the p-block heavy atoms. Large spin-orbit coupling contributions for the Po compounds are probably connected with the inverse of the lowest triplet excitation energy.

On the truncation of the number of excited states in density functional theory sum-over-states calculations of indirect spin spin coupling constants

It is investigated, whether the number of excited (pseudo)states can be truncated in the sum-over-states expression for indirect spin-spin coupling constants (SSCCs), which is used in the Contributions from Localized Orbitals within the Polarization Propagator Approach and Inner Projections of the Polarization Propagator (IPPP-CLOPPA) approach to analyzing SSCCs in terms of localized orbitals. As a test set we have studied the nine simple compounds, CH4, NH3, H2O, SiH4, PH3, SH2, C2H2, C2H4, and C2H6. The excited (pseudo)states were obtained from time-dependent density functional theory (TD-DFT) calculations with the B3LYP exchange-correlation functional and the specialized core-property basis set, aug-cc-pVTZ-J. We investigated both how the calculated coupling constants depend on the number of (pseudo)states included in the summation and whether the summation can be truncated in a systematic way at a smaller number of states and extrapolated to the total number of (pseudo)states for the given one-electron basis set. We find that this is possible and that for some of the couplings it is sufficient to include only about 30% of the excited (pseudo)states.

Natural bond orbital/natural J-coupling study of vicinal couplings

NBO-NJC decomposition of vicinal (3)J HH spin-spin coupling constants into Lewis, delocalization, and repolarization contributions are presented. A deep study allows to assign the main contributions to specific orbitals or electron delocalizations between two orbitals. (3)J HH torsional dependence and the substituent effect are analyzed according to the main orbital contributions for ethane and fluoroethane molecules using different basis sets. The torsional dependence for the energies corresponding to electron delocalization is also studied.

1JCH couplings in Group 14/IVA tetramethyls from the gas-phase NMR and DFT structural study: a search for the best computational protocol

The gas-phase ¹J_0,CHs in ‘isolated’ molecules of EMe₄ were determined and discussed in terms of their geometric/electronic properties obtained from DFT calculations.

1H NMR spectra of alkane-1,3-diols in benzene: GIAO/DFT shift calculations

The proton nuclear magnetic resonance (NMR) spectra of propane-1,3-diol, 2-methylpropane-1,3-diol, 2,2-dimethylpropane-1,3-diol, butane-1,3-diol, 3-methylbutane-1,3-diol, pentane-2,4-diols (dl and meso), 2-methylpentane-2,4-diol and cyclohexane-1,3-diols (cis and trans) in benzene have been analysed. The conformer distribution and the NMR shifts of these diols have been computed on the basis of density functional theory, the solvent being included by means of the integral equation formalism phase continuum model (IEFPCM) implemented in Gaussian 09. Relative Gibbs energies of all conformers are calculated at the Perdew, Burke and Ernzerhof (PBE)0/6-311 + G(d,p) level, and NMR shifts by the gauge-including atomic orbital method with the PBE0/6-311 + G(d,p) geometry and the cc-pVTZ basis set. Vicinal coupling constants for 1,2- and 1,3-diols are rationalised in terms of relative conformer populations and geometries. The NMR shifts of hydrogen-bonded protons in individual conformers of alkane-1,n-diols show a very rough correlation with the OH⋯OH distances. The computed overall NMR shifts for CH protons in 1,2- and 1,3-diols are systematically high but correlate very well with the experimental values, with a gradient of 1.07 ± 0.01. Some values for nonequivalent methylene protons in 1,3-diols are reversed, calculation giving enhanced values for the proton anti to the COH bonds. Errors in the NMR shifts computed for the OH protons of nonsymmetrical diols appear to be related to relative populations of conformers where one or other of the OH groups is the donor. Some results based on the second-order Møller-Plesset approach, the Becke three-parameter Lee-Yang-Parr method and on the IEFPCM solvation model implemented in Gaussian 03 are included.

DFT studies of the conformational/structural dependencies of geminal 1H,1H scalar coupling 2J(H,H') in substituted methanes

A study is presented of the structural dependencies for scalar, interproton J-coupling across two bonds in a series of substituted methanes. The coupled perturbed, density functional theory method with a B3PW91 functional and aug-cc-pVTZ-J basis sets is used to examine coupling between geminal protons (2)J(H,H') in methane and a series of substituted compounds CH(3)X (X = CH3, CH(2)CH(3), CH=CH2, CH=O, and NH2) as functions of the dihedral angle phi measured about the C1-X2 bonds. All four contributions are obtained but all conformational effects are dominated by the Fermi contact term. Simple linear combination of atomic orbitals (LCAO)-molecular orbital (MO) sum-over-states methods are used to examine the relationships of the coupling constants with dihedral angles as well as internal H-C-H and H-C1-X2 angles. This study explores some novel aspects of geminal H-H coupling including an analysis of the asymmetry in the conformational dependencies arising from non-next-nearest neighbor interactions. For each of the substituted methanes, explicit trigonometric/exponential expressions are given and these accurately reproduce the (2)J(H,H') structural dependencies with standard deviations usually less than 0.03 Hz. The molecular structures for representative bicyclic molecules were fully optimized, and DFT results for (2)J(H,H') reproduce all the trends in the experimental data. A discussion is given on the applicability of the equations for H--H coupling in the substituted methanes to coupling in the bicyclic molecules.

Complete Prediction of the1H NMR Spectrum of Organic Molecules by DFT Calculations of Chemical Shifts and Spin-Spin Coupling Constants

1H NMR chemical shifts and coupling constants for several aromatic and aliphatic organic molecules have been calculated with DFT methods. In some test cases (furan, o-dichlorobenzene and n-butyl chloride) the performance of several functionals and basis sets has been analyzed, and the various contributions to spin-spin coupling (Fermi-contact, diamagnetic and paramagnetic spin-orbit) have been evaluated. The latter two components cancel each other, so that the calculation of the contact term only is sufficient for an accurate evaluation of proton-proton couplings. Such calculated values are used to simulate the 1H NMR spectra of organic molecules with complicated spin systems (e.g. naphthalene, o-bromochlorobenzene), obtaining a generally very good agreement with experimental spectra with no prior knowledge of the involved parameters.