Correlations of coupling constants with alkyl substituent constants

The one-bond coupling constants (¹J_XY) of nine series of organometallic compounds with the part structure R-X-Y (R is an alkyl group attached directly to the metal atom X) have been correlated with two sets of alkyl substituent constants, TAFT's σ* and our σ_a. The latter gives better correlations and is more consistent with the experimental data.     

The relationship between proton-proton NMR coupling constants and substituent electronegativities—I : An empirical generalization of the karplus equation

A new coupling constant-torsion angle relation for the three-bond ¹H-¹H spin-spin coupling constant is formulated. The relation includes a correction for the electronegativity of substituents. The correction term is written as a function of the electronegativity, the H-C-C-H torsion angle, and the orientation of each substituent relative to the coupled protons. A dataset consisting of 315 experimental coupling constants was used to derive six empirical parameters by means of an iterative least-squares minimization procedure. The precision of the proposed equation, expressed as the root-mean-square deviation (0.48 Hz), is superior to any hitherto reported. It is shown that separate treatment of CH₂CH₂, CH₂CH and CHCH fragments even improves this precision. An application in the field of monosubstituted cyclohexanes is given.     

System of metal electronegativities calculated from the force constants of the bonds

It is proposed to use a unified one-parameter equation which relates the force constants in molecules and crystals to the electronegativities of atoms. The results obtained for all currently studied compounds depend little on the ligand type, which made it possible to construct a complete electronegativity scale for elements in molecular and crystalline states equivalent in known cases to the thermochemical Pauling scale. An attention is paid to the chemical bond characteristics in some molecules and crystals of b subgroups.     

Additivity of substituent effects on the proton-proton coupling constants of disubstituted pyridines

From the spin-spin coupling constants of pyridine and monosubstituted pyridines the effects of several substituents have been calculated- Assuming an additivity relationship when two of these substituents are present in the same molecule, the spin-spin coupling constants for 11 disubstituted pyridines have been calculated.The NMR spectra of 13 disubstituted pyridines have been studied to obtain accurate values of their coupling constants. The experimental values of these constants are in very good agreement with those calculated using the additivity relationships.     

Pseudo-exact force constants from coriolis coupling constants

A method has been developed to calculate Pseudo-Exact Force Constants from ζ-constants after truncating a higher dimensional secular equation with the High Low Frequency Separation Method (PE-HFS and PE-LFS method). A procedure to reduce the C-matrix is presented and the conditions for the applicability will be discussed. The present method is applicable to many cases where only one ζ-constant can be measured (e.g., if some band contours are disturbed and prevent the determination of all ζ-values). It is also possible to get excellent starting force constants for a least square programme in cases where sufficient data are known. The effectiveness of the different first order ζ-constants in fixing the force constants has been pointed out. The pseudo ζ sum rule for the truncated secular equation is also discussed. A proof for the sign rule for ζ-constants for xzy₃ (C_3v) type molecules [ζ(δ_as(XY₃)) < 0 and ζ(?_r(XY₃)) > 0] is given.     

Influence of substituent electronegativities on conformational preferences in 1,2-disubstituted ethanes

Ab initio molecular orbital theory has been used to examine internal rotation in the 1,2-disubstituted ethanes, CH₃CH₂—CH₂Li, LiCH₂—CH₂Li and LiCH₂—CH₂F. Interpretation of the rotational potential function is facilitated by decomposition into Fourier components. The rotational potential functions for the three molecules are representative of three distinct classes of conformational behaviour which are characterized by the electronegativities of the substituents. A striking individual result is the preferred cis eclipsed conformation for LiCH₂—CH₂F.     

Calculation of proton coupling constants for dibenzothiophene radical anion

The calculation of proton hyperfine coupling constants for dibenzothiophene radical anion is reported.     

Calculation of nuclear quadrupole coupling constants in pyrazole and imidazole from ab initio wavefunctions

The nuclear quadrupole coupling constants and asymmetry parameters of ¹⁴N and ²H of pyrazole and imidazole have been computed from ab initio SCF wavefunctions of double-zeta quality. The results agree with the recent experimental values obtained from microwave studies and are much better than those of previous theoretical studies.     

The relationship between proton–proton NMR coupling constants and substituent electronegativities. II—conformational analysis of the sugar ring in nucleosides and nucleotides in solution using a generalized Karplus equation

The relationship between vicinal NMR proton–proton coupling constants and the pseudorotational properties of the sugar ring in nucleosides and nucleotides is reinvestigated. Compared with our earlier study several important improvements are introduced: first, a new empirical generalization of the classical Karplus equation is utilized, which allows an accurate correction for the effects of electronegativity and orientation of substituents on ³J(HH); second, empirical correlations between the parameters governing the conformation of β-D -furanosides (taken from an analysis of 178 crystal structures) were used to define proton–proton torsion angles as a function of the pseudorotation parameters P and Φ_m; and, third an iterative least-squares computer program was devised to obtain the best fit of the conformational parameters to the experimental coupling constants. NMR data for the sugar ring in the following compounds were taken from the literature and analysed: 3′,5′-cyclic nucleotides, a base-stacked ribonucleotide, 2′-anhydroarabinonucleosides, α-D -2′,2-O-cyclouridine, 2′- and 3′-aminosubstituted ribonucleosides, 2′- and 3′-deoxyribonucleosides. The present results confirm that the conformational properties found in the solid state are, on the whole, preserved in solution.     

Calculation of 14N hyperfine coupling constants by the INDO-MO method

The proportionality constant between the hyperfine splitting constant for ¹⁴N and the spin density calculated by the INDO-MO method has been redetermined using a larger sample of results. A revised value of 332 Gauss is suggested.     

Prediction of anti and gauche vicinal proton-proton coupling constants in carbohydrates: A simple additivity rule for pyranose rings

The effect of the relative orientation and electronegativity of substituents on the magnitude of ³J(aa), ³J(ae) and ³J(ee) is well predicated by a simple set of additivity constants, valid for pyranose rings in carbohydrates. The proposed set of parameters is used to calculate 327 coupling constants [³J(HH)] in a variety of pyranosides and related compounds. A comparison with experimental values taken from the literature shows that couplings in molecules which are conformationally pure and underformed can be predicted with a surprising accuracy. An overall root-mean-square agreement of 0.29 Hz is attained for a selected group of 305 coupling values. A statistical breakdown of ΔJ(aa) and ΔJ(ae) [ΔJ=J(exp)-J(calc)] along each carbon-carbon bond in the pyranose systems reveals an unexpected degree of geometrical homogeneity.     

Inductive substituent constants σji from olefinic geminal 1h, 1h nmr coupling constants

Theoretical and experimental arguments are presented supporting the postulate that olefinic NMR coupling constants ²J_HH are insensitive to β-substituent influences on the olefinic π orbital. A new set of substituent constants, σ^J_I, is proposed to measure directly the inductive substituent effect transmitted by σ-bonds. The previously available range of inductive substituent constants can be appreciably extended in this way. Comparisons of σ^J_I with other observables and parameters for selected substituents are made as a test of consistency.     

Calculation of stability constants from photometric data

A method originally designed for the determination of stability constants by using a wedge calorimeter has been adapted to normal photometric measuring techniques. The calculations are simple and the method is especially suitable for use when the absorptivities of the absorbing species are unavailable because these species cannot be obtained alone in solution. Results of some determinations are given.     

13C nuclear magnetic resonance—XIV : Carbon-fluorine coupling constants as probes for intramolecular substituent interactions

It is shown that one-bond carbon-fluorine coupling constants sensitively reflect intramolecular n,σ^* interactions of substituents separated by four bond and being in W-arrangement (Fig.1).     

Possible connection between lipophilicity and steric substituent constants

The steric constants E_s and ν have the greatest chance of being independent of lipophilic substituent constants. There is a high probability among hydrocarbon substituents that the steric constants correlate with the lipophilic ones, while in the case of polar substituents the probability is low.     

Calculation of indirect nuclear spin-spin coupling constants within the regular approximation for relativistic effects

A new method for calculating the indirect nuclear spin-spin coupling constant within the regular approximation to the exact relativistic Hamiltonian is presented. The method is completely analytic in the sense that it does not employ numeric integration for the evaluation of relativistic corrections to the molecular Hamiltonian. It can be applied at the level of conventional wave function theory or density functional theory. In the latter case, both pure and hybrid density functionals can be used for the calculation of the quasirelativistic spin-spin coupling constants. The new method is used in connection with the infinite-order regular approximation with modified metric (IORAmm) to calculate the spin-spin coupling constants for molecules containing heavy elements. The importance of including exact exchange into the density functional calculations is demonstrated.