Relationship between torsional angles and ring-puckering coordinates

A relationship between torsional angles and ring puckering coordinates, applicable to any puckered ring, is deduced assuming infinitesimal displacements from a planar reference conformation. For equilateral polygons there is only one maximum torsion angle θ_m and one phase angle φ_m for each value of m, whereas for non-equilateral polygons there is one maximum θ_m^j,j+1 and a correction, ?_m^j,j+1, to be applied to the phase angle φ_m for each torsion angle θ_j,j+1. The resulting expressions for equilateral pentagons and hexagons are equivalent to the empirical ones proposed previously.     

Calculation of the torsional angles in alkyl vinyl ethers from the direct13C-1H spin-spin interaction constants

It is shown that the differences in the direct¹³C-¹H SSIC of the C atom of the vinyl groups in alkyl vinyl ethers depend on the value of the torsional angle about the C bond. This dependence is used to make quantitative estimates of the torsional angles in alkyl vinyl ethers with alkyl substituents ranging from CH₃ to t-C₄H₉ Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 390–393, February, 1991.The authors are grateful to S. V. Kirpichenko for providing the triethylvinylthiosilane, synthesized as described in [11].     

Palladium charcoal-catalyzed Suzuki-Miyaura coupling to obtain arylpyridines and arylquinolines

A phosphine ligand, such as PPh3 or 2-(dicyclohexylphosphino)biphenyl, is essential for the Pd/C-catalyzed Suzuki-Miyaura coupling of halopyridines and haloquinolines, although it has been reported that the reaction of phenyl chlorides can be catalyzed by nonprereduced Pd/C without any additives. In the reactions of bromopyridines, bromoquinolines, 2-chloropyridines, and 2-chloroquinolines, PPh3 was effective enough to provide coupling products in good yields. However, in the reactions of 3-chloropyridine, 4-chloropyridine, and 6-chloroquinoline, sterically hindered 2-(dicyclohexylphosphino)biphenyl was indispensable as a ligand.     

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.     

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.     

A theoretical study of nuclear spin coupling constants and hyperfine coupling constants of se-heterocyclics

MO calculations based on the finite perturbation theory in the INDO approximation have been carried out on selenophene, eighteen of its monosubstituted derivatives and benzo (b)selenophene. The calculated nuclear spin coupling constants satisfactorily reproduce signs, magnitudes, internal orders and some trends of the experimental values. Comparison of different ZDO calculations provides information on the relative importance of σ and π pathways for the various coupling constants in selenophene and benzo(b)selenophene. Unrestricted Hartree-Fock calculations at the INDO level have been performed on the radical anions of dibenzoselenophene and 2,1,3-benzoselenadiazole, the phenoselenazine radical cation, the phenoselenazine neutral radical and the phenoselenazine nitroxide. The isotropic hyperfine coupling constants have been found to be generally in satisfactory agreement with experiment.     

NDDO MO calculations: isotropic hyperfine coupling constants and nuclear spin-spin coupling constants

The nonempirical NDDO MO method in its unrestricted form has been used to evaluate isotropic hyperfine coupling constants and nuclear spin-spin coupling constants. Satisfactory agreement with INDO and experimental results is obtained.     

Characterization of coupling constants to nitrogen. Carbon-13 chemical shifts and carbon–nitrogen coupling constants of substituted benzaldoximes

Carbon-13 chemical shifts and ¹³C?¹⁵N coupling constants of substituted benzaldoximes and of two non-aromatic oximes have been determined. The coupling constants display little variation as a function of the nature or position of substitution, which suggests that perturbations in the π electron distribution have little effect. The values are compared with the markedly different ones reported for mesitaldoxime, and, in conjunction with the observed solvent effects, are discussed within the Schulman-Venanzi framework for assessing the role of the lone-pair.     

Spin-spin coupling constants in methylfuroxanes

Conclusions The possibility of determining the substituent positions in methylfuroxanes by NMR spectroscopy was demonstrated using the coupling constants of the methyl protons with the¹³C atoms of the heterocycle.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2852–2854, December, 1987.     

Proton spin coupling constants in methylcyclopentadienes

A detailed procedure for analysing the ¹H n.m.r. spectra of two isomeric methylcyclopentadienes, two dimethylcyclopentadienes and 3-methylindene is reported. Finite perturbation INDO calculations are used to discuss the conformational features of the long range proton coupling constants.     

Proton spin coupling constants in cycloalkenes

The proton-proton coupling constants in cyclopentadiene, cyclohexadiene, cycloheptatriene, cyclooctatetraene, cyclopentene and cyclohexene have been calculated using the finite perturbation INDO method. The carbon geometry was taken from the corresponding electron diffraction studies. The theory gives satisfactory agreement with experiment. In terms of the many factor structural model, distance and angle distortions have been studied in ethylene, cis-1,3-butadiene and cis-2-butene. It has been established that these distortions produce noticeable changes only for those protons whose co-ordinates are changed. The coupling constants across three, four and five bonds are discussed separately in connection with the structural factors and the σ–π contribution. Finally, variations in the proton co-ordinates in cyclopentadiene are used to optimize the proton couplings.     

The relation of directly bonded C-H coupling constants to s-characters revisited

Summary Following earlier theoretical studies of orbital delocalization effects on one-bond ¹³C-H coupling constants (¹ J _CH) an approximate proportionality of (¹ J _CH) to ^3/2, where is the s-character of the carbon hybrid orbital appropriate to the construction of the best localized molecular orbitals, is predicted and verified by using published data. It is proposed that this proportionality should replace the previous linear correlations of (¹ J _CH) with , which involve large additive constants.     

C,H coupling constants of propene

The proton coupled ¹³C NMR spectrum of propene has been analysed at 100.6 and 25.2 MHz and a complete set of C,H coupling constants is reported. In particular, vicinal and one-bond coupling constants are discussed with respect to substituent effects of alkyl groups. An incremental method allows us to predict ¹J(CH) in more highly methyl-substituted alkenes.     

Basis-set dependence of nuclear spin-spin coupling constants

The convergence of NMR indirect spin-spin coupling constants with the extension of the basis set is analyzed, based on calculations carried out at the multiconfigurational self-consistent-field level for the HF and H₂O systems. For the dominant and difficult Fermi-contact contribution, the standard correlation-consistent basis sets of electronic-structure theory are not suitable, lacking flexibility in the core region. Improved but not satisfactory convergence of the couplings is observed when decontracting the s functions of the correlation-consistent cc-pVXZ basis sets for 2≤X≤6. Next, by systematically extending these basis sets with tight s functions, new sets are obtained that are sufficiently flexible for accurate calculations of indirect nuclear spin-spin couplings, without sacrificing the smooth convergence behavior of the correlation-consistent basis sets. Received: 22 September 1997 / Accepted: 30 December 1997     

Sensitivity of monomer nuclear quadrupole coupling constants to hydrogen bond formation

Ab initio calculations on several representative complexes have been carried out to show the sensitivity of quadrupole coupling constants to hydrogen bond formation, and particularly the sensitivity to orientation in the complex. Spectroscopic structural determinations based on nuclear quadrupole coupling constants are examined for five representative systems. It appears that nuclear quadrupole coupling constants may give orientational angles accurately, but inaccuracy may amount to ≈ 15° in certain instances.     

The dependence of 13C-1H coupling constants on CCC bond angles

The relationship between one-bond ¹³C-¹H coupling constants and internuclear CCC bond angles (_n) in hydrocarbons of the type R₂CH₂ is best approximated by a quadratic expression.