A positive 6J(H,CHO) in some meta derivatives of benzaldehyde. A simple model

1989 ◽  
Vol 67 (5) ◽  
pp. 827-830 ◽  
Author(s):  
Ted Schaefer ◽  
Craig S. Takeuchi
Keyword(s):  
Chemical Shifts ◽  
Valence Bond ◽  
Spin Coupling ◽  
Coupling Constant ◽  
Parent Molecule ◽  
Ionic Valence ◽  
Valence Bond Structure ◽  
Substituent Constants ◽  
Negative Coupling ◽  
Derivatives Of

6J(H,CHO), the long-range coupling constant between the aldehydic and para protons in benzaldehyde, has not been detected, possibly because the σ–π interaction giving rise to a negative coupling is intrinsically rather small and because the internal barrier to rotation about the [Formula: see text] bond is large. However, 6J(H,CHO) in some meta substituted derivatives is actually positive and as large as 0.09 Hz in 3,5-difluorobenzaldehyde; the barrier to internal rotation in this molecule is some 4 kJ/mol lower than in the parent molecule. The magnitude of 6J(H,CHO) in these derivatives correlates well with σR.ST values, a recent set of substituent constants derived from 13C nuclear magnetic resonance chemical shifts of the β carbon in styrene derivatives. It is hypothesized that the substituents with negative σR.ST values stabilize an ionic valence bond structure that has a positive 6J(H,CHO). A brief discussion of 4J(H,CHO) in some of these molecules is also presented. Keywords: NMR, spin coupling NMR, benzaldehyde.

The negative 7J(CHO, CH3) in 4-methylbenzaldehyde. Ionicity of the carbonyl bond

1992 ◽  
Vol 70 (10) ◽  
pp. 2555-2557
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian
Keyword(s):  
First Principles ◽  
Valence Bond ◽  
Spin Coupling ◽  
Coupling Mechanism ◽  
Unexpected Result ◽  
Coupling Constant ◽  
Valence Bond Structure ◽  
Carbonyl Bond ◽  
Spin Spin Coupling ◽  
Benzaldehyde Derivatives

The spin–spin coupling constant over seven bonds between the formyl and methyl protons in 4-methylbenzaldehyde is −0.030 Hz in CS2/C6D12/TMS, and (−)0.035 Hz in acetone-d6, solutions at 297 K. This unexpected result is rationalized in terms of a spin–spin coupling mechanism attributed to the importance of a valence bond structure with an ionic carbonyl bond. The result again emphasizes the sensitivity to substituent perturbations of the six-bond coupling constant in quasi-planar benzaldehyde derivatives. It can have either sign and presents a challenge to its computation from first principles.

The proximate spin–spin coupling, 5J(F,CH3), as a quantitative conformational indicator in alkylfluorobenzenes and related compounds

1986 ◽  
Vol 64 (8) ◽  
pp. 1602-1606 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian ◽  
Glenn H. Penner ◽  
S. R. Salman
Keyword(s):  
Nuclear Spin ◽  
Spin Coupling ◽  
Torsional Angle ◽  
Rotational Behaviour ◽  
Coupling Constant ◽  
Spin Coupling Constant ◽  
Spin Spin Coupling ◽  
Torsional Potentials ◽  
Derivatives Of ◽  
Related Compounds

The through-space or proximate nuclear spin–spin coupling constant, 5J(F,CH3) = 5J, between methyl protons and ring fluorine nuclei in alkylfluorobenzenes is postulated as [Formula: see text] θ being the torsional angle for the [Formula: see text] bond. A and B are obtained from the known internal rotational behaviour in 2,6-difluoroethylbenzene and the corresponding cumene derivative. The parameterization is tested on the observed 5J in derivatives of 2,4,6-tri-tert-butyl- and 2,4,6-tri-isopropyl-fluorobenzene, in 2-chloro-6-fluoroisopropylbenzene, 2,6-difluoro-α-methylstyrene, and N-methyl-8-fluoroquinolinium halides. A prediction is made for 5J in 2,6-difluoro-tert-butylbenzene. It appears that the present parameterization allows the derivation of approximate torsional potentials from proximate couplings, for example in α,α-dimethyl-2,6-difluorobenzyl alcohol.

Reconnaissance of diverse structural and electronic environments in germanium halides by solid-state 73Ge NMR and quantum chemical calculations

2011 ◽  
Vol 89 (9) ◽  
pp. 1118-1129 ◽  
Author(s):  
Brandon J. Greer ◽  
Vladimir K. Michaelis ◽  
Victor V. Terskikh ◽  
Scott Kroeker
Keyword(s):  
Solid State ◽  
Quantum Chemical Calculations ◽  
Quantum Chemical ◽  
Chemical Shifts ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Coupling Constant ◽  
Electronic Environments ◽  
Isotropic Chemical Shifts ◽  
Spin Spin Coupling

Solid-state 73Ge nuclear magnetic resonance (NMR) is an attractive technique for the characterization of solid germanium-containing materials, but experiments can be exceedingly difficult in practice due to the unfavourable NMR properties of the 73Ge nucleus. Presented herein is a series of solid-state 73Ge NMR experiments on germanium halides (GeX4 and GeX2, where X = I, Br, and Cl) conducted at moderate (9.4 and 11.7 T) and ultrahigh (21.1 T) magnetic fields, intended to characterize the 73Ge NMR response in highly symmetric and asymmetric coordination environments. Quadrupole coupling constants range from 0.16 to 35 MHz. Isotropic chemical shifts for the GeX4 series trend with halide electronegativity, as found for the analogous silicon and tin halides. The indirect spin-spin coupling constant 1J(73Ge, 127I) is estimated from 73Ge MAS NMR to be 35 ± 10 Hz in GeI2, with the reduced coupling constant agreeing with those of other group 14 halides. Quantum chemical calculations using GIPAW DFT are in reasonable accord with experimental quadrupole couplings, but fail for chemical shielding. A preliminary NMR crystallographic study of GeI2 and GeCl2 incorporating 127I and 35Cl NMR spectra has led to plausible conclusions reflecting the structural homology of these compounds, although definitive characterization remains elusive.

Long-range 13C, 13C spin–spin coupling constants in anisole and some derivatives

1988 ◽  
Vol 66 (7) ◽  
pp. 1635-1640 ◽  
Author(s):  
Ted Schaefer ◽  
Glenn H. Penner
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Methoxy Group ◽  
Chemical Shifts ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Coupling Constant ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Nuclear Magnetic

13C nuclear magnetic resonance chemical shifts and nJ(13C,13C) are reported for anisole and 16 of its derivatives, all enriched with 13C in the methoxyl group. 5J(13C,13C) is directly proportional to sin2θ, where θ is the angle by which the methoxy group twists about the C(1)—O bond. In acetone-d6 solution, 5J(C,C) is not observable for a number of 4-substituted anisoles, except for 1,4-dimethoxybenzene. For the latter, 5J(C,C) is compatible with a twofold barrier of 19.3 ± 1.1 kJ/mol hindering rotation about the C(1)—O bond. However, it is unlikely that the barrier is purely twofold in nature. The observed 5J(C,C) is also compatible with 10.5 and 6.0 kJ/mol for the twofold and fourfold components, respectively, implying a dynamical nuclear magnetic resonance barrier of less than 13 kJ/mol. While phase and solvent effects on the internal barrier in anisole are certainly substantial, it appears that a fourfold component must also be present. The apparent twofold barrier in 2,6-difluoroanisole is 5.4 ± 0.9 kJ/mol, based on 5J(C,C) and 6J(H-4,13C). The latter coupling constant is also reported for 1,2,3-trimethoxybenzene and used to deduce its conformation. The θ dependence of 3J(C,C) and 4J(C,C) is briefly discussed for symmetrical anisole derivatives. Differential 13C, 13C isotope shifts are reported for 1,4-dimethoxybenzene.

Chemometrical analysis of substituent effects on 13C and 15N NMR chemical shifts in 1-aroyl-3-substituted thioureas

1989 ◽  
Vol 54 (9) ◽  
pp. 2399-2407 ◽  
Author(s):  
Oldřich Pytela ◽  
Josef Jirman ◽  
Antonín Lyčka
Keyword(s):  
Latent Variables ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
The Other ◽  
15N Nmr ◽  
Nmr Chemical Shifts ◽  
Additional Constant ◽  
Substituent Constants ◽  
Derivatives Of ◽  
C Nmr

The methods of conjugated deviations and regression analysis have been used to study the substituent effects on 13C and 15N NMR chemical shifts of 12 derivatives of 1-aroyl-3-phenylthiourea and 1-aroyl-3-methylthiourea. The 13C NMR chemical shifts can be described by two latent variables, one univocally correlated with the Hammett substituent constants (r = 0.993) and the other reflecting the increased shielding of the nuclei due to overlap of the adjacent bond electrons as a consequence of electron-donor or electron-acceptor character of the substituents.This effect is less pronounced with the 15N nuclei. Application of dual substituent constants σR, σF with the additional constant σα describing the polarization has failed in giving sufficiently close correlations and explanation of the substituent effect found.

Long-range spin-spin coupling in 1,4-benzoquinones and some related compounds

1966 ◽  
Vol 19 (4) ◽  
pp. 617 ◽  
Author(s):  
RK Norris ◽  
S Sternhell
Keyword(s):  
Long Range ◽  
Chemical Shifts ◽  
Point Of View ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Halogen Atoms ◽  
Methyl Methyl ◽  
Negative Coupling ◽  
Spin Spin Coupling ◽  
Related Compounds

N.m.r, data for fifteen 1,4-benzoquinones, four 1,4-naphthoquinones, and six cyclohex-2-ene-1,4-diones are tabulated. From these, and previously available data, it is possible to obtain characteristic ranges for methyl-allylic coupling constants (c 1.3 c/s) and methyl-methyl homoallylic coupling constants (c. 1.3 c/s) for this for this series of compounds as well as values for other long-range and vicinal interactions, including a negative coupling across five bonds. On the basis of both chemical shifts and coupling constants it was concluded that 1,4-benzoquinones have little aromaticity from the N.M.R. point of view. The halogen atoms in 5,6-dihalogenocyclohex-2-ene-1,4-diones appear to be tram diaxial.

The rotational angle dependence of 5JmH,SH in benzenethiol

1982 ◽  
Vol 60 (15) ◽  
pp. 1924-1927 ◽  
Author(s):  
Ted Schaefer ◽  
Timothy A. Wildman ◽  
Rudy Sebastian
Keyword(s):  
Spin Coupling ◽  
Benzene Derivatives ◽  
Coupling Constant ◽  
Spin Coupling Constant ◽  
Rotational Angle ◽  
Angle Dependence ◽  
Meta Position ◽  
Electron Contribution ◽  
Spin Spin Coupling ◽  
Derivatives Of

It is suggested that the five-bond spin–spin coupling constant between the sulfhydryl proton and the ring proton in the meta position is given by [Formula: see text]. Here θ is the angle by which the S—H bond twists out of the benzene plane and angular brackets indicate expectation or average values, determined by the twofold barrier to internal rotation about the C—S bond. [Formula: see text] is the π electron contribution and has a maximum at θ = 90° and [Formula: see text] is the σ electron contribution with a maximum at θ = 180° (zig-zag orientation). For eight para substituted derivatives of benzenethiol, the 5J numbers can be reproduced by this equation, which also agrees with the observed couplings in 2-hydroxybenzenethiol. In this compound θ lies near 90°. It appears that this approach will allow an extension of the J method to the evaluation of two-term potential functions in, for example, ortho substituted benzene derivatives.

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