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.

Proton magnetic resonance spectra of 3-fluorotoluene and 2-chloro-5-fluorotoluene. The sign and coupling mechanism of

1978 ◽  
Vol 56 (17) ◽  
pp. 2233-2236 ◽  
Author(s):  
Ted Schaefer ◽  
Werner Danchura ◽  
Walter Niemczura
Keyword(s):  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Spin Coupling ◽  
Coupling Mechanism ◽  
Coupling Constant ◽  
Electron Component ◽  
Electron Contribution ◽  
Full Analysis ◽  
Spin Spin Coupling ◽  
Magnetic Resonance Spectra

A full analysis of the proton magnetic resonance spectra of 3-fluorotoluene and of 2-chloro-5-fluorotoluene, as 10 mol% solutions in CS2, demonstrates that the long-range spin–spin coupling constant over five bonds between methyl protons and fluorine-19 is negative. The coupling mechanism consists of a large positive σ electron component and a negative π electron component. The negative sign of the π electron contribution arises from a spin density in the 2pz orbital at carbon-3, which is opposite in sign to that of the spin densities at C-2 and C-4. Combined with positive hyperfine interaction constants, QCCH and QCF, the consequence is a negative π electron component.

Long-range spin–spin coupling constants between ring protons and aldehydic protons in some para-substituted benzaldehydes

1969 ◽  
Vol 47 (21) ◽  
pp. 4005-4010 ◽  
Author(s):  
S. S. Danyluk ◽  
C. L. Bell ◽  
T. Schaefer
Keyword(s):  
Long Range ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Coupling Constant ◽  
Proton Proton ◽  
Proton Coupling ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Coupling Mechanisms ◽  
Benzaldehyde Derivatives

The long-range proton–proton coupling constants between the ring protons and the aldehydic proton are reported for a series of para-substituted benzaldehyde derivatives. It was found that JoH,CHO < 0 and JmH,CHO > 0. Furthermore, JoH,CHO increases in magnitude as the electron donating power of the sub-stituent increases. A similar trend is observed forJmH,CHO but the ratio of the increase to the magnitude of JmH,CHO is much less than for JoH,CHO. A good correlation is obtained between JoH,CHO and the sub-stituent parameters of Swain and Lupton.The coupling constant data are discussed in terms of σ and π coupling mechanisms and it is concluded that σ electron mechanisms are dominant for both JoH,CHO and JmH,CHO.

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.

An estimate of the spin–spin coupling constant, 1J(1H,13C), in gaseous benzene

1996 ◽  
Vol 74 (8) ◽  
pp. 1524-1525 ◽  
Author(s):  
Ted Schaefer ◽  
Guy M. Bernard ◽  
Frank E. Hruska
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Dielectric Constant ◽  
Magnetic Resonance ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Coupling Constant ◽  
Spin Coupling Constant ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Gaseous Benzene

An excellent linear correlation (r = 0.9999) exists between the spin–spin coupling constants 1J(1H,13C), in benzene dissolved in four solvents (R. Laatikainen et al. J. Am. Chem. Soc. 117, 11006 (1995)) and Ando's solvation dielectric function, ε/(ε – 1). The solvents are cyclohexane, carbon disulfide, pyridine, and acetone. 1J(1H,13C)for gaseous benzene is predicted to be 156.99(2) Hz at 300 K. Key words: spin–spin coupling constants, 1J(1H,13C) for benzene in the vapor phase; spin–spin coupling constants, solvent dielectric constant dependence of 1J(1H,13C) in benzene; benzene, estimate of 1J(1H,13C) in the vapor; nuclear magnetic resonance, estimate of 1J(1H,13C) in gaseous benzene.

Internal motion in benzylidene diacetate and its 2,6-dibromo derivative by DNMR and the J method

1989 ◽  
Vol 67 (6) ◽  
pp. 1022-1026 ◽  
Author(s):  
Ted Schaefer ◽  
Craig S. Takeguchi
Keyword(s):  
Double Resonance ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Maximum Magnitude ◽  
Coupling Constant ◽  
Spectral Parameters ◽  
Proton Proton ◽  
Ether Solution ◽  
Spin Spin Coupling ◽  
Dibromo Derivative

The 1H nuclear magnetic resonance spectral parameters are reported for benzylidene diacetate in CS2 and acetone-d6 solutions. The long-range spin–spin coupling constant over six formal bonds, 6J, is used to derive apparent twofold barriers to rotation about the exocyclic C(1)—C bond in the two solutions. The conformation of lowest energy has the α. C—H bond in the benzene plane. The barrier is higher in CS2 than in acetone-d6 solution, in contrast to a molecule like benzyl chloride. In the 2,6-dibromo derivative, the free energy of activation for reorientation about the bond in question is 36 kJ/mol at 165 K in dimethyl ether solution. Such a high barrier implies a very small six-bond proton–proton coupling constant for this derivative because 6J is proportional to the expectation value of sin2θ. The angle θ is zero when the α C—H bond lies in the benzene plane. 6J is −0.051 Hz in acetone-d6 solutions; its sign is determined by double resonance experiments. The question of an angle-independent component of 6J, that is, whether 6J is finite at θ = 0°, is addressed. A maximum magnitude of 0.02 Hz may be present at θ = 0° for the 2,6-dibromo derivative, although a zero magnitude is also compatible with the experimental data. In a compound with a higher internal barrier, α,α,2,6-tetrachlorotoluene, the experimental results are best in accord with a negligibly small 6J at θ = 0°. Keywords: 1H NMR of benzylidene diacetate, spin–spin coupling constants for benzylidene diacetate, DNMR, 2,6-dibromobenzylidene diacetate.

A solid-state 31P NMR study of 1:1 silver–triphenylphosphine complexes — Interpretation of 1J(107,109Ag,31P) values,

2009 ◽  
Vol 87 (7) ◽  
pp. 1090-1101 ◽  
Author(s):  
Fu Chen ◽  
Se-Woung Oh ◽  
Roderick E. Wasylishen
Keyword(s):  
Solid State ◽  
Spin Coupling ◽  
Coupling Mechanism ◽  
P Values ◽  
X Ray Crystallography ◽  
Phosphine Complexes ◽  
Nmr Study ◽  
Fermi Contact ◽  
Spin Spin Coupling ◽  
Cp Mas Nmr

High-resolution solid-state 31P NMR spectroscopy was used to investigate a series of 1:1 silver–triphenylphosphine complexes, [Ph3PAgX]n, where X is a monovalent anion and n = 1, 2, 3, 4, or ∞. The 31P CP MAS NMR spectra reveal the number of distinct phosphorus sites in these complexes as well as the |1J(109Ag,31P)| values, which range from 401 ± 10 Hz (X = N3–) to 869 ± 10 Hz (X = SO3CF3–). The data obtained here and in earlier investigations indicate that |1J(109Ag,31P)| values for silver–tertiary phosphine complexes decrease as Ag–P bond lengths increase. This experimental conclusion is supported by DFT calculations, which also indicate that the Fermi-contact mechanism is the only important spin–spin coupling mechanism for 1J(109Ag,31P) in these complexes. In addition, the crystal structure of a silver–triphenylphosphine trifluoroacetate tetramer was determined using X-ray crystallography, and the structure of a silver–triphenylphosphine chloride tetramer was reinvestigated.

Form of the dihedral angle dependence of the spin-spin coupling constant JHNNH

1989 ◽  
Vol 25 (3) ◽  
pp. 338-341
Author(s):  
L. M. Kapkan ◽  
A. Yu. Chervinskii ◽  
T. M. Pekhtereva ◽  
Yu. I. Smirnov ◽  
A. F. Dmitruk
Keyword(s):  
Dihedral Angle ◽  
Spin Coupling ◽  
Coupling Constant ◽  
Spin Coupling Constant ◽  
Angle Dependence ◽  
Spin Spin Coupling
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