Chemical Shifts and Pi-Electron Densities in Substituted Benzenes

Wu, Ting Kai; Dailey, B. P.

doi:10.1063/1.1726355

Cited by 99 publications

(15 citation statements)

References 35 publications

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“…The usual way to use (7) is to begin with each ~ put equal to e@ and to obtain the energies and charges in the molecular orbitals by solving the appropriate secular equations. These charges q~" lead, by (7), to a new set of ~i, and the process is now continued iteratively until the set of qi"'s is self-consistent.…”

Section: Calculation Of Charges and Bond Ordersmentioning

confidence: 99%

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Fluorine chemical shifts in aromatic systems

Caldow

1966

Molecular Physics

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The fluorine chemical shifts in halogen-substituted benzenes have been calculated and compared with experimental values. It is found that the co-called van der Waals shift makes a substantial contribution to the total chemical shift particularly when the substituent is ortho to the fluorine nucleus. This van der Waals shift is strongly dependent on the molecular geometry so that fluorine chemical shifts could be a possible source of information about overcrowding in molecules.

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Section: Calculation Of Charges and Bond Ordersmentioning

confidence: 99%

“…There have been several attempts to apply equation (2) to the calculation of fluorine chemical shifts, 8, in aromatic systems [5][6][7]. The most convenient ~-Present address: Department of Theoretical Chemistry University of Bristol, England.…”

Section: Introductionmentioning

confidence: 99%

Fluorine chemical shifts in aromatic systems

Caldow

1966

Molecular Physics

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“…There is little evidence that this goal will soon be reached. Some progress has been made in correlating the proton shifts in substituted benzenes with electron densities at the bonded carbon atoms (1)(2)(3) or with Hammett substituent constants (4,5). But there are also contributions2 to the shifts arising from substituent magnetic anisotropy, local van der Waals interactions, field effectsintroduced by substituent dipole moments, and from rather subtle ortho effects (7,8).…”

Section: Introductionmentioning

confidence: 99%

Proton chemical shifts of polyhalobenzenes in cyclohexane and carbon tetrachloride. An additivity scheme based on pairwise parameters

Richardson¹,

Schaefer²

1968

Can. J. Chem.

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An additivity scheme which takes into account the interactions between ortho substituents is applied to 39 proton shifts in 22 tri-, tetra-, and pentasubstitutcd halobenzenes as 3 mole % solutions in cyclohexane and carbon tetrachloride. The shift of any proton can be written as the sum of two parameters D ( X , Y ) and one parameter d,. D ( X , Y ) refers to a pair of substituents placed ortho ( X ) and meta ( Y )to the proton and d, refers to a substituent placed para to the proton. The mean deviation between observed and calculated shifts is found to be 0.010 p.p.m. for the cyclohexane solutions and 0.012 p.p.m. for the carbon tetrachloride solutions. The scheme also works reasonably well for mono and dihalobenzenes. The parameteris a measure of the stcric interaction between the X and Y substituents. It is also suggested that the steric environment of a proton is important in determining the shift in carbon tetrachloride solution.

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“…'.l3 This view is, however, incompatible with the normal p-H SCS for them. 4 Another phenomenon, more difficult to understand, is the very low m-H SCS values of 3 and 4 for halogens. One of the present authors52 attributed this to 'steric inhibition of diamagnetism', i.e.…”

Section: Substituent Diamagnetic Anisotropy Effectmentioning

confidence: 99%

Substituent effect in aromatic proton NMR spectra—IV Proton NMR spectra of monosubstituted (poly) methylbenzenes

Nomura

Takeuchi

1969

Org. Magn. Reson.

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Abstract-Proton N M R spectra of ]-substituted 2,4-dimethylbcnzenes (2). I-substituted 2.6-dimethylbenzenes (3) and 1-substituted 2,4,6-trimethylbenzenes (4) were determined and the SCS values compared with those of monosubstituted benzenes (1). SCS of 1 are assumed to be primarily due to the effects of x-electron charge density, substituent electric field and substituent diamagnetic anisotropy, and the van der Waals interaction: thus scs scs, f SCSZ 1 scs.4 + SCSV When, however, the substituent is sterically hindered, then:where c , ,~ may be constants predictable from the theories associated R i t h each component. By estimating SCS,,v or c,,~, a quantitative separation of SCS into their components was attempted. It is shown, however, that the data available, as well as the nature of this approach, cannot necessarily be sufficient for this purpose.Various effects which might also contribute to the proton N M R chemical shifts of sterically hindered molecules are also discussed. I N T R O D U C T I O NALTHOUGH considerable data has been reported on the proton NMR spectra of substituted benzenes, the nature of the substituent effect which determines the substituent chemical shift (SCS)t of monosubstituted benzenes (1) has not yet been fully understood.2 In principle, the calculation of chemical shifts is based on the Ramsey e q~a t i o n ,~" which in this case is practically impossible because of the lack of knowledge of the exact wave functions for ground and all excited states. In discussing substituent effects, the method of Saika and S1ichtersb is usually followed. This subdivides the chemical shifts into the diamagnetic and paramagnetic contributions from the atom in question and neighbouring atoms. Although there are several factors contributing to the local diamagnetic term, the most dominant factor4 for substituted benzenes seems to be the changes in .rr-electron charge density in the ring resulting from the substituent perturbation. Aromatic proton chemical shifts due to the diamagnetic term are thus most conveniently assumed to be proportional to Chem. SOC. 88, 3318 (1966): the difference in chemical shift for a proton in a substituted benzene and the appropriate proton shift when the substituent is hydrogen.

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Chemical Shifts and Pi-Electron Densities in Substituted Benzenes

Cited by 99 publications

References 35 publications

Fluorine chemical shifts in aromatic systems

Fluorine chemical shifts in aromatic systems

Proton chemical shifts of polyhalobenzenes in cyclohexane and carbon tetrachloride. An additivity scheme based on pairwise parameters

Substituent effect in aromatic proton NMR spectra—IV Proton NMR spectra of monosubstituted (poly) methylbenzenes

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