A pseudoatom molecular orbital approach to substituent effects in organic Compounds. III. HH coupling constants in substituted ethylenes, ethanes, and benzenes

The nature and types of lone-pair effects on nuclear spin coupling constants are reviewed in the context of a localized bond description of molecular electronic structure. Emphasis is placed on the importance of residual delocalization involving the otherwise lone-pair orbital, in terms of which the effect of an X lone-pair, when compared with an isoelectronic Y-H or X-H+ group or with an X-R group, and its orientational dependence can be interpreted. One-, two-and three-bond coupling constants are considered and the importance of lone-pair effects for configurational and conformational information is stressed and illustrated. They also serve for a better understanding of substituent inductive effects on coupling constants.A large collection of illustrative examples are presented, with particular attention paid to couplings involving H, C, N, F and P nuclei, organized in a systematic manner into nine categories. The signs of the lone-pair effects on the reduced coupling constants are found to be independent of the actual nuclei under study in the same category. This is taken as an indication that the electron lone-pairs mainly affect the Fermi contact contribution to the coupling and, accordingly, an interpretation is given in terms of simple sum-over-states models. In addition, symmetry-based relationship are established involving the sign of lone-pair effects in coupling constants between nuclei which are a different number of bonds apart.KEY WORDS NMR Multinuclear spin coupling Lone-pair effects Substituent effects Stereochemistry MO Sum-over-states models Electron delocalization CONTENTS

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Effect of electron lone‐pairs on nuclear spin–spin coupling constants

Gil

Philipsborn

1989

Magnetic Reson in Chemistry

220

107

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Angular dependence of geminal spin–spin coupling constants in a prototype CH₂ group. J(H,H) versus interbond angle in methane

Geertsen¹,

Oddershede

Raynes

1993

Magnetic Reson in Chemistry

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I N T R O D U C T I O NThat nuclear spin-spin coupling constants are often very dependent on interbond angles has been known for many years.' Attention has been largely devoted to vicinal coupling constants and there is still lively interest in this area., However, the angle dependence of geminal coupling constants has also been of interest. In this paper we report the results of accurate calculations of the dependence of the coupling constant 2J(H,, H2) of the methane molecule on the interbond angle HI-C-H,.Our primary aim was to carry out a definitive calculation of the angle dependence of this coupling, subsequently denoted J ( 1, 2), which would lead to a simple equation from which J(1, 2) could be predicted for any angle not only for methane but, hopefully to a high degree of accuracy, for any methylene group involving an sp3-hybridized carbon atom. In the process of calculation, however, we also obtained all other coupling constants of the methane molecule for a set of variations of the HI-C--H, angle which leaves the molecule with C,, symmetry. They are the carbonproton coupling constants J(C, 1) = J(C, 2) and J(C, 3) = J(C, 4) and the other two proton-proton coupling constants J(1, 3) = J(1, 4) = 4 2 , 3) = 4 2 , 4) and 4 3 , 4).

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A pseudoatom molecular orbital approach to substituent effects in organic Compounds. III. HH coupling constants in substituted ethylenes, ethanes, and benzenes

Cited by 2 publications

References 23 publications

Effect of electron lone‐pairs on nuclear spin–spin coupling constants

Effect of electron lone‐pairs on nuclear spin–spin coupling constants

Angular dependence of geminal spin–spin coupling constants in a prototype CH₂ group. J(H,H) versus interbond angle in methane

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A pseudoatom molecular orbital approach to substituent effects in organic Compounds. III. HH coupling constants in substituted ethylenes, ethanes, and benzenes

Cited by 2 publications

References 23 publications

Effect of electron lone‐pairs on nuclear spin–spin coupling constants

Effect of electron lone‐pairs on nuclear spin–spin coupling constants

Angular dependence of geminal spin–spin coupling constants in a prototype CH2 group. J(H,H) versus interbond angle in methane

Contact Info

Product

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Angular dependence of geminal spin–spin coupling constants in a prototype CH₂ group. J(H,H) versus interbond angle in methane