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Arithmetic manipulation of coupling constants to obtain torsional angles
Author(s) -
Lambert Joseph B.,
Sun Hsiangning
Publication year - 1977
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270091104
Subject(s) - electronegativity , substituent , simple (philosophy) , coupling constant , chemistry , polar , computational chemistry , value (mathematics) , function (biology) , coupling (piping) , range (aeronautics) , constant (computer programming) , mathematics , stereochemistry , physics , quantum mechanics , organic chemistry , materials science , statistics , philosophy , epistemology , evolutionary biology , computer science , metallurgy , composite material , biology , programming language
Various arithmetic combinations of J trans and J cis from the AA′BB′ analysis of CH 2 —CH 2 – fragments in 6‐membered rings have been examined for their relationship to the internal torsional angle Ψ. The simple ratio R ( J trans / J cis ) appears to offer the most complete removal of substituent electronegativity effects and hence the best values of Ψ. The Karplus equation is very inaccurate when the substituents are highly polar. The simple difference D ( J trans – J cis ) overcompensates for electronegativity effects and thus also gives relatively inaccurate torsional angles. An alternative, artificial linear combination of the coupling constants, such as D ′ ( J trans –2 J cis ), appears to be able to provide torsional angles that are almost as good as those from the R value, although the Karplus A value must be evaluated. Adjustment of the simple difference D for electronegativity is possible by including a function of the AA′ and BB′ chemical shifts. These results are fairly good, and the parameter A need not be evaluated. This approach may fail, however, with particularly anisotropic substituents, since the chemical shift may depend on factors other than electronegativity. Although all five methods give reasonably good results, the R value appears to offer the most reliable results for the full range of substituents.