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Can One Assess the π Character of a C–C Bond with the Help of the NMR Spin–Spin Coupling Constants?
Author(s) -
Cremer Dieter,
Kraka Elfi,
Wu Anan,
Lüttke Wolfgang
Publication year - 2004
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200300987
Subject(s) - chemistry , fermi contact interaction , spin (aerodynamics) , coupling constant , character (mathematics) , diamagnetism , coupling (piping) , bond order , paramagnetism , nuclear magnetic resonance , bond length , condensed matter physics , physics , thermodynamics , crystallography , quantum mechanics , materials science , geometry , mathematics , magnetic field , crystal structure , metallurgy
Measured one‐bond spin–spin coupling constants (SSCC) 1 J (CC) can be used to describe the nature of the C–C bond, provided one is able to separate the various coupling mechanisms leading to 1 J (CC). The Fermi‐contact (FC) term probes the first‐order density at the positions of the coupling nuclei, whereas the noncontact terms (the paramagnetic spin orbit (PSO) and the spin–dipole (SD) terms) probe the π character of the C–C bond (the diamagnetic spin orbit (DSO) term can mostly be neglected). A model is tested, in which the value of the FC(CC) term is estimated with the help of measured SSCCs 1 J (CH). The difference between the measured J (CC) and the estimated FC(CC) values, Δ(CC)=PSO(CC)+SD(CC)+DSO(CC), provides a semiquantitative measure of the π character of a C–C multiple bond. The applicability and limitations of this approach are discussed by partitioning the four Ramsey terms of the SSCC 1 J (CC) into one‐ and two‐orbital contributions. The FC, PSO, and SD terms of 1 J (CC) are explained and analyzed with regard to their relationship to other C–C bond properties. It is shown that empirical relationships between measured SSCCs and the s character of a bond need reconsideration.