Can one assess the pi character of a C-C bond with the help of the NMR spin-spin coupling constants?
Measured one-bond spin-spin coupling constants (SSCC) 1J(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 1J(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 pi 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 1J(CH). The difference between the measured J(CC) and the estimated FC(CC) values, delta(CC) = PSO(CC) + SD(CC) + DSO(CC), provides a semiquantitative measure of the pi 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 1J(CC) into one- and two-orbital contributions. The FC, PSO, and SD terms of 1J(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.
Cremer
Dieter
D
Department of Theoretical Chemistry, Göteborg University, Reutersgatan 2, S-413 20 Göteborg, Sweden. Dieter.Cremer@Theoc.gu.se
Kraka
Elfi
E
Wu
Anan
A
Lüttke
Wolfgang
W