Vicinal Coupling (3J, H-C-C-H)
Coupling is mediated by the interaction of orbitals within the bonding framework. It is therefore dependent upon overlap, and hence upon dihedral angle. The relationship between the dihedral angle and the vicinal coupling constant 3J (as observed from 1H NMR spectra) is given theoretically by the generalised Karplus equations:
3Jab = J0cos2f-0.28 (0o < f < 90o)
3Jab = J180cos2f-0.28 (90o < f < 180o)
where J0 = 8.5 and J180 = 9.5 are constants which depend upon the substituents on the carbon atoms and f is the dihedral angle. The dihedral angle is defined by:
An approximate calculated relationship (ignoring the small constant of 0.28 in this graph) between the dihedral angle and the coupling constant may be illustrated below:
In some cases the axial-axial coupling constant for an antiperiplanar 180o H-C-C-H configuration may be more than 9.5 Hz. Indeed for rigid cyclohexanes it is around 9-13 Hz, because the dihedral angle is close to 180o, where the orbitals overlap most efficiently.
Enter the coupling constant in box as a numeral (e.g. 2.38) to calculate the theoretical dihedral angle in the molecule to assist with molecular modelling.
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Although the generalised Karplus equations generally work well for rigid bicyclics (e.g. camphor and its derivatives), for other molecules the situation is sometimes better served by alternative equations, e.g. the Bothner-By equation (available on the Android App). As an illustrated example, the authors1 describe the near-axial and equatorial (ca. 180o and 60o) relationship between the 3J dihedral couplings between C-2 and C-3 respectively for the trans-1,4-benzoxazepine below (its lowest energy conformer A shows the axial and equatorial relationships). The 3J 2H-3Hax = 11.5 Hz and 3J 2H-3Heq = 3.2 Hz corresponding to a dihedral angle of ca. 158 and 66o using the Bothner-By equation; using the Karplus equations it is slightly out-of-range for the axial and the equatorial coupling is 50o. From XYZ co-ordinates from the geometry-optimised structures using computational chemistry calculations (DFT B3LYP 6-31G(d), Gaussian 09) the dihedral angles are 179.7 and 64.6o respectively, revealing the Bothner-By equation to be the better choice.
What's New: A book chapter has recently been published (Recent Advances in Asymmetric Diels-Alder Reactions; author J.P. Miller) that may be of interest in organic chemistry:
free open access.
1. L.Tóth, Y. Fu, H. Y. Zhang, A. Mándi, K. E. Kövér, T.-Z. Illyés, A. Kiss-Szikszai, B. Balogh, T. Kurtán, S. Antus, P. Mátyus, Beilstein J. Org. Chem. 2014, 10, 2594-2602.
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