We introduced the use of chemical shifts of geminal protons (H^a and H^b) of vinyl or isopropenyl substituents as experimental and theoretical probe of ring current induced anisotropies of aromatic systems [1].
HF, MP2 and DFT B3LYP calculations of rotational barriers of the vinyl substituents led for energies and GIAO NMR shieldings to problems around saddle points in the presence of freely rotating methyl groups observed for isopropenyl benzene (1) and for 2-methylstyrene (2).

Difficulties have been observed: 1) for NIMAG-values, 2) with adjustment of extrapolation curves by spline functions for energies and 3) for plots of NMR shieldings while fully optimising the geometry for the complete range of rotation (Φ) between the vinylic substituent and the phenyl system in steps of 15°. The calculation of a two-dimensional energy hypersurface in dependence on the torsional angle Φ and on the rotation of the methyl group φ (with 48 points for 1 and 104 points for 2 in steps of 15° in each dimension) shows how the rotational barrier behaves around the saddle points. The "true" rotational barrier may be extracted as the minimum path on this surface. The observed difficulty can be avoided by optimisations with use of the key words CalcAll (leading to evaluation of the Hessian matrix for each optimisation cycle) or by use of VeryTight (setting extremely tight optimisation convergence criteria). The torsional dependence of experimental and GIAO calculated chemical shifts for the geminal protons H^a and H^b have been compared to predictions by use of our APUDI model [2].

[1] G. Häfelinger, W. Knapp, T. Zuschneid and F. P. Dietrich, J. Phys. Org. Chem. 2005, 18, 800-817: Vinyl or isopropenyl substituents as experimental and theoretical probes for diamagnetic anisotropies of aromatic hydrocarbons.
[2] M. Westermayer, G. Häfelinger and C. Regelmann, Tetrahedron 1984, 40, 1845-1854: Additives atomares Punktdipol-(APUDI)- Modell zur Berechnung der 1H-NMR-Werte von benzoiden Kohlenwasserstoffen.

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