CONCERNING THE INTERNAL ROTATIONAL BARRIER AND THE EXPERIMENTAL AND THEORETICAL (N)J(C-13,C-13) AND (N)J(H-1,C-13) IN ETHYLBENZENE-BETA-C-13

The H-1 nuclear magnetic resonance spectra of ethylbenzene-beta-C-13 i n CS2/C6D12 and acetone-d(6) solutions yield long-range H-1,H-1 and H- 1, C-13 coupling constants. The C-13 {H-1} NMR spectra yield C-13,C-13 couplings. The conformational dependence of some of these coupling co nstants is compatible with two values of the barrier to internal rotat ion about the exocyclic Csp(2)-Csp(3) bond. If the fourfold component of the internal rotational potential is not larger than about 20% of t he twofold component and the perpendicular conformer is most stable, t hen the barrier height is probably less than 6 kJ/mol. However, if the stable conformer has a torsion angle of 60 degrees for the exocyclic C-C bond, then the coupling constants are consistent with a twofold ba rrier of about 23 kJ/mol. Experimental values of (sin(2) psi) and (sin (2) theta), where psi and theta are the torsion angles for the exocycl ic C-C and C-H bonds, respectively, are compared to those obtained fro m INDO MO FPT computations of the angular dependence of (n)J(H,C), and (n)J(H,H), (n)J(C,C) for n greater than or equal to 3. For example, t he computations very likely give the correct qualitative psi dependenc e of (5)J(C,C), yet overestimate its extremum by about a factor of two , either because of an overestimate of the sigma-pi exchange integrals or because of too large a valence orbital density at the carbon nucle us. Other (n)J values are discussed in a similar manner and, because o ptimized geometries are used in the computations, a somewhat more reli able treatment arises for some coupling constants; an example is (3)J( C,C) at small torsion angles.