Portal de Periódicos da CAPES

Concerning the internal rotational barrier and the experimental and theoretical n J ( 13 C, 13 C) and n J ( 1 H, 13 C) in ethylbenzene-β- 13 C

1994; NRC Research Press; Volume: 72; Issue: 9 Linguagem: Inglês

10.1139/v94-252

1480-3291

Ted Schaefer, Wing Keung Chan, Rudy Sebastian, Robert W. Schurko, Frank E. Hruska,

Analytical Chemistry and Chromatography

The 1 H nuclear magnetic resonance spectra of ethylbenzene-β- 13 C in CS 2 /C 6 D 12 and acetone-d 6 solutions yield long-range 1 H, 1 H and 1 H, 13 C coupling constants. The 13 C { 1 H} NMR spectra yield 13 C, 13 C couplings. The conformational dependence of some of these coupling constants is compatible with two values of the barrier to internal rotation about the exocyclic Csp 2 —Csp 3 bond. If the fourfold component of the internal rotational potential is not larger than about 20% of the twofold component and the perpendicular conformer is most stable, then the barrier height is probably less than 6 kJ/mol. However, if the stable conformer has a torsion angle of 60° for the exocyclic C—C bond, then the coupling constants are consistent with a twofold barrier of about 23 kJ/mol. Experimental values of [Formula: see text] and [Formula: see text], where ψ and θ are the torsion angles for the exocyclic C—C and C—H bonds, respectively, are compared to those obtained from INDO MO FPT computations of the angular dependence of n J(H,C), and n J(H,H), n J(C,C) for n ≥ 3. For example, the computations very likely give the correct qualitative ψ dependence of 5 J(C,C), yet overestimate its extremum by about a factor of two, either because of an overestimate of the σ–π exchange integrals or because of too large a valence orbital density at the carbon nucleus. Other n J values are discussed in a similar manner and, because optimized geometries are used in the computations, a somewhat more reliable treatment arises for some coupling constants; an example is 3 J(C,C) at small torsion angles.