Remarks on the barrier to rotation about the C sp 2—O bond in anisole
1989; NRC Research Press; Volume: 67; Issue: 7 Linguagem: Inglês
10.1139/v89-173
ISSN1480-3291
Autores Tópico(s)Various Chemistry Research Topics
ResumoMolecular orbital computations with the basis set 6-31G are reported for seven values of θ, the torsion angle about the [Formula: see text] bond in anisole. All bond angles and lengths are optimized but the atoms of the phenyl group are constrained to a plane. The relative energies are fit by V(θ)/kJ mol −1 = 7.78(5) sin 2θ + 2.41(5) sin 2 2θ − 0.54(5) sin 2 3θ, where θ is zero when the heavy-atom skeleton is planar. Computations with the basis set 6-31G*(5D) for three values of θ can be reproduced by V(θ)/kJ mol −1 = 6.07 sin 2 θ + 2.68 sin 2 2θ. These results are compared with experimental gas phase data from the literature. The analysis of the 1 H nuclear magnetic resonance spectrum of anisole-α- 13 C in aCS 2 /C 6 D 12 /TMS solvent mixture yields a value of 6 J( 1 H, 13 C), the long-range spin-spin coupling constant between the 13 C nucleus in the methyl group and thepara proton. Because this coupling constant is proportional to sin 2 θ, it is shown, together with previous dynamic nmr measurements, that the barrier to rotation about the [Formula: see text] bond in solution cannot be purely twofold. The internal potential must also contain a fourfold term of the same sign as that of the twofold component. If the V 2 /V 4 ratio given by the various molecular orbital computations holds in solution, then V 2 is 15.0 ± 2.0 kJ/mol and V 4 is 5.6 ± 2.2 kj/mol. The apparent doubling of the internal barrier in solution is perhaps unprecedented for such a simple molecule. Keywords: anisole, internal barrier in solution, anisole-α- 13 C, 1 H NMR, conformational behaviour, MO computations.
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