Relaxation of the acetyl group through an angle limited by steric hindrance has been observed in 2,4,6-trimethylacetophenone and in 9-acetylphenanthrene. The enthalpy of activation for this process is less than that in acetophenone. A similar type of motion is possible in 1-acetylnaphthalene but the overlapping absorptions due to group and molecular relaxations could not be separated by the technique employed. In 9-acetylanthracene only molecular relaxation appears to be detectable, indicating that any possible acetyl group relaxation in this compound may be limited to small angles.
I NTRODUCTI ON
A variety of techniques has been applied to the investigation of steric hindrance effects in organic molecules. In 2,4,6-ttimethylacetophenone, infrared data {I] suggest that resonance of the acetyl group with the aromatic ring is considerably reduced in comparison to acetophenone. Data for 13C shielding from n.m.r, work [2] indicates that the acetyl group in this compound is twisted out of the ring plane by 510 , while molar Kerr constants [3] suggest a value of go Β° for this dihedral angle. Mountain and Walker [4] have reported that the dipole moment of this compound is comparable to that of compounds containing non-conjugated acetyl groups, and were unable to detect relaxation of this substituent group from dielectric studies in the microwave region of a solution of this compound in non-polar p-xylene.
The ability to detect from dielectric data a short relaxation time