Structure and dynamics of 3'-aminoacetophenone and 4'-aminoacetophenone from rotational spectroscopy

The rotational spectra of 4'-aminoacetophenone, and those of two conformers (Z and E arrangement of the CO and NH₂ groups) of 3'-aminoacetophenone and their ¹³C and ¹⁵N isotopologues were investigated both in the microwave (2-8 GHz) and millimetre (59.6-74.4 GHz) frequency regions using chirped pulse Fourier transform and free-jet absorption techniques, respectively. The spectra consist of μ_a and μ_b type lines that show a hyperfine structure due to both the nuclear quadrupole coupling of the ¹⁴N nucleus and the methyl internal rotation. Relative intensity measurements show that the Z form in 3'-aminoacetophenone is favoured with respect to E and the measured energy difference upper limit is about 5.5(1) kJ mol^-1. Barriers to methyl internal rotation are V₃ = 7.04(2) and 6.530(6) kJ mol^-1 for 3'(Z)- and 4'-aminoacetophenone, respectively. Flexible model analyses of the amino inversion motion based on ab initio potential energy paths, suggest that the corresponding vibrational splitting increases up to 78% from aniline to 3'(E)-, 3'(Z), and 4-aminoacetophenone. However, due to supersonic expansion cooling, no splitting related to amine inversion is observed.