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 CO 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 mu a and mu 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. Molecules are flexible objects. Despite the rigidity of the phenyl ring, aminoacetophenones in the gas phase are prototype systems suitable for the study of several internal motions: acetyl torsion, methyl internal rotation and amine roto-inversion.