Two Conformers Observed and Characterized in Isobutylbenzene

Abstract

The microwave spectrum of isobutylbenzene (2-methyl-1-phenylpropane) reveals the presence of two conformers that are characterized by their microwave spectra and by quantum chemical calculations. The more stable conformer has a gauche configuration of the C(phenyl)-C(1)-C(2)-H chain coupled with a approximately 80 degrees dihedral angle between the phenyl group and the C(phenyl)-C(1)-C(2) plane with C(1) symmetry. The less stable conformer has a plane of symmetry, C(s), with an anti configuration of the C(phenyl)-C(1)-C(2)-H chain coupled with a 90 degrees dihedral angle between the phenyl group and the C(phenyl)-C(1)-C(2) plane. The rotational constant values are 3070.9273(4) MHz, 736.01980(6) MHz, and 680.92889(6) MHz for the C(1) species and 2500.780(8) MHz, 885.72743(10) MHz, and 770.42036(10) MHz for the C(s) species. Quantum chemical calculations are in agreement with these structures and predict a relative energy between those two conformers of 0.4 kcal/mol.

The shape of ibuprofen in the gas phase

“…For isobutyl benzene, i.e., ibuprofen's structural motif without the propanoic acid group, both the asymmetric and the symmetric conformers were detected in a recent cavity-based Fourier transform microwave (FTMW) spectroscopy study. 20 The transition intensities of the symmetric one were about one order of magnitude lower than of the asymmetric form. Based on their experimental data, the authors estimated an energy difference between the lower-lying asymmetric and the symmetric conformer of about 4 kJ mol À1 .…”

“…Recently, two different conformers were observed spectroscopically for ibuprofen's structural motif isobutyl benzene. 20 It will be interesting to compare these findings with the different conformers of ibuprofen, which is structurally more complex. Intramolecular interactions arising from the carboxy group can become important.…”

New insights into the degradation mechanism of ibuprofen in the UV/H2O2 process: role of natural dissolved matter in hydrogen transfer reactions

“…S1 (ESI †), which are consistent with those obtained by previous experimental and theoretical studies. 47,48 The structures were differentiated by R-S isomerism and the orientation of the carboxyl group and isopropyl group. As can be seen in Fig.…”