Two Coupled Low-Barrier Large Amplitude Motions in 3,5-Dimethylanisole Studied by Microwave Spectroscopy

Area of Science:

• Physical Chemistry
• Molecular Spectroscopy
• Quantum Chemistry

Background:

• Internal rotation of methyl groups in aromatic compounds influences molecular properties.
• Understanding torsional barriers is crucial for predicting molecular behavior and reactivity.
• Previous studies on toluene derivatives provide a basis for comparison.

Purpose of the Study:

• To investigate the internal rotation of methyl groups in 3,5-dimethylanisole using microwave spectroscopy.
• To determine the torsional barriers for the syn-m and anti-m methyl groups.
• To elucidate the role of electrostatic versus steric effects in low-barrier internal rotations.

Main Methods:

• Recording the microwave spectrum of 3,5-dimethylanisole (2.0–26.5 GHz) using a pulsed molecular jet Fourier transform microwave spectrometer.
• Analyzing spectral splittings attributed to internal methyl group rotations.
• Modeling the rotational and torsional parameters using the XIAM and ntop programs with 622 rotational lines.

Main Results:

• Observed and analyzed spectral splittings from internal rotations of syn-m and anti-m methyl groups.
• Determined torsional barriers for the syn-m methyl group (58.62367(53) cm⁻¹) and the anti-m methyl group (36.28449(69) cm⁻¹).
• Successfully assigned the complex spectrum by employing combination difference loops and fitting five torsional components.

Conclusions:

• The low torsional barriers for both methyl groups presented significant challenges in spectral analysis.
• Comparison with other toluene derivatives suggests electrostatic effects are more dominant than steric effects in low-barrier aromatic systems.
• The study provides valuable insights into the dynamics of methyl group rotation in substituted aromatic molecules.