Coupled methyl internal rotations with intermediate and low torsional barriers in 2,5-dimethylanisole investigated by microwave spectroscopy

The Journal of Chemical Physics, vol. 161

Abstract

We recorded and analyzed the microwave spectra of 2,5-dimethylanisole using a pulsed molecular jet Fourier transform microwave spectrometer and the newly developed Passage And Resonant-Impulse Synergy spectrometer across a frequency range of 2-26.5 GHz with support from quantum chemical calculations. Only one conformer was predicted and observed, where the methoxy group and its adjacent methyl group adopt anti-positions. The two methyl groups, located at the ortho- and meta-positions of the anisole ring, exhibit internal rotation, resulting in quintet splitting of all rotational transitions. The low torsional barrier of the m-methyl group, amounting to 65.723 611(84) cm-1, combined with the intermediate barrier of 451.664(19) cm-1 for the o-methyl group, presented challenges in the spectral analysis. Using the XIAM and BELGI-Cs-2Tops programs, we successfully fitted 460 torsional intrastate rotational transitions, allowing for precise determination of molecular parameters and internal rotation characteristics. The torsional barriers are compared to those in the related isomers 2,3-, 2,4-, and 3,4-dimethylanisole as well as other o- and m-substituted toluene derivatives.

Authors 4

1. Haoyue Sun

   Centre National de la Recherche Scientifique · Université Paris Cité · Université Paris-Est Créteil · Laboratoire Interuniversitaire des Systèmes Atmosphériques

   Affiliation as printed

   Université Paris Cité and Univ Paris Est Créteil, CNRS, LISA 1 , F-75013 Paris,

2. Lynn Ferres Aachen Institute of Physical Chemistry

   RWTH Aachen University

   Affiliation as printed

   Institute of Physical Chemistry, RWTH Aachen University 2 , D-52074 Aachen,

   IPC - Institute of Physical Chemistry [Aachen] (Landoltweg 2 52074 Aachen - Germany)

3. Isabelle Kleiner

   Centre National de la Recherche Scientifique · Université Paris Cité · Université Paris-Est Créteil · Laboratoire Interuniversitaire des Systèmes Atmosphériques

   Affiliation as printed

   Université Paris Cité and Univ Paris Est Créteil, CNRS, LISA 1 , F-75013 Paris,

4. Ha Vinh Lam Nguyen

   Centre National de la Recherche Scientifique · Université Paris Cité · Institut Universitaire de France · Université Paris-Est Créteil · Laboratoire Interuniversitaire des Systèmes Atmosphériques

   Affiliation as printed

   Institut Universitaire de France (IUF) 4 , F-75231 Paris,

   Univ Paris Est Créteil and Université Paris Cité, CNRS, LISA 3 , F-94010 Créteil,