Neighborhood matters: Steric effects on methyl internal rotation and chlorine nuclear quadrupole coupling in 2-fluoro-4-chlorotoluene

The rotational spectrum of 2-fluoro-4-chlorotoluene was studied in the frequency range 4 to 24 GHz by Fourier transform microwave spectroscopy. Fine torsional splittings into doublets were found for all rotational transitions due to methyl internal rotation hindered with a barrier of about 230 cm(-1). In combination with the quadrupole coupling hyperfine splittings on the same order of magnitude originating from the chlorine nucleus, rich spectral features were observed. Rotational and quadrupole coupling constants for the Cl-35 and Cl-37 isotopologues studied in natural abundance were precisely determined. Results from the experiment were supported by quantum chemical calculations at several levels of theory for the rotational constants and by Bailey's semi-experimental method for the quadrupole coupling constants. The chi(cc) quadrupole coupling tensor element and the barrier to methyl internal rotation were compared with those of other halogen-substituted toluene derivatives. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Rotational spectrum of 2-fluoro-4-chlorotoluene was studied using Fourier transform microwave spectroscopy, revealing fine torsional splittings and quadrupole coupling hyperfine splittings. Precise rotational and quadrupole coupling constants for Cl-35 and Cl-37 isotopologues were determined, supported by quantum chemical calculations and Bailey's semi-experimental method. Comparison with other halogen-substituted toluene derivatives was made in terms of quadrupole coupling tensor element and barrier to methyl internal rotation.