Internal rotation and chlorine nuclear quadrupole coupling of o-chlorotoluene studied by microwave spectroscopy and ab initio calculations

The microwave spectrum of o-chlorotoluene has been reinvestigated using molecular beam Fourier transform microwave (MB-FTMW) spectrometers in the frequency range of 4–23 GHz. Due to the high resolution of this molecular beam technique the analysis yielded improved rotational constants, centrifugal distortion constants, and, for the first time, the complete chlorine nuclear quadrupole coupling tensor. From the torsional fine structure the barrier to internal rotation of the methyl group was found to be 5.5798(52) kJ mol⁻¹. Experimental results and ab initio calculations are compared.