LAUSR

Abstract Supersonic expansion rotational spectroscopy at 4–18 GHz with rectangular pulse and chirp excitation was used to measure spectra of twelve isotopic species of 1,2-dichlorobenzene at conditions of completely resolved double chlorine hyperfine structure by using a newly constructed slit expansion nozzle. Four species: 37Cl13 and the three singly substituted 13C species have been assigned for the first time. Spectroscopic constants for six isotopic species with single deuterium substitution were considerably improved by using chemically synthesised samples. Hyperfine constants in the field gradient principal axes system were determined and used as a test of self-consistency of the analysis. Additional room-temperature measurements up to 275 GHz allowed assignment of rotational transitions in first excited states of the three lowest normal modes. The determined values of vibrational changes in rotational constants, inertial defects, and quartic and sextic centrifugal distortion constants, allowed calibration of harmonic and anharmonic force field calculations, resulting in determination, among others, of precise semi-experimental equilibrium, r e SE , geometry of 1,2-dichlorobenzene. It is found that r(C-C) and ∠ (C-C-C) in the phenyl ring of 1,2-dichlorobenzene show very small changes relative to benzene.