Rotational spectrum, structure, and chlorine nuclear quadrupole coupling constants of the van der Waals complex Ar–Cl2

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Abstract
The pure rotational spectrum of the van der Waals complex Ar–Cl2 has been observed between 5 and 14 GHz using a Balle–Flygare type pulsed molecular beam microwave Fourier transform spectrometer. Ten a-type rotational transitions of Ar–35Cl2, as well as five a-type rotational transitions of the mixed isotopomer Ar–35Cl37Cl, have been assigned. The rotational constants and quartic centrifugal distortion constants have been determined. Unlike its isovalent linear isomer Ar–ClF, Ar–Cl2 has been found to be a T-shaped complex. This confirms the result from electronic spectroscopy and is in accord with the atom–atom additive model. The distance from the Ar nucleus to the center of mass of the Cl2 subunit has been calculated to be 3.7190 Å for Ar–35Cl2 and 3.7184 Å for Ar–35Cl37Cl, respectively. The van der Waals stretch and bend force constants, as well as their corresponding harmonic vibrational frequencies, have been derived and compared with the theoretically predicted values. The nuclear hyperfine splittings due to the two chlorine nuclei have been resolved and the coupling constants have been accurately determined. For 35Cl these are (in MHz): χaa=54.8180(16), χbb=−110.7131(19), χcc=55.8951(19) in Ar–35Cl2; and χaa=54.8407(27), χbb=−110.706(20), χcc=55.866(20) in Ar–35Cl37Cl. For 37Cl the values are χaa=43.2111(29), χbb=−87.267(12), χcc=44.056(12) in Ar–35Cl37Cl. A value has been obtained for the nuclear quadrupole coupling constant in free diatomic chlorine.