Analysis of the Complex Quadrupole Hyperfine Patterns for Two Chlorine Nuclei in the Rotational Spectrum of 2,5-Dichlorothiophene.

The rotational spectrum of 2,5-dichlorothiophene (DCT) was measured for the first time using Fourier transform microwave spectroscopy from 5.5-19 GHz. Dense hyperfine splitting patterns due to the two quadrupolar chlorine nuclei (I = 3/2) were resolved and assigned for the 35Cl-35Cl, 37Cl-35Cl, and 37Cl-37Cl isotopologues and for the two 13C and one 34S analogues with two 35Cl atoms, allowing derivation of their respective nuclear quadrupole coupling tensors. The rotational constants obtained from fitting the spectra of the six isotopic species allowed derivation of the experimental geometry of DCT for comparison with the equilibrium structure computed at the MP2/aug-cc-pVTZ level. This revealed that the electron-withdrawing effect of chlorine causes small distortions in the ring geometry relative to thiophene, including a 1.1° increase in the two S-C-C angles and a 0.012 Å increase in the two S-C bond lengths.