The rotational spectrum and molecular structure of the furan–HCl complex

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Abstract
The microwave spectrum of the furan–HCl complex in the ground vibrational state has been measured and assigned using a Fourier‐transform microwave spectrometer employing a Fabry–Perot cavity and a pulsed supersonic nozzle as a molecular source. Furan–HCl is planar, with the axis of the HCl subunit oriented along the a axis of furan, bisecting the oxygen–carbon angle. A hydrogen bond is formed between the HCl proton and the lone electron pair of oxygen. The spectroscopic constants for furan–H 35Cl in MHz are A″=9421.3(39), B″=1004.2001(27), C″=904.5526(26), τ1=−0.9392(12), τ2=−0.0751(2), τbbbb =−0.004 01(19), τcccc=−0.002 17(17), χaa=−52.803(17), χbb=25.626(54), and χcc=27.177(54). Rotational transitions for the isotopic species furan–H 37Cl and furan–D 35Cl were also measured and assigned. The oxygen–chlorine distance is 3.26(1) Å. The binding of the HCl to oxygen, rather than to the π bonds between the β carbon atoms, was confirmed by measurements on the 2‐D furan–H 35Cl complex.