The hydrogen bonded heterodimer tetrahydrofuran⋯HCl has been investigated using molecular beam Fourier transform microwave spectroscopy in combination with ab initio calculations. The rotational spectrum, observed in the range 6–18 GHz, shows a complex pattern originated by the existence of small tunneling splittings in addition to the Cl-nuclear quadrupole coupling hyperfine structure. The vibrational energy difference between the members of the doublet, ΔE=3.550(25) MHz, has been determined from the analysis of the a-type Coriolis coupling interaction between them. Doublets of the same magnitude are also present in the spectra of the different HCl isotopomers analyzed. These tunneling splittings were not observed for the species C4D8O⋯H35Cl. The analysis of all the available data has allowed us to conclude that these splittings are due to pseudorotation within the tetrahydrofuran subunit. The spectroscopic constants have been interpreted in terms of a geometry in which tetrahydrofuran has a conformation close to the twisted ring-form with HCl lying on the plane bisector to the COC ring angle. The potential energy surface for the interaction between tetrahydrofuran and hydrogen chloride has been explored by using ab initio methodologies at the correlated level [MP2, MP4(SDTQ)] with Pople’s 6-31G** and Dunning’s aug-cc-pVDZ basis sets. One minimum and three transition structures were located and characterized at the MP2/6-31G** level. The geometry parameters and rotational constants of the minimum agree quite well with those determined from the spectroscopic data. The transition structures correspond to interconversions between equivalent conformations, the first one via an inversion motion and the remaining two via pseudorotation movements. One of these latter two is responsible for the splittings detected in the microwave spectroscopy study. The tetrahydrofuran⋯hydrogen chloride interaction can be seen as a combination of electrostatic and charge transfer contributions both consistent with the angular geometry exhibited by the complex.
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The rotational spectrum of the hydrogen bond heterodimer 2,5-dihydrofuran⋯HF has been investigated in the frequency region 6–18.5 GHz using molecular beam Fourier transform microwave spectroscopy. The R-branch μa-type spectra of the parent, 2,5-dihydrofuran⋯DF, Cα13-2,5-dihydrofuran⋯HF and Cβ13-2,5-dihydrofuran⋯HF species have been measured. All the observed lines are split into two components due to the tunneling of HF between the two equivalent hydrogen bonding sites at the oxygen atom. A value for the barrier to HF inversion of 100 cm−1 has been calculated from the analysis of the rotational constants of both tunneling states on the basis of a simple one-dimensional model. Ab initio calculations at the MP2/6-311+G(d,p) and MP2/6-311+G(2df,p) levels of theory have been carried out to complement the experimental data. The results on the HF inversion barrier and hydrogen bond angular geometry of 2,5-dihydrofuran⋯HF have been compared to those previously obtained for the H2O⋯HF complex.
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