The tetrahydrofuran center dot center dot center dot hydrogen chloride complex: Rotational spectrum and theoretical analysis

The hydrogen bonded heterodimer tetrahydrofuran ... HCl has been investigat ed using molecular beam Fourier transform microwave spectroscopy in combina tion with ab initio calculations. The rotational spectrum, observed in the range 6-18 GHz, shows a complex pattern originated by the existence of smal l tunneling splittings in addition to the Cl-nuclear quadrupole coupling hy perfine structure. The vibrational energy difference between the members of the doublet, Delta E=3.550(25) MHz, has been determined from the analysis of the a-type Coriolis coupling interaction between them. Doublets of the s ame magnitude are also present in the spectra of the different HCl isotopom ers analyzed. These tunneling splittings were not observed for the species C4D8O ... (HCl)-Cl-35. The analysis of all the available data has allowed u s to conclude that these splittings are due to pseudorotation within the te trahydrofuran subunit. The spectroscopic constants have been interpreted in terms of a geometry in which tetrahydrofuran has a conformation close to t he twisted ring-form with HCl lying on the plane bisector to the COC ring a ngle. The potential energy surface for the interaction between tetrahydrofu ran and hydrogen chloride has been explored by using ab initio methodologie s 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 parameter s and rotational constants of the minimum agree quite well with those deter mined from the spectroscopic data. The transition structures correspond to interconversions between equivalent conformations, the first one via an inv ersion motion and the remaining two via pseudorotation movements. One of th ese latter two is responsible for the splittings detected in the microwave spectroscopy study. The tetrahydrofuran ... hydrogen chloride interaction c an be seen as a combination of electrostatic and charge transfer contributi ons both consistent with the angular geometry exhibited by the complex. (C) 1999 American Institute of Physics. [S0021-9606(99)00637-6].