International Symposium on Molecular Spectroscopy 60 th Meeting

International Symposium on Molecular Spectroscopy 60 th Meeting - - June 20 -24, 2005 -RC 09 PSEUDOROTATION IN TETRAHYDROFURAN: SUPPLEMENTARY DATA FROM THE FREE JET ROTATIONAL SPECTRUM OF THE d 8 SPECIES Walther Caminati, Sonia Melandri, Assimo Maris, José L. Alonso, Juan C. López, Rolf Meyer Dipartimento di Chimica “G. Ciamician”- Universita’ Departamento di de. Bologna Química-Física y Química-Inorgánica, Universidad de Valladolid Sonnenbergstrasse 18, Zufikon, CH-5621, Switzerland. 1

Tetrahydrofuran: available potential energy surfaces Tetrahydrofuran (THF) is a model molecule for the investigation of the hindered pseudorotation in saturated five-membered rings. 2

Tetrahydrofuran: available potential energy surfaces Two recent papers, based on the vibrational spacings among the four lowest pseudorotation states of normal THF, reported contrastant results on the potential energy function: four equivalent minimaa, or 2 sets of two equivalent minima. MLAMCF R. Meyer, J. C. Lòpez, J. L. Alonso, S. Melandri, W. Caminati, and P. G. Favero, J. Chem. Phys. 1999, 111, 7871, following G. G. Engerholm, A. C. Luntz, W. D. Gwinn, and D. O. Harris, J. Chem. Phys. 1969, 50, 2446. . MGMd. L D. G. Melnik, S. Gopalakrishnan, T. A. Miller, F. C. D. Lucia, J. Chem. Phys. 2003, 118, 3589, 3 following A. H. Mamleev, L. N. Gunderova, and R. V. Galeev, J. Struct. Chem. 2001, 42, 365.

Tetrahydrofuran: more recent ab initio calculations PES: (a) MP 2/cc-p. VDZ, (b) MP 2/cc-p. VTZ and MP 2/aug-cc-p. VDZ, (c) MP 2/aug-cc-p. VTZ basis sets. The topology of the MP 2/aug-cc-p. VTZ potential-energy surface is fully consistent with the potential function experimentally derived. Indeed, in both cases two equivalent envelope, two equivalent twisted, and four equivalent asymmetric conformations are the stationary conformations on the tetrahydrofuran potential-energy surface. . . supports the superiority of the model employed by Mamleev et al. and Melnik et al. over the ones proposed by Engerholm et al. and Meyer et al. in order to lead to a physical interpretation of the microwave and farinfrared measurements. However, ab initio calculations predict the envelope Cs conformations to be the global minima structures in contrast with the potential functions of Mamleev et al. and Melnik et al. that lead to a twisted C 2 equilibrium geometry. Since the potential functions experimentally derived are very sensitive to the different models adopted for the fittings, we do believe that the high-level ab initio predictions reported in this work might well be useful to plan new experimental work on THF. 4 Víctor M. Rayón and Jose A. Sordo, J. Chem. Phys. 2005, 122,

Tetrahydrofuran: available potential energy surfaces MLAMCF 1) Inclusion of the Rotational constants in the fit 2) Instantaneous value of the reduced constant of the motion from the structural relaxations. 3) Not yet available the v: 2 1 splitting. 4) Correct symmetry of the wavefunctions? MGMd. L 1) Correct symmetry of wavefunctions 2) Inclusion of the v: 2 1 splitting. 3) Better agreement with ab initio calculations. 5

Tetrahydrofuran-d 8: Rotational spectrum Calculated PR splittings MLAMCF E 01/MHz MGMd. L* 97718560 Esplanations • • Needs an improved model? H D isotopic effects? 6

The spectrometer : mmw free jet absorption 7