1. DIMETHYL -ETHER THREE DIMENTIONAL SPECTRA M. VILLA U.A.M.-I. (México) and M. L. SENENT C.S.I.C. (Spain)

2. Objectives: We present preliminary results for DME 1) DME is an interstellar molecule detected by Snyder et al. Astrophys. J. (1974) 2) We are repeating a work that has been done previously. Theoretical study of the molecule Far-Infrared spectra : a) 2D (Two CH 3 torsions) Senent et al. Can. J. Phys., (1995). b) 3D (Both CH 3 torsions + COC bending) Senent et al, J.Chem.Phys. (1995).

3. Why to repeat? Both old studies have been performed using MP2/6-31G(d,p). Now we will use: State-of-the-art calculations: CCSD(T)/aug-cc-pVTZ DZ  Aug-TZ CH 3 interactions will be better described.

4. Why two preceding papers? 2D model is insufficient to reproduce properly the band positions 3D model allow us to describe well the interactions between H of the methyl groups The interaction between H of the methyl groups are quite strong Fermi interactions between the torsional modes and the COC bending mode. Conclusion: The three motions are not separable.

5. FIR RAMAN Experimental: Groner et al (1977), Calculated: Senent et al (1995)

6. Internal coordinates :  1, and  2 for the CH 3 rotation and the  for the COC bending.

7. a)the dipole moment has been calculated with MP2/aug-cc-pVTZ b)distances in A°; angles in degrees; dipole moment in Debyes; rotational constants in MHz Exp: = 1.302 D

9. Anharmonic analysis (3N-6 D) Force field determination Performed using second order perturbation theory (FIT-ESPEC) 1)Preliminary results for torsional frequencies 2)Description of interactions between torsions and remaining vibrational modes f ij CCSD/aug-cc-pVTZ f ijk and f ijkl MP2/aug-cc-pVTZ Force field calculated with GAUSSIAN

11. Perturbation theory A 0 = 38 788.2 MHz B 0 = 10 056.5 MHz C 0 = 8 886.8 MHz Lovas, et al.J. Phys. Chem. Ref. Data. (1979).

12. Fundamental frequencies (in cm-1) MP2/6-31G** (Variational) MP2/aug-cc-pVTZ (PT) MP2/aug-cc-pVTZ (Variational) CCSD/aug-cc-pVTZ (PT) Exp. COC Bending  7 430405420401412 CH 3 Torsion  15 245246234235241 CH 3 Torsion  11 193202191189

14. Future: variational calculation of isotopomers In progress : CCSD(T)/aug-cc-pVTZ PES

15. Conclusions With this calculations one obtains an improvement for the spectroscopic parameters and the transitions. By using perturbation theory one is able to see the Fermi interactions. Thank you for your attention