DIMETHYL -ETHER THREE

DIMENTIONAL SPECTRA

M. VILLA

U.A.M.-I. (México)

and

M. L. SENENT

C.S.I.C. (Spain)

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 CH3 torsions) Senent et al. Can. J. Phys., (1995).

b) 3D (Both CH3 torsions + COC bending) Senent et al,

J.Chem.Phys. (1995).

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-TZCH3 interactions will be better described.

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.

FIR RAMAN

Experimental: Groner et al (1977) , Calculated: Senent et al (1995)

Internal coordinates : 1 , and 2 for the CH3 rotation and the for the COC bending.

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

Structural parameters, dipole momenta and rotational constants corresponding to the

minimum energy geometry of DME calculated with CCSD/aug-cc-pVTZb.

O1C2 = O1C3 1.4090 H4C2 = H7C3 1.0879 H5C2 = H6C2 = H8C3 = H9C3 1.0962 C3O1C2 111.4 H4C2O1 = H7C3O1 107.5 H5C2O1 = H6C2O1 = H8C3O1 = H9C3O1 111.2 H4C2O1C3 = H7C3O1C2 180.0 H5C2O1H4 = H9C3O1H7 119.4 H6C2O1H4 = H8C3O1H7 -119.4 1.4882

Ae Be Ce

38973.4024 10145.2871 8961.8951

Exp: = 1.302 D

Rotational Barriers (cm-1)

MP2 CCSD CCSD+ZPVE

V060 922 863 883

V6060 1665 1592 1610

Rotational Barriers (cm-1)

MP2/6-31G**

V060 991

V6060 1780

Anharmonic analysis (3N-6 D)

Force field determination

Performed using second order perturbation theory (FIT-ESPEC)

1) Preliminary results for torsional frequencies2) Description of interactions between torsions and

remaining vibrational modes

fij CCSD/aug-cc-pVTZfijk and fijkl MP2/aug-cc-pVTZ

Force field calculated with GAUSSIAN

Fundamental frequencies (in cm-1) of various isotopomers of DME of C2V symmetry.

sym. mode

MP2 CCSD DME-h6 DME-h6 DME-

d6 DME-

d2

A1 1 3179 3043 3147 3014 2270 2864 2 3024 2827 3011 2766 2039 2148 3 1533 1489 1537 1492 1163 1491 4 1498 1461 1511 1474 1084 1377 5 1275 1244 1291 1260 1060 1114 6 958 931 971 946 838 845 7 417 405 421 401 331 373

A2 8 3090 2979 3059 2977 2203 3059 9 1507 1462 1511 1465 1069 1296 10 1173 1151 1180 1156 880 1108 11 210 202 202 189 138 176

B1 12 3082 2950 3055 2913 2204 2910 13 1518 1471 1521 1474 1076 1299 14 1201 1173 1212 1183 938 1140 15 260 246 252 235 183 207

B2 16 3178 3043 3145 3027 2262 2864 17 3019 2898 3002 2936 2051 2160 18 1519 1471 1522 1475 1184 1472 19 1467 1434 1478 1445 1083 1349 20 1211 1176 1231 1198 1070 1166 21 1131 1105 1139 1113 866 926

Rotational parameters (in MHz) MP2 CCSD DME-h6 DME-h6 DME-d6 J 0.0095 0.0091 0.0048 K 0.3346 0.3328 0.0663 JK -0.3134 -0.0292 0.0018 Ae Be Ce

38549.07 10192.86 8974.20

38973.13 10145.29 8961.41

25853.98 7533.13 6841.78

A0 B0 C0

38510.51 10358.27 9103.91

38764.72 10298.74 9083.22

25727.95 7630.60 6927.51

Perturbation theory

A0 = 38 788.2 MHz B0 = 10 056.5 MHzC0 =  8 886.8 MHzLovas, et al .J. Phys. Chem. Ref. Data. (1979).

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

430 405 420 401 412

CH3

Torsion 15

245 246 234 235 241

CH3

Torsion 11

193 202 191 189 189

Fermi displacements of the torsional levels (cm-1) MP2 CCSD

DME-h6 DME-h6 DME-d6 215 (torsion) 7 (COC bending)

491 498 410 405

467 479 413 401

471 482 341 331

𝐻ሺ𝛼,𝜃1,𝜃2ሻ= − ൬𝑑𝑑𝑞𝑖൰𝐵𝑞𝑖𝑞𝑗 ቆ 𝜕𝜕𝑞𝑗ቇ+ 𝑉ሺ𝛼,𝜃1,𝜃2ሻ𝑗𝑖

+𝑉𝑍𝑃𝐸ሺ𝛼,𝜃1,𝜃2ሻ

Future: variational calculation of isotopomers

In progress :

CCSD(T)/aug-cc-pVTZ PES

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