A%20Spectroscopic%20Study%20of%20Methyl%20Acetate%20Matthew%20J.%20Kelley%20-%20Division%20of%20Chemistry%20and%20Chemical%20Engineering%20%20Geoffrey%20A.%20Blake%20-%20Division%20of%20Geological%20and%20Planetary%20Sciences%20California%20Institute%20of%20Technology,%20Pasadena,%20CA%2091125,%20USA

1
A Spectroscopic Study of Methyl AcetateMatthew
J. Kelley - Division of Chemistry and Chemical
Engineering Geoffrey A. Blake - Division of
Geological and Planetary Sciences California
Institute of Technology, Pasadena, CA 91125, USA
Figure 1 The Orion Hot core in visible (left)
and IR (right), showing a cluster of molecules.

• 3.) Formation of Methyl Acetate in the
  Interstellar Medium
• Methyl acetate could be formed through
  photolysis pathways via acetaldehyde or methyl
  formate (both found in the ISM, see below).
• Of particular interest is the possible
  esterification reaction of methanol and acetic
  acid (also present in the ISM).

• 1.) Reasons for Study
• Astrochemical Importance
• Methyl acetate is a relatively complex molecule
  with many possible routes to formation in
  molecular clouds that has not yet been detected.
• Observation possible around regions of high-mass
  star formation in hot molecular cores (100-200 K)
• Spectroscopic Importance
• Methyl acetate is a double internal rotor due to
  the acetyl- and methoxy- methyl groups (-CH3),
  thus a test to current fitting techniques.
• The energy barrier height for internal rotation
  is imprecise and extending spectral coverage will
  improve the precision.

• This reaction mechanism has not yet been observed
  or attributed to chemistry in the ISM.
• This mechanism could also help to explain the
  unusually high observed abundances of methyl
  formate.
• Structural isomers Acetic Acid Glycolaldehyde
  Methyl Formate 1 0.5 26
  within LMH (Large Molecular Heimat in Sgr B2)3

• 4.) Experimental
• Flow cell experiments were performed at 3 mm
  (90-120 GHz coverage) and at 1 mm (225-360 GHz
  coverage)
• Frequencies were synthesized by a frequency
  generator and appropriate multiplier chains.
• Detection at 3 mm was done with a diode detector
  at room temperature. At 1 mm, a liquid helium
  cooled InSb hot electron bolometer was used.
• Linewidths were 1.0 MHz
• R-branches occur every 6.1 GHz (see below)

3.) Hollis, J. M. Voel, S. N., Snyder, L. E.,
Jewell, P. R. and Lovas, F. J. Ap. J. 2001, 554,
L81. 4.) Gibb, E., Nummelin, A., Irvine, W. M.,
Whitet, D. C. B., and Bergman, P. Ap. J. 2000,
545, 309. 5.) Ikeda, M., Ohishi, M., Nummelin,
A, Dickens, J.E., and Irvine, W. M. Ap. J. 2001.
560, 792. 6.) Liu, S. Y., Mehringer, D. M. and
Snyder, L. E., Ap. J. 2001. 552, 654. 7.)
Mehringer, D. M., Snyder, L. E., Mio, Y., and
Lovas, F. J. Ap. J. 1997, 480 L71. 8.) Remijan,
A. Snyder, L. E., Liu, S.-Y., Mehringer, D. and
Kuan, Y.-J. Ap. J., 2002, 576, 264. 9.) Remijan,
A. Snyder, L. E., Friedel, D. N., Liu, S.-Y., and
Shah, R. Y. Ap. J., 2003, 590, 314.
Figure 4 Possible routes to methyl acetate in
the ISM
Source N (cm-2) Acetaldehyde N (cm-2) Methyl Formate N (cm-2) Formic Acid N (cm-2) Acetic Acid
Sgr B2 N-LMH 4.0 2.0 x 1014 1.5 0.5 x 1017 11.0 2.7 x 1015 6.1 0.6 x 1015
W51e2 2.5 0.7 x 1014 9 6 x 1017 18.0 1.6 x 1015 1.7 0.5 x 1016
G34.30.2 2.4 0.2 x 1014 1.6 0.1 x 1016 lt 7.7 x 1015 1.2 0.4 x 1015
G327.3-0.6 5.0 0.8 x 1014 5.1 1.0 x 1017 8.5 4 x 1013 ..
Figures 2, 3 Grain surface chemistry involving
CO, single-atom addition to CO1, 2
1.) W.D. Langer et Al. Chemical Evolution of
Protostellar Matter. Protostars and Planets IV.
2000.
2.) Charnley, S., Interstellar organic
chemistry. The Bridge Between the Big Bang and
Biology Stars, Planetary Systems, Atmospheres,
Volcanoes Their Link to Life. 2001.
Table 1 Column Densities of Various Organic
Molecules in Hot Cores of Interest4,5,6,7,8,9

• 5.) Past Results, Fitting, and Discussion
• The microwave spectrum from 13 40 GHz was
  first studied in 1970 and analyzed with
  first-order predictions/assignments.10
• In 1980 the spectra were measured from 8-40 GHz
  using double-resonance techniques and assigned
  using a more advanced treatment.11
• µb1.64 debye
• µa0.06 debye

Parameter Current Fit Old Fit11
A (MHz) 10248.2148(96) 10246.60(10)
B (MHz) 4169.6771(33) 4170.278(59)
C (MHz) 3077.16886(39) 3076.527(52)
?J 0.731141(76)
?JK 6.027(26)
?K -0.92(73)
dJ 0.195003(40)
dK 0.70(13)
?01 -96.94(18)
?10 -11777.40(17)
?20 254.591(79)
(A-(BC)/2)10 1.6174(59)
((BC)/2)10 -0.09066(29)
((B-C)/4)10 0.010320(14)
?1 0.05857973(85)
?2 0.06239(11)
?1 12.2332(14)
?2 171.24(21)
It 3.1990292 3.2085(26)
I? 3.2515845 3.16(24)
Ft 28.0525050 27.982(23)
F? 20.724 25.3(30)
V3 t --- 99.559(83)
V3? --- 425(31)
S 1.331 -----
n/p 341/18 76/9
Only b-type transitions (?Ka 1, ?Kc 1) are
observed.

• Large, variable splitting exist corresponding to
  a doublet (due to G00 and G01 of isomeric group
  of order 18) and a triplet (G10, G11, G12) split
  by 250 MHz to 3 GHz
• Splittings within doublet and triplet 0 to 70
  MHz
• Success was obtained by fitting using a
  least-squares fitting program written by Peter
  Groner12
• Over 1000 lines were assigned.

Figure 5 Experimental Setup
Table 2 Spectroscopic constants for the
rotational ground state spectrum of methyl
acetate. (parameter not fit but derived from
the fit parameters)
Figure 7 Millimeter spectrum of methyl acetate
from 330-360 GHz.
Figure 6 Principal axes of methyl acetate.