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

• μb=1.64 debye

• μa=0.06 debye

Lock-in Amplifier

Frequency Generator

Multiplier Chain (x18 for 1 mm, x6 for 3 mm)

Flow Cell

Detector

2.) Basic Grain Chemistry

• Four Important Processes in Molecular Clouds:

1. Grain Surface Reactions of Accreted Species

2. Thermal Processing by Nearby Stars

3. Energetic Processing by High Energy Photons/Particles

4. Depletion of Molecules/Gas Grain Interaction

– Ices coating grain surfaces can be composed of H2O, CO, NH3, CH4, (CH3OH, etc.)

– UV photons can cause bond to break, creating radicals such a H, O, OH, N, NH, NH2, C, CH, CH2, CH3, (etc.)

– The HCO channel above is still applicable if the ices contain CO, though now more complexity is possible due to further photolysis of complex species

– Species on grains can be evaporated/sublimated if the cloud is surrounded by/forms new stars

– Provides means of observing/spectroscopy by using the light from stars behind the cloud.

– In the gas phase, CO is by far the dominant carbon containing species.– Hydrogenation to HCO (formyl radical) is an important first step– CH3O (methyoxy radical) chemistry is also important in the production

of more complicated molecules

– Due to a variety of factors, material accreted onto grain surfaces can be released into the gas phase.

– Particularly important in regions subject to shocks due to outflow from young stars.

Two models are considered:

– “Accretion-limited”: Reactions limited by rate of arrival on grain surface– “Reaction-limited”: Reactions limited by rate equations (commonly

considered)

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).

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

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.

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.

C

O

OHH

C

O

OHCH3

C

O

OCH3

CH3

C O

CH2 O C

O

HCH3

C

O

OH

CH3

CH3

CH3

CH3

CH3O

CH3O

CH3OHOH

formic acid

acetaldehyde

methyl formate

acetic acid

methyl acetate

CO

HCO

H2CO

CH3O

CH3OH

CO2

HCOO

HCOOH

CH3OO

CH3OC

CH3OCH3

CCHO

CH3CHO

CH3CH2O

HCON

HCONH2

O

O

O

H

H

H

H

3H

H ?

3H

C

N

H2

Figure 1: The Orion Hot core in visible (left) and IR (right), showing a cluster of molecules.

SourceN (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

G34.3+0.2 2.4 ± 0.2 x 1014 1.6 ± 0.1 x 1016 < 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 …..

– 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)34.) 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)

a

b

C

CH3

O

O

CH3

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

Table 1: Column Densities of Various Organic Molecules in Hot Cores of Interest4,5,6,7,8,9

Figure 5: Experimental Setup

Only b-type transitions (ΔKa= ±1, ΔKc= ±1) are observed.

• Large, variable splitting exist corresponding to a doublet (due to Γ00 and Γ01 of isomeric group of order 18) and a triplet (Γ10, Γ11, Γ12) 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.

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)*

ΔJ 0.731141(76)

ΔJK 6.027(26)

ΔK -0.92(73)

δJ 0.195003(40)

δK 0.70(13)

Є01 -96.94(18)

Є10 -11777.40(17)

Є20 254.591(79)

(A-(B+C)/2)10 1.6174(59)

((B+C)/2)10 -0.09066(29)

((B-C)/4)10 0.010320(14)

ρ1 0.05857973(85)

ρ2 0.06239(11)

Β1 12.2332(14)

Β2 171.24(21)

Iτ 3.1990292* 3.2085(26)

Iν 3.2515845* 3.16(24)

Φτ 28.0525050* 27.982(23)

Φν 20.724* 25.3(30)

V3 τ --- 99.559(83)

V3ν --- 425(31)

Σ 1.331 -----

n/p 341/18 76/9

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.