1. 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 μ b =1.64 debye μ a =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 H 2 O, CO, NH 3, CH 4, (CH 3 OH, etc.) –UV photons can cause bond to break, creating radicals such a H, O, OH, N, NH, NH 2, C, CH, CH 2, CH 3, (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 –CH 3 O (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 Acetate Matthew 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 (-CH 3 ), 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 CO 1, 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. Figure 1: The Orion Hot core in visible (left) and IR (right), showing a cluster of molecules. Source N (cm -2 ) Acetaldehyde N (cm -2 ) Methyl Formate N (cm -2 ) Formic Acid N (cm -2 ) Acetic Acid Sgr B2 N-LMH4.0 ± 2.0 x 10 14 1.5 ± 0.5 x 10 17 11.0 ± 2.7 x 10 15 6.1 ± 0.6 x 10 15 W51e22.5 ± 0.7 x 10 14 9 ± 6 x 10 17 18.0 ± 1.6 x 10 15 1.7 ± 0.5 x 10 16 G34.3+0.22.4 ± 0.2 x 10 14 1.6 ± 0.1 x 10 16 < 7.7 x 10 15 1.2 ± 0.4 x 10 15 G327.3-0.65.0 ± 0.8 x 10 14 5.1 ± 1.0 x 10 17 8.5 ± 4 x 10 13 ….. –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) a b 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 Interest 4,5,6,7,8,9 Figure 5: Experimental Setup Only b-type transitions (ΔKa= ±1, ΔKc= ±1) are observed. Large, variable splitting exist corresponding to a doublet (due to Γ 00 and Γ 01 of isomeric group of order 18) and a triplet (Γ 10, Γ 11, Γ 12 ) 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 Groner 12 Over 1000 lines were assigned. ParameterCurrent FitOld Fit 11 A (MHz)10248.2148(96)10246.60(10)* B (MHz)4169.6771(33)4170.278(59)* C (MHz)3077.16886(39)3076.527(52)* ΔJΔJ 0.731141(76) Δ JK 6.027(26) ΔKΔK -0.92(73) δJδJ 0.195003(40) δKδK 0.70(13) Є 01 -96.94(18) Є 10 -11777.40(17) Є 20 254.591(79) (A-(B+C)/2) 10 1.6174(59) ((B+C)/2) 10 -0.09066(29) ((B-C)/4) 10 0.010320(14) ρ1ρ1 0.05857973(85) ρ2ρ2 0.06239(11) Β1Β1 12.2332(14) Β2Β2 171.24(21) IτIτ 3.1990292*3.2085(26) IνIν 3.2515845*3.16(24) ΦτΦτ 28.0525050*27.982(23) ΦνΦν 20.724*25.3(30) V 3 τ ---99.559(83) V3νV3ν ---425(31) Σ1.331----- n/p341/1876/9 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.