Laboratory and Possible Interstellar Detection of trans-Methyl Formate MATT T. MUCKLE, JUSTIN L. NEILL, DANIEL P. ZALESKI, and BROOKS H. PATE University of Virginia, Chemistry Department S. SPEZZANO, V. LATTANZI and M.C. MCCARTHY Harvard- Smithsonian Center for Astrophysics, and School of Engineering and Applied Sciences, Harvard University And A.J. REMIJAN, National Radio Astronomy Observatory Background photo from :

Collaborative Effort Cavity FTMW up to 40 GHz and pulsed discharge source from CfA PRIMOS data from NRAO Microwave Microwave Double Resonance Gordon G. Brown, Brian C. Dian, Kevin O. Douglass, Scott M. Geyer, Steven T. Shipman, and Brooks H. Pate, Rev. Sci. Instrum. 79 Jens-Uwe Grabow, E. Samuel Palmer, Michael C. McCarthy, and Patrick Thaddeus, Rev. Sci. Instrum. 76, (2005) Masakazu Nakajima, Yoshihiro Sumiyoshi, and Yasuki Endo, Rev. Sci. Instrum. 73, 165 (2002), DOI: / Chirped Pulse Fourier Transform Spectroscopy

The Methyl Formate “Problem” High abundance interstellar species Mechanism for production under study Gas Phase – Horn et al (2004)*—considered many gas phase routes [CH 3 OH 2 ] + + H 2 CO  [HC(OH)OCH 3 ] + + H 2 H 2 C=O + [H 2 C=O-H] +  [HC(OH)OCH 3 ] + + hv [CH 3 OH 2 ] + + CO  [HC(OH)OCH 3 ] + + hv  CH HCOOH  [HC(OH)OCH 3 ] + + hv – Activation barriers all too high to explain current observed abundances Grain Surface Chemistry** Photoionized surface reactions Models create a diverse chemical environment calculating some structural isomer ratios better than previous attempts including methyl formate to acetic acid and glycolaldehyde – HCO + CH 3 O → CHOOCH 3 How can we test production mechanisms? *A. Horn et al., Ap.J., 611 (2004) *** R.T. Garrod, S.L. Widicus Weaver, and E.Herbst, Ap.J., 682 (2008)

Testing MF Production Mechanisms Light black lines: methyl formate Dark black lines: formic acid Suggests methyl formate abundance at the expense of formic acid High Resolution Spatial Mapping * * S.-Y. Liu, J.M. Girart, A. Remijan, and L.E. Snyder, Ap.J., 576 (2002)

Conformers of Methyl Formate Cis \ Trans

Conformational Properties of Methyl Formate Very high (5000 cm -1 /7200 K) isomerization barrier (cis to trans) Equilibrium cis/trans ratio ~14000:1 at 300K 3*10 12 :1 at 100K  Suggests “freezing” of cis/trans population ratio  Allows for non thermal distribution of methyl formate between the cis/trans conformers—insight into production mechanisms? trans µ a = 4.1 D (ab initio) µ b = 2.8 D (ab initio) A = MHz B = MHz C = MHz V 3 = 14.9 cm -1 cis µ a = 1.63 D (Bauder 1979) µ b = 0.68 D (Bauder 1979) A = MHz (Curl 1959) B = MHz (Curl 1959) C = MHz (Curl 1959) V 3 = cm-1 (Oesterling et al 1998) Senent et al., Ap.J., 627 (2005) Mp2/ g(d,p)

Example Mechanism (Fischer Esterification) 23kJ/mol barrier to reaction 4.1kJ difference in transition states Calculated cis/trans ratio CH 3 OH +HCOOH 2 +  CH 3 OCHOH + + H 2 0 Mp2/6-31 g(d,p)

The laboratory search for trans-methyl formate Difficult E state fit due to low V 3 Mp2/6-31++g(d,p)

The search for trans-methyl formate Difficult E state fit due to low V 3 Mp2/6-31++g(d,p)

Chirped Pulse Fourier Transform Microwave spectrometer GHz 1us Linear sweep Direct Digitization 10 FID's/ gas pulse  Reduces sample consuption 3 Gas Input Nozzles  Linear 3x signal/noise increase saving 9x in time

Broadband FTMW vs. Cavity FTMW Broadband Lower Resolution (100KHz KHz FWHM) Requires High Power (up to 1KW) High Bandwidth/acquisition (11GHz) No scanning required Accurate relative intensities to ~20% Cavity FTMW High Resolution ( ~5 KHz) Requires sub-mW MW power for most molecules (  ~ 0.1 D) Limited Bandwidth/acquisition (<1MHz) Slow scan speed!! 14 hours / 11 GHz Difficult to obtain accurate intensities

Acquisition Time Reduction

Sample Reduction 10 FID’s acquired per valve pulse 3 Pulsed Valve Nozzles for linear 3x signal gain Saves a factor of 30 in sample and 90 in time

Pulsed-Jet Methyl Formate Spectrum Observed:5500:1 cis/trans intensity ratio (30000:1 in population) x avgs (90min)

CfA Discharge Nozzle Enhancement 800V discharge increased trans signal by 100x Without this enhancement a confident fit would be much more difficult

Double Resonance Searches  A single transition is monitored in a Balle-Flaygare microwave cavity  Microwave horn orthogonal to the cavity removes coherence of a single transitions  A second frequency is scanned while monitoring the cavity transition for intensity depletion  All transitions connected by a quantum state to the resonant transition will be removed

Cavity Double Resonance Spectrometer INSERT SCHEMATIC HERE

Measurement Approach Survey Scans (36-40GHz) at Harvard-Smithsonian CfA to find B type transitions (strongly dependent on internal rotor) Precision FTMW frequencies for all transitions found by CfA spectrometer State connectivity confirmed by MW-MW double resonance at UVA

A+E Global Fit ParameterExperimentalAb Initio A (MHz)47357(320) B (MHz) (6) C (MHz) (1) Δ J (kHz)1.1(1) Δ JK (kHz)-124(9) δ J (kHz)0.108(5) Δ Km (MHz)-163(61) Δ Jm (MHz)0.92(8) δ m (MHz)-1.6(6) V 3 (cm -1 )14.9(6)22.6 θ top (deg) a 23.49(16)26.0 I α (amu Å 2 ) 3.18(6) Nlines28 rms error (kHz)35 Fit with XIAM H. Hartwig and H. Dreizler, Z. Naturforsch 51a (1996)

Tentative GBT Methyl Formate Detection TransitionA speciesE species a All interstellar data from publicly available PRIMOS website ± Data from 5 spectral regions in Sgr- B2N (64km/s Doppler shift)available that correspond with possible trans methyl formate transitions All 5 lines found – NO negative searches A-E splitting corresponds with lab data ±

Population Determination From rough column density calculations – 100:1 cis:trans Formed at 10K: – 32cm -1 (0.4kJ) difference in transition state Formed at 100K: – 321cm -1 (3.96kJ) difference in transition state J. M. Hollis, P. R. Jewell, F. J. Lovas, and A. Remijan. Apj. 613(2004) L45.Q

Conclusions Collaboration between UVA, CfA and NRAO yielded efficient assignment of trans methyl formate All 28 lines with appreciable intensity from 6- 40GHz assigned and state connectivity confirmed 5/5 lines searched for were found suggests trans methyl formate may exist in the interstellar medium Identification of isomeric species can aid in identification of interstellar production mechanisms

Acknowledgements Pate laboratory Brooks Pate Justin Neill Danny Zaleski Christoph E. Harvard-Smithsonian Cfa Mike McCarthy Silvia Spezzano Valerio Lattanzi NRAO Tony Remijan Centers for Chemical Innovation

Double Resonance Searches  A single transition is monitored in a Balle-Flaygare microwave cavity  Microwave horn orthogonal to the cavity removes coherence of a single transitions  A second frequency is scanned while monitoring the cavity transition for intensity depletion  All transitions connected by a quantum state to the resonant transition will be removed

Cavity Double Resonance Spectrometer INSERT SCHEMATIC HERE

Double Resonance Searches -Single strong candidate Methyl Formate A type Lines monitored in the cavity -Line width can be extrapolated to measure dipole moment due to power broadening

Energy Calculations