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 :http://antwrp.gsfc.nasa.gov/apod/ap090519.html

Collaborative Effort• Cavity FTMW up to 40 GHz and

pulsed discharge source from CfAPRIMOS 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, 093106 (2005) http://www.cv.nrao.edu/~aremijan/PRIMOS/

Masakazu Nakajima, Yoshihiro Sumiyoshi, and Yasuki Endo, Rev. Sci. Instrum. 73, 165 (2002), DOI:10.1063/1.1426230

404

505

3 03

2 02 2 12

3 13

4 14

5 15

4 13

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

[CH3OH2]+ + H2CO [HC(OH)OCH3]+ + H2

H2C=O + [H2C=O-H]+ [HC(OH)OCH3]+ + hv

[CH3OH2]+ + CO [HC(OH)OCH3]+ + hv

CH3+ + HCOOH [HC(OH)OCH3]+ + 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 + CH3O → CHOOCH3

How can we test production mechanisms?

*A. Horn et al., Ap.J., 611 (2004) 605-614 *** R.T. Garrod, S.L. Widicus Weaver, and E.Herbst, Ap.J., 682 (2008) 283-302

Testing MF Production Mechanisms

• Light black lines: methyl formateDark 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) 255-263.

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*1012:1 at 100K

Suggests “freezing” of cis/trans population ratio

Allows for non thermal distribution of methyl formate between the cis/trans conformers—insight intoproduction mechanisms?

transµa = 4.1 D (ab initio)µb = 2.8 D (ab initio)A = 47354.28 MHzB = 4704.440 MHz C = 4398.435 MHzV3 = 14.9 cm-1

cisµa = 1.63 D (Bauder 1979)µb = 0.68 D (Bauder 1979)A = 19985.71 MHz (Curl 1959)B = 6914.63 MHz (Curl 1959)C = 5304.47 MHz (Curl 1959)V3 = 398.76 cm-1 (Oesterling et al 1998)

Senent et al., Ap.J., 627 (2005) 567-576

Mp2/6-31 ++g(d,p)

Example Mechanism (Fischer Esterification)

23kJ/mol barrier to reaction

4.1kJ difference in transition states

Calculated cis/trans ratio ~120:1 @100K

CH3OH +HCOOH2+ CH

3 OCHOH+ + H20

Mp2/6-31 g(d,p)

The laboratory search for trans-methyl formate

• Difficult E state fit due to low V3

Mp2/6-31++g(d,p)

The search for trans-methyl formate

• Difficult E state fit due to low V3

Mp2/6-31++g(d,p)

Chirped Pulse Fourier Transform Microwave spectrometer

6.5-18.5GHz 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)

x4500

30000avgs (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

404

505

303

202212

313

414

515

413

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 state

7 0 7 6 1 6 8812.53281 0 1 0 0 0 9124.22097 1 6 7 1 7 9160.37804 1 4 5 0 5 11124.53528 1 7 8 1 8 11776.68169 1 8 9 1 9 14719.24502 1 2 1 1 1 17921.5076

10 1 9 10 1 10 17987.58322 0 2 1 0 1 18247.03842 1 1 1 1 0 18575.91643 1 3 4 0 4 20877.49583 1 3 2 1 2 26881.34213 0 3 2 0 2 27367.04803 1 2 2 1 1 27862.94052 1 2 3 0 3 30479.00044 1 4 3 1 3 35840.07824 0 4 3 0 3 36482.85044 1 3 3 1 2 37148.83401 1 1 2 0 2 39924.5432

E state1 0 1 0 0 0 9207.42742 1 2 1 1 1 17820.96172 0 2 1 0 1 18367.84812 0 2 3 1 3 26053.73803 1 3 2 1 2 26750.95973 0 3 2 0 2 27440.99221 0 1 2 1 2 34436.84934 1 4 3 1 3 35701.08554 0 4 3 0 3 36406.6717

A+E Global FitParameter Experimental Ab Initio

A (MHz) 47357(320) 46543.42

B (MHz) 4704.44(6) 4732.99

C (MHz) 4398.434(1) 4417.46

ΔJ(kHz) 1.1(1)

ΔJK (kHz) -124(9)

δJ (kHz) 0.108(5)

ΔKm (MHz) -163(61)

ΔJm (MHz) 0.92(8)

δm (MHz) -1.6(6)

V3 (cm-1) 14.9(6) 22.6

θtop (deg)a 23.49(16) 26.0

Iα (amu Å2) 3.18(6) 3.149

Nlines 28

rms error (kHz) 35

Fit with XIAMH. Hartwig and H. Dreizler, Z. Naturforsch 51a (1996) 923-932.

Tentative GBT Methyl Formate DetectionTransition A species E species

101-000 9124.21 9207.44

202-101 18247.03 18367.86

211-110 18575.95 ---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

±http://www.cv.nrao.edu/~aremijan/PRIMOS/

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 PateJustin NeillDanny ZaleskiChristoph E.

Harvard-SmithsonianCfa

Mike McCarthySilvia SpezzanoValerio Lattanzi

NRAOTony 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

404

505

303

202212

313

414

515

413

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