1. Laboratory Spectrum of the trans-gauche Conformer of Ethyl Formate Justin L. Neill, Matt T. Muckle, Daniel P. Zaleski, Brooks H. Pate Department of Chemistry, University of Virginia, McCormick Rd, P.O. Box 400319, Charlottesville, VA 22904 V. Lattanzi, S. Spezzano, M.C. McCarthy Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, and School of Engineering and Applied Sciences, Harvard University, 29 Oxford St., Cambridge, MA 02138.

2. Laboratory and Interstellar Detection of trans-Methyl Formate (2009) cis V 3 = 399.1 cm -1 trans V 3 = 14.9 cm -1 trans cis mp2/6-31++g(d,p) Barrier to conformer interconversion: 5000 cm -1 [60 kJ/mol, 14 kcal/mol, 7000K] Equilibrium population ratio: 16000:1 at 300 K, 3 x 10 12 at 100 K M.L. Senent et al., Ap.J. 627, 567 (2005). M.T. Muckle et al., 64 th International Symposium on Molecular Spectroscopy, RH15. Y. Karakawa et al., J. Mol Spectrosc. 210, 196 (2001).

3. Nucleophilic Substitution [CH 3 OH 2 ] + + HCOOH  [HC(OH)OCH 3 ] + + H 2 O [CH 3 OH 2 ] + + HCOOH trans-[HC(OH)OCH 3 ] + + H 2 O cis-[HC(OH)OCH 3 ] + + H 2 O trans transition state: -5.3 kJ/mol cis transition state: +13.3 kJ/mol Gas Phase Production of trans-Methyl Formate m06-2x/6-31+g(d,p) Gaussian 09, Revision A.02, M.J. Frisch et al., Gaussian Inc., Wallingford, CT, 2009. A.Horn et al., Ap.J. 611, 605-614 (2004). P. Ehrenfreund and S.B. Charnley, Annu. Rev. Astron. Astrophys., 38, 427 (2000). G. Bouchoux and N. Choret, Rapid Communications in Mass Spectrometry, 11, 1799 (1997). Adding to gas/grain reaction network models (S. Widicus Weaver, E. Herbst) Competing proton transfer reaction is endothermic Analogous to CH 3 OH + [CH 3 OH 2 ] +  [CH 3 (OH)CH 3 ] + + H 2 O (dimethyl ether production route)

4. Interstellar Detection of trans-Methyl Formate (2009) Sgr-B2(N) Green Bank Telescope PRIMOS Project, available on the Internet at http://www.cv.nrao.edu/~aremijan/PRIMOS. All features in absorption (cis-methyl formate in emission); different spatial distribution? Total column density ~1% that of cis-methyl formate Temperature (K)

5. Experimental Methods Chirped pulse FTMW spectroscopy (Virginia): 6.5-18.5, 25-40 GHz (~10 6 signal averages) Balle-Flygare-type FTM (Harvard-Smithsonian): 8-40 GHz, high resolution, MW-MW double resonance Pulsed discharge nozzles used to enhance population G.G. Brown et al., Rev. Sci. Instrum. 79, 053103 (2008). M.C. McCarthy, W. Chen, M.J. Travers, and P. Thaddeus, Ap. J. Supp. Series, 129, 611-623 (2000).

6. Conformers of Ethyl Formate J.M. Riveros and E.B. Wilson, J. Chem. Phys. 46, 4605 (1967). I.R. Medvedev, F.C. De Lucia, E. Herbst, Ap. J. Supp. Series 181, 433 (2009). A. Belloche et al., A&A 499, 215 (2009). cis (ester)-trans (ethyl) isomer recently detected in Sgr B2(N) trans-gauchetrans-trans

7. cis-trans global minimum cis-gauche E = 14.3 cm -1 (Riveros: 65+21 cm -1 ) trans-gauche E = 1917 cm -1 trans-trans (transition state) E = 2060 cm -1 Potential Energy Surface of Ethyl Formate mp2/6-31+g(d,p) Ester isomerization energy: 4760 cm -1

8. ester cis ester trans cis-gauche methyl V 3 1273 cm -1 cis-trans methyl V 3 1262 cm -1 trans-gauche methyl V 3 1324 cm -1 mp2/6-31g(d,p) (Ethyl) Calculates two tunneling subspecies split by 0.25 MHz (highly sensitive to barrier) Potential Energy Surface of Ethyl Formate

9. Two tunneling states (  =0,1) -a, b-type transitions:  = 0 -c-type transitions:  = +1 (across tunneling gap) a-type transitions split by <200 kHz c-type transitions split by ~20 MHz (not constant) b-type transitions not observed (low calculated dipole moment) Tunneling in trans-gauche Ethyl Formate

10. CP-FTMW Spectra J= 5-4 a-types trans-gauche simulation

11.  =0  =1 A (MHz)17402.39(24)17379.59(24) B (MHz)2652.67795(13)2652.68514(13) C (MHz)2531.99121(13)2532.01952(13)  J (kHz) 3.6596(15)3.6433(15)  JK (kHz) -112.29(9)-94.25(9)  J (kHz) 1.0044(13)1.0096(13) D a (MHz) * 10.219(6)  E 01 (MHz) 21.03(24) N lines 54 rms error (kHz)1.9 Effective K a =0, +1 fit J max = 7 Hamiltonian Parameters  =0  =1 A (MHz)17391.020(55) B (MHz)2652.714(9)2652.699(9) C (MHz)2531.985(9)2532.013(9)  J (kHz) 3.71(6)3.76(6)  JK (kHz) -101.7(7)-100.8(7)  J (kHz) 1.15(9)1.18(9)  E 01 (MHz) 9.67(7) N lines 70 rms error (kHz)242.6 Fit to full data set J max = 7, K a max = 4 Ab Initio A (MHz) 16342.16 B (MHz) 2721.03 C (MHz) 2567.39  A (D) 4.45  B (D) 0.08  C (D) 2.38 mp2/6-311g++(d,p)

12. Gas-Phase Production of trans-gauche ethyl formate Nucleophilic Substitution EtOH 2 + +HCOOH cis transition state: +12.0 kJ/mol trans transition state: -1.1 kJ/mol m06-2x/6-31+g(d,p) [HC(OH)OEt] + +H 2 O [EtOH 2 ] + +HCOOH Model of Belloche et al. proposed that ethyl formate production occurs through grain-surface processes Possible secondary gas-phase reaction in high-ionization regions A. Belloche et al., A&A 499, 215 (2009).

13. Conclusions trans-gauche-ethyl formate has been detected in the laboratory -all transitions with significant intensity at low T <40 GHz measured -effective fit to K a =0,1 transitions to experimental uncertainty -most useful for extrapolations/astronomical observations could be produced in the ISM via the barrierless reaction of formic acid with protonated ethanol (especially in high-ionization regions) -similar morphology to trans-methyl formate? Future work: -detection of protonated species in this reaction network -further observations/high resolution maps

14. Acknowledgements NSF Centers for Chemical Innovation (Chemistry of the Universe) University of Virginia http://www.virginia.edu/ccu

16. Nucleophilic Substitution [MeOH 2 ] + +HCOOH [CH 3 OH 2 ] + + HCOOH trans-[HC(OH)OCH 3 ] + + H 2 O cis-[HC(OH)OCH 3 ] + + H 2 O cis transition state: +13.3 kJ/mol trans transition state: -5.3 kJ/mol Fischer Esterification MeOH +[HC(OH) 2 ] + cis transition state: +17.4 kJ/mol trans transition state: +21.2 kJ/mol trans-[HC(OH)OCH 3 ] + + H 2 O cis-[HC(OH)OCH 3 ] + + H 2 O CH 3 OH 2 + [HCO(OH) 2 ] + Gas Phase Reactions to Produce Methyl Formate m062x/6-31+g(d,p) Gaussian 09, Revision A.02, M.J. Frisch et al., Gaussian Inc., Wallingford, CT, 2009. G. Bouchoux and N. Choret, Rapid Communications in Mass Spectrometry, 11, 1799 (1997). Adding to gas/grain reaction network models (S. Widicus Weaver, E. Herbst) blue=cis, red=trans

17. Reactions to form trans-gauche ethyl formate Nucleophilic Substitution EtOH 2 + +HCOOH Fischer Esterification EtOH +HC(OH) 2 + cis transition state: +12.0 kJ/mol trans transition state: -1.1 kJ/mol cis transition state: +7.8 kJ/mol trans transition state: +10.3 kJ/mol m062x/6-31+g(d,p) EtOH+[HC(OH) 2 ] + blue=cis, red=trans [HC(OH)OEt] + +H 2 O [EtOH 2 ] + +HCOOH