Laboratory Spectrum of the trans-gauche Conformer ofEthyl 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.

Laboratory and Interstellar Detection of trans-Methyl Formate (2009)

cisV3 = 399.1 cm-1

transV3 = 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 1012 at 100 K

M.L. Senent et al., Ap.J. 627, 567 (2005).M.T. Muckle et al., 64th International Symposium on Molecular Spectroscopy, RH15.Y. Karakawa et al., J. Mol Spectrosc. 210, 196 (2001).

Nucleophilic Substitution [CH3OH2]+ + HCOOH [HC(OH)OCH3]+ + H2O

[CH3OH2]+ + HCOOH

trans-[HC(OH)OCH3]+ + H2O

cis-[HC(OH)OCH3]+ + H2O

trans transition state: -5.3 kJ/molcis 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 transferreaction is endothermic

Analogous toCH3OH + [CH3OH2]+ [CH3(OH)CH3]+ + H2O(dimethyl ether production route)

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

Tem

pera

ture

(K

)

Experimental Methods

Chirped pulse FTMW spectroscopy (Virginia): 6.5-18.5, 25-40 GHz (~106 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).

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

cis-transglobal minimum

cis-gaucheE = 14.3 cm-1

(Riveros: 65+21 cm-1)

trans-gaucheE = 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

ester cis

ester trans

cis-gauchemethyl V3 1273 cm-1

cis-transmethyl V3 1262 cm-1

trans-gauchemethyl V3 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

Two tunneling states (=0,1) -a, b-type transitions: = 0 -c-type transitions: = +1 (across tunneling gap)

a-type transitions split by <200 kHzc-type transitions split by ~20 MHz (not constant)b-type transitions not observed (low calculated dipole moment)

Tunneling in trans-gauche Ethyl Formate

CP-FTMW SpectraJ= 5-4 a-types

trans-gauche simulation

=0 =1A (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)

Da (MHz)* 10.219(6)

E01 (MHz) 21.03(24)

Nlines 54

rms error (kHz) 1.9

Effective Ka=0, +1 fitJmax = 7

Hamiltonian Parameters

=0 =1A (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)

E01 (MHz) 9.67(7)

Nlines 70

rms error (kHz) 242.6

Fit to full data setJmax = 7, Ka max = 4

Ab InitioA (MHz) 16342.16

B (MHz) 2721.03

C (MHz) 2567.39A (D) 4.45

B (D) 0.08

C (D) 2.38

mp2/6-311g++(d,p)

1ˆ0 σ|P|σD aa

Gas-Phase Production of trans-gauche ethyl formate

Nucleophilic Substitution EtOH2

+ +HCOOH

cis transition state: +12.0 kJ/moltrans transition state: -1.1 kJ/mol

m06-2x/6-31+g(d,p)

[HC(OH)OEt]++H2O

[EtOH2]++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).

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 Ka=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

Acknowledgements

NSF Centers for Chemical Innovation (Chemistry of the Universe)University of Virginia

http://www.virginia.edu/ccu

Nucleophilic Substitution [MeOH2]+ +HCOOH

[CH3OH2]+ + HCOOH

trans-[HC(OH)OCH3]+ + H2O

cis-[HC(OH)OCH3]+ + H2O

cis transition state: +13.3 kJ/moltrans transition state: -5.3 kJ/mol

Fischer Esterification MeOH +[HC(OH)2]+

cis transition state: +17.4 kJ/moltrans transition state: +21.2 kJ/mol

trans-[HC(OH)OCH3]+ + H2O

cis-[HC(OH)OCH3]+ + H2O

CH3OH2 + [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

Reactions to form trans-gauche ethyl formateNucleophilic Substitution

EtOH2+ +HCOOH

Fischer Esterification EtOH +HC(OH)2

+

cis transition state: +12.0 kJ/moltrans transition state: -1.1 kJ/mol

cis transition state: +7.8 kJ/moltrans transition state: +10.3 kJ/mol

m062x/6-31+g(d,p)

EtOH+[HC(OH) 2]+

blue=cis, red=trans

[HC(OH)OEt]++H2O

[HC(OH)OEt]++H2O

[EtOH2]++HCOOH