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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
=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