ethyl formate v3

Report
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.
Laboratory and Interstellar Detection of trans-Methyl Formate (2009)
Barrier to conformer interconversion:
5000 cm-1 [60 kJ/mol, 14 kcal/mol, 7000K]
trans
cis
V3 = 399.1 cm-1
cis
mp2/6-31++g(d,p)
trans
V3 = 14.9 cm-1
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).
Gas Phase Production of trans-Methyl Formate
Nucleophilic Substitution
[CH3OH2]+ + HCOOH  [HC(OH)OCH3]+ + H2O
Competing proton transfer
reaction is endothermic
]+
[CH3OH2 + HCOOH
cis-[HC(OH)OCH3]+ + H2O
Analogous to
CH3OH + [CH3OH2]+ 
[CH3(OH)CH3]+ + H2O
(dimethyl ether production route)
trans-[HC(OH)OCH3]+ + H2O
m06-2x/6-31+g(d,p)
trans transition state: -5.3 kJ/mol
cis transition state: +13.3 kJ/mol
Adding to gas/grain reaction network models (S. Widicus Weaver, E. Herbst)
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).
Interstellar Detection of trans-Methyl Formate (2009)
Sgr-B2(N)
Temperature (K)
All features in absorption (cis-methyl formate in emission); different spatial distribution?
Total column density ~1% that of cis-methyl formate
Green Bank Telescope PRIMOS Project, available on the Internet at http://www.cv.nrao.edu/~aremijan/PRIMOS.
Experimental Methods
Pulsed discharge nozzles used
to enhance population
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
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
cis (ester)-trans (ethyl) isomer recently detected in Sgr B2(N)
trans-trans
trans-gauche
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).
Potential Energy Surface of Ethyl Formate
trans-trans
(transition state)
E = 2060 cm-1
trans-gauche
E = 1917 cm-1
cis-trans
global minimum
mp2/6-31+g(d,p)
Ester isomerization energy: 4760 cm-1
cis-gauche
E = 14.3 cm-1
(Riveros: 65+21 cm-1)
Potential Energy Surface of Ethyl Formate
Calculates two tunneling subspecies split by 0.25 MHz
(highly sensitive to barrier)
ester trans
trans-gauche
methyl V3 1324 cm-1
ester cis
cis-trans
methyl V3 1262 cm-1
cis-gauche
methyl V3 1273 cm-1
(Ethyl)
mp2/6-31g(d,p)
Tunneling in trans-gauche 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 kHz
c-type transitions split by ~20 MHz (not constant)
b-type transitions not observed (low calculated dipole moment)
CP-FTMW Spectra
J= 5-4 a-types
trans-gauche simulation
Hamiltonian Parameters
Effective Ka=0, +1 fit
Jmax = 7
=0
17402.39(24)
A (MHz)
=1
17379.59(24)
Fit to full data set
Jmax = 7, Ka max = 4
=0
A (MHz)
B (MHz)
C (MHz)
J (kHz)
JK (kHz)
J (kHz)
Da (MHz)*
E01 (MHz)
2652.67795(13)
2652.68514(13)
2531.99121(13)
2532.01952(13)
3.6596(15)
3.6433(15)
-112.29(9)
-94.25(9)
1.0044(13)
1.0096(13)
10.219(6)
21.03(24)
B (MHz)
C (MHz)
J (kHz)
JK (kHz)
J (kHz)
E01 (MHz)
Nlines
Nlines
rms error (kHz)
54
1.9
rms error (kHz)
Da   σ  0|Pˆa | σ  1 
A (MHz)
B (MHz)
C (MHz)
A (D)
B (D)
C (D)
Ab Initio
16342.16
2721.03
2567.39
mp2/6-311g++(d,p)
4.45
0.08
2.38
=1
17391.020(55)
2652.714(9)
2531.985(9)
3.71(6)
-101.7(7)
1.15(9)
2652.699(9)
2532.013(9)
3.76(6)
-100.8(7)
1.18(9)
9.67(7)
70
242.6
Gas-Phase Production of trans-gauche ethyl formate
Nucleophilic Substitution
EtOH2+ +HCOOH
[EtOH2]++HCOOH
[HC(OH)OEt]++H2O
Model of Belloche et al. proposed that ethyl
formate production occurs through
grain-surface processes
Possible secondary gas-phase reaction
in high-ionization regions
m06-2x/6-31+g(d,p)
cis transition state: +12.0 kJ/mol
trans transition state: -1.1 kJ/mol
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
Gas Phase Reactions to Produce Methyl Formate
Fischer Esterification
MeOH +[HC(OH)2]+
Nucleophilic Substitution
[MeOH2]+ +HCOOH
CH3OH2 + [HCO(OH)2]+
[CH3OH2]+ + HCOOH
trans-[HC(OH)OCH3]+ + H2O
cis-[HC(OH)OCH3]+ + H2O
cis-[HC(OH)OCH3]+ + H2O
trans-[HC(OH)OCH3]+ + H2O
m062x/6-31+g(d,p)
cis transition state: +17.4 kJ/mol
trans transition state: +21.2 kJ/mol
cis transition state: +13.3 kJ/mol
trans transition state: -5.3 kJ/mol
blue=cis, red=trans
Adding to gas/grain reaction network models (S. Widicus Weaver, E. Herbst)
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).
Reactions to form trans-gauche ethyl formate
Fischer Esterification
EtOH +HC(OH)2+
EtOH+[HC(OH) 2]+
Nucleophilic Substitution
EtOH2+ +HCOOH
[EtOH2]++HCOOH
[HC(OH)OEt]++H2O
[HC(OH)OEt]++H2O
m062x/6-31+g(d,p)
cis transition state: +7.8 kJ/mol
trans transition state: +10.3 kJ/mol
blue=cis, red=trans
cis transition state: +12.0 kJ/mol
trans transition state: -1.1 kJ/mol

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