trans-gauche
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
V
3
= 399.1 cm -1 cis mp2/6-31++g(d,p) trans
V
3
= 14.9 cm -1
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).

Gas Phase Production of transMethyl Formate
Nucleophilic Substitution
[CH
3
OH
2
] + + HCOOH  [HC(OH)OCH
3
] + + H
2
O
[CH
3
OH
2
] + + HCOOH cis[HC(OH)OCH
3
] + + H
2
O
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) trans[HC(OH)OCH
3
] + + H
2
O 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)
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 (~10 6 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).

trans-trans
(transition state)
E = 2060 cm -1
Potential Energy Surface of Ethyl Formate cis-trans global minimum trans-gauche
E = 1917 cm -1 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 V
3
1324 cm -1 ester cis cis-trans methyl V
3
1262 cm -1 cis-gauche methyl V
3
1273 cm -1
(Ethyl) mp2/6-31g(d,p)

trans-gauche
Two tunneling states (

a, b -type transitions:

c -type transitions:

=0,1)


= +
= 0
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)

J= 5-4 a -types trans-gauche simulation

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

Gas-Phase Production of trans-gauche ethyl formate
Nucleophilic Substitution
EtOH
2
+ +HCOOH
[EtOH
2
] + +HCOOH
[HC(OH)OEt] + +H
2
O
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).

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

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
[MeOH
2
] + +HCOOH
CH
3
OH
2
+ [HCO(OH)
2
] +
[CH
3
OH
2
] + + HCOOH trans[HC(OH)OCH
3
] + + H
2
O cis[HC(OH)OCH
3
] + + H
2
O cis[HC(OH)OCH
3
] + + H
2
O trans[HC(OH)OCH
3
] + + H
2
O m062x/6-31+g(d,p) cis transition state: +17.4 kJ/mol cis transition state: +13.3 kJ/mol trans transition state: +21.2 kJ/mol blue=cis, red=trans trans transition state: -5.3 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.
G. Bouchoux and N. Choret, Rapid Communications in Mass Spectrometry , 11 , 1799 (1997).

trans-gauche
Fischer Esterification
EtOH +HC(OH)
2
+
Nucleophilic Substitution
EtOH
2
+ +HCOOH
EtOH+[HC(OH)
2
] +
[EtOH
2
] + +HCOOH
[HC(OH)OEt] + +H
2
O
[HC(OH)OEt] + +H
2
O 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