Jasmine Mays, Arianna Seabrooks, Anthony Speas, Jasmine Drake, Daniel Zaleski, Amanda Steber
Reaction Pathway : Methyl Transfer
Methods
Motivation
The goal was to gain an understanding of the reactive chemistry
in the interstellar medium through the use of rotational
spectroscopy to study methyl and ethyl formate.
Broadband
Spectrometer
Introduction
The main focus of the experiment was to identify trans-methyl
formate in the interstellar medium. There are two conformers
of methyl formate— cis and trans-- which are based upon its
rotation about the heavy atom dihedral angle. For a long time,
the more abundant cis conformer of methyl formate had been
known to exist in the interstellar medium.1 Until recently, the
interstellar detection of the higher energy trans-methyl
formate had not been seen because its laboratory spectrum
was unknown. Based on terrestrial abundances of cis and trans
methyl formate, its interstellar presence would be doubtful.
With a laboratory spectrum in place, Muckle et al. posed a
question on
the interstellar formation of trans-methyl
formate.2 Based on two proposed theoretical pathways, it was
determined that when using one reaction pathway (the Fischer
Esterification), cis methyl formate was more favorable than
trans; however, another possible barrierless reaction pathway
(the methyl transfer reaction) helped explain the abundance of
trans-methyl. Based on correlations between experimental
and theoretical evidence, the GBT was used in the detection of
interstellar trans-methyl formate.
Theory
Analysis
• Assemble various components regarding the
spectrum to be ran
• Adjust Necessary Controls to Desired settings
• Test Set Up to ensure no leaks , correct
pressure, and proper controls
• Run Spectrometer overnight
•
•
•
•
Gaussian 09
Spartan
QST3 for calculations to find transition states
Optimizations and frequency analysis to determine
geometric optimization
• Which gives rotational constants and energies
• A 2D scan requires a special type of optimization
known as the Z-matrix function.
Previous theoretical and experimental observations were undertaken to find possible gas-phase reactions
producing methyl formate6. The chemical processes studied that led to the formation of methyl formate are
included in a chemical model of hot cores, but these models vastly underpredict the observed abundances of
methyl formate.
New Way to Form Protonated Methyl Formate:
The reaction between protonated methanol and formic acid is similar to Fischer Esterification; however, the
reaction mechanism differs by the exchange of only the methyl group. The methyl group in protonated methanol
attaches itself to the carbonyl oxygen. When the methyl group in protonated methanol transfers to formic acid, a
water molecule is formed and the methyl group attaches to the carbonyl oxygen that is in formic acid. During the
reaction, the two transition states are observed. In the trans-methyl formate transition state, there is no activation
barrier, unlike in the cis where there is an activation barrier. Steric hindrance and electronegativity are possible
reasons the methyl group bonds to the double bonded oxygen in formic acid and not the oxygen in the hydroxyl
group. The carbon becomes positively charged at the end of the reaction because of the double bond that is
broken. The product is protonated methyl formate due to the additional hydrogen atom.
• Identify lines by comparing frequencies with those
found in Splatalogue Database 5; those not known
in database are considered U-lines and put aside for
later determination
• JB95 was used for spectral fitting
Transition state of methyl transfer reaction
Potential Energy Surface
cis-methyl formate
trans-methyl formate
The arrangement of the methyl group
along the dihedral at which aligns the
methyl group in the same plane
The arrangement of the methyl group
along the dihedral at which aligns the
methyl group in the opposite plane
Theoretical calculations identify the potential energy
surface and rotational constants of a molecule. The potential
energy surface of a molecule is the energy of a molecule as
a function of its structure. As the parameters of the
molecule are adjusted, the energy levels change. In a 2Dimensional Scan, two parameters are adjusted, such as
two dihedral angles. The maximum states on these energies
maps (the red areas) represent the instability at the
transitional state. The minima on these maps (the blue
areas) represent the stable forms called conformers.
Reading along one side of a two dimensional scan would
show the change in energy if only one parameter were to
change.
Broadband Spectrometer
10 MHz Rb
Standard
Chirped Pulse
Conclusion
Based on the data, it has been confirmed that both cis and trans methyl formate are, in fact, in the interstellar medium.
The cis was determined as the more energetically favorable molecule. The spectra identified the two states, A and E, that
were due to the internal rotation of the methyl group. Furthermore, it was possible to conclude that based on results
obtained in the lab, and based on comparisons in the GBT primos data7 trans-methyl formate was detected in the
interstellar medium. In fact, the amount of trans methyl formate seen in the interstellar medium seems to suggest kinetic
control rather than thermodynamics.
2D Potential energy surface scan for methyl formate
24 Gs/s
AWG
18.95 GHz
PDRO
TWT
Amplifier
Lactic Acid and Water Clusters
Pulsed Valve
x3
nozzles
20 GHz
Oscilloscope
(50 Gs/s)
Free Induction Decay
FID acquisition and FT
Schematic drawing of a broadband spectrometer
3
The Fischer Esterification reaction mechanism produces an acid
with an ester and water. The Fischer Esterification mechanism is
an energetically unfavorable reaction process. It requires energy
to be put into a system to allow the reaction to occur. It is usually
found in biological systems and the energy input may be
controlled.
Experimental spectrum collected for lactic acid. In this spectrum,
both lactic acid monomer and water dimer were able to be
identified. More calculations are still being performed in efforts to
identify larger clusters.
Microwave spectra of both the cis- and trans- methyl formate
conformers were taken. All transitions across a bandwidth of 6.5 to 18.5
GHz were polarized, which allowed both conformers to be seen
simultaneously. For the J = 1-0 transition a signal to noise ratio of
80,000:1 was achieved for the cis- methyl formate conformer and 130:1
for the trans- methyl formate conformer. Over 300,000 averages were
taken in order to achieve aforementioned signal to noise ratios. The
trans-methyl formate conformer is higher in energy, which caused the
need for a pulse discharge to be applied in order to bring up its weaker
signal intensities. A 1.6 kV DC discharge was applied synchronously
with the gas pulse in order increase the spectral lines by a factor of four.
Experimental Rotational
Constants (MHz)
Computational Calculation s of
Rotational Constants (MHz)
4965.987 (48)
4480.76
1336.9117 (12)
1362. 39
1127.4466 (10)
1149.89
Illustration of Fischer Esterification process
Reaction pathway for Fischer Esterification reaction
4
In addition to the experiments that were performed on methyl
formate, the spectrometer is useful for a variety of other
experiments. For example, a portion of the summer research
program was also spent researching, running calculations, and
performing experiments on lactic acid in efforts to help other
colleagues. Pictured below is a spectrum that was collected. The
microwave spectrum of lactic acid had previously been reported by
van Eijck.7 A theoretical investigation of lactic acid plus water had
been performed by Smaga, et al.8
Reaction Pathway: Fischer Esterification
Pulse
Monitor
Schematic of pulse discharge nozzle
Reaction pathway of methyl transfer reaction
Illustration of methyl transfer reaction
The process occurs in several steps, which begins with formic acid
accepting a proton. The new resonance stabilized structure forms a
bond with methanol. An activated complex is formed via proton
transfer from the oxoniom ion to another molecule. Protonation of the
hydroxyl group in the activated complex yields a new oxoniom ion,
which is later lost as water and results in protonated methyl formate.
Lactic acid monomer and water molecule
Basis level: 6-311 ++ G (d, p)/M05-2X
References
Funding
1.
2.
This work was supported in part by the National Science Foundation Centers for Chemical Innovation through award CHE-0847919, the
University of Virginia, and the Virginia-North Carolina LSAMP Program.
3.
4.
5.
6.
7.
8.
9.
Ellder. J, et al. ApJ, 242, L93, (1980).
Laboratory and possible interstellar detection of trans-methyl formate. Matt Muckel, Justin Neill, Daniel Zaleski, Brooks Pate. The Ohio State
University 64th International Symposium on Molecular Spectroscopy, (2009).
Brown ,Dian, Douglass, Geyer , Shipman, Pate. Rev. Sci. Instrum.,79, 053103, (2008).
Grabow, Palmer, McCarthy, Thaddeus, Rev. Sci. Instrum., 76, 093106, (2005).
www.Splatalogue.net
Horn, et al. The ApJ. 611, 605-614, (2004).
GBT Primos survey, www.cv.nrao.edu/~aremijan/primos.
Van Eijck. J. Mol. Spec., 101, 133-138, (1983).
Smaga, Sadlej. J. Phys. Chem. A, 114, 4427-4436, (2010).
Acknowledgments
Anthony Remijan – National Radio Astronomy Observatory,
Valerio Lattanzi, Silvia Spezzano, Michael McCarthy – Harvard-Smithsonian Center for Astrophysics
Justin Neill, Matt Muckle – University of Virginia
Zbigniew Kisiel – Institute of Physics, Polish Academy of Sciences
Rick Suenram – National Institute of Standards and Technology