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Title: Pyrolysis of the Simplest Carbohydrate, Glycolaldehyde (CHO-CH2OH), and Glyoxal in a Heated Microreactor

Abstract

Both glycolaldehyde and glyoxal were pyrolyzed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from CHO-CH2OH and HCO-CHO were detected and identified by vacuum ultraviolet (VUV) photoionization mass spectrometry. Complementary product identification was provided by argon matrix infrared absorption spectroscopy. Pyrolysis pressures in the microreactor were about 100 Torr, and contact times with the microreactors were roughly 100 μs. At 1200 K, the products of glycolaldehyde pyrolysis are H atoms, CO, CH2=O, CH2=C=O, and HCO-CHO. Thermal decomposition of HCO-CHO was studied with pulsed 118.2 nm photoionization mass spectrometry and matrix infrared absorption. Under these conditions, glyoxal undergoes pyrolysis to H atoms and CO. Tunable VUV photoionization mass spectrometry provides a lower bound for the ionization energy IE(CHO-CH2OH) ≥ 9.95 ± 0.05 eV. The gas-phase heat of formation of glycolaldehyde was established by a sequence of calorimetric experiments. The experimental result is ΔfH298(CHO-CH2OH) = -75.8 ± 1.3 kcal mol-1. Fully ab initio, coupled cluster calculations predict ΔfH0(CHO-CH2OH) of -73.1 ± 0.5 kcal mol-1 and ΔfH298(CHO-CH2OH) of -76.1 ± 0.5 kcal mol-1. The coupled-cluster singles doubles and noniterative triples correction calculations also lead to a revision of the geometry of CHO-CH2OH. We have found that the O-H bond length differsmore » substantially from earlier experimental estimates, due to unusual zero-point contributions to the moments of inertia.« less

Authors:
 [1];  [1];  [2];  [2];  [3];  [4];  [5];  [6];  [6];  [1]
  1. Univ. of Colorado, Boulder, CO (United States). Dept. of Chemistry and Biochemistry
  2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Sciences Division
  3. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
  4. Xavier Univ. of Louisiana, New Orleans, LA (United States). Dept. of Chemistry
  5. Univ. of Colorado, Boulder, CO (United States). Center for Combustion and Environmental Research and Dept. of Mechanical Engineering,
  6. Univ. of Texas, Austin, TX (United States). Dept. of Chemistry
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division; National Science Foundation (NSF); Univ. of New Orleans, New Orleans, LA (United States). Advanced Materials Research Inst. (AMRI) Fabrication, Analysis, and Consulting Services (FACS); Robert A. Welch Foundation
OSTI Identifier:
1464135
Grant/Contract Number:  
AC02-05CH11231; CBET-1403979; CHE-1361874; CHE-1361031; F-1283; FG02-07ER15884
Resource Type:
Accepted Manuscript
Journal Name:
Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory
Additional Journal Information:
Journal Volume: 120; Journal Issue: 14; Related Information: © 2016 American Chemical Society.; Journal ID: ISSN 1089-5639
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 47 OTHER INSTRUMENTATION

Citation Formats

Porterfield, Jessica P., Baraban, Joshua H., Troy, Tyler P., Ahmed, Musahid, McCarthy, Michael C., Morgan, Kathleen M., Daily, John W., Nguyen, Thanh Lam, Stanton, John F., and Ellison, G. Barney. Pyrolysis of the Simplest Carbohydrate, Glycolaldehyde (CHO-CH2OH), and Glyoxal in a Heated Microreactor. United States: N. p., 2016. Web. doi:10.1021/acs.jpca.6b00652.
Porterfield, Jessica P., Baraban, Joshua H., Troy, Tyler P., Ahmed, Musahid, McCarthy, Michael C., Morgan, Kathleen M., Daily, John W., Nguyen, Thanh Lam, Stanton, John F., & Ellison, G. Barney. Pyrolysis of the Simplest Carbohydrate, Glycolaldehyde (CHO-CH2OH), and Glyoxal in a Heated Microreactor. United States. https://doi.org/10.1021/acs.jpca.6b00652
Porterfield, Jessica P., Baraban, Joshua H., Troy, Tyler P., Ahmed, Musahid, McCarthy, Michael C., Morgan, Kathleen M., Daily, John W., Nguyen, Thanh Lam, Stanton, John F., and Ellison, G. Barney. Wed . "Pyrolysis of the Simplest Carbohydrate, Glycolaldehyde (CHO-CH2OH), and Glyoxal in a Heated Microreactor". United States. https://doi.org/10.1021/acs.jpca.6b00652. https://www.osti.gov/servlets/purl/1464135.
@article{osti_1464135,
title = {Pyrolysis of the Simplest Carbohydrate, Glycolaldehyde (CHO-CH2OH), and Glyoxal in a Heated Microreactor},
author = {Porterfield, Jessica P. and Baraban, Joshua H. and Troy, Tyler P. and Ahmed, Musahid and McCarthy, Michael C. and Morgan, Kathleen M. and Daily, John W. and Nguyen, Thanh Lam and Stanton, John F. and Ellison, G. Barney},
abstractNote = {Both glycolaldehyde and glyoxal were pyrolyzed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from CHO-CH2OH and HCO-CHO were detected and identified by vacuum ultraviolet (VUV) photoionization mass spectrometry. Complementary product identification was provided by argon matrix infrared absorption spectroscopy. Pyrolysis pressures in the microreactor were about 100 Torr, and contact times with the microreactors were roughly 100 μs. At 1200 K, the products of glycolaldehyde pyrolysis are H atoms, CO, CH2=O, CH2=C=O, and HCO-CHO. Thermal decomposition of HCO-CHO was studied with pulsed 118.2 nm photoionization mass spectrometry and matrix infrared absorption. Under these conditions, glyoxal undergoes pyrolysis to H atoms and CO. Tunable VUV photoionization mass spectrometry provides a lower bound for the ionization energy IE(CHO-CH2OH) ≥ 9.95 ± 0.05 eV. The gas-phase heat of formation of glycolaldehyde was established by a sequence of calorimetric experiments. The experimental result is ΔfH298(CHO-CH2OH) = -75.8 ± 1.3 kcal mol-1. Fully ab initio, coupled cluster calculations predict ΔfH0(CHO-CH2OH) of -73.1 ± 0.5 kcal mol-1 and ΔfH298(CHO-CH2OH) of -76.1 ± 0.5 kcal mol-1. The coupled-cluster singles doubles and noniterative triples correction calculations also lead to a revision of the geometry of CHO-CH2OH. We have found that the O-H bond length differs substantially from earlier experimental estimates, due to unusual zero-point contributions to the moments of inertia.},
doi = {10.1021/acs.jpca.6b00652},
journal = {Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory},
number = 14,
volume = 120,
place = {United States},
year = {Wed Mar 16 00:00:00 EDT 2016},
month = {Wed Mar 16 00:00:00 EDT 2016}
}

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Works referenced in this record:

Organocatalytic conversion of cellulose into a platform chemical
journal, January 2013

  • Caes, Benjamin R.; Palte, Michael J.; Raines, Ronald T.
  • Chem. Sci., Vol. 4, Issue 1
  • DOI: 10.1039/C2SC21403B

Quantitative IR Spectrum and Vibrational Assignments for Glycolaldehyde Vapor: Glycolaldehyde Measurements in Biomass Burning Plumes
journal, May 2013

  • Johnson, Timothy J.; Sams, Robert L.; Profeta, Luisa T. M.
  • The Journal of Physical Chemistry A, Vol. 117, Issue 20
  • DOI: 10.1021/jp311945p

An FTIR study of mechanisms for the HO radical initiated oxidation of C2H4 in the presence of NO: detection of glycolaldehyde
journal, June 1981


Laboratory and Theoretical Study of the Oxy Radicals in the OH- and Cl-Initiated Oxidation of Ethene
journal, September 1998

  • Orlando, John J.; Tyndall, Geoffrey S.; Bilde, Merete
  • The Journal of Physical Chemistry A, Vol. 102, Issue 42
  • DOI: 10.1021/jp981937d

Emissions from smoldering combustion of biomass measured by open-path Fourier transform infrared spectroscopy
journal, August 1997

  • Yokelson, Robert J.; Susott, Ronald; Ward, Darold E.
  • Journal of Geophysical Research: Atmospheres, Vol. 102, Issue D15
  • DOI: 10.1029/97JD00852

Rate and mechanism of the reactions of OH and Cl with 2-methyl-3-buten-2-ol
journal, October 1998

  • Ferronato, C.; Orlando, J. J.; Tyndall, G. S.
  • Journal of Geophysical Research: Atmospheres, Vol. 103, Issue D19
  • DOI: 10.1029/98JD00528

Development and evaluation of a photooxidation mechanism for isoprene
journal, January 1992

  • Paulson, Suzanne E.; Seinfeld, John H.
  • Journal of Geophysical Research, Vol. 97, Issue D18
  • DOI: 10.1029/92JD01914

Development and evaluation of a detailed mechanism for the atmospheric reactions of isoprene and NOx
journal, January 1996


A product study of the gas-phase reaction of methyl vinyl ketone with the OH radical in the presence of NOx
journal, December 1989

  • Tuazon, Ernesto C.; Atkinson, Roger
  • International Journal of Chemical Kinetics, Vol. 21, Issue 12
  • DOI: 10.1002/kin.550211207

Lignin fast pyrolysis: Results from an international collaboration
journal, May 2010

  • Nowakowski, D. J.; Bridgwater, A. V.; Elliott, D. C.
  • Journal of Analytical and Applied Pyrolysis, Vol. 88, Issue 1, p. 53-72
  • DOI: 10.1016/j.jaap.2010.02.009

Mechanism of the OH-Initiated Oxidation of Glycolaldehyde over the Temperature Range 233−296 K
journal, November 2006

  • Butkovskaya, Nadezhda I.; Pouvesle, Nicolas; Kukui, Alexandre
  • The Journal of Physical Chemistry A, Vol. 110, Issue 50
  • DOI: 10.1021/jp064993k

Formation and Reaction of Hydroxycarbonyls from the Reaction of OH Radicals with 1,3-Butadiene and Isoprene
journal, June 2005

  • Baker, Jillian; Arey, Janet; Atkinson, Roger
  • Environmental Science & Technology, Vol. 39, Issue 11
  • DOI: 10.1021/es047930t

Global budgets of atmospheric glyoxal and methylglyoxal, and implications for formation of secondary organic aerosols
journal, January 2008

  • Fu, Tzung-May; Jacob, Daniel J.; Wittrock, Folkard
  • Journal of Geophysical Research, Vol. 113, Issue D15
  • DOI: 10.1029/2007JD009505

The glyoxal budget and its contribution to organic aerosol for Los Angeles, California, during CalNex 2010: GLYOXAL BUDGET FOR LOS ANGELES
journal, October 2011

  • Washenfelder, R. A.; Young, C. J.; Brown, S. S.
  • Journal of Geophysical Research: Atmospheres, Vol. 116, Issue D21
  • DOI: 10.1029/2011JD016314

Relatively Selective Production of the Simplest Criegee Intermediate in a CH 4 /O 2 Electric Discharge: Kinetic Analysis of a Plausible Mechanism
journal, November 2014

  • Nguyen, Thanh Lam; McCarthy, Michael C.; Stanton, John F.
  • The Journal of Physical Chemistry A, Vol. 119, Issue 28
  • DOI: 10.1021/jp510554g

HCO formation in the thermal unimolecular decomposition of glyoxal: rotational and weak collision effects
journal, January 2008

  • Friedrichs, Gernot; Colberg, Mark; Dammeier, Johannes
  • Physical Chemistry Chemical Physics, Vol. 10, Issue 43
  • DOI: 10.1039/b809992h

Thermal decomposition of CH 3 CHO studied by matrix infrared spectroscopy and photoionization mass spectroscopy
journal, October 2012

  • Vasiliou, AnGayle K.; Piech, Krzysztof M.; Reed, Beth
  • The Journal of Chemical Physics, Vol. 137, Issue 16
  • DOI: 10.1063/1.4759050

Polarized Matrix Infrared Spectra of Cyclopentadienone: Observations, Calculations, and Assignment for an Important Intermediate in Combustion and Biomass Pyrolysis
journal, January 2014

  • Ormond, Thomas K.; Scheer, Adam M.; Nimlos, Mark R.
  • The Journal of Physical Chemistry A, Vol. 118, Issue 4
  • DOI: 10.1021/jp411257k

Pyrolysis of Cyclopentadienone: Mechanistic Insights from a Direct Measurement of Product Branching Ratios
journal, February 2015

  • Ormond, Thomas K.; Scheer, Adam M.; Nimlos, Mark R.
  • The Journal of Physical Chemistry A, Vol. 119, Issue 28
  • DOI: 10.1021/jp511390f

Isomerization and Fragmentation of Cyclohexanone in a Heated Micro-Reactor
journal, December 2015

  • Porterfield, Jessica P.; Nguyen, Thanh Lam; Baraban, Joshua H.
  • The Journal of Physical Chemistry A, Vol. 119, Issue 51
  • DOI: 10.1021/acs.jpca.5b10984

The properties of a micro-reactor for the study of the unimolecular decomposition of large molecules
journal, September 2014

  • Guan, Qi; Urness, Kimberly N.; Ormond, Thomas K.
  • International Reviews in Physical Chemistry, Vol. 33, Issue 4
  • DOI: 10.1080/0144235X.2014.967951

The products of the thermal decomposition of CH 3 CHO
journal, July 2011

  • Vasiliou, AnGayle; Piech, Krzysztof M.; Zhang, Xu
  • The Journal of Chemical Physics, Vol. 135, Issue 1
  • DOI: 10.1063/1.3604005

Thermochemical Studies of Epoxides and Related Compounds
journal, April 2013

  • Morgan, Kathleen M.; Ellis, Jamie A.; Lee, Joseph
  • The Journal of Organic Chemistry, Vol. 78, Issue 9
  • DOI: 10.1021/jo4002867

On the Design and Efficiency of Isothermal Reaction Calorimeters.
journal, January 1959


Thermochemical studies of carbonyl compounds. 5. Enthalpies of reduction of carbonyl groups
journal, April 1991

  • Wiberg, Kenneth B.; Crocker, Louis S.; Morgan, Kathleen M.
  • Journal of the American Chemical Society, Vol. 113, Issue 9
  • DOI: 10.1021/ja00009a033

A complete basis set model chemistry. V. Extensions to six or more heavy atoms
journal, February 1996

  • Ochterski, Joseph W.; Petersson, G. A.; Montgomery, J. A.
  • The Journal of Chemical Physics, Vol. 104, Issue 7
  • DOI: 10.1063/1.470985

Microwave spectra of isotopic glycolaldehydes, substitution structure, intramolecular hydrogen bond and dipole moment
journal, May 1973


The pure rotational spectrum of glycolaldehyde isotopologues observed in natural abundance
journal, February 2013

  • Brandon Carroll, P.; McGuire, Brett A.; Zaleski, Daniel P.
  • Journal of Molecular Spectroscopy, Vol. 284-285
  • DOI: 10.1016/j.jms.2013.01.002

The Submillimeter Spectrum of Glycolaldehyde
journal, October 2010

  • Carroll, P. Brandon; Drouin, Brian J.; Widicus Weaver, Susanna L.
  • The Astrophysical Journal, Vol. 723, Issue 1
  • DOI: 10.1088/0004-637X/723/1/845

The submillimeter spectrum of deuterated glycolaldehydes
journal, March 2012


Millimeter and submillimeter wave spectra of 13 C-glycolaldehydes
journal, January 2013


Determination of Molecular Structures from Ground State Rotational Constants
journal, October 1958

  • Costain, C. C.
  • The Journal of Chemical Physics, Vol. 29, Issue 4
  • DOI: 10.1063/1.1744602

Determination of Molecular Structure from Microwave Spectroscopic Data
journal, January 1953

  • Kraitchman, J.
  • American Journal of Physics, Vol. 21, Issue 1
  • DOI: 10.1119/1.1933338

Spectroscopic studies of glycolaldehyde
journal, October 1969


Infiiared and raman studies of the structure of crystalline glygolaldehyde
journal, June 1976

  • Kobayashi, Yoko; Takahara, Hirofumi; Takahashi, Hiroaki
  • Journal of Molecular Structure, Vol. 32, Issue 2
  • DOI: 10.1016/0022-2860(76)85002-8

Glycolaldehyde dimer in the stable crystal phase has axial OH groups: Raman, infrared and X-ray data analysis
journal, September 2013


Lactones. 2. Enthalpies of hydrolysis, reduction, and formation of the C4-C13 monocyclic lactones. Strain energies and conformations
journal, September 1991

  • Wiberg, Kenneth B.; Waldron, Roy F.
  • Journal of the American Chemical Society, Vol. 113, Issue 20
  • DOI: 10.1021/ja00020a036

Accuracy of Calculations of Heats of Reduction/Hydrogenation: Application to Some Small Ring Systems
journal, October 2012

  • Wiberg, Kenneth B.
  • The Journal of Organic Chemistry, Vol. 77, Issue 22
  • DOI: 10.1021/jo302118b

Phase Transition Enthalpy Measurements of Organic and Organometallic Compounds. Sublimation, Vaporization and Fusion Enthalpies From 1880 to 2010
journal, December 2010

  • Acree, William; Chickos, James S.
  • Journal of Physical and Chemical Reference Data, Vol. 39, Issue 4
  • DOI: 10.1063/1.3309507

Vapor Pressure Measurements of Hydroxyacetaldehyde and Hydroxyacetone in the Temperature Range (273 to 356) K
journal, September 2009

  • Petitjean, M.; Reyès-Pérez, E.; Pérez, D.
  • Journal of Chemical & Engineering Data, Vol. 55, Issue 2
  • DOI: 10.1021/je9004905

Structure, Thermochemistry and Reactivity of Protonated Glycolaldehyde
journal, August 2001

  • Bouchoux, Guy; Penaud-Berruyer, Florence; Bertrand, William
  • European Journal of Mass Spectrometry, Vol. 7, Issue 4-5
  • DOI: 10.1255/ejms.445

Theoretical enthalpies of formation for atmospheric hydroxycarbonyls
journal, January 2005


Electron impact ionization of glycolaldehyde
journal, May 2005

  • Ptasińska, Sylwia; Denifl, Stephan; Scheier, Paul
  • International Journal of Mass Spectrometry, Vol. 243, Issue 2
  • DOI: 10.1016/j.ijms.2005.02.007

The Description of Chemical Bonding From AB Initio Calculations
journal, October 1978


Bond Dissociation Energies of Organic Molecules
journal, April 2003

  • Blanksby, Stephen J.; Ellison, G. Barney
  • Accounts of Chemical Research, Vol. 36, Issue 4
  • DOI: 10.1021/ar020230d

Biomass pyrolysis: Thermal decomposition mechanisms of furfural and benzaldehyde
journal, September 2013

  • Vasiliou, AnGayle K.; Kim, Jong Hyun; Ormond, Thomas K.
  • The Journal of Chemical Physics, Vol. 139, Issue 10
  • DOI: 10.1063/1.4819788

VUV spectroscopy and photochemistry of five interstellar and putative prebiotic molecules
journal, January 2012

  • Schwell, M.; Gaie-Levrel, F.; Bénilan, Y.
  • EAS Publications Series, Vol. 58
  • DOI: 10.1051/eas/1258050

Metastable ions characteristics. XVIII. Enolic C3H6O+ ion formed from aliphatic ketones
journal, July 1971

  • McLafferty, F. W.; McAdoo, D. J.; Smith, James S.
  • Journal of the American Chemical Society, Vol. 93, Issue 15
  • DOI: 10.1021/ja00744a028

Role of Hydrogen-Bonded Intermediates in the Bimolecular Reactions of the Hydroxyl Radical
journal, May 2002

  • Smith, Ian W. M.; Ravishankara, A. R.
  • The Journal of Physical Chemistry A, Vol. 106, Issue 19
  • DOI: 10.1021/jp014234w

Infrared study of glycolaldehyde isolated in parahydrogen matrix
journal, September 2010

  • Ceponkus, Justinas; Chin, Wutharath; Chevalier, Michèle
  • The Journal of Chemical Physics, Vol. 133, Issue 9
  • DOI: 10.1063/1.3474994

Tables of molecular vibrational frequencies. Consolidated volume II
journal, July 1977

  • Shimanouchi, T.
  • Journal of Physical and Chemical Reference Data, Vol. 6, Issue 3
  • DOI: 10.1063/1.555560

A matrix isolation study of the water-formaldehyde complex. The far-infrared region
journal, January 1992


Spectroscopy of methylene: Einstein coefficients for CH 2 ( b ̃  1 B 1 – a ̃  1 A 1 ) transitions
journal, November 1993

  • García‐Moreno, I.; Bradley Moore, C.
  • The Journal of Chemical Physics, Vol. 99, Issue 9
  • DOI: 10.1063/1.465882

Chromophore Systeme, 1. Konformation und Lichtabsorption von Hexa-1,5-dien-3,4-dion (Divinylglyoxal)
journal, April 1991

  • Bühl, Michael; Kramme, Roland; Martin, Hans-Dieter
  • Chemische Berichte, Vol. 124, Issue 4
  • DOI: 10.1002/cber.19911240423

Infrared spectrum of cis-glyoxal
journal, July 1988


Infrared Spectra of Carbon Monoxide in an Argon Matrix
journal, April 1964

  • Leroi, George E.; Ewing, George E.; Pimentel, George C.
  • The Journal of Chemical Physics, Vol. 40, Issue 8
  • DOI: 10.1063/1.1725508

Chemiluminescent recombination of formyl radicals
journal, January 1967


Rotationally resolved threshold photoelectron spectra of OH and OD
journal, July 1992

  • Wiedmann, R. T.; Tonkyn, R. G.; White, M. G.
  • The Journal of Chemical Physics, Vol. 97, Issue 2
  • DOI: 10.1063/1.463179

Rotationally resolved pulsed field ionization photoelectron study of CO+(X 2Σ+,v+=0–42) in the energy range of 13.98–21.92 eV
journal, November 1999

  • Evans, M.; Ng, C. Y.
  • The Journal of Chemical Physics, Vol. 111, Issue 19
  • DOI: 10.1063/1.480259

The formaldehyde cation: Rovibrational energy level structure and Coriolis interaction near the adiabatic ionization threshold
journal, July 2008

  • Schulenburg, A. M.; Meisinger, M.; Radi, P. P.
  • Journal of Molecular Spectroscopy, Vol. 250, Issue 1
  • DOI: 10.1016/j.jms.2008.04.005

Rotationally resolved photoionization of O2+ near threshold
journal, December 1989

  • Tonkyn, Russell G.; Winniczek, Jerry W.; White, Michael G.
  • Chemical Physics Letters, Vol. 164, Issue 2-3
  • DOI: 10.1016/0009-2614(89)85005-5

High resolution photoelectron spectroscopy and femtosecond intramolecular dynamics of H 2 CCO + and D 2 CCO +
journal, August 1993

  • Niu, Baohua; Bai, Ying; Shirley, David A.
  • The Journal of Chemical Physics, Vol. 99, Issue 4
  • DOI: 10.1063/1.465215

Enols Are Common Intermediates in Hydrocarbon Oxidation
journal, June 2005


Ionization energies of formic and acetic acid monomers
journal, February 1974

  • Knowles, D. J.; Nicholson, A. J. C.
  • The Journal of Chemical Physics, Vol. 60, Issue 3
  • DOI: 10.1063/1.1681132

Complexes of Atmospheric α-Dicarbonyls with Water:  FTIR Matrix Isolation and Theoretical Study
journal, March 2007

  • Mucha, Malgorzata; Mielke, Zofia
  • The Journal of Physical Chemistry A, Vol. 111, Issue 12
  • DOI: 10.1021/jp066685s