Isomeric effects on fragmentations of crotonaldehyde and methacrolein in low energy electron–molecule collisions Arup K. Ghosh, Aparajeo Chattopadhyay.

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Isomeric effects on fragmentations of crotonaldehyde and methacrolein in low energy electron–molecule collisions Arup K. Ghosh, Aparajeo Chattopadhyay and Tapas Chakraborty Physical Chemistry Department Indian Association for the Cultivation of Science Jadavpur, Kolkata 700032, India 69th International Symposium on Molecular Spectroscopy 17th June, 2014

(methacryloyl peroxy nitrate) Aim of the study hν 313 nm >  > 200 nm CO + CH4 ● + H● + HCO● Crotonaldehyde Isoprene OH/O2 NO/O2 Cigarette Smoke Biogenic Emission from forests Wheat Straw Solar UV radiation produce photoelectrons in the atmosphere. Primary auroral electrons ionize the atmospheric gases producing secondary electrons. β α UV radiation OH/O3/NOx e- OH/O2 NO2/O2 PAN (methacryloyl peroxy nitrate) OOH  Hydroxyacetone VOCs Methacrolein β α E. R. Allen, J. N. Pitts Jr., J. Am. Chem. Soc., 91, 3135, 1969. Physics and chemistry of the upper atmosphere, M. H. Rees, 1989. Volatile Organic Compounds in the Atmosphere, R. Koppmann, 2007. J.D. Crounse, H. C. Knap, K. B. Ørnsø, S. Jørgensen, F. Paulot, H.G. Kjaergaard, P.O. Wennberg, J. Phys. Chem. A, 116, 5756, 2012.

Reaction Scheme and Product Analysis Reaction Zone Analysis Zone Channeltron detector - -5 kV -2 kV e Quadrupole mass analyzer Electron Gun Sample Turbo pump Diffusion pump Signal Computer Sample Inlet

Products formed after collisions with electrons having kinetic energy of 12 eV Crotonaldehyde (I.E. = 9.73 eV) Methacrolein (I.E. =9.92 eV) CO loss channel CO loss channel Molecular ion Molecular ion HCO loss channel HCO loss channel H loss channel H loss channel + ● + ● ●+ ●+ + 70 + + 42 70 30 40 50 60 70 m/z + 41 42 69 CH3CO+ 41 69 12 eV 30 40 50 60 70 m/z A. K. Ghosh, A. Chattopadhyay, A. Mukhopadhyay, T. Chakraborty, Chem. Phy. Lett., 561-562, 24-30, 2013.

A comparison of the dissociation channels… Crotonaldehyde Methacrolein ●+ ●+ + ● + ● 10 eV + + + CH3CO+ 10 eV + 12 eV 12 eV 15 eV 30 40 50 60 70 m/z 15 eV 30 40 50 60 70 m/z

Variation of ion yield with the kinetic energy of electrons: e-KE (eV) Intensity (arb. units) Molecular Ion H loss channel + ●+ Intensity (arb. units) e-KE (eV) CO loss channel HCO loss + ● +

Transition states for the H loss channels: e-KE (eV) Intensity (arb. units) + 2.027 1.418 1.347 1.123 1.401 1.354 1.02 eV 1.461 1.352 1.113 Methacrolein ion 1.13 eV 1.964 1.136 1.393 1.359 1.419 1.365 1.112 1.126 1.378 1.366 1.06 eV 0.94 eV Crotonaldehyde ion Energy (eV) DFT/B3LYP/6-311++G(d,p) Reaction co-ordinate

Transition states for the CO loss channels: + ● Intensity (arb. units) e-KE (eV) 2.007 2.524 1.097 1.47 eV 1.461 1.113 1.092 -0.1 eV Methacrolein ion 1.103 2.493 1.971 1.419 1.112 1.089 0.13 eV Crotonaldehyde ion Energy (eV) 1.98 eV DFT/B3LYP/6-311++G(d,p) Reaction co-ordinate

The HCO loss channel of crotonaldehyde and methacrolein cations: + Intensity (arb. units) e-KE (eV) + Relative Energy (eV) C1 C2 C3 C4 Crotonaldehyde cation C1 C2 C3 C4 Methacrolein cation C1-C2 Bond Length (Å) DFT/B3LYP/6-311++G(d,p)

The acetyl cation (CH3CO+) at m/z 43 1.166 1.310 1.518 1.534 EK (0.17) 1.376 1.133 1.493 DMK (0.15) 1.372 1.483 1.558 EK + (8.92) 1.131 1.311 1.169 1.514 DMK + (8.63) Relative Energy (eV) 1.223 1..537 1.460 1.340 MVK+ (9.43) Ionization 1.233 1.419 1.365 1.470 CA+ (9.49) Ha 1.217 1.461 1.504 1.352 MA+ (9.68) 1.213 1.467 CA (0.0) 1.211 1.484 1.501 1.339 MA (0.03) We suggest that MA+ undergoes isomerization to form methyl vinyl cation (MVK+) and dissociation of MVK+ produces the acetyl cation.

Summary: The two isomeric compounds studied here show similar fragmentation channels, however, the relative yields of the fragmentation are different. The differences in product yields can be explained qualitatively from their relative stabilities. Electronic structure calculation predicts similar trends, and the transition states being structurally similar to the products, the relative TS energies correspond to the product stabilities.

Acknowledgements: All my labmates Funding: Council of Scientific and Industrial Research (CSIR), India

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