Abstract
A previous study (Ahmed, A. et al., Anal. Chem. 84, 1146–1151( 2012) reported that toluene used as a solvent was the proton source for polyaromatic hydrocarbon compounds (PAHs) that were subjected to (+)-mode atmospheric-pressure photoionization. In the current study, the exact position of the hydrogen atom in the toluene molecule (either a methyl hydrogen or an aromatic ring hydrogen) involved in the formation of protonated PAH ions was investigated. Experimental analyses of benzene and anisole demonstrated that although the aromatic hydrogen atom of toluene did not contribute to the formation of protonated anthracene, it did contribute to the formation of protonated acridine. Thermochemical data and quantum mechanical calculations showed that the protonation of anthracene by an aromatic ring hydrogen atom of toluene is endothermic, while protonation by a methyl hydrogen atom is exothermic. However, protonation of acridine by either an aromatic ring hydrogen or a methyl hydrogen atom of toluene is exothermic. The different behavior of acridine and anthracene was attributed to differences in gas-phase basicity. It was concluded that both types of hydrogen in toluene can be used for protonation of PAH compounds, but a methyl hydrogen atom is preferred, especially for non-basic compounds.
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References
Ahmed, A., Choi, C.H., Choi, M.C., Kim, S.: Mechanisms Behind the Generation of Protonated Ions for Polyaromatic Hydrocarbons by Atmospheric Pressure Photoionization. Anal. Chem. 84, 1146–1151 (2012)
Kauppila, T.J., Kuuranne, T., Meurer, E.C., Eberlin, M.N., Kotiaho, T., Kostiainen, R.: Atmospheric Pressure Photoionization Mass Spectrometry. Ionization Mechanism and the Effect of Solvent on the Ionization of Naphthalenes. Anal. Chem. 74, 5470–5479 (2002)
Tubaro, M., Marotta, E., Seraglia, R., Traldi, P.: Atmospheric Pressure Photoionization Mechanisms. 2. The Case of Benzene and Toluene. Rapid Commun. Mass Spectrom. 17, 2423–2429 (2003)
Kauppila, T.J., Kostiainen, R., Bruins, A.P.: Anisole, a new dopant for atmospheric pressure photoionization mass spectrometry of low proton affinity, low ionization energy compounds. Rapid Commun. Mass Spectrom. 18, 808–815 (2004)
Syage, J.A.: Mechanism of [M + H] + Formation in Photoionization. Mass Spectrom. J. Am. Soc. Mass Spectrom. 15, 1521–1533 (2004)
Kauppila, T.J., Bruins, A.P., Kostiainen, R.: Effect of the Solvent Flow Rate on the Ionization Efficiency in Atmospheric Pressure Photoionization-Mass Spectrometry. J. Am. Soc. Mass Spectrom. 16, 1399–1407 (2005)
Purcell, J.M., Hendrickson, C.L., Rodgers, R.P., Marshall, A.G.: Atmospheric Pressure Photoionization Proton Transfer for Complex Organic Mixtures Investigated by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. J. Am. Soc. Mass Spectrom. 18, 1682–1689 (2007)
Kamel, A., Jeanville, P., Colizza, K., J-Riverac, L.E.: Mechanism of [M + H] + Formation in Atmospheric Pressure Photoionization Mass Spectrometry: Identification of Propionitrile in Acetonitrile with High Mass Accuracy Measurement and Tandem Mass Spectrometry and Evidence for Its Involvement in the Protonation Phenomenon. J. Am. Soc. Mass Spectrom. 19, 1579–1589 (2008)
Mayer, J.M.: Understanding Hydrogen Atom Transfer: From Bond Strengths to Marcus Theory. Acc. Chem. Res. 44, 36–46 (2011)
Sirjoosingh, A., Hammes-Schiffer, S.: Proton-Coupled Electron Transfer versus Hydrogen Atom Transfer: Generation of Charge-Localized Diabatic States. J. Phys. Chem. A 115, 2367–2377 (2011)
Becke, A.D.: Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 98, 5648–5652 (1993)
Lee, C., Yang, W., Parr, R.G.: Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B Condens. Matter 37, 785–789 (1988)
Hehre, W.J., Ditchfield, R., Pople, J.A.: Self-Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules. J. Chem. Phys. 56, 2257–2261 (1972)
Gonzalez, C., Schlegel, H.B.: Improved algorithms for reaction path following: Higher-order implicit algorithms. J. Chem. Phys. 95, 5853–5860 (1991)
Schmidt, M.W., Baldridge, K.K., Boatz, J.A., Elbert, S.T., Gordon, M.S., Jensen, J.H., Koseki, S., Matsunaga, N., Nguyen, K.A., Su, S., Windus, T.L., Dupuis, M., Montgomery, J.A.: General atomic and molecular electronic structure system. J. Comput. Chem. 14, 1347–1363 (1993)
Pang, F., Boggs, J.E., Pulay, P., Fogarasi, G.: The molecular structure of toluene. J. Mol. Struct. 66, 281–287 (1980)
Lifshitz, C.: Tropylium Ion Formation from Toluene: Solution of an Old Problem in Organic Mass Spectrometry. Acc. Chem. Res. 27, 138–144 (1994)
NIST Chemistry Webbook, NIST Standard Reference Database Number 69; Mallard, W. G., Linstrom, P. J., Eds.; National Institute of Standards and Technology: Gaithersburg, MD, 20899, July 2001 Available at: http://webbook.nist.gov). Accessed 7 July 2012
Acknowledgments
The authors acknowledge that this research was supported by the Korea Basic Science Institute, Kyungpook National University Research Fund, 2012 and project no. PM56951 (grants-in-aid from the Ministry of Land, Transport and Maritime Affairs, ROK).
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Ahmed, A., Ghosh, M.K., Choi, M.C. et al. Which Hydrogen Atom of Toluene Protonates PAH molecules in (+)-Mode APPI MS Analysis?. J. Am. Soc. Mass Spectrom. 24, 316–319 (2013). https://doi.org/10.1007/s13361-012-0520-x
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DOI: https://doi.org/10.1007/s13361-012-0520-x