title: Correlation between experimental data of protonation of aromatic compounds at (+) atmospheric pressure photoionization and theoretically calculated enthalpies.
abstract: BACKGROUND
  The theoretical enthalpy calculated from the overall protonation reaction (electron transfer plus hydrogen transfer) in positive-mode (+) atmospheric-pressure photoionization (APPI) was compared with experimental results for 49 aromatic compounds. A linear relationship was observed between the calculated ΔH and the relative abundance of the protonated peak. The parameter gives reasonable predictions for all the aromatic hydrocarbon compounds used in this study.
  
  
  METHODS
  A parameter is devised by combining experimental MS data and high-level theoretical calculations. A (+) APPI Q Exactive Orbitrap mass spectrometer was used to obtain MS data for each solution. B3LYP exchange-correlation functions with the standard 6-311+G(df,2p) basis set was used to perform density functional theory (DFT) calculations.
  
  
  RESULTS
  All the molecules with ΔH <0 kcal/mol for the overall protonation reaction with toluene clusters produced protonated ions, regardless of the desolvation temperature. For molecules with ΔH >0, molecular ions were more abundant at typical APPI desolvation temperatures (300°C), while the protonated ions became comparable or dominant at higher temperatures (400°C). The toluene cluster size was an important factor when predicting the ionization behavior of aromatic hydrocarbon ions in (+) APPI.
  
  
  CONCLUSIONS
  The data used in this study clearly show that the theoretically calculated reaction enthalpy (ΔH) of protonation with toluene dimers can be used to predict the protonation behavior of aromatic compounds. When compounds have a negative ΔH value, the types of ions generated for aromatic compounds could be very well predicted based on the ΔH value. The ΔH can explain overall protonation behavior of compounds with ΔH values >0. Copyright © 2017 John Wiley & Sons, Ltd.
authors.affiliation: Department of Chemistry, Kyungpook National University, Daegu, 702-701, Republic of Korea.
authors.forename: Arif
authors.initials: A
authors.lastname: Ahmed
authors.affiliation: Department of Chemistry, Kyungpook National University, Daegu, 702-701, Republic of Korea.
authors.forename: Dongwon
authors.initials: D
authors.lastname: Lim
authors.affiliation: Department of Chemistry, Kyungpook National University, Daegu, 702-701, Republic of Korea.|Department of Chemistry, Green Nano Center, Daegu, 702-701, Republic of Korea.
authors.forename: Cheol Ho
authors.initials: CH
authors.lastname: Choi
authors.affiliation: Department of Chemistry, Kyungpook National University, Daegu, 702-701, Republic of Korea.|Department of Chemistry, Green Nano Center, Daegu, 702-701, Republic of Korea.
authors.forename: Sunghwan
authors.identifier: http://orcid.org/0000-0002-3364-7367
authors.initials: S
authors.lastname: Kim