Proton affinity of para-substituted acetophenones in gas phase and in solution: a theoretical study
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ORIGINAL PAPER
Proton affinity of para-substituted acetophenones in gas phase and in solution: a theoretical study Abir Haloui & Ezzeddine Haloui
Received: 28 March 2012 / Accepted: 28 August 2012 / Published online: 21 September 2012 # Springer-Verlag 2012
Abstract The gas phase proton affinities PA and basicities GB for a series of para-substituted acetophenones weak bases (B) p.X−C6H4CO*CH3 with X0H, F, Cl, Br, I, Me, CF3, CN, NO2, OCH3, NH2, CH2OH, N (CH 3 ) 2 , OH, NHþ 3 , … have been calculated at 298.15 K at the density functional theory DFT/B3LYP level with a 6-311++G (2d,2p) basis set. Conformational results lead to only one stable planar conformer for both unprotonated compounds and their O*-protonated forms. Satisfactory accuracy and computational efficiency could be reached if the computed PAs are scaled by a factor 0.983. Protonation at more than one site is discussed and the carbonyl oxygen atom is found to be the preferential protonated site rather than the substituent X. The calculated gas phase PAs show a good agreement with the experimental available data. The electrondonating/electron-withdrawing nature of the substituents has an enormous influence upon the thermochemical and structural properties. The influence of environment on the proton affinity has been studied by means of SCRF solvent effect computations using PCM solvation model for two solvents: water and SO2CI2. Confrontation between computed and experimental pK(B) values exhibits better agreement in aqueous solution than in organic solvent. Keywords DFT(B3LYP) . Para-substituted acetophenones . pKb . Proton affinity . Solvent effect . Substituent effect A. Haloui (*) : E. Haloui Department of Chemistry, Faculty of Sciences, Manar 2, 2092 Tunis, Tunisia e-mail: [email protected]
Introduction Over the last three decades, protonation reactions have been the subject of extensive investigations in physical organic chemistry and biochemistry from both an experimental and theoretical point of view [1–22]. Protonation consists of the addition of a proton H+ to an atom, molecule or ion. The proton affinity (PA) and the gas-phase basicity (GB) are the two most pivotal quantities used to understand the proton transfer process and are directly related to the acid–base approach [2–4, 22–25]. A great number of experimental studies have measured the basicities of organic compounds in both gas phase and solution-phase using modern mass spectrometric techniques. Measurements were performed by spectroscopic methods in acid media [9–14, 16], kinetic method with a tandem mass spectrometer [17], Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry [17, 23, 26], etc. Despite these numerous experimental studies, a detailed characterization of structural and thermochemical parameters is still largely unknown, especially in solution. In the last few years, several experimental studies have been combined with theoretical approaches to provide a full partnership with experiment. Ab initio (HF, MPn [4, 6, 27–34] and G3 [23, 26, 35–37]) as wel
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