The impact of environment and resonance effects on the site of protonation of aminobenzoic acid derivatives

The charge distribution in a molecule is crucial in determining its physical and chemical properties. Aminobenzoic acid derivatives are biologically active small molecules, which have two possible protonation sites: the amine ( N -protonation) and the carbonyl oxygen ( O -protonation). Here, we empl...

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Bibliographic Details
Published in:Physical chemistry chemical physics : PCCP 2016, Vol.18 (36), p.25474-25482
Main Authors: Seo, Jongcheol, Warnke, Stephan, Gewinner, Sandy, Schöllkopf, Wieland, Bowers, Michael T, Pagel, Kevin, von Helden, Gert
Format: Article
Language:English
Subjects:
Aqueous environments
Charge distribution
Derivatives
Ion mobility
Mass spectrometry
Mathematical analysis
Protonation
Sheds
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Summary:The charge distribution in a molecule is crucial in determining its physical and chemical properties. Aminobenzoic acid derivatives are biologically active small molecules, which have two possible protonation sites: the amine ( N -protonation) and the carbonyl oxygen ( O -protonation). Here, we employ gas-phase infrared spectroscopy in combination with ion mobility-mass spectrometry and density functional theory calculations to unambiguously determine the preferred protonation sites of p -, m -, and o -isomers of aminobenzoic acids as well as their ethyl esters. The results show that the site of protonation does not only depend on the intrinsic molecular properties such as resonance effects, but also critically on the environment of the molecules. In an aqueous environment, N -protonation is expected to be lowest in energy for all species investigated here. In the gas phase, O -protonation can be preferred, and in those cases, both N - and O -protonated species are observed. To shed light on a possible proton migration pathway, the protonated molecule-solvent complex as well as proton-bound dimers are investigated. Ion mobility- and m / z -selected infrared spectroscopy unambiguously determines the site of protonation of p -, m -, and o -aminobenzoic acid derivatives.
ISSN:1463-9076
1463-9084
DOI:10.1039/c6cp04941a