title: The impact of environment and resonance effects on the site of protonation of aminobenzoic acid derivatives.
abstract: 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.
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. helden@fhi-berlin.mpg.de.
authors.forename: Jongcheol
authors.identifier: http://orcid.org/0000-0001-5844-4585
authors.initials: J
authors.lastname: Seo
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. helden@fhi-berlin.mpg.de.
authors.forename: Stephan
authors.initials: S
authors.lastname: Warnke
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. helden@fhi-berlin.mpg.de.
authors.forename: Sandy
authors.initials: S
authors.lastname: Gewinner
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. helden@fhi-berlin.mpg.de.
authors.forename: Wieland
authors.identifier: http://orcid.org/0000-0003-0564-203X
authors.initials: W
authors.lastname: Schöllkopf
authors.affiliation: Department of Chemistry and Biochemistry, University of California Santa Barbara, Santa Barbara, California 93106, USA.
authors.forename: Michael T
authors.initials: MT
authors.lastname: Bowers
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. helden@fhi-berlin.mpg.de and Freie Universität Berlin, Department of Biology, Chemistry and Pharmacy, Takustrasse 3, 14195 Berlin, Germany. kevin.pagel@fu-berlin.de.
authors.forename: Kevin
authors.initials: K
authors.lastname: Pagel
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. helden@fhi-berlin.mpg.de.
authors.forename: Gert
authors.identifier: http://orcid.org/0000-0001-7611-8740
authors.initials: G
authors.lastname: von Helden