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Benzocaine, protomers, ion mobility-mass spectrometry, IRMPD spectroscopy, gas-phase structure
Abstract:
The immediate environment of a molecule can have a profound influence on its properties. Benzocaine, the ethyl
ester of para-aminobenzoic acid, which finds an application as a local anesthetic (LA), is found to adopt in its protonated form at
least two populations of distinct structures in the gas phase and their relative intensities strongly depend on the properties of the
solvent used in the electrospray ionization (ESI) process. Here we combine IR-vibrational spectroscopy with ion mobility-mass
spectrometry (IM-MS) to yield gas-phase IR spectra of simultaneously m/z and drift-time resolved species of benzocaine. The
results allow for an unambiguous identification of two protomeric species - the N- and O-protonated form. Density functional
theory (DFT) calculations link these structures to the most stable solution and gas-phase structures, respectively, with the electric
properties of the surrounding medium being the main determinant for the preferred protonation site. The fact that the N-protonated
form of benzocaine can be found in the gas phase is owed to kinetic trapping of the solution phase structure during transfer into the
experimental setup. These observations confirm earlier studies on similar molecules where N- and O-protonation has been
suggested.
