Mid-IR spectroscopy of protonated leucine methyl ester performed with an FTICR or a Paul type ion-trap

The protonated leucine methyl ester structure was probed using mid-infrared multiphoton dissociation (IRMPD) spectroscopy performed at CLIO, the Orsay Free Electron Laser facility. A first experimental spectrum was obtained with a Fourier-transform ion-cyclotron-resonance mass spectrometer with ions generated through a MALDI process. A second spectrum was recorded with ions generated using ElectroSpray Ionisation and trapped in a Paul ion-trap. These two experimental spectra are analyzed and compared with infrared absorption spectra derived from hybrid density functional theory calculations. The two IRMPD spectra are in excellent agreement, although the one recorded with the Paul ion-trap presents a better resolution with an fwhm of the IRMPD bands of 20–25 cm−1. Comparison with the calculated IR absorption spectra clearly shows that only the lowest energy isomer is formed under both experimental conditions. The CO stretch, together with two vibrational modes of the ammonium group were the three infrared signatures identified in the photon energy range explored here.