Infrared multiple photon dynamics and spectroscopy of cationic PABA and its dehydroxylated fragment ion

Abstract

The gas-phase infrared spectra of cationic para-amino benzoic acid (PABA, H 2 N-C 6 H 4 -COOH) and its dehydroxylated fragment ion, the para-amino benzoyl (PABz, H 2 N-C 6 H 4 -CO) cation, are presented in the 500-2500 cm À1 range. The spectra are obtained via mass-selective infrared multiple photon dissociation (IRMPD) using a continuously tunable free electron laser. Although both spectra are in good overall agreement with results from density functional theory, some interesting discrepancies are also observed. Particularly in the spectrum of the PABz cation, the strongly anharmonic NH 2 out-of-plane bending mode causes substantial deviations from theory. The relative intensity of this out-of-plane bending mode in both the PABA and PABz cation spectra is much lower than calculated, similar to what has been reported for cationic aniline. For the PABz cation, two distinct fragmentation products are observed and recording the infrared spectra on these individual decay channels yields information on the dissociation mechanism. Moreover, it is experimentally shown that the decay rate into the two competing channels can qualitatively be described using statistical models.

Structural characterization of gas-phase cysteine and cysteine methyl ester complexes with zinc and cadmium dications by infrared multiple photon dissociation spectroscopy

“…This vibrational mode is known to have strong anharmonic character in amino acids, an effect that is often manifested as a shift in IRMPD spectra. [41][42][43] None of the other conformers shown in Fig. 3 have predicted spectra that match the experimental spectrum.…”

Infrared Spectroscopy of Discrete Uranyl Anion Complexes

“…If the latter is true, then it suggests that the ν 3 frequency in the spectrum of the OCH 3 loss channel is redshifted by about 20 cm -1 due to the higher energetic requirements for that channel; shifts of this magnitude have been observed previously. 58 The IR spectra generated from the photodissociation channels corresponding to the loss of either H or H 2 bore strong similarities to one another in that they contained a peak with a maximum at 987 cm -1 and a second peak at about 887 cm -1 . The higher-frequency peak probably contains components from the unresolved uranyl asymmetric stretch and C-O stretching, and the lower-frequency peak corresponds to the symmetric UO 2 stretch.…”

Molecular Complexes of Simple Anions with Electron‐Deficient Arenes: Spectroscopic Evidence for Two Types of Structural Motifs for Anion–Arene Interactions

“…The observed ion abundance ratio for the photofragment ions conforms to the general observation that the IRMPD process activates the fragmentation of the absorbing species along the low-energy dissociation channels. [61,73,74] When both fragmentation paths are taken into account, the plot of the IRMPD yield (R = ÀlogA C H T U N G T R E N N U N G (I parent /(I parent +SI fragment )) as a function of the radiation wavenumber generates the IRMPD spectrum reported in Figure 3. The most pronounced band in the spectrum is a broad feature centered at 1234-1268 cm…”