Photoionization proton transfer

All the reactions are dependent on the ionization energies and proton affinities of the analyte, solvent, and dopant. Thus there are three possibilities for ionization in the positive mode, direct photoionization, proton transfer, and charge exchange. 

Robb, D. B., and Blades, M. W. (2005). Effects of solvent flow, dopant flow, and lamp current on dopant-assisted atmospheric pressure photoionization (DA-APPI) for LC-MS. Ionization via proton transfer.. Am. Soc. Mass Spectrom. 16, 1275—1290. 

Ionization reactions can occur under vacuum conditions at any time during this process but the origin of ions produced in MALDI is still not fully understood [27,28], Among the chemical and physical ionization pathways suggested for MALDI are gas-phase photoionization, excited state proton transfer, ion-molecule reactions, desorption of preformed ions, and so on. The most widely accepted ion formation mechanism involves proton transfer in the solid phase before desorption or gas-phase proton transfer in the expanding plume from photoionized matrix molecules. The ions in the gas phase are then accelerated by an electrostatic field towards the analyser. Figure 1.15 shows a diagram of the MALDI desorption ionization process. 

Thus, besides the direct photoionization, the analytes in positive APPI mode are ionized either by charge exchange or by proton transfer. The direct ionization and the charge exchange processes allow the ionization of non-polar compounds. This is not possible either with APCI or ESI. 

M. Miyazaki, A. Fujii, T. Ebata, and N. Mikami, Infrared spectroscopy of hydrated benzene cluster cations, [CgHg—(H20) ] (n = 1-6) structural changes upon photoionization and proton transfer reactions, Phys. Chem. Chem. Phys. 5, 1137-1148 (2003). 

The efficient photodecarboxylation of the keto acids (77) has been studied. The reactions involve the formation of the carbanions (78). Aqueous solutions of fenofibric acid (79) at pH 7.4 show the formation of two intermediates when subjected to laser excitation. The study has indicated that the triplet state of the acid in water is of a jtji type. Photoionization is an important process in the aqueous medium. New photoreactive phenylalanine analogues (80) and (81) have been prepared. These were incorporated into position 5 of the pentapeptide, thymopentin. The resultant derivatives were photolabile and underwent decomposition on irradiation at 365 nm. Computational methods have been used to analyse the photoreactivity of the tryptophan derivative (82). The calculations were directed towards an understanding of the quenching of the fluorescence. The results indicate that hydrogen transfer alone does not quench the fluorescence, but that an aborted decarboxylation path is involved. Proton transfer... 

Owing to the relatively mild ionization process, the dominant ions formed include protonated molecular ions, molecular ion adducts, and cluster ions similar to those described for FAB (section 9.2.1.6). A number of different chemical and physical mechanisms have been proposed to explain ion formation, including gas-phase photoionization, ion-molecule reactions, disproportionation, excited state proton transfer, energy pooling, thermal ionization, and desorption of preformed ions [37]. The choices of ma-... 

Dopants (D) are photoionized first, followed by charge exchange with the analytes (M). The ionization of analytes can also be achieved by means of proton transfer processes ... 

Another ionization technique, which seems closely related to TSI and ESI, is atmospheric-pressure chemical ionization (APCI). In APCI, the solvent stream, e.g., the effluent from an LC column, is pneumatically nebuhzed into a heated vaporizer zone, where (almost) complete evaporation of the aerosol droplets is achieved [71-73]. Analyte ionization is initiated by electrons from a downstream corona discharge needle. The electrons act as primary source of ionization of the solvent or mobile-phase constituents, which in turn by gas-phase ion-molecule reactions in the API source ionize the analyte molecules, mostly by proton-transfer reactions, i.e., formation of [M-hH]+ in positive-ion and [M-H] in negative-ion mode. There are also some results, indicating the Na -cationization can take place under APCI conditions. Atmospheric-pressure photoionization (APPI) is an ionization technique closely related to APCI. In APPI, the analyte ionization is initiated by light from a vacuum-ultraviolet lamp, e.g., a Kr-lamp, instead of by means of a corona discharge. Next to direct photoionization of the analytes, gas-phase ion-molecular reactions greatly contribute to the ionization in APPI [74,75]. 

The other method involves direct ionization of the molecule to the ion energy of interest. This can be done optically by photoelectron-photoion coincidence (described below), or by charge exchange, which is a form of chemical ionization. Common chemical ionization methods include charge transfer and proton transfer. 

The excited state properties of hydroxyaromatic compounds (phenols, naphthols, etc) are of interest to a wide audience in chemistry, including those interested in the environmental decomposition of phenols, chemical physicists interested in the very fast dynamics of excited-state proton transfer (ESPT) and excited-state intramolecular proton transfer (ESIPT), physical chemists interested in photoionization and the photochemical pathways for phenoxyl radical formation, and organic photochemists interested in the mechanisms of phenol and hydroxyarene photochemistry. Due to space limitations, this review is restricted to molecular photochemistry of hydroxyaromatic compounds reported during the last three decades that are of primary interest to organic photochemists. It also includes a brief section on the phenomenon of enhanced acidity of phenols and other hydroxyaromatics because this is central to hydroxyarene photochemistry and forms the basis of much of the mechanistic photochemistry to be discussed later on. Several reviews that offer related coverage to this work have also appeared recently. This review does not cover aspects of electron photoejection from phenols or phenolate ions (and related compounds such as tyrosine) or phenol OH homolysis induced photochemically, as shown in Eq. (39.1), as these are adequately covered elsewhere ... 

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