Gas-phase ion structures

Ionisation processes in IMS occur in the gas phase through chemical reactions between sample molecules and a reservoir of reactive ions, i.e. the reactant ions. Formation of product ions in IMS bears resemblance to the chemistry in both APCI-MS and ECD technologies. Much yet needs to be learned about the kinetics of proton transfers and the structures of protonated gas-phase ions. Parallels have been drawn between IMS and CI-MS [277]. However, there are essential differences in ion identities between IMS, APCI-MS and CI-MS (see ref. [278]). The limited availability of IMS-MS (or IMMS) instruments during the last 35 years has impeded development of a comprehensive model for APCI. At the present time, the underlying basis of APCI and other ion-molecule events that occur in IMS remains vague. Rival techniques are MS and GC-MS. There are vast differences in the principles of ion separation in MS versus IMS. 

A lot of information regarding identification, structural characterization, quantitation, gas phase ion chemistry and thermodynamics can be obtained by MS. 

In the gas phase, ions may be isolated, and properties such as stability, metal-ligand bond energy, or reactivity determined, full structural characterization is not yet possible. There are no complications due to solvent or crystal packing forces and so the intrinsic properties of the ions may be investigated. The effects of solvation may be probed by studying ions such as [M(solvent) ]+. The spectroscopic investigation of ions has been limited to the photoelectron spectroscopy of anions but other methods such as infrared (IR) photodissociation spectroscopy are now available. 

The dissociation of gas-phase ions can be a guide to the structure of the ions. There are two ways to dissociate gas-phase ions, either by collision-induced dissociation (CID) sometimes termed collision-activation dissociation (CAD) or by photodissociation. In each method, a mass selected ion is dissociated and the fragment ion (often called a daughter ion) is measured the neutral fragment cannot be experimentally observed. 

Mass spectrometers use the difference in mass-to-charge ratio (m/z) of ionized atoms, molecular fragments, or whole molecules to differentiate between them. Mass spectrometry is therefore useful for quantitation of atoms or molecules and also for determining chemical and structural information about them [329, 531-533]. Molecules have distinctive fragmentation patterns which provide information to identify structural components. The general operation of a mass spectrometer is to (1) create gas-phase ions, (2) separate the ions in space or time based on their mass-to-charge ratio, and (3) measure the quantity of ions of each mass-to-charge ratio. The ion separation power of a mass spectrometer is described by the resolution, which is defined as ... 

A comparison is made between the gas phase and solution phase reaction pathways for a wide range of organic reactions. Examples are presented in which the gas phase and solution phase mechanisms are the same for a given set of reactants in which they differ, but attachment of the first molecule of solvent to the bare gas phase ionic reactant results in the solution phase products and in which the bare, monosolvated, and bulk-solvated reactions proceed by three different pathways for the same reactants. The various tools available to the gas phase ion chemist are discussed, and examples of their use in the probing of ionic structures and mechanisms are reported. 

Recent developments in instrument design have led to lower limits of detection, while new ion activation techniques and improved understanding of gas-phase ion chemistry have enhanced the capabilities of tandem mass spectrometry for peptide and protein structure elucidation. Future developments must address the understanding of protein-protein interactions and the characterization of the dynamic proteome (Chalmers and Gaskell 2000). 

The situation in solution is quite different. The difficulty of stabilizing charge in the gas phase is well known and in solution smaller differences between structures are expected. There should also be less dependence on the size of the ion, which is a well-recognized feature of gas-phase ion stabilization, but does not appear to be significant in solution.41... 

Versatile Pulse Sequence One of the great strengths of FTMS is the flexibility to selectively accelerate, activate, and eject ions in any combination and any sequence without hardware modifications. This versatility makes FTMS the method of choice for MS/MS and hence for establishing pathways and rate constants for gas-phase ion-molecule reactions, and to correlate this data with structural information. Recently up to (MS)5 has been demonstrated (18). 

Three main topics relevant to the gas-phase chemistry of Ge, Sn and Pb derivatives are discussed in the present chapter (a) the mass spectrometry related to organometallic compounds of group 14 with particular emphasis on the more general aspects (b) the gas-phase ion chemistry comprising the thermochemistry, structure and reactivity of ions and (c) gas-phase reactions involving neutral species. 

The fundamental aspects related to the thermochemistry, structure and reactivity of gas-phase ions are usually considered the domain of gas-phase ion chemistry. By extension, some of these same properties are often obtained for simple neutrals and radicals from methods used in gas-phase ion chemistry. A wide range of experimental techniques can be used for this purpose, and instrumental developments have contributed a great deal to our knowledge of gas-phase ions. Theoretical calculations have also played an important role and gas-phase ion chemistry has witnessed a very lively interplay between experiment and theory in recent years. 

In summary, the gas-phase ion chemistry of Ge, Sn and Pb positive ions reveals some very unusual reactivity and will probably witness important advances in the near future regarding structural aspects of these ions. 

The energy with the polarization of the ion removed, ET this is achieved by only including the same number of natural orbitals in the basis as occupied orbitals for the given ion, hence no relaxation from the gas-phase electronic structure is possible. 

Generation of unusual molecules by NRMS typically, but not entirely, relies on collisional reduction of cation-radicals produced by dissociative ionization of stable molecular precursors. Owing to the advanced state of gas-phase ion chemistry, ion structures can often be unambiguously assigned to products of ion dissociations and the ions then used to generate unusual neutral molecules. 

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