Structural studies induced dissociation

The structures of protonated azoles in the gas phase (equilibrium ) can be determined by mass spectrometry in a chemical ionization experiment followed by collision-induced dissociation. The method has been used to study the protonation of benzimidazole (5), indazole (7), and 1-ethylimidazole (179) (all, as expected, on the pyridinelike nitrogen atoms) (80OMSI44) of 1-ethylpyrrole (probably at the -position) (80OMS144) and indole (at the -position) (85IJM49) (see Section IV.A). 

The reactions of Mg+ with cyanoacetylene are remarkable in that while Mg is unreactive towards HCN, multiple ligation occurs for cyanoacetylene . Furthermore, there is evidence from collision induced dissociation (CID) studies that ligand-ligand interactions occur. In fact, the Mg(NC3H)4+ is especially stable, being resistant to CID. DFT calculations suggest that the semibulvalene-type structure, 4, is around 12 kcalmoU more... 

The collision-induced dissociations of several triatomic and polyatomic ions have been studied and the participation of excited states such as COS+(/42II) has been detected.224 Such experiments225 227 have also provided insight into the structures of isomeric ions involved. All these studies were conducted with incident ions of relatively high (kiloelectron volt) kinetic energies and also entailed determination of the kinetic energies of the product ions and/or the fragmentation patterns. 

Energy-resolved rate constant measurements near the threshold for diplet methylene formation from ketene have been used to provide confirmation of the fundamental hypothesis of statistical transition state theory (that rates are controlled by the number of energetically accessible vibrational states at the transition state).6 The electronic structure and aromaticity of planar singlet n2-carbenes has been studied by re-election coupling perturbation theory.7 The heats of formation of three ground-state triplet carbenes have been determined by collision-induced dissociation threshold analysis.8 The heats of formation of methylene, vinylcarbene (H2C=CHCH), and phenylcarbene were found to be 92.2 3.7, 93.3 3.4, and 102.8 33.5 kcal mol-1, respectively. 

Data obtained from collision-induced dissociation experiments did not allow for a distinction of the isomeric metal-silene and-silylene species however, structure-specific ion-molecule reactions of the complexes with labeled ethene were used to clearly differentiate between the metal silene and the silylene. In this intriguing study, Jacobson and coworkers also bracketed the bond dissociation energies of the isomeric ions. 

Because of the importance of the biphenyl scaffold, as a structure of pharmaceutical and active therapeutic compounds, the mass spectrometric behavior of a series of 6,6-disubstituted dibenzo[r//][l,3]dioxepins 36 has been studied. The electron ionization-induced fragmentation patterns were discussed based on use of labeled compounds, accurate mass measurements, and collisionally induced dissociation experiments using an ion trap <2006JMP577>. 

Other means of manipulating ions trapped in the FTMS cell include photodissociation (70-74), surface induced dissociation (75) and electron impact excitation ("EIEIO")(76) reactions. These processes can also be used to obtain structural information, such as isomeric differentiation. In some cases, the information obtained from these processes gives insight into structure beyond that obtained from collision induced dissociation reactions (74). These and other processes can be used in conjunction with FTMS to study gas phase properties of ions, such as gas phase acidities and basicities, electron affinities, bond energies, reactivities, and spectroscopic parameters. Recent reviews (4, 77) have covered many examples of the application of FTMS and ICR, in general, to these types of processes. These processes can also be used to obtain structural information, such as isomeric differentiation. 

Methylhydroxycarbene, MeC(OH), has been generated in one of the three reaction pathways of the collision-induced dissociation of protonated butane-2,3-dione.13 Its enthalpy of formation was found to be 16 4 kcalmol-1. Fluorophenoxycarbene (PhOCF) has been generated inside a hydrophobic hemicarcerand (1) by irradiation of the corresponding confined diazirine.14 Its reactivity (especially dimerization and reaction with water) was significantly lowered by the incarceration, allowing its persistence for days at room temperature. New (amino)(silyl)carbenes (2) have been generated and their structure-activity relationship studied. They showed behaviour similar to those of previously reported (amino)(alkyl)carbene.15... 

Although collision induced dissociation (CID) is a well-known method for investigating the structures of cations in the gas phase (McLafferty, 1983), it has been applied much less to anions (Bowie, 1986). Actually, in some cases CID has been used to study the fragmentation mechanisms of anions, such as the elimination of molecular hydrogen from alkoxide ions (Hayes el al., 1984) or the primary fragmentation routes of ester enolate ions (Froelicher et al., 1985). 

MALDI has also been successfully used to study a number of intact non-covalent complexes such as protein quaternary structures [132-135]. However, MALDI has not yet contributed widely to the study of these complexes because it requires the crystallization of the sample with the matrix. Furthermore, the energy deposited to desorb the ions is not clearly known. In consequence, MALDI generally induces dissociation of the non-covalent interactions and leads to the formation of non-specific aggregates. Nevertheless, weak non-covalent interactions can survive during the MALDI process to allow the direct detection of intact complexes if specific methods, which have been developed to preserve these interactions, are followed [136],... 

Because the soft ionization methods used for oligosaccharides produce few fragments, collision-induced dissociation (CID) or post-source decay (PSD) must be used for structural study. These two techniques have been applied to deprotonated, protonated or alkaliated... 

Electrospray mass spectrometry has developed into a well-established method of wide scope and potential over the past 15 years. The softness of electrospray ionization has made this technique an indispensable tool for biochemical and biomedical research. Electrospray ionization has revolutionized the analysis of labile biopolymers, with applications ranging from the analysis of DNA, RNA, oligonucleotides, proteins as well as glycoproteins to carbohydrates, lipids, gly-colipids, and lipopolysaccharides, often in combination with state-of-the-art separation techniques like liquid chromatography or capillary electrophoresis [1,2]. Beyond mere analytical applications, electrospray ionization mass spectrometry (ESMS) has proven to be a powerful tool for collision-induced dissociation (CID) and multiple-stage mass spectrometric (MSn) analysis, and - beyond the elucidation of primary structures - even for the study of noncovalent macromolecular complexes [3]. 

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