Pulsed-laser-ionization high-pressure mass spectrometry: a new route to gas-phase metal ion-ligand thermochemistry

Aluminum monocation basicity and affinity scales

The experimental aspects of the determination of thermochemical data for the attachment of the aluminum monocation Al(+) to neutral atoms and molecules are reviewed. Literature aluminum cation affinities (enthalpy scale) and basicities (Gibbs energy scale) are tabulated and discussed. Ab initio quantum chemical calculations at the G4 level on 43 adducts provide a consistent picture of the energetics of the adducts and their structures. The Al(+)-ligand bonding is analyzed in terms of natural bond orbital and atom-in molecule analyses. A brief comparison of the Al(+) basicity scales and other gas- phase cation basicities is presented.

Blackbody infrared radiative dissociation of bradykinin and its analogues: energetics, dynamics, and evidence for salt-bridge structures in the gas phase

Blackbody infrared radiative dissociation (BIRD) spectra of singly and doubly protonated bradykinin and its analogues are measured in a Fourier-transform mass spectrometer. Rate constants for dissociation are measured as a function of temperature with reaction delays up to 600 s. From these data, Arrhenius activation parameters in the zero-pressure limit are obtained. The activation parameters and dissociation products for the singly protonated ions are highly sensitive to small changes in ion structure. The Arrhenius activation energy (E(a)) and pre-exponential (or frequency factor, A) of the singly protonated ions investigated here range from 0.6 to 1.4 eV and 10(5) to 10(12) s(-1), respectively. For bradykinin and its analogues differing by modification of the residues between the two arginine groups on either end of the molecule, the singly and doubly protonated ions have average activation energies of 1.2 and 0.8 eV, respectively, and average A values of 10(8) and 10(12) s(-1), respectively, i.e., the presence of a second charge reduces the activation energy by 0.4 eV and decreases the A value by a factor of 10(4). This demonstrates that the presence of a second charge can dramatically influence the dissociation dynamics of these ions. The doubly protonated methyl ester of bradykinin has an E(a) of 0.82 eV, comparable to the value of 0.84 eV for bradykinin itself. However, this value is 0.21 +/- 0.08 eV greater than that of singly protonated methyl ester of bradykinin, indicating that the Coulomb repulsion is not the most significant factor in the activation energy of this ion. Both singly and doubly protonated Lys-bradykinin ions have higher activation energies than the corresponding bradykinin ions indicating that the addition of a basic residue stabilizes these ions with respect to dissociation. Methylation of the carboxylic acid group of the C-terminus reduces the E(a) of bradykinin from 1.3 to 0.6 eV and the A factor from 1012 to 105 s(-1). This modification also dramatically changes the dissociation products. Similar results are observed for [Ala(6)]-bradykinin and its methyl ester. These results, in combination with others presented here, provide experimental evidence that the most stable form of singly protonated bradykinin is a salt-bridge structure.

Structures, energetics, and dynamics of gas phase ions studied by FTICR and HPMS

Both Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and high-pressure mass spectrometry (HPMS) are very powerful tools in the field of gas phase ion chemistry. Many experimental method developments based on FTICR-MS and HPMS are summarized, including the coupling of a high-pressure external ion source to a FTICR mass spectrometer, blackbody infrared radiative dissociation (BIRD), coupling laser desorption ionization with HPMS, infrared multiple photon dissociation (IRMPD), radiative association and bimolecular routes to gas phase cluster ion formation. An abundance of thermochemical data, such as proton affinities, gas phase acidities, methyl cation affinities and metal cation affinities, have been obtained. Some of these data are the basis of the standard data listed in the NIST thermochemical databases. Ion-molecule interactions, energetics, reactivities, and structures of molecules have been extensively investigated using the methods developed based on HPMS and FTICR mass spectrometric techniques.

Spectroscopy of metal ion complexes: gas-phase models for solvation

Weakly bound metal ion complexes are produced in molecular beams and studied with mass-selected laser photodissociation spectroscopy. The metal ions Mg+ and Ca+ are the focus of these studies because they have a single valence electron and strong atomic resonance lines in convenient wavelength regions. Weakly bound complexes of these ions with rare-gas atoms and small molecules are prepared with laser vaporization in a pulsed nozzle cluster source. The vibrationally and rotationally resolved electronic spectra obtained for these complexes help to determine the complexes' structures and bonding energetics. Observations from these studies have provided many new insights into the fundamental interactions in electrostatic bonding.

Noncovalent metal-ligand bond energies as studied by threshold collision-induced dissociation

This review focuses on noncovalent metal ion-ligand complexes and measurements of the bond energies of such species. The method utilized in this work is threshold collision-induced dissociation (CID), as achieved using a guided ion beam tandem mass spectrometer. Accurate determination of bond energies requires attention to many details of the experiments and data analysis. These details are discussed thoroughly and compared to other methods. A comprehensive listing of metal-ligand bond dissociation energies determined by threshold CID is provided. This list includes a variety of metals (alkalis, magnesium, aluminum, and first and second row transition metals), many different types of ligands, and variations in the number of ligands. The trends in these values are discussed, and we elucidate the importance of ion-dipole and ion-induced dipole interactions, chelation, different conformers and tautomers, steric interactions, solvation phenomena, and electronic effects such as hybridization and promotion.

Collision-induced and infrared multiphoton dissociation studies on M(acetone)2 (+) (M=Al, Fe, Co, Cu, ScO) in the gas phase

In this paper we report an extension of our earlier study on the structure of Alfacetone)2 (+) Collision-induced dissociation (CID) on MfacetoneXacetone-d6)(+) for M = Al, Fe, Co, and Cu yields primarily, if not exclusively, nearly equal amounts of acetone and acetone-d6. Likewise, infrared multiphoton dissociation (IRMPD) at 10.6 μm yields, exclusively, nearly equal losses of the labeled and unlabeled acetones. These results suggest that the two acetone ligands bind in an equivalent fashion. Sc(+) was also studied, which proved to be the most interesting. Sc(+) reacts with acetone to form primarily ScO(+), which undergoes higher order reactions leading to several products including ScO(acetone)2 (+). IRMPD on this ion produces ScO(acetone-d6)(CD2CO)(+), while its perdeuterated analog also produces ScO(acetone-d6)(+) in addition to ScO(acetone-d6(CD2CO)(+). The IRMPD results are supplemented by studying the primary and higher order reactions of Sc(+) with acetone, as well as the CID of ScO(acetone)2 (+). Finally, a qualitative assessment of the infrared photodissociation cross sections is given. It is found that the relative photodissociation cross sections follow the orders Co(acetone-d6)2 (+) > Co(acetone)(acetone-d6) > Co(acetone)2 (+) and Co(acetone-d6)(+) > Co(acetone)(+).