M. T. Rodgers

STATISTICAL MODELING OF COLLISION-INDUCED DISSOCIATION THRESHOLDS

Analysis of the energy dependence of the cross sections for collision-induced dissociation reactions has permitted the determination of quantitative thermodynamic information for a variety of ionic clusters. As such clusters become larger, the rate at which the decomposition occurs becomes comparable to the instrumental time available for observing the reaction. A method for incorporating statistical theories for energy-dependent unimolecular decomposition in this threshold analysis is reviewed and updated. The revision relies on the fact that for most ionic clusters, the transition state is a loose association of the products that can be located at the centrifugal barrier. This permits a straightforward estimation of the molecular parameters needed in statistical theories for the transition state. Further, we also discuss several treatments of the adiabatic rotations of the dissociating cluster. The various models developed here and previously are compared and used to analyze a series of data for Li+(ROH...

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.

Statistical modeling of competitive threshold collision-induced dissociation

Collision-induced dissociation of (R1OH)Li+(R2OH) with xenon is studied using guided ion beam mass spectrometry. R1OH and R2OH include the following molecules: water, methanol, ethanol, 1-propanol, 2-propanol, and 1-butanol. In all cases, the primary products formed correspond to endothermic loss of one of the neutral alcohols, with minor products that include those formed by ligand exchange and loss of both ligands. The cross-section thresholds are interpreted to yield 0 and 298 K bond energies for (R1OH)Li+–R2OH and relative Li+ binding affinities of the R1OH and R2OH ligands after accounting for the effects of multiple ion–molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. We introduce a means to simultaneously analyze the cross sections for these competitive dissociations using statistical theories to predict the energy dependent branching ratio. Thermochemistry in good agreement with previous work is obtained in all cases. In essence, this statistical approach prov...

Low-energy collision-induced dissociation of deprotonated dinucleotides: determination of the energetically favored dissociation pathways and the relative acidities of the nucleic acid bases

Abstract Fourier transform ion cyclotron resonance mass spectroscopy has been used to examine the collision-induced dissociation pathways of all 16 of the possible deprotonated dinucleotides. These quasimolecular ions were generated by cesium ion bombardment of a mixture of triethanolamine, ammonium hydroxide and the dinucleotide. Collisional activation using continuous off-resonance excitation permits observation of energetically-favorable dissociation pathways. Dissociation products were examined over the range of center of mass energies from 0 to the minimum energy required to bring about complete dissociation of the reactant ion which did not exceed 7.7eV for deprotonated parent ions and 8.8eV for fragment ions in any of the systems. Semiempirical calculations were performed using the PM3 method, a variant of the AM1 method, to obtain gas-phase model structures and energies of the deprotonated dinucleotides and their collision-induced dissociation fragments. The acidities of the nucleic acid bases and dimethyl phosphate were calculated using the AM1 method. The deprotonated quasimolecular ions dissociate to yield several characteristic products. The major products formed in all systems are the deprotonated 5′-terminus base, the ion resulting from loss of the neutral 5′-terminus base, or the metaphosphate anion, PO−3. Insight into the relative stabilities of the fragment ions is gained by comparing the product distributions observed in each of the systems. The relative yields of products involving either the 3′- or 5′-end of the molecule suggest the 3′-terminus base is stabilized through hydrogen bonding interaction with the phosphate group. The relative strength of this stabilization follows the order guanine > thymine > cytosine > adenine. Additionally, the relative abundances of the deprotonated nucleic acid fragments suggest that the relative acidities of the nucleic acid bases follow the order adenine > thymine > guanine > cytosine. Only minor yields of sequence ions in which one of the phosphate diester linkages is cleaved are observed with these quasimolecular ions. Reaction mechanisms which account for the observed products are proposed.

Infrared Multiphoton Dissociation Spectroscopy of Cationized Threonine : Effects of Alkali-Metal Cation Size on Gas-Phase Conformation

The gas-phase structures of alkali-metal cation complexes of threonine (Thr) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser in conjunction with quantum chemical calculations. Spectra of Li+(Thr) and Na+(Thr) are similar and relatively simple, whereas K+(Thr), Rb+(Thr), and Cs+(Thr) include distinctive new IR bands. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level to identify the structures present in the experimental studies. For the smaller metal cations, the spectra match those predicted for charge-solvated structures in which the ligand exhibits tridentate coordination, M1[N,CO,OH], binding to the amide and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain. K+(Thr), Rb+(Thr), and Cs+(Thr) exhibit evidence of the charge-solvated complex, M3[COOH], in which the metal cation binds to the carboxylic acid group. Evidence for a small population of the zwitterionic analogue of this structure, ZW[CO2-], is also present, particularly for the Cs+ complex. Calculations indicate that the relative stability of the M3[COOH] structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally. The present results are similar to those obtained previously for the analogous M+(Ser) complexes, although there are subtle distinctions that are discussed.

Statistical Rate Theory and Kinetic Energy-Resolved Ion Chemistry: Theory and Applications†

Ion chemistry, first discovered 100 years ago, has profitably been coupled with statistical rate theories, developed about 80 years ago and refined since. In this overview, the application of statistical rate theory to the analysis of kinetic-energy-dependent collision-induced dissociation (CID) reactions is reviewed. This procedure accounts for and quantifies the kinetic shifts that are observed as systems increase in size. The statistical approach developed allows straightforward extension to systems undergoing competitive or sequential dissociations. Such methods can also be applied to the reverse of the CID process, association reactions, as well as to quantitative analysis of ligand exchange processes. Examples of each of these types of reactions are provided and the literature surveyed for successful applications of this statistical approach to provide quantitative thermochemical information. Such applications include metal-ligand complexes, metal clusters, proton-bound complexes, organic intermediates, biological systems, saturated organometallic complexes, and hydrated and solvated species.

Influence of substituents on cation–π interactions: 5. Absolute binding energies of alkali metal cation–anisole complexes determined by threshold collision-induced dissociation and theoretical studies

Threshold collision-induced dissociation of M+(C6H5CH3)x with Xe is studied using guided ion beam mass spectrometry. M+ include the following alkali metal ions:  Li+, Na+, K+, Rb+, and Cs+. Both mono- and bis-complexes are examined (i.e., x = 1 and 2). In all cases, the primary and lowest energy dissociation channel observed is endothermic loss of an intact toluene ligand. Sequential dissociation of a second toluene ligand is observed at elevated energies in the bis-complexes. Minor production of ligand exchange products, M+Xe and M+(C6H5CH3)Xe, is also observed. The cross section thresholds for the primary dissociation channel are interpreted to yield 0 and 298 K bond dissociation energies for (C6H5CH3)x-1M+−C6H5CH3, x = 1−2, after accounting for the effects of multiple ion−neutral collisions, the kinetic and internal energies of the reactants, and dissociation lifetimes. Density functional theory calculations at the B3LYP/6-31G* level of theory are used to determine the structures of these complexes and...

Structural and Energetic Effects in the Molecular Recognition of Protonated Peptidomimetic Bases by 18-Crown-6

Absolute 18-crown-6 (18C6) affinities of nine protonated peptidomimetic bases are determined using guided ion beam tandem mass spectrometry techniques. The bases (B) included in this work are mimics for the n-terminal amino group and the side chains of the basic amino acids, i.e., the favorable sites for binding of 18C6 to peptides and proteins. Isopropylamine is chosen as a mimic for the n-terminal amino group, imidazole and 4-methylimidazole are chosen as mimics for the side chain of histidine (His), 1-methylguanidine is chosen as a mimic for the side chain of arginine (Arg), and several primary amines including methylamine, ethylamine, n-propylamine, n-butylamine, and 1,5-diamino pentane as mimics for the side chain of lysine (Lys). Theoretical electronic structure calculations are performed to determine stable geometries and energetics for neutral and protonated 18C6 and the peptidomimetic bases, as well as the proton bound complexes comprised of these species, (B)H(+)(18C6). The measured 18C6 binding affinities of the Lys side chain mimics are larger than the measured binding affinities of the mimics for Arg and His. These results suggest that the Lys side chains should be the preferred binding sites for 18C6 complexation to peptides and proteins. Present results also suggest that competition between Arg or His and Lys for 18C6 is not significant. The mimic for the n-terminal amino group exhibits a measured binding affinity for 18C6 that is similar to or greater than that of the Lys side chain mimics. However, theory suggests that binding to n-terminal amino group mimic is weaker than that to all of the Lys mimics. These results suggest that the n-terminal amino group may compete with the Lys side chains for 18C6 complexation.

Theoretical studies of the unimolecular and bimolecular tautomerization of cytosine

Computational investigations of the unimolecular and bimolecular tautomerization of isolated and dimeric cytosine have been performed. Stationary and transition states of the isolated and dimeric cytosine systems were characterized at the MP2(full)/6-311+G(2d,2p)//MP2(full)/6-31G* and MP2(full)/6-311+G(2d,2p)//B3LYP/6-31G* levels of theory, respectively. In the solid phase, cytosine exists in a single tautomeric state. In contrast, experiments conducted in the gas phase find that cytosine exists as a mixture of several tautomeric forms. The energy barriers for unimolecular tautomerization of the tautomeric form found in solids to those observed in the gas phase are high and vary between 142.2 and 169.9 kJ mol−1. The formation of dimers with dual hydrogen bonding interactions results in a significant lowering of the barriers to tautomerization, thus facilitating tautomerization during the sublimation process. Based on such bimolecular tautomerization mechanisms, we believe that the relative populations of the cytosine tautomers produced in the gas phase via thermal vaporization cannot be accurately predicted without considering intermolecular hydrogen bonding interactions present in the condensed phase.

Reactions of Cu+(1S and 3D) with O2, CO, CO2, N2, NO, N2O, and NO2 studied by guided ion beam mass spectrometry

Abstract Reactions of Cu + ( 1 S and 3 D ) with O 2 , CO, CO 2 , N 2 , NO, N 2 O, and NO 2 are studied using guided ion beam mass spectrometry. Cross sections as a function of kinetic energy are measured for each system to over 17 eV. In all cases, the observed reactions of Cu + ( 1 S ) are endothermic. Because of the closed shell character of ground state Cu + ( 1 S , 3 d 10 ), most of these systems exhibit cross sections with onsets and peaks at much higher energies than expected from the known thermochemistry. Such behavior indicates that the reactions occur on relatively repulsive potential energy surfaces and by impulsive processes. Reliable thermodynamic information is obtained primarily from the NO 2 system where an analysis of the kinetic energy dependence of the reaction cross sections is used to obtain D 0 (Cu + –O)= 1.35± 0.12 eV and D 0 (Cu–O)= 2.94± 0.12 eV. Although speculative, the threshold for an excited state product asymptote in the N 2 O system also allows the derivation of D 0 (Cu + –N 2 )= 0.92± 0.31 eV. Reactions of the Cu + ( 3 D , 4 s 1 3 d 9 ) excited state are generally more efficient than those of the ground state and are exothermic in several cases.