Showing papers by "P. B. Armentrout published in 2004"

A thermodynamic "vocabulary" for metal ion interactions in biological systems.

Abstract: This Account focuses on metal ion-ligand complexes of biological relevance and measurements of the bond dissociation energies (BDEs) of such species. These complexes yield thermochemistry that begins to provide a thermodynamic "vocabulary" for thinking quantitatively about the strength of interactions in biological systems. The method utilized is threshold collision-induced dissociation in a guided ion beam tandem mass spectrometer. Accurate determination of BDEs requires attention to many details of the experiments and data analysis, as outlined here. Trends in metal ion-ligand BDEs are examined as a function of the metal ion, ligand, and extent of ligation. We elucidate the importance of ion-dipole and ion-induced dipole interactions, chelation, conformation, tautomeric form, steric interactions, and electronic effects such as hybridization and promotion. Interactions of metal ions with nucleobases and amino acids are quantified and the effects of hydration on these values are explored for the amino acid systems. Although data limitations restrict the present discussion to monocations, the trends elucidated here should be relevant to multiply charged metal ions, for which data is forthcoming.

Systematic and random errors in ion affinities and activation entropies from the extended kinetic method.

Abstract: An evaluation of the extended kinetic method with full entropy analysis was conducted using RRKM theory to simulate data for collision-induced dissociation under single-collision conditions. A rigorous method for analyzing kinetic method data, orthogonal distance regression, is introduced and compared with previous methods in the literature. The results demonstrate that the use of the extended kinetic method is definitely superior to the standard kinetic method, but final ion affinities and activation entropies differ intrinsically from the correct values. Considering the effects of both systematic and random error in Monte Carlo simulations of the full entropy analysis, error distributions of ±4 to ±12 kJ mol−1 for ion affinities and of ±9 to ±30 J mol−1 K−1 for activation entropy differences are found (±2 standard deviations of the sample populations). The systematic errors in ion affinities are larger for systems with large activation entropy differences. These uncertainties do not include any error in the absolute calibration of the reference ion affinity scale. We argue that application of an empirical correction factor is inadvisable. Copyright © 2004 John Wiley & Sons, Ltd.

Fundamentals of ion–molecule chemistry

Abstract: The dependence of ion–molecule reactions on kinetic and internal energy is explored from both experimental and theoretical points of view. The conversion of raw data (ion intensities versus laboratory energies) to instrument independent information (reaction cross sections versus center-of-mass energies) is explained in detail. The characteristic experimental behavior of ion–molecule reactions as a function of kinetic energy is illustrated and then theoretically characterized. The conversion and relationship between reaction cross sections and rate constants is provided along with examples. The changes in reaction efficiencies with changes in internal excitation are examined, with appropriate examples. Various explanations for why exothermic ion–molecule reactions are not always efficient are then provided along with several illustrative experimental cases. The factors that influence the efficiency of dissociative processes are also elucidated. The relevance of these various considerations for application to atomic mass spectrometry is explored throughout.

An experimental and theoretical dissection of potassium cation/glycine interactions

Abstract: The binding of K+ to glycine is examined in detail by studying the interaction of the potassium cation with glycine and ligands that contain the functional components of glycine both singly and in pairs. Bond dissociation energies of M+–L where L = glycine, ethanol amine, propionic acid, methyl ethyl ketone, 1-propanol, and 1-propylamine are reported. Experimentally, the bond energies are determined using threshold collision-induced dissociation of the M+–L complexes with Xe using a guided ion beam tandem mass spectrometer. Analyses of the energy dependent cross sections provide 0 K bond energies for the M+–L complexes. All bond energy determinations include consideration of unimolecular decay rates, internal energy of reactant ions, and multiple ion–molecule collisions. Ab initio calculations, including those for a 1-amino-2-propanone ligand, using MP2, density functional, and compound methodologies show good agreement with the experimental bond energies and with the few previous experimental values available. The combination of this series of experiments and calculations allows the binding strength of individual functional groups and the influence of chelation to be thoroughly explored. This permits a detailed understanding of the driving forces for the interaction of K+ with glycine and of the differences between the theoretical structures of Na+(glycine) and K+(glycine).

Is Spin Conserved in Heavy Metal Systems? Experimental and Theoretical Studies of the Reaction of Re+with Methane†

Abstract: A guided ion beam tandem mass spectrometer is used to study the reactions of atomic 187Re+ with CH4 and CD4 and collision-induced dissociation (CID) of ReCH4+ with Xe These studies examine the activation of methane by Re+ in a low-pressure environment free of ligand supports or other reactive species In the bimolecular reaction, ReCH2+ is efficiently produced in a slightly endothermic process and is the only ionic product observed at low energies, whereas at higher energies, ReH+ dominates the product spectrum Other products observed include ReC+, ReCH+, and ReCH3+ Modeling of these endothermic reactions yields 0 K bond dissociation energies in eV of D0(Re+−C) = 512 ± 004, D0(Re+−CH) = 584 ± 006, D0(Re+−CH2) = 414 ± 006, D0(Re+−CH3) = 222 ± 013 Analysis of the behavior of the cross sections suggests that formation of ReH+, ReCH2+, and ReCH3+ occurs via an H−Re+−CH3 intermediate CID of ReCH4+ reveals a bond energy of 053 ± 015 eV for Re+−CH4 The experimental bond energies compare favorably

The mechanism of proton exchange: guided ion beam studies of the reactions, H(H2O)n+ (n=1-4)+D2O and D(D2O)n+ (n=1-4)+H2O.

Abstract: Reactions of protonated water clusters, H(H(2)O)(n) (+) (n=1-4) with D(2)O and their "mirror" reactions, D(D(2)O)(n) (+) (n=1-4) with H(2)O, are studied using guided-ion beam mass spectrometry. Absolute reaction cross sections are determined as a function of collision energy from thermal energy to over 10 eV. At low collision energies, we observe reactions in which H(2)O and D(2)O molecules are interchanged and reactions where H-D exchange has occurred. As the collision energy is increased, the H-D exchange products decrease and the water exchange products become dominant. At high collision energies, processes in which one or more water molecules are lost from the reactant ions become important, with simple collision-induced dissociation processes, i.e., those without H-D exchange, being dominant. Threshold energies of endothermic channels are measured and used to determine binding energies of the proton bound complexes, which are consistent with those determined by thermal equilibrium measurements and previous collision-induced dissociation studies. A kinetic scheme that relies only on the ratio of isomerization and dissociation rate constants successfully accounts for the kinetic energy dependence observed in the branching ratios for H-D and water exchange products in all systems. Rice-Ramsperger-Kassel-Marcus theory and ab initio calculations confirm the feasibility and establish the details of this kinetic model.

Methane activation by cobalt cluster cations, Con+ (n=2–16): Reaction mechanisms and thermochemistry of cluster-CHx (x=0–3) complexes

Abstract: The kinetic energy dependences of the reactions of Con+ (n=2–16) with CD4 are studied in a guided ion beam tandem mass spectrometer over the energy range of 0–10 eV. The main products are hydride formation, ConD+, dehydrogenation to form ConCD2+, and double dehydrogenation yielding ConC+. These primary products decompose to form secondary and higher order products, ConCD+, Con−1D+, Con−1C+, Con−1CD+, and Con−1CD2+ at higher energies. Adduct formation of ConCD4+ is also observed for the largest cluster cations, n≥10. In general, the efficiencies of the single and double dehydrogenation processes increase with cluster size, although the hexamer cation shows a reduced reactivity compared to its neighbors. All reactions exhibit thresholds, and cross sections for the various primary and secondary reactions are analyzed to yield reaction thresholds from which bond energies for cobalt cluster cations to D, C, CD, CD2, and CD3 are determined. The relative magnitudes of these bond energies are consistent with simp...

Probes of spin conservation in heavy metal reactions: Experimental and theoretical studies of the reactions of Re+ with H2, D2, and HD

Abstract: A guided ion beam tandem mass spectrometer is used to examine the kinetic energy dependence of reactions of the third-row transition metal cation, Re(+), with molecular hydrogen and its isotopologues. A flow tube ion source produces Re(+) in its (7)S(3) electronic ground state. Reaction with H(2), D(2), and HD forms Re H(+)(Re D(+)) in endothermic processes. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energy of D(0)(Re(+)-H)=2.29+/-0.07 eV (221+/-6 kJ/mol). The experimental thermochemistry is consistent with ab initio calculations, performed here and in the literature. Theory also provides the electronic structures of these species and is used to examine the reactive potential energy surfaces. Results from reactions with HD provide insight into the reaction mechanisms and indicate that the late metal ion, Re(+), reacts largely via a statistical mechanism. This is consistent with the potential energy surfaces which locate a stable Re H(2) (+)((5)B(2)) complex. Results for this third-row transition metal system are compared with the first-row congener (Mn(+)) and found to have much higher reactivity towards dihydrogen and stronger M(+)-H bonds. These differences can be attributed to efficient coupling among surfaces of different spin along with lanthanide contraction and relativistic effects.

Ligand Exchange Reactions of Sodium Cation Complexes Examined Using Guided Ion Beam Mass Spectrometry: Relative and Absolute Dissociation Free Energies and Entropies

Abstract: Guided ion beam mass spectrometry is used to study the ligand exchange reactions of Na+L1 with L2, where L1, L2 = H2O, C6H6, CH3OH, CH3OCH3, NH3, and C2H5OH, as a function of kinetic energy. For the endothermic ligand exchange reactions, reaction endothermicities are obtained by analyzing the kinetic energy dependence of the cross sections using our empirical threshold modeling equation. The thresholds are found to be systematically higher than values previously determined using competitive CID experiments by 0.07−0.2 eV. An analysis of the endothermic cross sections using a bimolecular, polyatomic phase theory model and a competitive, bimolecular RRKM model demonstrates that the systematic deviations result from a competitive shift between the thermoneutral reactions back to the reactants and the endothermic reactions to the ligand exchange products. For all reactions, thermal rate constants, k(298), are determined by modeling the cross sections in the low-energy region and integrating the model over a M...

Reaction of Cu+ with dimethoxyethane: competition between association and multiple dissociation channels.

Abstract: The reaction of Cu+ with dimethoxyethane (DXE) is studied using kinetic-energy dependent guided ion beam mass spectrometry. The bimolecular reaction forms an associative Cu+(DXE) complex that is long-lived and dissociates into several competitive channels: C4H9O2++CuH, Cu+(C3H6O)+CH3OH, back to reactants, and other minor channels. The kinetic-energy dependences of the cross sections for the three largest product channels are interpreted with several different models (including rigorous phase space theory) to yield 0 K bond energies after accounting for the effects of multiple ion–molecule collisions, internal energy of the reactant ions, Doppler broadening, and dissociation lifetimes. These values are compared with bond energies obtained from collision-induced dissociation (CID) studies of the Cu+(DXE) complex and found to be self-consistent. Although all models provide reasonable thermochemistry, phase space theory reproduces the details of the cross sections most accurately. We also examine the dynamics...

Guided Ion Beam Study of Potential Energy Surfaces for Platinum Interactions with Nitrogen Oxides

Abstract: A guided ion beam tandem mass spectrometer is used to examine the kinetic energy dependence of reactions of Pt+ with NO, NO2 and N2O, and PtO+ with NO and NO2, as well as collision-induced decomposition (CID) of PtO+ with N2 and PtNO+ with Xe. Pt+ reacts with NO and NO2 most efficiently by charge transfer, although oxygen atom transfer is also prominent for NO2 and the dominant reaction with N2O. Charge transfer is also the most efficient process observed for PtO+ + NO, whereas this ion reacts with NO2 by inefficient oxygen atom transfer, and in the reaction of PtO+ with N2, only simple CID and ligand exchange are observed. Analysis of the kinetic energy dependences for endothermic reaction cross sections yields the 0 K bond dissociation energies (BDEs) in eV (kJ mol−1) of D0(Pt+–N) = 3.35 ± 0.08 (323 ± 8), D0(Pt–N) = 3.84 ± 0.10 (371 ± 10), D0(Pt+–NO) = 3.13 ± 0.07 (302 ± 7) and D0(Pt+–O) = 3.26 ± 0.07 (315 ± 7). The bond energy of D0(Pt+–O) is consistent with our previous results and confirmed by result...