Department of Chemistry & Biochemistry

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Characterization of the single photon initiated dissociative rearrangement reaction (SPIDRR) technique and its application in the study of transition metal cation bound organic cluster reactions.
(2018-07-09) Gutierrez, Michael G., 1992-; Bellert, Darrin Joseph, 1968-
The study of gas-phase ion-molecule reactions has been influential in the investigation of transition metal mediated bond activation and catalysis. This research has recently been advanced by a new method designated the single photon initiated dissociative rearrangement reaction (SPIDRR) technique. SPIDRR measures the microcanonical kinetics for reactions between transition metal cations and neutral organic molecules. By assessing the energy dependence of the microcanonical rate constant, as well as determining product branching fractions and kinetic isotope effects, SPIDRR can establish unique mechanistic and dynamic information. This dissertation will provide a detailed overview of the SPIDRR technique and demonstrate its application in the study of transition metal cation bound organic cluster reactions.
Chemical kinetics and dynamics of M+(organic) molecules using single photon initiated dissociative rearrangement reactions (SPIDRR) measurements.
(2018-07-17) Theis, Zachry C., 1988-; Bellert, Darrin Joseph, 1968-
Gas-phase studies are important to many areas of science and technology. The ability to prepare molecular species in an environment devoid of complexities present in the condensed phase allows for high resolution measurement of molecular level details. This is particularly important when studying ion-molecule reactions where transition metals are involved. The open-shell, radical nature of the transition metal in combination with its charge promotes a host of low-energy chemical transformations with multiple reactive pathways open within a few eV of the zero point level of the encounter complex ground state. Moreover, barriers along these pathways are often submerged with respect to the separated reactant limit making both temporal and energetically resolved kinetic measurement challenging. However, it is precisely these qualities of a transition metal cation (a chemically reactive center that mediates low energy chemical transformations) that make it a desired catalytic active site and thus demands high resolution study. To this end, the single photon initiated dissociative rearrangement reaction (SPIDRR) technique was developed. This dissertation details the use of this novel tool toward measurement of the kinetics and dynamics of the Ni+ and Co+ mediated decomposition of several organic molecules. Ab initio quantum chemical calculations were performed to compliment these experimental studies. The potential energy surface has been determined at the DFT level to suggest the mechanistic features that occur during the metal mediated decomposition of an organic molecule. The combination of experiment and theory has permitted a far deeper understanding of the reaction dynamics and has proven integral towards explaining experimental observation. For example, emerging concepts that guide hydrogen transfers on Ni+ centered catalytic reactions have evolved from this synergistic combination.
Construction and calibration of a custom time-of-flight mass spectrometer and its use in measuring the reaction kinetics of transition metal ion-organic interactions.
(2009-09-03) Castleberry, Vanessa A.; Bellert, Darrin Joseph, 1968-; Chemistry and Biochemistry.; Baylor University. Dept. of Chemistry and Biochemistry.
A unique instrument was constructed and used to generate and interrogate jet cooled neutrals, ions and their respective clusters. The instrument is a result of the artful marriage of supersonic expansions combined with time of flight spectroscopy. Ionization occurs in a large main chamber. The ions are separated via a kinetic energy pulse from a custom built linear particle accelerator. The revelatory hardware for our instrument is a microchannel plate detector (MCP). The MCP is mounted on the exit of a custom designed and built hemispherical energy analyzer (sector), which acts as an energy filter. This filtering characteristic of the sector permits study of selected ionized fragments. To test the instrument, the 2-photon resonant, 3-photon ionization spectrum of gaseous atomic copper was measured. The (n = 9 – 21) Rydberg series was observed in 2-photon excitation. The term energies of this series converged to copper's lowest ionization threshold with an apparent quantum defect of 0.92. The state which couples the ground state of copper to the Rydberg series is a non-stationary state composed primarily of the spin-orbit components of the lowest 2P° atomic states. Additionally, the time dependence of the gaseous unimolecular decomposition of the jet-cooled adduct ion, Ni⁺•Acetone was monitored by selective detection of the daughter fragment, Ni⁺CO. Various photon energies were supplied to initiate dissociation of the adduction. The energies employed in this reaction were well below that required to fragment C-C ς-bonds. First-order unimolecular decomposition rate constants, k(E) ranged from 55000 – 113000 s⁻¹. The rate constants decreased with decreasing amounts of internal excitation. Ni⁺ cation is implicated as a catalytic necessity to activate the bond and cause molecular fragmentation. These experiments represent the first direct kinetic study of such catalytic type reactions.
Kinetics and mechanistic detail of Ni⁺ assisted organic decomposition reaction at low internal energies.
(2011-12-19) Laboren, Ivanna E.; Bellert, Darrin Joseph, 1968-; Chemistry and Biochemistry.; Baylor University. Dept. of Chemistry and Biochemistry.
The unimolecular decomposition kinetics of jet-cooled cluster ions have been monitored over ranges of internal energies. The clusters are formed by the combination of Ni cation with and organic ketone, aldehyde or ether molecules. The internal energy delivered to the clusters is provided through laser photon absorption. The quantum of photon energy approximates the total energy content of the reacting species as the clusters are jet cooled prior to photon absorption. The interaction of the organic substrate with the transition metal cation lowers the kinetic barriers to the activation of σ-bond. Thus the cation activates organic bonds and mediates the formation of products. The unimolecular decomposition products in these studies are a stable neutral with the corresponding ion. This dissertation will focus on the unimolecular decomposition kinetics of Ni⁺- Butanone. First order rate constants are acquired for the precursor ion dissociation into three product channels. The temporal growth of each fragment ion is selectively monitored and yields similar valued rate constants. The common-valued rate constants, comparisons to earlier studies, and the results of DFT calculations reveal the dissociation dynamics. This unimolecular decomposition reaction is proposed to proceed along two parallel reaction coordinates that originate with the rate-limiting Ni⁺ oxidative addition into either the OC-CH₃ or OC-C₂H₅ σ-bond in the butanone molecule. Rate constant values for the activation of both bonds are determined.
Measurement and kinetic analysis of complex ion-molecule reactions influenced by multiple electronic surfaces.
(August 2022) Lewis, Tucker William Richard, 1994-; Bellert, Darrin Joseph, 1968-
Energetically and temporally resolved gas phase studies of ion-molecule reactions, particularly those featuring open-shell transition metal ions, are fundamental to understanding complex chemical reactivity paradigms that can help shed light on the behavior of bulk and extended materials. Kinetic studies are particularly revealing as these elucidate mechanistic and dynamic features that would otherwise be impossible to determine. The unique single photon initiated dissociative rearrangement reactions (SPIDRR) technique measures the time dependence of product formation in ion-molecule reactions and is applied here to elucidate the mechanism of several ion-molecule systems. Moreover, the role of the transition metal /metal oxide ion’s electronic structure toward chemical reactivity is determined.
Metal ion assisted unimolecular decomposition of gaseous organometallic complexes : acquisition of reaction rate constants and dynamics of the dissociative mechanism.
(2011-12-19) Villarroel, Otsmar J.; Bellert, Darrin Joseph, 1968-; Chemistry and Biochemistry.; Baylor University. Dept. of Chemistry and Biochemistry.
Reaction rate constants have been acquired for the transition metal ion assisted decomposition of various organic molecules, and their deuterium labeled analogs in the gas phase. The metal ion activates organic bonds and mediates the formation of products. Thus, the transition metal cation lowers the bond activation energy requirements in these decomposition reactions making these systems model for catalysis. Catalytic reaction kinetics are not well understood and it is hoped that the resolved study of simpler catalytic models will further the development of the theoretical tools necessary to describe such mechanistic behavior at the molecular level. Reaction rate constants for these model systems are measured using a custom-built molecular beam apparatus. The clusters are formed under supersonic expansion conditions and are bound by the charge-dipole electrostatic interaction between a transition metal cation and a polar organic molecule. The unimolecular decomposition occurs upon laser photon absorption by the jet-cooled cluster yielding a stable neutral molecule and corresponding ion. This dissertation will focus on the unimolecular decomposition kinetics of the Co⁺-Acetone cluster and its deuterium labeled analog. Rate constants are measured at well resolved cluster internal energies. The kinetic isotope effect (KIE) for each measurement was determined. Results are compared to the similar Ni⁺-Acetone decomposition reactions, where the KIE was also measured. These two similar systems present rather different dissociation dynamics. Arguments based on the electronic structure of each ion explain this unique behavior between these similar systems. DFT calculations are made on most systems presented in this dissertation. The most likely geometries and relative energies of the reactants, intermediates and products are determined. Such information specifies aspects of the reaction coordinate and leads to suggestions of mechanisms. This was primarily applied in the final chapter of this dissertation where preliminary results of Ni⁺-assisted decomposition of cyclopentanone are presented. This system represents the group’s first study of a ring-opening reaction.
Rates of photon induced, metal Ion assisted, unimolecular decomposition of gaseous organometallic complexes using a custom built molecular beam apparatus.
(2010-10-08T16:16:55Z) Dee, S. Jason (Scott Jason); Bellert, Darrin Joseph, 1968-; Chemistry and Biochemistry.; Baylor University. Dept. of Chemistry and Biochemistry.
This dissertation will focus on the development of a novel technique to study the kinetics and dynamics of transition metal ion assisted σ-bond activation of gaseous organic molecules. The measurements reported represent the first direct experimental determination of reaction kinetics of type: M⁺ + CnHmCO → M⁺CO + CnHm where M⁺ represented a transition metal ion and the organic molecule is often a ketone or aldehyde. The dissociation process is activated by the metal ion and such reactions occur with internal energies significantly less than that required to break C-C or C-H single bonds in isolated organic molecules. Determination of the ion/molecule (and the corresponding isotopic variants) reaction rate constants, over ranges of internal energies, yield information about the dynamics of the reaction mechanism. Rate constants are extracted from the photon-induced decomposition of Ni⁺(Acetone), Ni⁺(d₆-Acetone), and Ni⁺(Acetaldehyde) into Ni⁺CO. A custom-built molecular beam apparatus is constructed to study rates of photon-induced unimolecular decomposition of jet-cooled ionic molecular complexes. The complexes are generated in a pulsed system in which a seeded carrier gas entrains liberated metal ions in a supersonic expansion. The metal ions are created through laser ablation of a metal rod at the throat of a supersonic expansion using photons from a KrF GAM Ex5 Excimer laser. The jet-cooled molecular beam is collimated to extract the coldest portion of the molecular beam. The ions in the molecular beam are then orthogonally accelerated down a TOF-MS. Photons intersect the molecular beam prior to orthogonal extraction, resulting in fragmentation of the ionic complexes. A hemispherical kinetic energy analyzer is used to separate these fragmented complexes from their respective precursor complex. Rates of decomposition are extracted from these studies. Direct measurement of the reaction kinetics over a range of energies yields information about the rate-limiting step in the dissociative mechanism. Optimization of the parameters involved in the construction of the molecular beam apparatus will also be discussed.
Unimolecular decomposition kinetics at resolved internal energies.
(2016-12-01) Mansell, Adam, 1986-; Bellert, Darrin Joseph, 1968-
The kinetic behavior of microcanonical unimolecular decomposition reactions involving clusters formed by the electrostatic interaction between transition metal cations and simple organic molecules has been measured at multiple internal energies. The presence of the metal lowers the energy necessary for the reactions to take place. Thus, these reactions serve as a useful model for catalysis. These measurements were made in a custom built molecular beam apparatus, in which the experimental environment is free of the many complicating effects which are inherent to condensed phase measurements. The reacting clusters are formed in a supersonic expansion and bound by the charge-dipole interaction between the organic and the transition metal cation. Rearrangement reactions are initiated by absorption of a visible photon from a dye laser. The most widely used formalism for unimolecular kinetic calculations is Rice- Ramsperger-Kassel-Marcus (RRKM) theory, which states that the rate constant for a unimolecular reaction is dependent on the way in which the internal energy is distributed to the various modes of motion of the molecule. The rate constant is then proportional to the ratio of the sum of states of the transition state to the density of states of the reactant. This theory is utilized by our group to determine the reaction activation energies. This dissertation will focus on the unimolecular decomposition of Ni+propanal and Ni+acetic acid. The two reactions involve competition between C-C bond activation and either C-H or C-O bond activation.

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