Statistical modeling of competitive threshold collision-induced dissociation

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Guided ion beam tandem mass spectrometry has proved to be a robust tool for the measurement of thermodynamic information. Over the past twenty years, we have elucidated a number of factors necessary to make such thermochemistry accurate. Careful attention must be paid to the reduction of the raw data, ion intensities versus laboratory ion energies, to a more useful form, reaction cross sections versus relative kinetic energy. Analysis of the kinetic energy dependence of cross sections for endothermic reactions can then reveal thermodynamic data for both bimolecular and collision-induced dissociation (CID) processes. Such analyses need to include consideration of the explicit kinetic and internal energy distributions of the reactants, the effects of multiple collisions, the identity of the collision partner in CID processes, the kinetics of the reaction being studied, and competition between parallel reactions. This work provides examples illustrating the need to consider this multitude of effects along with details of the procedures developed in our group for handling each of them. ( W hen does a mass spectrometer become an ion beam instrument? Is this defined by the instrument, by whether the operator is an analytical or physical chemist, or by the experiments done? All mass spectrometers utilize ion beams, but the unspoken definition of an ion beam instrument is an apparatus that can be used to make quantitative physical measurements. In this regard, the capabilities of the instrumentation and the intent and training of the operator are all important factors. Whereas a mass spectrometer is often used as an identification tool through the use of mass spectral patterns, by taking advantage of the ability to easily change the kinetic energy of ions, it can also be a powerful instrument for the determination of thermodynamic data.In this article, I lay down some of the founding principles behind our use of tandem mass spectrometry, and in particular, so-called guided ion beam mass spectrometry to elucidate thermochemical information. This is achieved primarily by examining the kinetic energy dependence of endothermic ion-molecule reactions and determining their energy thresholds.Although many types of instruments have been used to perform such measurements, the link between such studies and guided ion beam techniques is a strong one. Indeed, the NIST Webbook [1] states in its section on Determinations of Reaction Endothermicity that "more commonly a so-called guided-beam apparatus is utilized for such determinations." Considering that there are only about a dozen such laboratories worldwide, this is a testament to the ability of this particular kind of apparatus to provide a plethora of high quality information. However, such information requires attention to myriad details, which are outlined in this article.Two fundamental types of reactions can be used to acquire thermodynamic information: Bimolecular exchange reactions, process 1, and collision-induced dissociation (CID), process 2.Reactions 1 can be either ex...

“…We first studied such processes in bisligated Li ϩ complexes of alcohols and water [76]. An example is shown in Figure 11.…”

Section: Competitive Shiftsmentioning

confidence: 99%

Section: Competitive Shiftsmentioning

confidence: 99%

Section: Competitive Shiftsmentioning

confidence: 99%

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Mass spectrometry—Not just a structural tool: The use of guided ion beam tandem mass spectrometry to determine thermochemistry

Armentrout

2002

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“…Activation entropies for direct dissociation reactions are generally ϩ10 -20 cal/ mol · K, indicating loose transition states. Data analysis and modeling were carried out using the CRUNCH 4D program developed by Armentrout and coworkers [59,60,[65][66][67]. In the absence of reverse activation barriers, threshold energies can be equated with 0 K ⌬E values and converted to 298 K bond dissociation enthalpies by using the integrated heat capacities of reactants and products.…”

Section: Instrumental Description and Data Analysismentioning

confidence: 99%

Fluorotrimethylsilane affinities of anionic nucleophiles: A study of fluoride-induced desilylation

Krouse

Wenthold

2005

In this study, preparation and decomposition of five novel pentavalent fluorosiliconates, RSi(CH 3 ) 3 F Ϫ (R ϭ CH 3 CH 2 O, CF 3 CH 2 O, (CH 3 ) 2 CHO, (CH 3 ) 3 SiO, and (CH 3 ) 3 SiNH) is used to investigate the process of fluoride-induced desilylation. The siliconates were characterized by collision-induced dissociation and energy-resolved mass spectrometry. Decomposition of RSi(CH 3 ) 3 F Ϫ leads to loss of the nucleophile R Ϫ and FSi(CH 3 ) 3 , except in the case of (CH 3 ) 3 SiNHSi(CH 3 ) 3 F Ϫ , where HF loss is also observed. Ion affinities for FSi(CH 3 ) 3 have been measured for all five nucleophiles, and compare well with computational predictions. The observed trend of the bond dissociation energies resembles the trend of ⌬H acid values for the corresponding conjugate acids, RH. Additionally, this data has been incorporated with existing thermochemistry to derive fluoride affinities for four of the silanes (R ϭ CH 3 CH 2 O, (CH 3 ) 2 CHO, (CH 3 ) 3 SiO, and (CH 3 ) 3 SiNH). We use the fluoride affinity of the silanes and the FSi(CH 3 ) 3 affinity of the departing nucleophilic anion to assess the feasibility of fluorideinduced desilylation of the silanes examined in this work. (J Am Soc Mass Spectrom 2005, 16, 697-707)

“…[25,26] describes a number of validation criteria for determining the kinetic energy threshold for CID reactions [27][28][29]. These validation experiments are particularly important here because the present work is done with a commercial triple quadrupole MS built for chemical analysis, not for accurate thermochemical measurements [30 -34].…”

Section: Ms and Cid Of Nitrate Complexes Of Group 1 And 2 Metalsmentioning

confidence: 99%

Tandem mass spectrometry of metal nitrate negative ions produced by electrospray ionization

Byers

Houk

2003

M(NO 3 ) xϪ ions are generated by electrospray ionization (ESI) of metal solutions in nitric acid in negative ion mode. Collision-induced reactions of these ions are monitored in a tandem mass spectrometer (MS) of quadrupole-octopole-quadrupole (QoQ) geometry. For Group 1and 2 elements, the M(NO 3 ) x Ϫ ions dissociate into NO 3 Ϫ and neutral metal nitrate molecules. These elements also form some M x (NO 3 ) xϩ1 Ϫ clusters, especially Li ϩ . Metal nitrate ions from transition elements and Group 13 elements fragment into oxo products and also undergo internal electron transfer to leave the M atom in a lower oxidation state. To calibrate the collision energy, the dissociation energy of O-NO 2 Ϫ is found to be 5.55 eV, about 0.76 eV lower than a value derived from thermochemistry. Most studies of small inorganic ions have been done in positive ion mode [14 -18]. Many metal-solvent clusters, e.g., M xϩ (H 2 O) n , n ϭ 1 to 6, are typically formed under "soft" ion extraction conditions [7,17,18]. Highly charged metal ions tend to react with solvent molecules to yield hydroxy or methoxy ions when energetic collision conditions are used to remove solvent during ion extraction. Preserving the original form of the metal ion in positive mode generally requires element-specific optimization of ion extraction conditions and leads to a variety of ion-solvent clusters.Previous work from our group [19] showed that an excess of nitric acid can be used to produce M(NO 3 ) x Ϫ ions in negative ion mode. A single set of ion extraction conditions yields these ions for many elements. This early work was done with only one stage of MS. The present work reports collision-induced dissociation (CID) reactions of these M(NO 3 ) x Ϫ ions in a triple "quadrupole" MS, actually a QoQ instrument. Experimental Samples and Sample PreparationConcentrated nitric acid was purchased from J. T. Baker (Phillipsburg, NJ) and used without further purification. Aliquots of the concentrated acid were mixed with HPLC grade methanol and deionized water (18 M⍀, Barnstead Nanopure, Newton, MA) to form a solvent of 0.05% nitric acid in 25% water:75% methanol. Aliquots of aqueous, acidic metal stock solutions (1000 ppm, Spex Certiprep, Metuchen, NJ ) were diluted with solvent to the desired concentrations.