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Accurate ionic bond energy measurements with TCID mass spectrometry and imaging PEPICO spectroscopy

Author

Tyson G. Rowland, University of the Pacific

Date of Award

2012

Document Type

Thesis - Pacific Access Restricted

Degree Name

Master of Science (M.S.)

Department

Chemistry

First Advisor

Balint Sztaray

First Committee Member

Mike McCallum

Second Committee Member

Ryan Moffet

Third Committee Member

Jim Hetrick

Abstract

Two projects are presented here. In the first, metal-cyclopentadienyl bond dissociation energies (BDEs) were measured for seven metallocene ions (Cp₂M⁺, Cp = η⁵-cyclopentadienyl = c-C₅H₅, M = Ti, V, Cr, Mn, Fe, Co, Ni) using threshold collision-induced dissociation (TCID) performed in a guided ion beam tandem mass spectrometer. For all seven room temperature metallocene ions, the dominant dissociation pathway was simple Cp loss from the metal. Traces of other fragment ions were also detected, such as C₁₀H₁₀⁺, C₁₀H₈⁺, C₈H₈⁺, C₃H₃⁺, H₂M⁺, C₃H₃M⁺, C₆H₆M⁺, and C₇H₆M⁺, depending on the metal center. Statistical modeling of the Cp-loss TCID experimental data, including consideration of energy distributions, multiple collisions, and kinetic shifts, allow the extraction of 0 K [CpM⁺ - Cp] BDEs. These are found to be 4.95 ± 0.15, 4.02 ± 0.14, 4.22 ± 0.13, 3.51 ± 0.12, 4.26 ± 0.15, 4.57 ± 0.15, and 3.37 ± 0.12 eV for Cp₂To⁺, Cp₂V⁺, Cp₂Cr⁺, Cp₂Mn⁺, Cp₂Fe⁺, Cp₂Co⁺, and Cp₂Ni⁺, respectively. The measured BDE trend is largely in line with arguments based on a simple molecular orbital picture, with the exceptions of a reversal in Cp₂Mn⁺ and Cp₂Ni⁺ BDEs (although within uncertainty), and the exceptional case of titanocene, most likely attributable to its bent structure. The new results presented here are compared to previous literature values and are found to provide a more complete and accurate set of thermochemical parameters.

In the second project, imaging photoelectron photoion coincidence (iPEPICO) spectroscopy has been used to determine 0 K appearance energies for the unimolecular dissociation reactions of several energy selected 1-alkyl iodide cations n-C_nH_2n+1I⁺ → C_nH_2n+1⁺ + I, (n = 2-5). The 0 K appearance energies of the iodine-loss fragment ions were determined to be 9.836 ± 0.010, 9.752 ± 0.010, 9.721 ± 0.010, and 9.684 ± 0.010 eV for n-C₃H₇I, n-C₄H₉I, n-C₅H₁₁I, and n-C₆H₁₃I molecules, respectively. Isomerization of then-alkyl iodide structures into 2-iodo species adds complexity to this study. Using literature adiabatic ionization energies, ionic bond dissociation energies were calculated for the four modeled iodoalkyl cations and it was shown that as the alkyl chain length increases, the carbon-halogen bond strength decreases, supporting the suggestions set forth by inductive effects. In the modeling with statistical energy distributions and rate theory, the role of hindered rotors was also evaluated and no strong experimental evidence was found either way. The heaviest species in the series, heptyl iodide (C₇H₁₅I) was also measured via iPEPICO and showed to have a greater complexity of fragmentation than the lighter analogs. Sequential dissociation of the first fragment ion, C₇H₁₅⁺ leads to C₄H₉⁺, C₅H₁₁⁺, and C₃H₇⁺ ions in competitive dissociation processes, dominated at low energies by the C₄H₉⁺ cation.

Pages

84

Recommended Citation

Rowland, Tyson G.. (2012). Accurate ionic bond energy measurements with TCID mass spectrometry and imaging PEPICO spectroscopy. University of the Pacific, Thesis - Pacific Access Restricted. https://scholarlycommons.pacific.edu/uop_etds/809