Sequential Bond Dissociation Energies of Fe + (CO 2 ) n (n=1-5) Meghan MacKenna, Hideya Koizumi, and P.B. Armentrout Department of Chemistry, University of Utah

Abstract The sequential bond energies of Fe + with carbon dioxide are determined using collision-induced dissociation (CID) with xenon gas in a guided ion beam mass spectrometer. The kinetic energy dependences of the CID cross sections are analyzed to give 0 and 298 K bond energies for the successive loss of ligands after accounting for multiple collisions, internal energy, and lifetime effects. Experimental bond energies of Fe + (CO 2 ) n are determined for n=1-5, and theoretical values are determined for these systems as well.

Thermochemical Analysis Kinetic energy dependence of product cross sections is analyzed to determine E 0 Zero pressure extrapolations of Xe Modeling using:    g i (E + E rot + E vib + E i – E 0 ) n / E Equation is convoluted with kinetic energy distributions of product ions and Xe at 300 K before comparison with experimental data.

Geometries

THE GIBMS (Guided Ion Beam Mass Spectrometer) 1 Gas phase ions are created by associative reactions in a 1 meter long flow tube. Sodium ions are generated in a DC discharge by argon ion sputtering in a He bath gas. Ligands with a vapor pressure enter the flow tube directly via a leak valve. Complexes are thermalized by ~10 4 collisions with the buffer gas. 2 The ions are focused by a number of electrostatic lenses into a magnetic momentum analyzer which may be tuned to effectively select a single species or "parent" ion for further analysis. 3 The parent ions pass into an rf octopole ion guide with a well defined kinetic energy and then enter the collision cell where they collide with a neutral gas (Xe) and fragment. All reactant and product ions continue to drift to the end of the octopole. 4 Unreacted parent ions and any product ions are mass analyzed using a quadrupole mass spectrometer (QMS), counted, and recorded He and Ar Inlet ~ 1 torr ~ torr ~ torr ~ torr ~ torr

CID-THE BASICS The metal-ligand complex is collided at a well defined kinetic energy with a neutral and inert gas, usually xenon. An rf octopole consists of 8 equally spaced stainless steel rods. Opposite phases of an rf signal are applied to alternating rods. This creates a potential well, which traps the ions in the radial direction. The efficient trapping allows for all product and parent ions to be detected regardless of the fragmentation direction. WHY AN OCTOPOLE? CID or Collision Induced Dissociation is a method for accurately determining binding energies in the gas phase. In this example the association of a metal ion (M + ) with a neutral ligand (L) will be examined. When just enough energy is supplied to break the metal- ligand bond, the metal ion will dissociate. The strength of this bond will be determined Fragmentation products are detected as a function of collision energy. From this the Threshold Energy for the M + -L dissociation may be determined.

CID-THE NOT SO BASICS The results for the CID of the CO 2 -Iron cation complex with xenon are shown to the right. The threshold binding energy, E 0, was determined to be 0.77 eV. This is quite different from what appears to be the threshold of ~0.50 eV (1 eV = 96 kJ/mol). Why are they different? In order to accurately model and extract the true threshold energy from laboratory data, a number of factors must be taken into account. MULTIPLE COLLISIONS FINITE EXPERIMENTAL LIFETIME INTERNAL ENERGY If multiple collisions occur, an erroneously low threshold energy will be observed. In order to ensure that only single collisions occur between the complex and Xe, all reactions are run at a number of Xe pressures. The results are extrapolated to zero pressure, truly single collision conditions. Given an infinite amount of time, an energized molecule will dissociate into products. However, in the GIBMS the energized ions have a finite time to be detected. Statistical RRKM theory is used to estimate this lifetime effect. Reactant ions are thermalized (at 300 K) prior to collision with Xe. Therefore, they already possess a finite amount of internal energy (in rotations and vibrations) prior to colliding with the neutral gas. Ab- initio theory is used to determine the vibrational frequencies and rotational constants of the dissociating ions. The energy distribution of the Xe is also taken into account during analysis.

Species CO 2 N2N2 CO Fe + (L) + Xe Fe + (L) 2 + Xe Fe + (L) 3 + Xe Fe + (L) 4 + Xe Fe + (L) 5 + Xe Bond Dissociation Energies at 0 K (eV)

Summary Fe + (CO 2 ) 2 and Fe + (CO 2 ) 4 exhibit the highest BDE while Fe + (CO 2 ) 3 exhibits the lowest BDE. Fe + (CO 2 ) n exhibits a trend comparable to other weakly bound transition metal complexes. Experimental values for primary and secondary thresholds are in agreement with theoretical values.

Experiment Iron ions are created by a DC discharge source, extracted from the source, accelerated, and passed through a magnetic sector for mass analysis. The mass-selected ions are decelerated to the desired kinetic energy and focused into an rf octopole beam guide. The ions then reach a collision cell where they collide with a neutral gas (Xe) at pressures ranging from and fragment. The unreacted parent and product ions drift to the end of the octopole. They are then mass analyzed using a quadrupole mass spectrometer, counted, and recorded.

Important Considerations Provide well-defined collision energies: use an rf octopole longer than the collision cell. Thermalize ions: use a flow tube source to provide ions with a well characterized internal energy distribution. Use a collision gas that provides efficient kinetic to internal energy transfer: generally use Xe, which is heavy and polarizable, unless ligand exchange reactions interfere. Extrapolate to zero pressure cross sections: eliminate the effects of secondary collisions that reduce the observed threshold energy. Include all sources of energy in the data analysis: explicit kinetic and internal energy distributions of the reactants. Lifetime of dissociating ions: incorporate RRKM theory into the analysis of the dissociating species (kinetic shifts).

Thermochemical Analysis The kinetic energy dependence of product cross sections is analyzed to determine E 0, the energy threshold for product formation at 0 K. Endothermic reaction cross sections are extrapolated to zero pressures of Xe and modeled using the following equation:    g i (E + E rot + E vib + E i – E 0 ) n / E The internal energy of the Fe + (CO 2 ) n reactant ion is included explicitly as a summation over vibrational energy levels, i, with energies Ei and relative populations g i (  g i = 1). Here,   is an energy-independent scaling factor, E is the relative translational energy of the reactants, Erot is the average rotational energy of the reactants, Evib is the vibrational energy of the neutral reactant, E0 is the threshold for reaction of the ground vibrational and electronic state, and n is an adjustable parameter. Before comparison with the experimental data, the equation is convoluted with the kinetic energy distributions of the product ions and neutral collision partner at 300 K. The, n, and E0 parameters are then optimized by using a nonlinear least-squares analysis to give the best reproduction of the data.

Thermochemical Analysis 1.Zero pressure extrapolation 2.Fit zero pressure cross section:    g i (E + E rot + E vib + E i – E 0 ) n / E a.Kinetic energy spread of ion beam / neutral motion b.Internal energy available for reaction c.Lifetime effect using RRKM statistical theory

Special Thanks…

And especially…

Topics to Consider: Experiment Geometries Theoretical calculations performed Comparison to other small ligands