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 * REU Program

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 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 Modeling using:    g i (E + E rot + E vib – E 0 ) n / E Equation is convoluted with kinetic energy distributions of product ions and Xe at 300 K before comparison with experimental data. Zero pressure extrapolations of Xe Life Time Effect (RRKM)

Geometries Fe + (CO 2 ) 2 quartet state Å Å Å Å Å Å Fe + (CO 2 ) quartet state Fe + (CO 2 ) sexted state Å Å Å Fe + (CO 2 ) 2 sexted state Å Å Å

2.077 Å Å Å Å Å Å Å Å Å Å Fe + (CO 2 ) 5 CO 2 Fe + (CO 2 ) 3 quartet state Fe + (CO 2 ) 4 quartet state 98.3° 100.6° 81.4 °

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 (CO 2 ) 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.

Summary Fe + (CO 2 ) 2 and Fe + (CO 2 ) 4 exhibit the highest BDE while Fe + (CO 2 ) 3 exhibits the lowest BDE. BDEs of Fe + (CO 2 ) n exhibit a trend comparable to those of other Fe + (L) complexes. L =  accepting ligand Experimental values for primary and secondary thresholds are in agreement with theoretical values.