Counter‐Intuitive Gas‐Phase Reactivities of [V2]+ and [V2O]+ towards CO2 Reduction: Insight from Electronic Structure Calculations

Abstract [V2O]+ remains “invisible” in the thermal gas‐phase reaction of bare [V2]+ with CO2 giving rise to [V2O2]+; this is because the [V2O]+ intermediate is being consumed more than 230 times faster than it is generated. However, the fleeting existence of [V2O]+ and its involvement in the [V2]+ → [V2O2]+ chemistry are demonstrated by a cross‐over labeling experiment with a 1:1 mixture of C16O2/C18O2, generating the product ions [V2 16O2]+, [V2 16O18O]+, and [V2 18O2]+ in a 1:2:1 ratio. Density functional theory (DFT) calculations help to understand the remarkable and unexpected reactivity differences of [V2]+ versus [V2O]+ towards CO2.


Experimental Details
The ion/molecule reactions were performed in a Spectrospin CMS 47X Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with an external ion source as described elsewhere. [1] Briefly, [V2] + was generated by laser ablation of a vanadium target using a Nd:YAG laser operating at 532 nm; helium mixed with trace amount of N2 served as a cooling and carrier gas. It is important that the helium pipe has been baked beforehand to remove impurities on the inner walls of the feeding pipes and thus significantly improve the production of [V2] + . Using a series of ion lenses, the ions were transferred into the ICR cell, which is positioned in the bore of a 7.05 T superconducting magnet. After thermalization by about 1×10 5 collisions with pulsed-in argon, the reactions of mass-selected [V2] + were studied by introducing isotopologues of carbon dioxide (CO2 and C 18 O2) and a 1 : 1 mixture of CO2 The rate constants have been determined following the detailed protocol documented in the PhD Thesis of K. Koszinowski. [2] Typically, the pressures are determined with an uncalibrated Bayard-Alpard ion gauge whose reading differs depending on the kind of the gas.
As the concentration of the ionic reactant A + is small compared to the neutral substrate B, a pseudo first-order reaction can be assumed as a good approximation, here k is the true bimolecular and kobs represents the apparent pseudo-unimolecular rate constant. Recording a time-dependent profile of the natural logarithm of the normalized intensity of the educt ions delivers the decline of the reactant ions whose negative slope corresponds to kobs. For a general procedure to determine reaction-rate constants of ions with neutrals in the diluted gas phase, see reference. [3]

Computational Details
The calculations of the electronic structures were performed with Gaussian and ORCA. [4] S3 Quite elaborate multireference (MR) calculations were conducted to determine the relative energies of low-lying electronic states of [V2] + . To treat dynamic correlation without the problems of intruder states or level shifts, [5] n-electron valence perturbation theory (NEVPT2) [6] in conjunction with the ZORA-def2-QZVPP basis set, [7] as implemented in ORCA, was employed to optimize the geometries. Active space (9e,12o) has been considered in these calculations. The energetic information is given in Table S1; for the selection of the active space, see Figure S1.
Harmonic vibrational frequencies were computed to verify the nature of the stationary points.
The minimum structures reported in this paper show only positive eigenvalues of the Hessian matrix, whereas the transition states (TSs) have only one negative eigenvalue. Intrinsic reaction coordinate [26] calculations were also performed to confirm that the transition states correlate with designated intermediates. The thermodynamic functions (ΔH) were estimated within the ideal gas, rigid-rotor, and harmonic oscillator approximations at 298 K and 1 atm.
As commonly accepted, the geometries of molecular structures are less dependent on the level of theory than the energies thus obtained. For further energetic refinements, singlepoint energy (SPE) calculations at the ZORA-M06L/ZORA-def2-QZVPP level of theory were performed for the structures as optimized by the aid of the M06L functional.