Surprising Homolytic Gas Phase Co-C Bond Dissociation Energies of Organometallic Aryl-Cobinamides Reveal Notable Non-Bonded Intramolecular Interactions

Robustness of Threshold Collision-Induced Dissociation Simulations for Bond Dissociation Energies

The threshold collision-induced dissociation (T-CID) method is the workhorse for gas-phase bond dissociation energy (BDE) measurements. However, T-CID does not measure BDEs directly; instead, BDEs are obtained by fitting simulated data to the experimental data. We previously observed several large discrepancies between the computed and experimental BDEs. To analyze the reliability of the experimental values, we previously reported a study of the dissociation rate models in the simulation. Here, we report a study of the collision simulation part, specifically in the L-CID (ligand CID) program. We show that the BDE values are robust even to intentionally introduced mistakes in the simulations, varying in most cases by less than 3 kcal mol-1. The most significant exception is the collisional energy transfer (CET) simulation, which led to deviations larger than 10 kcal mol-1. However, we found that the BDEs obtained with explicitly simulated CET distributions deviated by only 3 kcal mol-1 from those simulated with the original model. Collectively, our results suggest that the T-CID-derived BDE values are robust and are likely to be accurate.

Cryogenic Ion Vibrational Predissociation (CIVP) Spectroscopy of Aryl Cobinamides in the Gas Phase: How Good Are the Calculations for Vitamin B12 Derivatives?

Aryl corrins represent a novel class of designed B12 derivatives with biological properties of "antivitamins B12". In our previous study, we experimentally determined bond strength in a series of aryl-corrins by the threshold collision-induced dissociation experiments (T-CID) and compared the measured bond dissociation energies (BDEs) with those calculated with density functional theory (DFT). We found that the BDEs are modulated by the side chains around the periphery of the corrin unit. Given that aryl cobinamides have many side chains that increase their conformational space and that the question of a specific structure, measured in the gas phase, was important for further evaluation of our T-CID experiment, we proceeded to analyze structural properties of aryl cobinamides using cryogenic ion vibrational predissociation (CIVP) spectroscopy, static DFT, and Born-Oppenheimer molecular dynamic (BOMD) simulations. We found that none of the examined DFT models could reproduce the CIVP spectra convincingly; both "static" DFT calculations and "dynamic" BOMD simulations provide a surprisingly poor representation of the vibrational spectra, specifically of the number, position, and intensity of bands assigned to hydrogen-bonded versus non-hydrogen-bonded NH and OH moieties. We conclude that, for a flexible molecule with ca. 150 atoms, more accurate approaches are needed before definitive conclusions about computed properties, specifically the structure of the ground-state conformer, may be made.

Surprising Homolytic Gas Phase Co-C Bond Dissociation Energies of Organometallic Aryl-Cobinamides Reveal Notable Non-Bonded Intramolecular Interactions

Aryl-cobalamins are a new class of organometallic structural mimics of vitamin B12 designed as potential 'antivitamins B12 '. Here, the first cationic aryl-cobinamides are described, which were synthesized using the newly developed diaryl-iodonium method. The aryl-cobinamides were obtained as pairs of organometallic coordination isomers, the stereo-structure of which was unambiguously assigned based on homo- and heteronuclear NMR spectra. The availability of isomers with axial attachment of the aryl group, either at the 'beta' or at the 'alpha' face of the cobalt-center allowed for an unprecedented comparison of the organometallic reactivity of such pairs. The homolytic gas-phase bond dissociation energies (BDEs) of the coordination-isomeric phenyl- and 4-ethylphenyl-cobinamides were determined by ESI-MS threshold CID experiments, furnishing (Co-Csp 2 )-BDEs of 38.4 and 40.6 kcal mol-1 , respectively, for the two β-isomers, and the larger BDEs of 46.6 and 43.8 kcal mol-1 for the corresponding α-isomers. Surprisingly, the observed (Co-Csp 2 )-BDEs of the Coβ -aryl-cobinamides were smaller than the (Co-Csp 3 )-BDE of Coβ -methyl-cobinamide. DFT studies and the magnitudes of the experimental (Co-Csp 2 )-BDEs revealed relevant contributions of non-bonded interactions in aryl-cobinamides, notably steric strain between the aryl and the cobalt-corrin moieties and non-bonded interactions with and among the peripheral sidechains.