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.

Increasing Complexity in a Conformer Space Step-by-Step: Weighing London Dispersion against Cation-π Interactions.

We report an evaluation of the importance of London dispersion in moderately large (up to 36 heavy atoms) organic molecules by means of a molecular torsion balance whose conformations "weigh" one interaction against another in the absence of solvents. The experimental study, with gas-phase cryogenic ion vibrational predissociation (CIVP) spectroscopy, solid-state Fourier transfer infrared (FT-IR), and single-crystal X-ray crystallography, is accompanied by density functional theory calculations, including an extensive search and analysis of accessible conformations. We begin with the unsubstituted molecular torsion balance, and then step up the complexity systematically by adding alkyl groups incrementally as dispersion energy donors (DEDs) to achieve a degree of chemical complexity comparable to what is typically found in transition states for many regio- and stereoselective reactions in organic and organometallic chemistry. We find clear evidence for the small attractive contribution by DEDs, as had been reported in other studies, but we also find that small individual contributions by London dispersion, when they operate in opposition to other weak noncovalent interactions, produce composite effects on the structure that are difficult to predict intuitively, or by modern quantum chemical calculations. The experimentally observed structures, together with a reasonable value for a reference cation-π interaction, indicate that the pairwise interaction between two tert-butyl groups, in the best case, is modest. Moreover, the visualization of the conformational space, and comparison to spectroscopic indicators of the structure, as one steps up the complexity of the manifold of noncovalent interactions, makes clear that in silico predictive ability for the structure of moderately large, flexible, organic molecules falters sooner than one might have expected.

Application of continuous wave quantum cascade laser in combination with CIVP spectroscopy for investigation of large organic and organometallic ions.

Rapidly developing mid-infrared quantum cascade laser (QCL) technology gives easy access to broadly tunable mid-IR laser radiation at a modest cost. Despite several applications of QCL in the industry, its usage for spectroscopic investigation of synthetically relevant organic compounds has been limited. Here, we report the application of an external cavity, continuous wave, mid-IR QCL to cryogenic ion vibrational predissociation spectroscopy to analyze a set of large organic molecules, organometallic complexes, and isotopically labeled compounds. The obtained spectra of test molecules are characterized by a high signal-to-noise ratio and low full width at half-maximum-values, allowing the assignment of two compounds with just a few wavenumber difference. Data generated by cw-QCL and spectra produced by another standard Nd:YAG difference-frequency generation system are compared and discussed.

Cryogenic ion vibrational predissociation (CIVP) spectroscopy of a gas-phase molecular torsion balance to probe London dispersion forces in large molecules.

We report a gas-phase molecular torsion balance that uses a conformational equilibrium to "weigh" London dispersion against a competing cation-π interaction, for which the readout is the shift in an N-H stretching frequency measured by cryogenic ion vibrational predissociation (CIVP) spectroscopy of electrosprayed pyridinium cations in a Fourier-transform ion cyclotron resonance trap. While frequency calculations with DFT, within the harmonic approximation, assist in the interpretation of the spectra, the observed complex spectrum most likely comes from a Fermi resonance of the N-H stretch with otherwise "dark" overtones of in-plane C-H wagging modes, as argued on the basis of comparison of the spectrum to those for a range of related cations with systematically varied substitution. An equilibrium in favor of the asymmetric conformer would suggest that the dispersion-corrected DFT calculations tested in this work appear to overestimate significantly the stability of the compact conformations favored by London dispersion in the gas phase, which would then pertain to the use of dispersion energy donors in the design of stereoselective reactions.

A 4 K FT-ICR cell for infrared ion spectroscopy.

We present the design of the newly constructed cryogenic Fourier-transform ion cyclotron resonance (FT-ICR) ion trap for infrared ion spectroscopy. Trapped ions are collisionally cooled by the pulsed introduction of buffer gas into the cell. Using different buffer gases and cell temperatures, we record action spectra of weakly bound neutral gas-analyte complexes with an IR laser source. We show for the first time that ion-He complexes can be observed in an ICR cell at temperatures around 4 K. We compare the experimental vibrational spectra of Ag(PPh3)2+ obtained by tagging with different neutral gases: He, Ne, Ar, H2, and N2 to computed vibrational spectra. Furthermore, the conditions necessary for the formation of neutral tags within an ICR ion trap are studied.