The meta and para OH Substitution Effect on C-Phenyl-Nitrilimine Bond-Shift Isomers

The geometric and electronic structure of 1,3-dipolar species, in particular of nitrile imines, can be surprisingly intricate. A particular example is the C-phenyl-nitrilimine, which exists as two energy minima that constitute bond-shift isomers. To examine the effect of substituents in the phenyl ring, here we investigate the meta and para OH substituted derivatives. These two nitrile imines were generated in an argon matrix by UV-irradiation of 2H-tetrazole precursors and found to photoisomerize to carbodiimides via 1H-diazirines. The different effects of the OH substitution in meta and para positions on the bond-shift isomerism are rationalized with the support of Natural Resonance Theory and Hirshfeld atomic charges. The understanding of how substitution affects the structural characteristics of C-phenyl-nitrilimines, opens a door to modulate the chemistry of those compounds (e. g. in cycloaddition reactions) by appropriate tuning of their substitution (substituent type and position).     

The Crystal Structure of α-F2: Solving a 50 Year Old Puzzle Computationally

The cohesive energy of α-fluorine, with C2/c space group symmetry, was calculated at benchmark quality by applying the method of increments. The known experimental X-ray structure data needed to be refined, since the reported intramolecular bond length was unrealistically large. At the CCSD(T) level, including corrections for zero-point energy, the basis set superposition error, and extrapolated to the complete basis set limit, a cohesive energy of −8.72 kJ mol⁻¹ was calculated, which agrees well with the 0 K-extrapolated experimental value of −8.35 kJ mol⁻¹. 1 Comparison of the C2/c structure with a Cmca structure, isotypic to that of chlorine, bromine, and iodine reveals that the origin of the different structure of solid fluorine, compared to the heavier halogens, is the lack of significantly stabilizing σ-hole interactions. In addition, the wave numbers of the stretching mode in solid fluorine were calculated at coupled cluster level and compared to newly recorded Raman spectra of condensed fluorine. Both experiment and calculation indicate a slight up-shift for the stretching mode by 2 or 5 cm⁻¹, respectively, with respect to a free F₂ molecule in the gas phase.     

Iron Halide Species Produced by Laser-Evaporation

Iron halide species were produced by the reaction of laser-evaporated iron atoms with halogen-containing reactant gas, and isolated in low-temperature matrices to obtain their Mössbauer spectra. Iron fluoride (Fe₂F₆, FeF₃ and Fe₂F₄) and iron iodide (FeI₂ and Fe₂I₄) were produced by the reaction of laser-evaporated iron atoms with sulfur hexafluoride SF₆ and methyl iodide CH₃I, respectively. The yields of the products varied depending on the concentration of reactant gas in the Ar matrix. Molecular orbital calculations were performed in order to confirm their assignments.     

Investigation of Molecular Alkali Tetrafluorido Aurates by Matrix-Isolation Spectroscopy

Molecular alkali tetrafluorido aurate ion pairs M[AuF₄] (M=K, Rb, Cs) are produced by co-deposition of IR laser-ablated AuF₃ and MF in solid neon under cryogenic conditions. This method also yields molecular AuF₃ and its dimer Au₂F₆. The products are characterized by their Au–F stretching bands and high-level quantum-chemical calculations at the CCSD(T)/triple-ζ level of theory. Structural changes in AuF₄⁻ associated with the coordination of the anion to different alkali cations are proven spectroscopically and discussed.     

Extending the Row of Lanthanide Tetrafluorides: A Combined Matrix‐Isolation and Quantum‐Chemical Study

Only the neutral tetrafluorides of Ce, Pr, and Tb as well as the [LnF₇]^3? anions of Dy and Nd, with the metal in the +IV oxidation state, have been previously reported. We report our attempts to extend the row of neutral lanthanide tetrafluorides through the reaction of laser‐ablated metal atoms with fluorine and their stabilization and characterization by matrix‐isolation IR spectroscopy. In addition to the above three tetrafluorides, we found two new tetrafluorides, ³NdF₄ and ⁷DyF₄, both of which are in the +IV oxidation state, which extends this lanthanide oxidation state to two new metals. Our experimental results are supported by quantum‐chemical calculations and the role of the lanthanide oxidation state is discussed for both the LnF₄ and [LnF₄]^? species. Most of the LnF₄ species are predicted to be in the +IV oxidation state and all of the [LnF₄]^? anions are predicted to be in the +III oxidation state. The LnF₄ species are predicted to be strong oxidizing agents and the LnF₃ species are predicted to be moderate to strong Lewis acids.     

Rotational Isomerism in O-Methylphenol: Matrix-Isolation Infrared Study and MNDO Calculations

Rotational isomerism in o-methylphenol was studied by matrix-isolation infrared spectroscopy and MNDO calculations. Monomer molecules were isolated both in the argon matrix and in the nitrogen matrix near 10°K. The resolution of one weak band from the main hydroxyl absorption in the stretching and torsional regions evidently indicates the existence of two stable rotational isomers. Results of MNDO semi-empirical molecular orbital calculations with full geometry optimization indicate that only two stable conformations can exist with the trans (stag) conformation being more stable than the cis (stag) conformation by 3.32 KJ/mole. The present experimental data are interpreted with the aid of theoretical MNDO calculations. The agreement between experiment and theoretical calculations is excellent. However, it was found that the CNDO/2 calculations would give misleading predictions on the relative stabilities of rotational isomers in the present case.     

IR-Laser Ablation of Potassium Cyanide: A Surprisingly Simple Route to Polynitrogen and Polycarbon Species

Pulsed laser irradiation of solid potassium cyanide (KCN) produces, besides free nitrogen and carbon atoms, the molecular species KN and KC which are potential candidates for interstellar species of potassium. Additionally, N₃, N₃⁻, KN₃, C₃, C₃⁻, and KC₃ are produced and isolated in solid noble gases as well as in solid N₂. Molecular potassium nitrene (KN) reacts with dinitrogen in neon and argon matrices after photochemical excitation (λ=470 nm) forming molecular end-on (C_∞v) and side-on (C_2v) potassium azide isomers. The side-on isomer (C_2v) is thermodynamically favored at the CCSD(T)/ma-def2-TZVP level of theory. It can be obtained from the end-on isomer by UV-irradiation (λ=273 nm).     

Differential Tunneling-Driven and Vibrationally-Induced Reactivity in Isomeric Benzazirines

Quantum mechanical tunneling of heavy-atoms and vibrational excitation chemistry are unconventional and scarcely explored types of reactivity. Once fully understood, they might bring new avenues to conduct chemical transformations, providing access to a new world of molecules or ways of exquisite reaction control. In this context, we present here the discovery of two isomeric benzazirines exhibiting differential tunneling-driven and vibrationally-induced reactivity, which constitute exceptional results for probing into the nature of these phenomena. The isomeric 6-fluoro- and 2-fluoro-4-hydroxy-2H-benzazirines ( 3-a and 3′-s ) were generated in cryogenic krypton matrices by visible-light irradiation of the corresponding triplet nitrene ³ 2-a , which was produced by UV-light irradiation of its azide precursor. The 3′-s was found to be stable under matrix dark conditions, whereas 3-a spontaneously rearranges (τ_1/2 ∼64 h at 10 and 20 K) by heavy-atom tunneling to ³ 2-a . Near-IR-light irradiation at the first OH stretching overtone frequencies (remote vibrational antenna) of the benzazirines induces the 3′-s ring-expansion reaction to a seven-member cyclic ketenimine, but the 3-a undergoes 2H-azirine ring-opening reaction to triplet nitrene ³ 2-a . Computations demonstrate that 3-a and 3′-s have distinct reaction energy profiles, which explain the different experimental results. The spectroscopic direct measurement of the tunneling of 3-a to ³ 2-a constitutes a unique example of an observation of a species reacting only by nitrogen tunneling. Moreover, the vibrationally-induced sole activation of the most favorable bond-breaking/bond-forming pathway available for 3-a and 3′-s provides pioneer results regarding the selective nature of such processes.     

Matrix Isolation-Vibrational Circular Dichroism Spectroscopic Study of Conformations and Non-Covalent Interactions of Tetrahydro-2-Furoic Acid

The conformational landscape and aggregation behaviour of tetrahydro-2-furoic acid (THFA) were investigated by using matrix isolation-vibrational circular dichroism (MI-VCD). The well-resolved experimental MI-IR and MI-VCD features in an argon matrix at 10 K allow one to identify two dominant monomeric conformations as trans-THFA where the hydroxyl and carbonyl groups of COOH are at opposite sides, as well as one cis-conformer. At 24 K and 30 K deposition temperatures, the experimental IR and VCD spectral features reveal further growth of the binary THFA aggregates. Systematic conformational searches identified three vastly different binary binding topologies, resulting in a few hundred stable (THFA)₂ conformers. Interestingly, the main binary structures observed correspond to an unusual type of structure which is made of two trans-THFA subunits, in contrast to the usual double H-bonded ring binary structures, identified in a previous solution study. The present work showcases the power of MI-VCD spectroscopy in revealing unusual structures formed in a cold rare gas matrix.     

Formation and Characterization of a BeOBeC Multiple Radical Featuring a Quartet Carbyne Moiety

Through reaction of beryllium dimers with carbon monoxide, a carbonyl complex BeBeCO is formed in solid neon. Upon visible light excitation, the BeBeCO complex rearranges to a BeCOBe isomer, which further isomerizes to a low‐energy BeOBeC species under UV‐visible light excitation. These species are identified on the basis of infrared absorption spectroscopy with isotopic substitutions and quantum chemical studies. The BeOBeC molecule is characterized to be a multiple radical species having an electronic quintet ground state featuring an unusual quartet carbyne unit with three unpaired electrons on the carbon center. Bonding analysis indicates that the strong Pauli repulsion between carbon 2s lone pair electrons and the σ electrons of the BeOBe fragment significantly weakens the Be?C bonding and destabilizes the triplet state of the BeOBeC radical with a doublet carbyne unit. The three‐center π‐bonding of BeOBe is also found to play a role in stabilizing the quartet carbyne.