Unveiling the Neutral Forms of Glutamine

Neutral glutamine has been evaporated by laser ablation of its solid sample to seed a rare gas carrier prior to a supersonic expansion and proved by Fourier transform microwave techniques. We report on three distinct neutral conformers that show a singular non‐interacting and flexible amide sidechain in contrast with the other proteinogenic aliphatic amino acids. It could explain the essential biological role of glutamine as a nitrogen source, and its unique ability to form a variety of hydrogen bonds with peptide backbones. Common computational methods fail to predict the delicate balance of intramolecular interactions controlling the geometry of the most stable conformer. The spectroscopic data here reported can be used to benchmark novel computational methods in quantum chemistry.     

Selective and Reversible Fluoride Complexation from Water by a Cyclic Tri(phosphonio)methanide Dication

Tri(phosphonio)methanide dication 3 ²⁺, prepared from a trifluoromethylsulfanylphosphonium dication ( 1 ²⁺) via an intramolecular electrophilic aromatic substitution reaction, is an unexpected P‐based, water‐resistant Lewis acid that is capable to selectively and reversibly bind fluoride ions from organic/aqueous biphasic solution. The formed complex is an unusual fluorophosphorane ( [3‐F] OTf). The multiple donor–acceptor interactions of 3 ²⁺ that are crucial for the fluoride fixation have been elucidated by quantum chemical calculation. Compound [3‐F] OTf can also be used as a convenient anhydrous fluoride ion source and was probed as a suitable catalyst of the silylotrifluoromethylation of various aldehydes.     

Rotational Spectroscopy Probes Water Flipping by Full Fluorination of Benzene

The topology of the interaction of water with benzene changes drastically upon full H→F substitution on the aromatic ring: the weak O−H⋅⋅⋅π hydrogen bond is replaced by a O⋅⋅⋅π linkage, of about the same strength. Hexafluorobenzene–water appears to be the prototype system to investigate this kind of weak bond. The pulsed Fourier transform microwave technique has been used for the detection of the rotational spectra of the normal species and five isotopologues which unambiguously led to the identification of the geometry. Quantum mechanical calculations have been performed to interpret the experimental evidence.