Excitation spectra of dibenzoborole containing pi-electron systems: controlling the electronic spectra by changing the p(pi)-pi* conjugation

Stabilization of the Elusive 9-Carbene-9-Borafluorene Monoanion

Two-electron reduction of carbene-supported 9-bromo-9-borafluorenes with excess KC₈ , Na, or Li-naphthalenide affords six N-heterocyclic carbene (NHC)- or cyclic(alkyl)(amino) carbene (CAAC)-stabilized borafluorene anions (3-8)-the first isolated and structurally authenticated examples of the elusive 9-carbene-9-borafluorene monoanion. The electronic structure, bonding, and aromaticity of the boracyclic anions were comprehensively investigated via computational studies. Compounds 5 and 8 react with metal halides via salt elimination to give new B-E (E=Au, Se, Ge)-containing materials (9-12). Upon reaction with diketones, the carbene ligand cleanly dissociates from 5 or 8 to yield new B-O containing spirocycles (13-14) that cannot be easily obtained using "normal" valent borafluorene compounds. Collectively, these results support the notion that carbene-stabilized monoanionic borafluorenes may serve as a new platform for the one-step construction of higher-value boracyclic materials.

Synthesis of 9-borafluorene analogues featuring a three-dimensional 1,1'-bis(o-carborane) backbone

The synthesis of [1,1'-bis(o-carboranyl)]boranes was achieved through the deprotonation of 1,1'-bis(o-carborane) reagents followed by salt metathesis with (iPr)2NBCl2. X-ray crystallography confirms planar central BC4 rings and Gutmann-Beckett studies reveal an increase in Lewis acidity at the boron center in comparison to their biphenyl congener, 9-borafluorene.

Successive carbene insertion into 9-phenyl-9-borafluorene

The reactions of 9-phenyl-9-borafluorene with trimethylsilyldiazomethane in a 1 : 1 and 1 : 2 stoichiometry furnished the corresponding BC5 and BC6 heterocycles via the formal insertion of one and two carbene units.

TD-DFT based fine-tuning of molecular excitation energies using evolutionary algorithms

An evolutionary de novo design method is presented to fine-tune the excitation energies of molecules calculated using time-dependent density functional theory (TD-DFT).

Density functional studies on photophysical properties and chemical reactivities of the triarylboranes: effect of the constraint of planarity

The geometric and electronic structures, absorption spectra, transporting properties, chemical reactivity indices and electrostatic potentials of the planar three-coordinate organoboron compounds 1-2 and twisted reference compound Mes(3)B, have been investigated by employing density functional theory (DFT) and conceptual DFT methods to shed light on the planarity effects on the photophysical properties and the chemical reactivity. The results show that the planar compounds 1-2 exhibit significantly lower HOMO level than Mes(3)B, owing to the stronger electronic induction effect of boron centers. This feature conspicuously induces a blue shifted absorption for 1, although 1 seemingly possesses more extended conjugation framework than Mes(3)B. Importantly, the reactivity strength of the boron atoms in 1-2 is much lower than that in Mes(3)B, despite the fact that the tri-coordinate boron centers of 1-2 are completely naked. The interesting and abnormal phenomenon is caused by the strong p-π electronic interactions, that is, the empty p-orbital of boron center is partly filled by π-electron of the neighbor carbon atoms in 1-2, which are confirmed by the analysis of Laplacian of the electron density and natural bond orbitals. Furthermore, the negative electrostatic potentials of the boron centers in 1-2 also interpret that they are not the most preferred sites for incoming nucleophiles. Moreover, it is also found that the planar compounds 1-2 can act as promising electron transporting materials since the internal reorganization energies for electron are really small.

Calculations of relative intensities of fragment ions in the MSMS spectra of a doubly charged penta-peptide

BACKGROUND

Currently, the tandem mass spectrometry (MSMS) of peptides is a dominant technique used to identify peptides and consequently proteins. The peptide fragmentation inside the mass analyzer typically offers a spectrum containing several different groups of ions. The mass to charge (m/z) values of these ions can be exactly calculated following simple rules based on the possible peptide fragmentation reactions. But the (relative) intensities of the particular ions cannot be simply predicted from the amino-acid sequence of the peptide. This study presents initial work towards developing a theoretical fundamental approach to ion intensity elucidation by utilizing quantum mechanical computations.

METHODS

MSMS spectra of the doubly charged GAVLK peptide were collected on electrospray ion trap mass spectrometers using low energy modes of fragmentation. Density functional theory (DFT) calculations were performed on the population of ion precursors to determine the fragment ion intensities corresponding to a Boltzmann distribution of the protonation of nitrogens in the peptide backbone amide bonds.

RESULTS

We were able to a) predict the y and b ions intensities order in concert with the experimental observation; b) predict relative intensities of y ions with errors not exceeding the experimental variation.

CONCLUSIONS

These results suggest that the GAVLK peptide fragmentation process in the ion trap mass spectrometer is predominantly driven by the thermodynamic stability of the precursor ions formed upon ionization of the sample. The computational approach presented in this manuscript successfully calculated ion intensities in the mass spectra of this doubly charged tryptic peptide, based solely on its amino acid sequence. As such, this work indicates a potential of incorporating quantum mechanical calculations into mass spectrometry based algorithms for molecular identification.

9-Borafluorenes – NMR spectroscopy and DFT calculations. Molecular structure of 1,2-(2,2′-diphenylylene)-1,2-diethyldiborane

9-Borafluorene derivatives 1 (9-R = Et (a), Ph (b), Cl (c), NEt2 (d)), the pyridine adduct 1py+ and 1,2-(2,2′-biphenylylene)-1,2-diethyldiborane(6) (3), were studied by 11B and 13C NMR spectroscopy to obtain a fairly complete data set for the first time. The molecular structure of the doubly hydrogen-bridged 1,2-diphenylenediborane 3 was determined by X-ray diffraction. The gas-phase structures of the compounds 1, related derivatives, and of some doubly hydrogen-bridged 1,2-diphenylenediboranes were optimized by quantum chemical calculations (B3LYP/6-311+G(d,p) level of theory) and NMR parameters, such as chemical shifts, 11B chemical shift tensors and indirect nuclear 13C–11B spin–spin coupling constants were calculated at the same level of theory and compared with experimental data.