Frank Blockhuys

Activation and de-activation of fluorine synthons by nitrogen substitution in fluorinated aza-distyrylbenzenes

Three derivatives of E,E-1,4-bis[2-(2,3,4,5,6-pentafluorophenyl)ethenyl]benzene, two of which bear nitrogen atoms in the ethenyl spacers, while in a third the central benzene ring is replaced by a pyrazine moiety, have been synthesized. The supramolecular structures of the resulting set of four compounds have been studied using single-crystal X-ray diffraction to gauge the influence of the position of the nitrogen atoms on the organisation of the molecules in the solid state. The crystal packing patterns were analyzed in terms of intermolecular interactions involving the fluorine and nitrogen atoms, i.e., CH⋯F, F⋯F, F⋯π and CH⋯N interactions. The analysis shows that in two of the three solid-state structures the main effect of the nitrogen atoms is an indirect one: they do not participate in intermolecular contacts themselves, but activate nearby hydrogen atoms and phenyl rings in fluorine synthons to form new interactions.

2-Hydroxyterpenylic Acid: An Oxygenated Marker Compound for α-Pinene Secondary Organic Aerosol in Ambient Fine Aerosol

An oxygenated MW 188 compound is commonly observed in substantial abundance in atmospheric aerosol samples and was proposed in previous studies as an α-pinene-related marker compound that is associated with aging processes. Owing to difficulties in producing this compound in sufficient amounts in laboratory studies and the occurrence of isobaric isomers, a complete assignment for individual MW 188 compounds could not be achieved in these studies. Results from a comprehensive mass spectrometric analysis are presented here to corroborate the proposed structure of the most abundant MW 188 compound as a 2-hydroxyterpenylic acid diastereoisomer with 2R,3R configuration. The application of collision-induced dissociation with liquid chromatography/electrospray ionization-ion trap mass spectrometry in both negative and positive ion modes, as well as chemical derivatization to methyl ester derivatives and analysis by the latter technique and gas chromatography/electron ionization mass spectrometry, enabled a comprehensive characterization of MW 188 isomers, including a detailed study of the fragmentation behavior using both mass spectrometric techniques. Furthermore, a MW 188 positional isomer, 4-hydroxyterpenylic acid, was tentatively identified, which also is of atmospheric relevance as it could be detected in ambient fine aerosol. Quantum chemical calculations were performed to support the diastereoisomeric assignment of the 2-hydroxyterpenylic acid isomers. Results from a time-resolved α-pinene photooxidation experiment show that the 2-hydroxyterpenylic acid 2R,3R diastereoisomer has a time profile distinctly different from that of 3-methyl-1,2,3-butanetricarboxylic acid, a marker for oxygenated (aged) secondary organic aerosol. This study presents a comprehensive chemical data set for a more complete structural characterization of hydroxyterpenylic acids in ambient fine aerosol, which sets the foundation to better understand the atmospheric fate of α-pinene in future studies.

(E,E)-2,5-Dipropoxy-1,4-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]benzene.

The title compound, C30H34O8, crystallizes in the space group P-1 with one-half of a molecule in the asymmetric unit. A three-dimensional network is generated by OCH3...pi and CH...pi interactions. The conformation of the C-C bond exocyclic to the central benzene ring is different from that of every other known derivative. A comparison of the geometry of the title molecule and of its solid-state structure with other 2,4,6-trimethoxy-substituted PPV [i.e. poly(p-phenylenevinylene)] oligomers, in particular the isopropoxy-substituted compound, is provided.

(E, E)-2,5-Dimethoxy-1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl]benzene

In the title compound, C30H34O8, molecular symmetry is coincident with crystallographic inversion symmetry. A three-dimensional network is generated containing both C-H...pi and C-H...n(O) interactions. A comparison of the geometry of this molecule and the structure of a number of 2,4,6-trimethoxy-substituted analogues is provided.

Roesky’s ketone: a spectroscopic study

We present a joint experimental-theoretical spectroscopic study of 5-oxo-1,3,2,4-dithiadiazole, also known as Roesky’s ketone. The theoretical results of a vibrational analysis, calculated at the DFT/B3LYP/6-311+G* level of theory, of the title compound have been compared with experimental data, consisting of Raman and IR frequencies in different phases, and the bands have been assigned to the normal vibrations of the molecule. Additionally, an analysis of the origin of the high intensity of the band assigned to the CO stretching mode was performed based on calculated stockholder charges and atomic dipoles. The results of theoretical calculations of the 13C and 14N NMR chemical shifts are compared to experimentally obtained shifts.

Experimental and Computational Study on the Structure and Properties of Herz Cations and Radicals: 1,2,3-Benzodithiazolium, 1,2,3-Benzodithiazolyl, and Their Se Congeners

Salts of 1,2,3-benzodithiazolium (1), 2,1,3-benzothiaselenazolium (3), and 1,2,3-benzodiselenazolium (4) (Herz cations), namely, [1][BF4], [1][SbCl6], [3][BF4], [3][GaCl4], [3][SbCl6], and [4][GaCl4], were prepared from the corresponding chlorides and NaBF4, GaCl3, or SbCl5. It was found that [1][SbCl6] and [3][SbCl6] spontaneously transform in MeCN solution to [1]3[SbCl6]2[Cl] and [3]3[SbCl6]2[Cl], respectively. [1][BF4], [1]3[SbCl6]2[Cl], [3][BF4], [3]3[SbCl6]2[Cl], and [4][GaCl4] were structurally characterized by X-ray diffraction (XRD). In solution, these [BF4](-) and [GaCl4](-) salts as well as [1][GaCl4], [2][GaCl4], [3][GaCl4], [3][Cl], and [4][Cl] were characterized by multinuclear nuclear magnetic resonance (NMR). The corresponding Herz radicals 1(•)-4(•) were obtained in toluene and DCM solutions by the reduction of the appropriate salts with Ph3Sb and characterized by EPR. Cations 1-4 and radicals 1(•)-4(•) were investigated computationally at the density functional theory (DFT) and second-order Møller-Plesset (MP2) levels of theory. The B1B95/cc-pVTZ method was found to satisfactorily reproduce the experimental geometries of 1-4; an increase in the basis set size to cc-pVQZ results in only minor changes. For both 1-4 and 1(•)-4(•), the Hirshfeld charges and bond orders, as well as the Hirshfeld spin densities for the radicals, were calculated using the B1B95/cc-pVQZ method. It was found for both the cations and the radicals that replacing S atoms with Se atoms leads to considerable changes in the atomic charges, bond lengths, and bond orders only at the involved and the neighboring sites. According to the calculations, 60% of the positive charge in the cations and 80% of the spin density in the radicals is localized on the heterocycles, with the spin density distributions being very similar for all radicals 1(•)-4(•). For the cations 1-4, the NICS values (B3LYP/cc-pVTZ for B1B95/cc-pVTZ geometries) lie in the narrow range from -5.5 ppm to -6.6 ppm for the carbocycles, and from -14.4 ppm to -15.5 ppm for heterocycles, clearly indicating the aromaticity of the cations. Calculations on radical dimers [1(•)]2-[4(•)]2 revealed, with only one exception, positive dimerization energies, i.e., the dimers are inherently unstable in the gas phase.

Racemic 1,2,3,4,7,8,9,10-octa-fluoro-6H,12H-5,11-methano-dibenzo[b,f][1,5]diazo-cine: an octa-fluorinated analogue of Tröger's base.

The title compound, C15H6F8N2, possesses a non-crystallographic twofold axis. The dihedral angle between the two benzene rings is 98.4 (2)°. The crystal structure involves intermolecular C—H⋯F hydrogen bonds.

Structures of tetrabromothiophene and tetrabromoselenophene: the influence of the heteroatom on the heterophene packing

The crystal structures of C(4)Br(4)S and C(4)Br(4)Se have been determined from X-ray powder diffraction data, using direct-space search techniques. In the case of C(4)Br(4)S two crystalline phases occur, a stable orthorhombic and a metastable monoclinic phase. For the orthorhombic phase two different structural models were found that fit the experimental data equally well. The diversity in crystal structure models and packings of C(4)Br(4)S is explained.

Quantum Chemical Mass Spectrometry: Verification and Extension of the Mobile Proton Model for Histidine

AbstractThe quantum chemical mass spectrometry for materials science (QCMS2) method is used to verify the proposed mechanism for proton transfer – the Mobile Proton Model (MPM) – by histidine for ten XHS tripeptides, based on quantum chemical calculations at the DFT/B3LYP/6-311+G* level of theory. The fragmentations of the different intermediate structures in the MPM mechanism are studied within the QCMS2 framework, and the energetics of the proposed mechanism itself and those of the fragmentations of the intermediate structures are compared, leading to the computational confirmation of the MPM. In addition, the calculations suggest that the mechanism should be extended from considering only the formation of five-membered ring intermediates to include larger-ring intermediates. Graphical Abstractᅟ

Asymmetrically substituted distyrylbenzenes and their polar crystal structures

The synthesis of twelve asymmetric donor–acceptor distyrylbenzene derivatives with either one nitrile group or one, two or three nitro groups as electron acceptors, and one, two or three methoxy groups as electron donors is reported. Peak potentials obtained from cyclic voltammetry were combined with experimental UV/Vis data and molecular dipole moments obtained from quantum chemical calculations, yielding insight into the influence of the positions of the substituents on the electronic structure and charge distribution of this as yet unexplored class of organic semiconductors. The supramolecular structures of five of these compounds have been studied using single-crystal X-ray diffraction to monitor the influence of the positions of donor and acceptor groups on the organisation of the molecules in the solid state, and three crystal structures have been identified in which the molecular dipoles do not organize themselves in a centrosymmetric lattice. Analysis of the dipoles in the unit cell yields further insight into the possible non-linear optical properties of these three polar structures.