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Role of OH-stretch/torsion coupling and quantum yield effects in the first OH overtone spectrum of cis-cis HOONO

A joint theoretical and experimental investigation is undertaken to study the effects of OH-stretch/HOON torsion coupling and of quantum yield on the previously reported first overtone action spectrum of cis-cis HOONO (peroxynitrous acid). The minimum energy path along the HOON dihedral angle is computed at the coupled cluster singles and doubles with perturbative triples level with correlation consistent polarized quadruple zeta basis set, at the structure optimized using the triple zeta basis set (CCSD(T)/cc-pVQZ//CCSD(T)/cc-pVTZ). The two-dimensional ab initio potential energy and dipole moment surfaces for cis-cis HOONO are calculated as functions of the HOON torsion and OH bond length about the minimum energy path at the CCSD(T)/cc-pVTZ and QCISD/AUG-cc-pVTZ (QCISD-quadratic configuration interaction with single and double excitation and AUG-augmented with diffuse functions) level of theory/basis, respectively. The OH-stretch vibration depends strongly on the torsional angle, and the torsional potential possesses a broad shelf at approximately 90 degrees , the cis-perp conformation. The calculated electronic energies and dipoles are fit to simple functional forms and absorption spectra in the region of the OH fundamental and first overtone are calculated from these surfaces. While the experimental and calculated spectra of the OH fundamental band are in good agreement, significant differences in the intensity patterns are observed between the calculated absorption spectrum and the measured action spectrum in the 2nu(OH) region. These differences are attributed to the fact that several of the experimentally accessible states do not have sufficient energy to dissociate to OH+NO(2) and therefore are not detectable in an action spectrum. Scaling of the intensities of transitions to these states, assuming D(0)=82.0 kJ/mol, is shown to produce a spectrum that is in good agreement with the measured action spectrum. Based on this agreement, we assign two of the features in the spectrum to Deltan=0 transitions (where n is the HOON torsion quantum number) that are blue shifted relative to the origin band, while the large peak near 7000 cm(-1) is assigned to a series of Deltan=+1 transitions, with predominant contributions from torsionally excited states with substantial cis-perp character. The direct absorption spectrum of cis-cis HOONO (6300-6850 cm(-1)) is recorded by cavity ringdown spectroscopy in a discharge flow cell. A single band of HOONO is observed at 6370 cm(-1) and is assigned as the origin of the first OH overtone of cis-cis HOONO. These results imply that the origin band is suppressed by over an order of magnitude in the action spectrum, due to a reduced quantum yield. The striking differences between absorption and action spectra are correctly predicted by the calculations.     

SNAr reactions of β-substituted porphyrins and the synthesis of meso substituted tetrabenzoporphyrins

Reaction of 2,3,7,8,12,13,17,18-octaethylporphyrin with LiR reagents containing functional groups readily yields meso substituted derivatives suitable for further transformations with residues such as -p-C₆H₅Br, -p-C₆H₅-CCH, -p-C₆H₅-NH₂ or -(CH₂)₃-CHCH₂. Similar reactions of tetrabenzoporphyrin with alkyllithium reagents afforded the first entry into meso mono- and dialkylsubstituted tetrabenzoporphyrins while reaction of bicyclo[2.2.2]oct-type masked isoindole precursors with LiR followed by in situ retro-Diels-Alder reaction also afforded the 5-phenyl and 5,10-diphenyltetrabenzoporphyrins in high purity.     

The Sialons MLn[Si4−xAlxOxN7−x] with M = Eu,Sr, Ba and Ln = Ho‐Yb – Twelve Substitution Variants with the MYb[Si4N7] Structure Type

The oxonitridoalumosilicates (so‐called sialons) MLn[Si_4?xAl_xO_xN_7?x] with M = Eu, Sr, Ba and Ln =Ho, Er, Tm, Yb were obtained by the reaction of the respective lanthanoid metal, the alkaline earth carbonates or europium carbonate, resp., AlN, “Si(NH)₂” and MCl₂ as a flux in a radiofrequency furnace at temperatures around 2100 °C. The compounds MLn[Si_4?xAl_xO_xN_7?x] are relevant for the investigation of substitutional effects on the materials properties due to their ability of tolerating a comparatively large phase width up to x ≈ 2.0(5). The crystal structures of the twelve compounds were refined from X‐ray single crystal data and X‐ray powder data and are found to be isotypic to the MYb[Si₄N₇] structure type. The compounds crystallize in space group P6₃mc (no. 186, hexagonal) and are made up of chains of so‐called starlike units [N^[4](SiN₃)₄] or [N^[4]((Si,Al)(O,N)₃)₄], respectively. These units are formed by four (Si,Al)(N/O)₄ tetrahedra sharing a common central nitrogen atom. The structure refinement was performed utilizing an O/N‐distribution model according to Paulings rules, i.e. nitrogen was positioned on the four‐fold bridging site and nitrogen and oxygen were distributed equally on both of the two‐fold bridging sites, resulting in charge neutrality of the compound. The Si and Al atoms were distributed equally on their two crystallographic sites, referring to their elemental proportion in the compound, due to being poorly distinguishable by X‐ray methods. The chemical compositions of the compounds were derived from electron probe micro analyses (EPMA).