Grenzen der Anwendbarkeit regionaler Symmetrieregeln: Der aromatische1Lb-Cottoneffekt

A critical examination concerning the validity of symmetry rules, which correlate theCD of the inherently achiral, chirally perturbed benzene chromophore in the¹L_b wavelength region with the absolute configuration is presented. It is shown, that the derivation of generally applicable symmetry rules is interdicted by fundamental arguments as is deduced by studying the origin of optical activity in the above mentioned electronic transition. Further evidence is given, that the generation of parallel components of the electric and magnetic moment is due to a vibronic perturbation. While the electric transition moment can roughly be estimated using spectroscopic partial moments, the magnetic transition moment will depend on the properties of the ?-skeleton including both conformational rigidity and the mass distribution (which latter involves the substitution pattern). Thus the uncertainity concerning the direction of the magnetic moment prevents the deduction of a symmetry rule applicable to different molecular frames. Moreover, the relevance of such rules is shown to be limited even within distinct frames. A method is developed to find the conditions which imply the establishment of an unequivocal sector rule for a definite substitution pattern. Moreover, this may afford a tool for a rough localization of the magnetic transition moment.     

Reactions ofOrtho-quinones and the model compound 4-t-butyl-1,2-benzo-quinone in alkaline peroxide

The literature on the occurence and reactions of quinone structures in mechanical pulps has been reviewed. A study on the reactions of a quinone model compound (4-t-butyl-1,2-benzoquinone) in alkaline hydrogen peroxide has been reported, with detailed analysis of reaction products and kinetic phenomena. The study reveals that the diversity of products formed is much more complex than that obtained using an α,β-unsaturated aldehyde as a model, a simple second-order expression can be used to describe the kinetics. The kinetics representing chromophore removal for the two classes of model compound are compared with previously reported studies of kinetic phenomena during bleaching of mechanical pulps.     

Compositions and structures of nanoparticles of trigonal symmetry <Emphasis Type="Italic">D</Emphasis><Subscript>3<Emphasis Type="Italic">d</Emphasis></Subscript>: Nanotubes

A method has been developed for the determination of the structure and number of atoms in the shells of nanoparticles as a function of the arrangement of atoms at the symmetry elements of a symmetry group. The formulas for calculation of the number of particles of symmetry D _3d have been reported. It has been shown that the number of atoms in trigonal shells is determined by three structurally invariant numbers and the quantum number of the group order n. All possible nanostructures of symmetry D _3d have been classified: C_{θ + 6z} , z = 0, 1, 2, ..., where the basic shells are C_θ = C₆, C₈, and C₁₄. A sum rule has been obtained for the coordination numbers of the shell sites located on symmetry axes. Trigonal nanoparticles are parent ones for obtaining (3,0), (6,0), and (9,0) nanotubes of trigonal type. The general formulas of these nanotubes with icosahedral, dodecahedral, and cubic caps are N_{8 + 12p} , N_{20 + 24p} , and N_{60 + 36p} (p = 1, 2, ...), respectively. The graphical constructions of all classes of trigonal nanoparticles and nanotubes are reported.     

Synthesis of new crosslinkable co-polymers containing a push-pull zinc porphyrin for non-linear optical applications

In this paper, the synthesis of a crosslinkable co-polymer containing new push-pull arylethynyl zinc porphyrins is described. The synthesis of porphyrin chromophores, analogous to Therien's porphyrin (J. Am. Chem. Soc.1996, 118, 1497-1503) functionalized with a methacrylic polymerizable group and a carboxylic acid crosslinking group was achieved with a new synthetic procedure leading to a higher overall yield compared to what was previously reported in the literature for similar and simpler structures. Radical copolymerization of the porphyrin chromophore with glycidyl methacrylate has then been carried out with success. This work opens a perspective on the possibility to integrate porphyrinic chromophore with high first-order molecular quadratic hyperpolarizability coefficient in opto-electronic devices.     

The infrared and Raman spectra of pyromellitic dianhydride

The infrared and Raman spectra of solid state samples of pyromellitic dianhydride have been measured. The infrared—Raman mutual exclusion rule has been observed and the frequencies have been tentatively assigned on the basis of D_2h symmetry. The values of the CO and skeletal ring stretching frequencies have been interpreted in terms of a conjugated π-system.     

Ab initio studies of the 3nπ* states of glyoxal and methyl glyoxal

Results of ab initio calculations on the ³nπ* states of glyoxal and methyl glyoxal are reported. The symmetry unconstrained ΔE_SCF transition energies, unlike the symmetry constrained results, are found to be very similar. These calculations explain some anomalies which have appeared in the literature recently.     

Quadratic nonlinear optical and preliminary piezoelectric investigation of crosslinked samples obtained from a liquid chromophore

A push–pull chromophore has been synthesized, which is liquid at room temperature and can be crosslinked owing to the presence of two methacrylate moieties. Thin films of the chromophore have been prepared by spin coating, and they have been simultaneously crosslinked and poled under strong electric field. On the poled crosslinked films, the quadratic nonlinear optical (NLO) characterization was performed through nonresonant second harmonic generation measurements at 1368 nm as the fundamental wavelength, yielding a fairly good d₃₃ value of 46 pm/V, with retention of 80% of that value after 2 months at 85 °C. Following the theoretical issue that the quadratic NLO and piezoelectric tensors of a material have the same symmetry properties, and exploiting the easy processing of the chromophore in the liquid phase, we have prepared poled crosslinked samples of the chromophore suitable for piezoelectric tests that were performed using a commercial piezoceramic sample as the reference. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Chiroptical spectra and molecular geometry of bicyclic dithioimides

Several chiral bicyclic dithioimides were prepared by thionation of the parent imides with Lawesson's reagent. Due to a flexibility of the dithioimide chromophore, their CD spectra show significant solvent dependence. A substantial deviation of the chromophore from planarity was confirmed by the MNDO calculations and crystallographic structures of 4, 5b, 6 and 7. The observed CE sign corresponding to the lowest energy n–π* transition can be predicted, upon careful examination of the molecular geometry, by the antioctant rule with sector signs opposite to those known for ketones.     

Circular dichroism—LXIII : Third-sphere contribution to the R-band cotton effect of lactones

The Cotton effect within the n→π* band of the lactone chromophore of the halo lactones 3a-c, prepared from (1S)-2-bromo adamantanedione-4.8 (1) is positive and can be used to determine unequivocally the sign of third-sphere contributions of the appropriate sector rule. An “axial” halogen (Cl, Br, I) substituent in a β-position to the keto group of these halogeno oxa-homoadamantanediones gives an antioctant contribution to the Cotton effect of the carbonyl chromophore as observed earlier for the corresponding halogeno-adamantanones.     

Spontaneous generation of chirality and chiroptical spectra of N-nitroso-2,4-diaryl-3-azabicyclo[3.3.1]nonanes

The crystal structures of several bicyclic N-nitrosamines indicate that they crystallize in the chiral (Sohncke) space group P2₁2₁2₁ as conglomerates. This allows the resolution of these compounds by manual picking of the enantiomorphous crystals. The optical activity of the single crystals was confirmed by their CD spectra taken in KBr disks. The absolute configurations of the title nitrosamines were assigned by crystallographic measurements and by a comparison of their CD spectra with those of a reference compound resolved by classical methods. The observed Cotton effect signs, corresponding to the n–π¹ transition, were correlated with the helicity of the inherently chiral nitrosamine chromophore.     

Compositions and structures of nanoparticles of tetrahedral symmetry

The formulas for calculation of the composition of nanoparticles with symmetry group T _d are reported. The numbers of atoms in the shells of tetrahedral symmetry are determined by three structurally invariant numbers and the “quantum number” of the group order n. Four classes of all possible nanostructures with symmetry group T _d are revealed: C _{? + 12z} , where z = 0, 1, 2, ..., and C _? is C ₄, C ₆, C ₈, C ₁₀, or C ₁₄. The sum rule for the coordination numbers of the shell sites located on the axes of symmetry is obtained. The six-shell periodicity of the number of occupied sites located on the symmetry axes and in symmetry planes is revealed.     

Conformation and chiroptical properties of mandelic acids

Using CD data (solution, solid state, various temperatures), X-ray data and MO calculations for a number of substituted mandelic acids (phenylglycolic acids) an attempt was made to explain the different results for the relation between sign of the ¹L_b CD band and substitution pattern as described in the literature for various types of aromatic compounds. Moreover the reported sector rule has been reconsidered taking into account the sign and magnitude of the spectroscopic moments. It has been found that in any explanation of the signs of the ¹L_b CD bands of substituted aromatic compounds both the conformational behaviour and the spectroscopic moments should be taken into account.     

Compositions and structures of nanoparticles of pentagonal axial symmetry <Emphasis Type="Italic">D</Emphasis><Subscript>5<Emphasis Type="Italic">d</Emphasis></Subscript>: Nanotubes

The method of determination of the structure and the number of atoms in the shells of nanoparticles as a function of the arrangement of atoms at the symmetry elements of a symmetry group has been developed. The formulas for the calculation of the number of particles with symmetry group D _5d are reported. The number of particles in these shells is determined by three structurally invariant numbers and the “quantum number” of the group order n. The classification of all possible nanostructures with symmetry group D _5d is given: C _θ+10z , z = 0, 1, 2, …, where the basic shells are C _θ = C ₂, C ₁₀, C ₁₂. The sum rule has been obtained for the coordination numbers of shell sites located at symmetry axes. Pentagonal axial nanoparticles are shown to be the initial shells for obtaining (5,5) and (10,10) armchair nanotubes or (5,0) and (10,0) zigzag nanotubes. The general formula of these nanotubes closed with icosahedral and dodecahedral caps is N _20+10p , N _60+10p (p = 1, 2, …). The graphical constructions of all classes of nanoparticles and nanotubes of the pentagonal axial type are reported.     

Compositions of the shells of nanoparticles with pentagonal symmetry

A method of calculation of the number of atoms in the structure of nanoforms with pentagonal symmetry (fullerenes, nanoparticles, clusters) depending on the arrangement of atoms on the symmetry elements of the I _h group has been developed. The formulas for calculation of the number of particles in all possible shells, including multilayer ones, are reported. The numbers of atoms in the shells of pentagonal symmetry are determined by four structurally invariant numbers and the “quantum number” of the order n of the group. The classification of all possible atomic shells S _{? + 60z} (z = 0, 1, ...) is presented, and the constructions of the basic shells S _? (? = 12, 20, 30, 50, 60) are given. For each basic shell, the sum rule is met: the sum of the coordination numbers of the elements of subshells is equal to 60. In clusters with magic numbers, basic shells are periodically repeated. In addition to the known shells of nanostructures, the formulas of new structures that are expected to be stable—B₂₀O₃₀, B₆₀O₉₀ (B₂O₃), and B₉₀O ₁₃₀ ¹⁰⁺ (borate)—are reported for the first time. The same is valid for similar compounds of Group III elements.     

Some characteristic features of shpolskii spectra. The key and hole rule for shpolskii systems

The key and hole rule for Shpolskii systems has been re-examined by studying fluorescence spectra of naphthalene, anthracene and naphthacene in several n-alkane polycrystals (20 K). It is demonstrated that the key and hole rule has no significance for the systems under study. From our results and literature data the Shpolskii effect appears to be a rather general phenomenon for aromatic hydrocarbons in frozen n-alkanes, provided a correct concentration is chosen and a proper cooling rate is applied.     

Control of Chromophore Symmetry by Positional Isomerism of Peripheral Substituents

The first example of the control of porphyrinoid chromophore symmetry based on the positional isomerism of peripheral substituents has been achieved by preparing tetraazaporphyrins (TAPs) with C_4h, D_2h, C_2v, and C_s symmetry due to the relative arrangement of peripheral tert‐butylamino and cyano groups as push and pull substituents, respectively. The four structural isomers were successfully isolated and characterized by ¹H NMR spectroscopy and X‐ray crystallography. The band morphology in the Q‐band region varies depending on the molecular symmetry due to the significant perturbation introduced into the chromophore by the push and pull substituents. The C_4h and C_2v isomers exhibit a single Q band, whereas the Q bands of the D_2h and C_s isomers show a marked splitting. The magnetic circular dichroism spectra indicate that the push–pull TAPs retain the properties of the 16‐membered 18π‐electron perimeter generally observed for porphyrinoids. Theoretical calculations have demonstrated that the perturbation introduced by the substituents lowers the D_4h symmetry of the parent TAP π‐conjugated system, and this results in significant spectral changes. A novel approach to the fine‐tuning of the spectral properties of porphyrinoids based on changes in the chromophore symmetry is described.     

The vibrational spectra of 1,1,4,4-tetrafluoro- and 1,1,4,4-tetrachlorobutadiene

Raman data are reported for gas, liquid and solid 1,1,4,4-tetrafluorobutadiene, F₂CCHCHCF₂, and IR data for gas and solid. The molecule has a planar trans conformation of C_2h symmetry. With the aid of Raman depolarization ratios and IR band contours, twenty of the twenty-four spectroscopically-active fundamentals can be assigned with assurance. Frequencies are suggested for the remaining four modes.Raman and IR data are reported for the liquid and solid 1,1,4,4-tetrachlorobutadiene. Mid-IR gas phase data are also reported. Again the data can be satisfactorily explained under C_2h symmetry. Fourteen fundamentals can be assigned with confidence. Suggestions have been made for the frequencies of nine other fundamentals.     

Synthesis,structure and chiroptical spectra of the bicyclic α-diketones,imides and dithioimides related to santenone

Isomeric α-diketones, imides and dithioimides related to santenone were prepared in a multistep synthesis from (+)-camphor and their CD spectra compared with those of analogous chromophoric systems related to camphor. In the case of conformationally rigid α-diketones the methyl substituents at C-7, lying on the symmetry plane of the chromophore, exert only a weak contribution to the n–π* Cotton effects. In contrast, the Cotton effect magnitudes of the anhydrides, imides and dithioimides are significantly affected by the substituents at C-8. The steric interaction of these chromophores with the syn-methyl group at C-8, leading to the chromophore distortion, was confirmed by X-ray crystallographic studies.     

Assessment of the absolute configuration of a series of (3R)-3-hydroxy-3-alkyl-β-lactams

The absolute stereochemistry of a series of (3R)-3-hydroxy-3-alkyl-β-lactams has been determined by circular dichroism and NMR spectroscopies. The sign of the circular dichroism band between 250 and 220 nm was related to the stereochemistry by applying the β-lactam sector rule. The NMR analysis unambiguously determines the relative configuration at C3 and C4 of the β-lactam ring. The reliability of the method has been proved by X-ray analysis of two of the examined compounds. The obtained results are in agreement with the mechanism proposed for the employed synthetic route.     

Hyperfine structure studies of Tantalum

A narrow bandwidth ring dye laser pumped by an argon ion laser has been used to investigate the hyperfine structure of the even and odd parity levels of tantalum by optogalvanic spectroscopy in the wavelength range 5640 to 6050 Å. Seventeen transitions have been observed and eight of these have not been reported in the literature so far. These transitions involve 27 levels with 15 odd and 12 even parity configurations. The magnetic dipole hyperfine interaction constants A and the electric quadrupole interaction constants B for these levels have been computed and compared with the data available in literature. The results for the levels at 34799.71 cm^?1, 26960.46 cm^?1 and 19657.78 cm^?1 are reported for the first time.