Die Photoelektronen-Spektren des Tricyclo[4.2.1.02,5]nonadiens und seines 3,4-Diaza-Analogons. Ein Beitrag zur Kenntnis der Wechselwirkung zwischen den einsamen Elektronenpaaren der cis-konfigurierten Azogruppe

A comparative photoelectron spectroscopical investigation of the title compound ( 11 ) and its 3,4- and 7,8-dihydro derivatives ( 9 and 10 ) indicates that a considerable ‘through bond’ interaction exists between the π-orbitals in 11 . The PE. spectra of the 3,4-diaza-analogue of 10 and 11 , which contain a cis azo group in a four-membered ring, yield a splitting Δ_n (4-memb. ring) = 1.55–1.60 eV between the nitrogen lone-pair orbital energies. This value contrasts with those obtained for a three-membered ring analogue (3,3-dimethyldiazirine ( 5 ), Δ_n (3-memb. ring) = 3.55 eV) and for a five-membered ring analogue (2,3-diazanorbornene ( 7 ) Δ_n(5-memb. ring) = 3.10 eV). The sequence Δ_n (3-memb. ring) > Δ_n (4-memb. ring) Δ_n (5-memb. ring) is satisfyingly reproduced by MINDO/2- and EHT-calculations for the model systems with n = 3,4,5. A similar trend can be deduced from MINDO/2-calculations for cis-diimid where Δ_n becomes minimal for a N?N? H angle φ ≈ 100°, whereas Δ_n for the corresponding trans-structure goes through a maximum in this region. The experimental finding as well as the calculated results confirm the predictions made by Gimarc [15] who attributes the behaviour of Δ_n for cis-azo groups to a ‘through-bond’ interaction of the n ₊-orbital with a lowerlying N? N σ-orbital; this interaction becomes maximal for N?N? R angles of the size present in a fourmembered ring, e.g. in 12 or 13 .     

Synthesis and properties of some novel high birefringence phenylacetylene liquid crystal materials with lateral substituents

We have synthesized and investigated new 3-ring phenylacetylene liquid crystals with high birefringence values (Δn) to improve the nematic temperature range while retaining a high optical anisotropy. In the case of modifying the terminal ring, the introduction of branched alkoxy chains, fluorine or methyl groups shifted the nematic phase to lower temperatures. In order to minimize the influence of the substituent on Δn, the incorporation of lateral methyl groups was chosen as the most profitable method for obtaining wide and low temperature nematic phases. With these results in mind, we studied the effect of the position of the methyl group on the physical properties. The most effective methyl position for improving the nematic temperature range was on the central ring. From the standpoint of obtaining high Δn values and low viscosity, the central ring was also the best position.     

Liquid crystalline perfluoroazomethines and perfluoropropenes exhibiting large dielectric anisotropies

Novel liquid crystalline compounds containing phenylperfluoroazomethine and cyclohexylperfluoropropene moieties have been synthesized. The perfluoroazomethine compounds exhibited large Δ? values of up to 24.3, which is greater than those of the trifluorobenzene or trifluoromethoxybenzene derivatives that are used in TN‐TFT LCDs. The perfluoropropene compounds exhibited low values of Δn (0.044–0.057) as well as large Δ? values (6.3–8.1), which are the properties required for reflection‐type LCDs.     

A Remarkable Solvent Effect on the Nuclearity of Neutral Titanium(IV)‐Based Helicate Assemblies

The spontaneous self‐assembly of a neutral circular trinuclear Ti^IV‐based helicate is described through the reaction of titanium(IV) isopropoxide with a rationally designed tetraphenolic ligand. The trimeric ring helicate was obtained after diffusion of n‐pentane into a solution with dichloromethane. The circular helicate has been characterized by using single‐crystal X‐ray diffraction study, ¹³C CP‐MAS NMR and ¹H NMR DOSY solution spectroscopic, and positive electrospray ionization mass‐spectrometric analysis. These analytical data were compared with those obtained from a previously reported double‐stranded helicate that crystallizes in toluene. The trimeric ring was unstable in a pure solution with dichloromethane and transformed into the double‐stranded helicate. Thermodynamic analysis by means of the PACHA software revealed that formation of the double‐stranded helicates was characterized by ΔH(toluene)=?30 kJ mol^?1 and ΔS(toluene)=+357 J K^?1 mol^?1, whereas these values were ΔH(CH₂Cl₂)=?75 kJ mol^?1 and ΔS(CH₂Cl₂)=?37 J K^?1 mol^?1 for the ring helicate. The transformation of the ring helicate into the double‐stranded helicate was a strongly endothermic process characterized by ΔH(CH₂Cl₂)=+127 kJ mol^?1 and ΔH(n‐pentane)=+644 kJ mol^?1 associated with a large positive entropy change ΔS=+1115 J K^?1?mol^?1. Consequently, the instability of the ring helicate in pure dichloromethane was attributed to the rather high dielectric constant and dipole moment of dichloromethane relative to n‐pentane. Suggestions for increasing the stability of the ring helicate are given.     

Linear free energy ortho-correlations in the thiophene series. Part IX . Kinetics of esterification with diazodiphenylmethane of some 3-, 4-, and 5-substituted thiophene-2-carboxylic acids in methanol

The rate constants for the esterification of some 3-, 4-, and 5-substituted thiophene-2-carboxylic acids with diazodiphenylmethane in methanol at 25° have been measured. The reactivity of some para- and ortho-substituted benzoic acids has also been determined. Logarithmic kinetic constants for ortho-, meta-, and para-like substituted thiophene-2-carboxylic acids furnish an excellent linear free energy relationship when plotted versus Δpk_a (β 0.89, r 0.989, C.L. > 99.9%, n 18, i 0.04), thus confirming the peculiar behaviour of five-membered ring derivatives. The correlation with σ^H values offers an additional proof of the hyper-ortho character of the 2,3-relation in thiophene derivatives. para- and ortho-Substituted benzoic acids show the usual behaviour of six-membered ring derivatives.     

Separation and Thermal Racemization of the Enantiomers of Spiro[cyclohexadiene-dihydroacridines]

 The enantiomers of substituted spiro[cyclohexadiene-dihydroacridines] were separated by enantioselective liquid chromatography with the sorbent/solvent systems triacetylcellulose/methanol, tris-(3,5-dimethylphenylcarbamoyl)-cellulose/silica gel (Chiralcel OD^TM)/n-heptane/2-propanol, and (+)-poly-(trityl methacrylate)/silica gel/n-heptane/2-propanol. Interconversion barriers of the enantiomers were determined for a series of derivatives by thermal racemization. Electrocyclic ring opening/ring closure in terms of the Woodward-Hoffmann rules is discussed for the enantiomerization mechanism; the interconversion of the enantiomers by enolization is ruled out by deuterium exchange experiments.     

Development of ferroelectric liquid crystals with low birefringence

We propose an approach for development of ferroelectric liquid crystals (FLC) with low birefringence Δn. Two basic principles have been used to get lowering of Δn: selection of molecules with short chains of conjugation as components for achiral matrix and averaging of local refractive indices by FLC helical structure. FLC mixtures with low birefringence (0.07 < Δn < 0.10 at wavelength 589.3 nm of sodium line) were elaborated and investigated. They consist of an achiral matrix including both nematic and smectic liquid crystal components and of phenylpyrimidine derivatives as chiral dopants. The materials developed can be used for all basic electro-optical FLC modes such as surface stabilised FLC (SSFLC), deformed helix ferroelectrics (DHF) and electrically suppressed helix (ESH). The mixtures developed allow to reduce the FLC cells chromatic retardance variation due to the weaker birefringence dispersion as compared with the known FLC materials to date.     

Use of Chemically Bonded β‐Cyclodextrin Silica Stationary Phase for Liquid Chromatographic Separation of Structural Isomers

The retention behavior of five disubstituted benzene derivatives and two naphthalene derivatives is examined by using a chemically bonded β‐cyclodextrin silica stationary phase with the moiety containing the s‐triazine. The chromatographic results of five disubstituted benzene derivatives and two naphthalene derivatives show that effective separation is achieved on this stationary phase by high‐performance liquid chromatography. The results of the present investigation indicate that the formation of inclusion complexes plays a dominant role in the separation mechanism. However, the selectivity can be significantly enhanced by the n‐n interactions between the s‐triazine ring of the chemically bonded β‐cyclodextrin silica stationary phase and the aromatic ring of solutes. For example, the effective separation of the o‐, m‐, and p‐toluidine isomers on this stationary phase with the moiety containing the s‐triazine ring was better than on that of some β‐cyclodextrin bonded stationary phases without the moiety containing s‐triazine ring.     

Strain induced bond localization in strained aromatic compounds with extended π systems

The influence of small rings annulated to aromatic rings is usually additive. However, in some systems the behavior is nonadditive, for example, in tricarbonyliron‐cyclobutadienobenzene versus tris(tricarbonylironcyclobutadieno)benzene. A study of dimethylenecyclobutabenzene, 1,3‐bis(dimethylenecyclobuta)benzene, tris(dimethylenecyclobuta)benzene, and their bora, imino, and oxo derivatives is presented. The behavior of these systems ranges from an almost perfect additivity (in the bora derivatives) to nonadditive behavior (in the oxo derivatives) where the single ring annulated system shows a negative difference between the bonds endocyclic and exocyclic to the annulated small ring (negative ΔR; anti‐Mills–Nixon distortion), the doubly annulated system shows ΔR=0, and the triply annulated system shows positive ΔR (Mills–Nixon distortion). The geometrical properties of the systems are analyzed in terms of the strain, bond curvature, and electronegativity of the exocyclic substituent. These atomic and σ properties are found to be responsible for the structural properties whereas π factors are of minor or no importance. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1377–1386, 2001     

Separation and Thermal Racemization of the Enantiomers of Spiro[cyclohexadiene-dihydroacridines]

Summary.  The enantiomers of substituted spiro[cyclohexadiene-dihydroacridines] were separated by enantioselective liquid chromatography with the sorbent/solvent systems triacetylcellulose/methanol, tris-(3,5-dimethylphenylcarbamoyl)-cellulose/silica gel (Chiralcel OD^TM)/n-heptane/2-propanol, and (+)-poly-(trityl methacrylate)/silica gel/n-heptane/2-propanol. Interconversion barriers of the enantiomers were determined for a series of derivatives by thermal racemization. Electrocyclic ring opening/ring closure in terms of the Woodward-Hoffmann rules is discussed for the enantiomerization mechanism; the interconversion of the enantiomers by enolization is ruled out by deuterium exchange experiments. Received April 3, 2000. Accepted April 12, 2000     

Liquid crystal birefringence for millimeter wave radar

Abstract

Many liquid crystals are found to have relatively high birefringence (Δn) values in the microwave and millimeter wave regions, as calculated from the phase shift induced by their reorientation by magnetic or electric fields. At 30 GHz, Δn values were obtained in the range of 0.08 to 0.18 for eleven liquid crystal mixtures of various types. The most favourable liquid crystal structures for high millimeter wave birefringence are highly conjugated rod-like molecules containing biphenyl, terphenyl, phenylpyrimidine, biphenylpyrimidine, and tolane groups in nematics of positive dielectric anisotropy (Δε). However, other liquid crystal structures including Schiffs base, azoxybenzene, and aromatic ester groups also have substantial birefringence, including nematics with negative and crossover Δε, as well as cholesteric nematics. The Δn varied only slightly at different frequencies of microwave millimeter wave in the 15–94 GHz range. Studies on magnetic and electrical field liquid crystal orientation in specially designed waveguides provide a basis for new types of modulators and scanning array antennae in the millimeter wave region, where more compact liquid crystal modulation media can be used than in the microwave region. These scanners can be used for both sending and receiving radar signals for potentially low cost radar systems.     

Synthesis and characterisation of asymmetrical mesogenic materials based on 2,5-disubstituted-1,3,4-oxadiazole

New mesogenic homologous series bearing 1,3,4-oxadiazole ring with a nitro terminal group, 4-(5-(4-nitrophenyl)-1,3,4-oxadiazol-2-yl)phenyl 4-((4-methoxybenzylidene)amino)benzoate (G₁–G₁₁), were synthesised. Their chemical structures are identified by fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (¹H-NMR) and elemental analysis. The liquid crystalline properties of the series G_n and their precursory F_n were screened by differential scanning calorimetry and optical polarising microscopy (OPM). The compounds of the series G_n were screened by thermogravimetric analysis to observe their thermal stability. The target compounds (G_n) in this study, were displayed different liquid crystalline mesophase, the first two homologous (G₁ and G₂) did not show any liquid crystalline behaviour, the homologous (G₃–G₁₀) which have an alkoxy terminal group (n = 3–10) exhibited nematic phase, whilst the last derivative of the series (G₁₁), n = 12, displayed SmA phase. The mesomorphic properties of these derivatives were affected by the presence of the nitro group at the end of the molecules which was classified as a strong polar group. Also, the role of alkoxy terminal chain and the bent heterocyclic ring (1,3,4-oxadiazole) in the liquid crystalline properties of these molecules were debated.     

Substituentenabhängigkeit der Ringinversion des Ungesättigten Siebenringes bei Benzocycloheptenderivaten. Konformative Beweglichkeit flexibler Ringsysteme—XIV: Untersuchungen mit Hilfe der Protonenresonanzspektroskopie

NMR spectroscopy has been used to investigate the ring inversions of the unsaturated seven membered ring system in a total of 20 benzocycloheptene derivatives with 1, 2 and 3 pairs of geminal substituents. For all compounds the inversion of the ring at ? 80°C is ‘frozen’ and at this temperature only one conformation is present in detectable quantity, presumably that of the chair form. The free activation enthalpies ΔG^≠ for the chair inversions lie between 9·9 and 13·7 kcal/mole. For disubstituted and tetrasubstituted benzocycloheptenes the ΔG^≠ values vary according to the positions of the ligands: for disubstituted derivatives ΔG^≠ is largest for the 5-position and smallest for the 3-position. For the tetrasubstituted derivatives the inversion of the ring—compared to that in the comparable dimethyl derivatives—is made more difficult when the ligands are in the 3,6- or 3,7- positions, but is facilitated when in the 3,5- or 4,6- positions. The effect observed in the 3,5- and 4,6- substituted rings is due to transanular repulsion of synaxial substituents, which leads to a flattening of the ring. Such a repulsion does not occur when the ligands are in the 3,6- positions. On the other hand, when the ligands are in 3,7- positions the transanular repulsion leads to a stronger puckering of the chair; the inversion could be hindered by this. For benzocycloheptene the activation energies for the inversions between chair, boat and twist (S, W, T) conformations were determined from model calculations. The best route for the inversion of the chair is the version way S → W via the transitional conformation V₄₅ and V₅₆. The calculated activation energy for this (14·6 kcal/mole) agrees well with the experimentally determined value (13 ± 1·5 kcal/mole). For the pseudorotation W → T a slightly lower calculated value of 11·1 kcal/mole was found.     

13C NMR chemical shifts of N-unsubstituted- and N-methyl-pyrazole derivatives

¹³C shielding data for 100 derivatives of pyrazole are reported. These include methyl, ethyl, n-propyl, tert-butyl, phenyl, hydroxymethyl, carboxyl, ethoxycarbonyl, cyano, amino, hydrazino, nitro, azido, chloro, bromo and iodo groups as substituents on the ring carbon atoms.     

Synthesis of certain N- and C-alkyl purine analogs

A number of N- and C-alkyl derivatives of selected guanine analogs have been synthesized as potential antiviral agents. n-Pentyl, n-hexyl and 6-hydroxyhexyl derivatives in the imidazo[1,2-α]-s-triazine, 9–11 , imid-azo[1,2-α]pyrimidine, 13–17 , and thiazolo[4,5-d]pyrimidine, 19–21, ring system have been prepared by the direct alkylation of the sodium salt of the appropriate aglycon with the respective alkylbromides. Dehydra-tive coupling of 3-amino-6-hydrazino-1,2,4-triazin-5(4H)-one ( 22 ) with either hexanoic acid or heptanoic acid, and further ring closure of the reaction products 24a and 24b provided the n-pentyl and n-hexyl derivatives of 6-amino-1,2,4-triazolo[3,4-f][1,2,4]triazin-8(7H)-one 25a and 25b , respectively. A similar condensation of 3-amino-6-aminomethyl-1,2,4-triazin-5(4H)-one ( 23 ) with heptanoic acid, followed by ring annulation, readily gave 2-amino-7-n-hexylimidazo[5,1-f][1,2,4]triazin-4(3H)-one ( 25c ). Bromination of 25c with N-bromosuccini-mide afforded the corresponding 5-bromo derivative 26 . Alkylation of the in situ generated sodium salt of 4-methoxycarbonylmethyl-5-methoxycarbonyl-2-oxo-1H,3H-imidazole ( 27 ) with 1-bromohexane gave the N-1 alkylated product 31 . Manipulation of the functional groups in 31 and further hydrazine mediated ring annulation furnished 5,6-diamino-1-n-hexyl-3-methylimidazo[4,5-c]pyridine-2,4-dione ( 39 ). Catalytic hydrogena-tion of 39 gave 7-methyl-8-oxo-9-hexyl-3-deazaguanine ( 40 ), a congener of the immunostimulator 7-methyl-8-oxoguanosine.     

Basicity of the nitrogen atom and of the carbonyl function of methyl isonicotinate and 4-acetylpyridine. An infrared study of the interaction with proton donors

The hydrogen bonded complexes between methyl isonicotinate and 4-acetylpyridine and phenol derivatives acting as proton donors have been investigated by ir spectroscopy. The thermodynamic parameters (K, — ΔH°, — ΔS°) have been determined. The ir data show that the hydrogen bond interaction occurs at the carbonyl function and at the nitrogen atom of the ring. When the acidity of the proton donor increases, N-complexation is favoured over carbonyl complexation and with the stronger acid hydrochloric acid, only N-protonated species are observed. The data are compared with those obtained for closely related pyridine derivatives, bearing a X-C=O substituent and it is shown that the proportion of OH···N and OH···O=C species is related to the basicity of the nitrogen atom of the heterocyclic ring and to inductive and mesomeric effects depending on the nature of X.     

Density and refractive index measurements in hexaheptyloxytriphenylene,a discotic liquid crystal

Density and refractive index measurements in the discotic liquid crystal hexaheptyloxytriphenylene were performed in the hexagonal columnar Col_ho and isotropic I phases. The temperature dependence of the density ρ(T) for this compound was obtained by combining small angle X-ray data and capillary methods. The ordinary n _o extraordinary n _e and isotropic liquid n _i refractive indices were measured using a modified Abbe refractometer to an accuracy of about 10^?3. To check the consistency of the density and the refractometry experiments we used the Lorentz–Lorenz relation. An anomaly in the empirical relationship at the Col_ho–I phase transition, which holds for many uniaxial liquid crystals, was detected. A discrepancy between low birefringence Δn～0.109 and a relatively high local electric field anisotropy for hexaheptyloxytriphenylene is discussed.     

Synthesis and mesogenic properties of a novel family of oligothiophene derivatives

With the aim of developing new oligothiophene-based liquid crystals involving hydrogen bonding, new terthiophene derivatives containing an alkylamide group, N,N'-distearyl-2,2' : 5',2"-terthiophene-5,5"-dicarboxamide (DNC₁₈3T) and N,N'-dialkyl-5,5"-dibromo-2,2' : 5',2"-terthiophene-4,4"-dicarboxamides (DNC_nDBr3T, n = 5, 8, 16, 18), were designed and synthesized, and their thermal behaviour examined. While DNC₁₈3T did not exhibit liquid crystallinity, DNC_nDBr3T compounds with n = 8, 16, 18 were found to form smectic A phases. In addition to liquid crystal behaviour, crystal polymorphism was also observed for DNC₁₆DBr3T. It is shown that both the position of the amido group and the alkyl chain length greatly affect liquid crystal phase formation. The absence of liquid crystallinity in the corresponding ester derivatives suggests that intermolecular hydrogen bonding also plays a role in the formation of liquid crystal phases in the DNC_nDBr3T system.     

Computational study of alkyllithium/pyridine derivative systems as initiators for the living anionic polymerization of methyl methacrylates

The reactivity of n‐butyllithium (n‐BuLi) toward pyridine derivatives (pyridine, pyridazine, pyrimidine, and 1,3,5‐triazine) was subjected to a computational study to determine the most suitable n‐BuLi/heterocyclic ring system as an initiator for the anionic polymerization of methyl methacrylate (MMA). These systems were suggested to prevent side reactions occurring through n‐BuLi attack on the carbonyl carbon of MMA by sterically blocking the initiator. The initiation reaction was modeled with the B3LYP methodology 6‐31+G*. Activation barriers were used to analyze the reactivity of each n‐BuLi/heterocyclic ring system. Computational results showed that n‐BuLi/triazine had a significantly lower activation barrier. Therefore, n‐BuLi/triazine was the suggested initiator system for the anionic polymerization of poly(methyl methacrylate). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 455–467, 2005     

Liquid Structure and Thermodynamics of Alkane Mixtures

Spectroscopic investigations and light scattering experiments with saturated, liquid hydrocarbons and their mixtures indicate a specific and distinct influence of the constitution, conformation, and flexibility of the molecule on the structure and macroscopic behavior of such liquids. Orientational order present in pure liquid n-alkanes, for example, characteristically affects the thermodynamic mixing properties, such as the enthalpy of mixing ΔH_M and the entropy of mixing ΔS_M , when these liquids are mixed with each other, or with other liquids. Nowadays it is possible to determine thermodynamic mixing properties experimentally with such precision that systematic investigations of these properties allow the behavior of liquids to be studied qualitatively and–with molecular theories of liquids–to some extent also quantitatively. The latest results in this respect, exemplified by mixtures of alkanes, are discussed. These results not only demonstrate the progress made in understanding the relations between molecular (microscopic) and macroscopic properties, but are also of importance for industrial applications (e.g. separation processes) in which mixtures of hydrocarbons are involved.