Thermogravimetric analysis of inclusion compounds of perhydrotriphenylene with highern-alcohols

The vaporization of perhydrotriphenylene and its inclusion compounds with higher evennumberedn-alcohols from C₈ to C₁₆ has been studied using thermogravimetric analysis. For the inclusion compounds of perhydrotriphenylene with the C₈, C₁₀ and C₁₂ n-alcohols, kinetic analysis shows that the vaporization processes of then-alcohol and the perhydrotriphenylene are partially superimposed, while for the inclusion compound perhydrotriphenylene-hexadecanol-1, the above processes are totally superimposed. From the experimental kinetic parameters the values of the preexponential factors corresponding to the vaporization of then-alcohols and perhydrotriphenylene were calculated, as were the compositions of the inclusion compounds of perhydrotriphenylene with the C₈-C₁₂ n-alcohols.     

Time-resolved fluorescence depolarization studies of rotational relaxation in viscous media

Rotational relaxation times (τ_roi) were measured for xanthene dyes in several n-alcohols, ethylene glycol and glycerol by following the fluorescence depolarization using a single photon counting technique. Additional studies were made of the fluidity of the hydrocarbon micellar interior using pyrene derivatives as fluorescent probes. While the correlation between τ_roi and viscosity for the smaller n-alcohols is good, as anticipated by the models of Stokes—Einstein—Debye and later Perrin, a saturation in this correlation is observed in more viscous systems in accordance with recent molecular dynamics calculations. The influence of molecular structure on both viscosity and rotational relaxation times is also discussed. Pyrene, 1-pyrene suphonic acid and 1-pyrene butyeric acid exhibit rapid reorientation τ_roi ? 10^?9 s, in glycerol and micellar systems.     

Observation of transient resonance raman spectra of the S1 state of trans-stilbene

Transient resonance Raman spectra of trans-stilbene in n-hexane have been obtained using two pulsed lasers at 266 and 585 nm. The former was used to pump the molecule to the first excited singlet state (S₁) and the latter to proble spontaneous Raman scattering in resonance with the S_n ← S₁ electronic transition. The dependence of the pump and probe laser power, the temporal behavior, and the excitation profile of the spectra clearly indicate that they are due to the S₁ state of trans-stilbene.     

Study of the effect of the solvent nature on cis-trans isomerization in Bis(allyl)nickel by the density functional theory method

The cis-trans isomerization of the bis(η³-allyl)nickel complex in different solvents (n-hexane, benzene, toluene, THF) has been modeled by the DFT-PBE method. According to calculations, in all cases, isomerization occurs in three stages by the η³-η¹-η³ mechanism. The rate-limiting stage is the stage of rotation of the vinyl moiety in the η¹-allyl intermediate.     

Photoisomerization dynamics of alkyl-substituted stilbenes in supersonic jets

We report time-resolved fluorescence decay lifetimes from photoselected states of jet-cooled 4-ethyl-trans-stilbene (4ETS) and 4-n-propyl-trans-stilbene (4PTS) in the energy range E_v = 0–2800 cm⁻¹ above the S₁ origin. Alkyl substitution enhances the non-radiative decay rates. These data can be fitted by the statistical RRKM theory in conjunction with the reduction of the photoisomerization threshold from E_t = 1300±50 cm⁻¹ for trans-stilbene to E_t = 1100±100cm⁻¹ for 4ETS, and E_t = 1000±100 cm⁻¹ for 4PTS.     

Liquid densities from 293 to 490 K of nine aliphatic alcohols

The results of measurements of liquid densities within the temperature range 293 to 490 K are presented for highly pure samples of n-alcohols C1 to 5, 8 and 12, and of propan-2-ol and butan-2-ol. Measurements were made by means of a magnetically controlled float which could be held in the centre of the sample with the sample cell suspended from an automatic balance. The results are considered to have an accuracy of ±0.15 kg m^?3 over this range of temperature. Some large disagreements with published data are identified. Densities at temperatures above the upper limit of the measurements can not be estimated reliably.     

New atropisomeric biaryl derivatives of 4-aminopyridine—identification of an improved nucleophilic catalyst for asymmetric acylation of sec-alcohols

The synthesis, CSP-HPLC resolution, and absolute configuration assignment of a series of 4-dialkylaminopyridine-based atropisomeric biaryls are described. Screening of these enantiomerically pure catalysts, which differ only in the nature of the 4-dialkylamino substituent, for the kinetic resolution of 1-(1-naphthyl)ethanol reveals the importance of this group on the selectivity of catalysis. The di-n-butylamino derivative displays the most favourable catalytic profile. The utility of this catalyst for the kinetic resolution of a selection of sec-alcohols, including a precursor for the synthesis of the antidepressant fluoxetine hydrochloride (Prozac^®) are reported. The possible role of the dialkylamino group in chirality transfer is discussed.     

Gas-phase and supercritical <Emphasis Type="Italic">n</Emphasis>-pentane isomerization on H-mordenite

The skeletal isomerization of supercritical n-pentane on the H form of mordenite under flow conditions was studied for the first time. It was found that the conversion of supercritical n-pentane was 30–35% at 90% selectivity for isopentane at 260°C, 130 atm, and a liquid hourly space velocity of 30 h^?1. The catalyst was deactivated as the temperature was increased above 280°C. According to differential thermal analysis data, the deactivation was related to the deposition of condensation products on the surface. The resistance of the H form of the zeolite to poisoning in n-pentane isomerization in a gas phase at 1–8 atm was lower than that under supercritical conditions. It was found that H-mordenite deactivated under gas-phase reaction conditions at 260°C and 8 atm can be regenerated by passing to supercritical isomerization conditions (260°C and 130 atm).     

Alkyl and dialkylammonium tetrafluoroborate catalyzed cis-trans isomerization of 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane

Alkyl and dialkylammonium tetrafluoroborate promoted cis-trans isomerization of 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane (1) in DMSO-d₆ were studied. The isomerization equilibrium constant K are within the range of 3.74-3.30 from 22 to 47 °C. Thermodynamic parameters of ΔH° and ΔS° for the isomerization were −0.95 kcal/mol and −0.59 cal/mol-K respectively. The isomerization rate is first order in [cis-1] and second order in [R_nNH_4−nBF₄]. Both components of R_nNH_4−n⁺ and BF₄⁻ are essential for the catalytic cis-trans isomerization. The catalytic strength follows the decreasing order of ⁺H₃N(CH₂)₆NH₃⁺>n-C₈H₁₇NH₃⁺>n-C₁₆H₃₃NH₃⁺>Me₃CNH₃⁺>PhCH₂NH₃⁺>Et₂NH₂⁺?Ph₂CHNH₃⁺, Et₃NH⁺. Inversion region was observed in the plot of ln(k_f/T) versus (1/T) with the ceiling located at around 38 °C. The positive activation enthalpy of 9 kcal/mol was estimated at 22-32 °C. The activation enthalpy turns to be slightly negative at T>38 °C.     

Adsorption properties of complexes of chitosan with copper and zinc chlorides and copper sulfate

The adsorption properties of complexes of chitosan with copper and zinc chlorides and copper sulfate with respect to n-alkanes (C₆H₁₄-C₉H₂₀) and n-alcohols (CH₃OH-C₃H₇OH) were investigated by means inverse gas chromatography in the Henry region. It was found that the capability of the chitosan complexes for dispersion interaction is somewhat less pronounced in comparison with the initial chitosan. It was shown that complexation give rises to more ordered supramolecular structures, which make for an increase in the adsorption capacity of complexes with respect to n-alcohols. The small adsorption values measured were explained by the interaction of the n-alcohols only with weakly specific OH groups, since the NH₂ group is inaccessible because of its being involved in the coordination bonding with the metal ion.     

Calculation of the anisotropy of molecular polarizability of liquid <Emphasis Type="Italic">n</Emphasis>-alkanes and <Emphasis Type="Italic">n</Emphasis>-alcohols

Light scattering from liquid n-alkanes and n-alcohols in a strong electric field is measured by a photoelectric colorimeter at various wavelengths. The anisotropy of molecular polarizability of the substances is calculated by the Rayleigh formula.     

Valence isomerization in dendrimers by photo-induced electron transfer and energy transfer from the dendrimer backbone to the core

A series of poly(aryl ether) dendrimers with a norbornadiene (NBD) group attaching to the core (Gn-NBD), generations 1–4, were synthesized and characterized, and their photophysical and photochemical properties were examined. The fluorescence of the dendrimer backbone is quenched by the norbornadiene group as a result of the electron transfer and energy transfer from the dendrimer backbone to the norbornadiene group in Gn-NBD. Selective excitation of the dendrimer backbone results in an isomerization of the norbornadiene group to the quadricyclane (QC) group. The intramolecular electron transfer and energy transfer efficiencies are ca. 0.93, 0.73, 0.54, 0.30 in dichloromethane, and ca. 0.90, 0.70, 0.55, 0.34 in tetrahydrofuran for generations 1–4, respectively, with the rate constant ca. 10¹⁰ s⁻¹. The light-harvesting ability of these dendritic molecules is demonstrated by the enhanced valence isomerization rate of NBD to QC with increasing generation.     

Electron transfer,equilibrium, and protonation in the system of cis- and trans-stilbene in 2-propanol

Spectrophotometric pulse radiolysis experiments with cis- and trans-stilbene (S_c and S_t) in 2-propanol show that both isomers react with the solvated electron with a rate constant of 4.5 × 10⁹ M^?1 s^?1. The absorption spectra of the two anion radicals have maxima at 496 and 486 nm, respectively. The absorbances at 400–550 nm disappear exponentially corresponding to a pseudo first order protonation of the anion radicals. The rate constants for the protonation of the cis isomer is 6.4 × 10⁵ and of the trans isomer 0.7 × 10⁵ s^?1. In mixtures of cis- and trans-stilbene the electron transfer

has a forward rate constant of 9 × 10⁷ M^?1 s^?1 while the back reaction has a rate constant of 2.15 × 10⁷ M^?1 s^?1. An equilibrium constant K = 4.2 is calculated.     

π-Allymetal chemistry 8. Uncatalyzed cis-trans isomerization of n3-allyl (pentachloropehnyl) palladium (II) complex

Two geometrical isomers of an n³-crotyhlpalladium(II) complex, Pd(n³-crotyl)(C₆Cl₅)(PPh₃) have been found to undergo uncatalyzed mutual interconversion in benzene above room temperature. The rate constants of the isomerization have been determined by the ¹H NMR method at 45–67°C, and thermodynamic data calculated. The large decrease of the rate on adding free tripheylphosphine ligand is attributed to occurrence of a dissciative isomerization path involving a 3-coordinate intermediate. Reductive elimination of the complex takes place at much slower rates than the isomerization to afford MeCHCHCH₂C₆Cl₅ selectively.     

Synthesis and characterization of dendrons and dendrimers skeleton-constructed with azobenzene moiety

A series of dendrons and dendrimers skeleton-constructed with azobenzene moiety based upon 4-carboxy-4′-(1,2-propanediolether)-azobenzene as an AB₂ monomer, via a convergent approach, proceeding in a repeated stepwise growth manner starting from 4-carboxy-4′-(n-butylether)-azobenzene as a peripheral monomer, were synthesized, and characterized by NMR, FTIR, and MALDI-TOF-MS analysis. Their regular molecular architecture and thus monodispersed molecular weights were confirmed by GPC. The UV-Vis absorbance and ¹H NMR spectrum study indicated that the azobenzene moieties in CHCl₃ solution took fully trans-cis isomerization under UV irradiation, and reversely isomerization back to the trans by visible light irradiation or by heat.     

Radiation-induced Cis to Trans isomerization of poly(n-butylpropiolate) prepared with a [Rh(norbornadiene)Cl]2 complex as a stereospecific catalyst

Normal alkylpropiolate, i.e., n-butylpropiolate: HCCCOOnC₄H₉ was polymerized with [Rh(norbornadiene)Cl]₂ catalyst in methanol solvent to produce poly(n-butylpropiolate), (PBPA) polymer called polyacetylene esters in relatively high yields. The obtained polymer was characterized in detail using ¹HNMR, gel permeation chromatography (GPC), diffuse reflective UV methods. Consequently, the data showed that the resulting polymers selectively have cis-transoid form. We also found that γ-ray irradiation of the cis isomer resulted in isomerization to the trans isomer even at room temperature in air to generate the extended trans sequences which can stabilize mobile unpaired electrons as π-radicals called solitons.     

Mode-selective broadening in low-frequency vibrational modes of trans-stilbene van der waals complexes

Both RE2PI and LIF excitation spectra have been taken of several van der Waals complexes of trans-stilbene, p-methyl-, and m-methyl-trans-stilbene. The t-stilbene spectra reveal a strongly mode-dependent broadening of the van der Waals complex transitions associated with the low-frequency vibrations 82 and 95 cm⁻¹ above the origin. The fwhm of these peaks is three times as broad as any other feature in the spectrum. The source of the broadening appears to be fast vibrational relaxation from these modes to the van der Waals modes of the complex.     

Rhizopus arrhizus-mediated asymmetric reduction of arylalkanones: unusual anti-Prelong products with benzyl alkyl ketones

Rhizopus arrhizus-mediated microbial reduction of various aryl alkyl ketones afforded chiral carbinols in good yields and high enantiomeric purity. The most striking feature was the formation of the anti-Prelog (R)-alcohols with the benzyl alkyl ketones, while the other ketones ArXCOR (X = (CH₂)_n, n = 0 or 2, OCH₂ or SCH₂ and R = Me/Et/n-Bu) furnished (S)-alcohols.     

Viscosity dependent radiationless relaxation rate of cyanine dyes. A picosecond laser spectroscopy study

The viscosity dependent radiationless relaxation of several cyanine dyes has been studied by picosecond laser spectroscopy. It was found that the relaxation rate is proportional to η^?α. The value of α, however, is not constant for a certain dye molecule, but is strongly dependent on the kind of solvent used. In n-alcohols for instance α is typically about 1. In glycerol/methanol or glycerol/water mixtures on the other hand α ≈ 0.5. A comparison is made with literature data on orientational relaxation lifetimes of some dyes in similar solvents. It is shown that the radiationless relaxation of cyanine dyes and the orientational relaxation of for instance xanthene dyes changes in roughly the same way as the solvent is changed. This is taken as proof of the proposal that a torsional motion of the heterocyclic quinolyl rings is the main course of the viscosity dependent relaxation of the cyanine dyes studied.