The cooperative model for orientational defects in ice: Continuum approach

A kink-solitonic model of orientational defects in the hydrogen-bonded network of ice is proposed within the framework of the continuum approach for an infinite quasi-one-dimensional chain, (H₂O)∞, of water molecules.     

Structure and dynamics of orientational defects in ice I

Orientational defects in hexagonal ice were investigated using molecular dynamics simulations. Energy relaxation during L- and D-defect migration was shown to be associated with improved alignment of water molecules along the local electric fields. Two new forms of defects, an "L+D complex," and a "5+7 defect," were characterized. These forms appear in ice trajectories close to the melting point, and in the course of L- and D-pair recombination process. Defect pair recombination was shown to be a complex process, involving collective H-bond changes in groups of molecules.     

The photodimerization of aceanthrylene

Sensitized photodimerization of aceanthrylene (AA) with Rose Bengal in methanol produced both syn and anti head-to-head and head-to-tail stereoisomers with slight preference for syn adducts. The stereochemical configurations were assigned by deuterium labeling and by high field homonuclear shift correlation (COSY) experiments and these compare favorably with the assignments made for the photodimers of 1-substituted acenaphthylenes. Currently, AA has photodimerized under direct irradiation only in carbon disulfide.     

Studies on anthracenes. 1. Human telomerase inhibition and lipid peroxidation of 9-acyloxy 1,5-dichloroanthracene derivatives

The synthetically useful approaches to 9-acyloxy 1,5-dichloroanthracene derivatives are reported. The system selectively reduces the carbonyl group flanked by the peri substituents of the anthracenediones to give the corresponding 1,5-dichloro-9(10H)-anthracenone. Simple regioselective acylation of anthracenone is applied with appropriate acyl chlorides in CH2Cl2 with catalytic amount of pyridine to give the novel 9-acyloxy 1,5-dichloroanthracene derivatives. Considerable interest has developed in the mechanism of how anthracenone achieves this desirable selectivity. In an attempt to understand the mechanism of this reaction, solid-state structures of anthracene derivatives have been obtained. In addition, the inhibition of lipid peroxidation in model membranes was determined as was their ability to inhibit the telomere-addition function of the human telomerase enzyme together with their inhibition of the Taq polymerase enzyme. In contrast to (+)-alpha-tocopherol, 3b, 3c, 3d, 3g, and 3i do not enhance lipid peroxidation in model membranes. Implications for 9-acyloxy 1,5-dichloroanthracene analogues as potential anticancer agents are discussed.     

Selectivity in the photodimerization of 6-alkylcoumarins

Coumarin and 6-alkylcoumarins (alkyl = C(1) to C(16)) were photodimerized in homogeneous solvents differing in polarity and in aqueous micellar solutions. The four possible photodimers, syn head-to-head (hh), anti head-to-head, syn head-to-tail (ht), and anti head-to-tail, were identified through a combination of X-ray analysis and NMR spectroscopy. In 6-methylcoumarin the concentration-corrected dimerization (quantum) yield increases with decreasing concentration of the educt; anti-hh was formed exclusively in nonpolar solvents and upon triplet sensitization and was the main product under all conditions except for ionic micellar systems, which direct to preferred syn-hh dimerization. Long alkyl substituents, however, lead to anti-hh in polar solvents and in micelles, too. Predominating ht dimer formation was observed for nonsubstituted coumarin in polar solvents only. Thus, syn/anti and hh/ht selectivity can be steered by varying the 6-alkyl substituent. Syn-hh photodimers of 6-methylcoumarin can be photochemically split into the monomers; they partly proved thermally unstable against acids, bases, methanol, and on SiO(2) surfaces.     

Investigation of the excimer dynamics in 9,10-dichloroanthracene crystals using picosecond spectroscopy

The dynamics of excimer formation, excimer migration, and excimer dissociation in β-9,10-dichloroanthracene crystals were investigated by means of picosecond time-resolved fluorescence spectroscopy. In the temperature range from T = 20 K to T = 40 K we were able to temporally resolve the relaxation into the excimer state. The excimer formation rates were determined to be k ≈ 1.8 × 10¹¹ s^?1 at T = 40 K and k ≈ 2.0 × 10¹⁰ s^?1 at T<30 K. The excimer migration was investigated by measuring the excimer annihilation rate as a function of temperature. At room temperature 20% of the excimers are dissociated. The excimer binding energy is estimated to be B = 1360 cm^?1. The experimental results are explained in terms of a kinetic scheme comprising the population and depopulation of exciton, trap, and excimer states. The nature of the trap state is identified and it is shown that thermal activation of a 25 cm^?1 librational mode induces the relaxation of the trap into the excimer state.     

The role of neutral defects in the structural chemistry of liquid water

For defective water associates with an extra hydrogen atom (n-defective associates), size and structure effects on ionization potentials compared to defectless (normal) associates of similar structures have been investigated by quantum chemical and molecular mechanics methods. The ionization potentials of small n defective water associates increase (from fractions of eV to 7 eV–8 eV) with the number of water molecules in the associate. These are most probably the source of a hydrated electron that are responsible for the ensuing equilibrium between all defects in liquid water: neutral n and p defects and ion defects (H _aq ⁺ , OH _aq ^– , e _aq ^– ). Delocalization of an odd electron in defective associates stabilizes the latter and promotes their recombinations, forming hydrated water molecules, hydrogen peroxide, and gaseous hydrogen. Structural instability of fullerene (H₂O)₂₀ relative to normal associates has been found. This compound is stabilized by the endo inclusion of a hydrogen atom, and exo fixation of the hydrogen atom gives rise to an extra source of hydrated electrons.Original Russian Text Copyright © 2004 by G. A. Domrachev, D. A. Selivanovskii, E. G. Domracheva, L. G. Domracheva, A. I. Lazarev, P. A. Stunzhas, S. F. Shishkanov, and V. L. VaksTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 4, pp. 670–677, July–August, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Effect of pressure on the luminescence of 9,10-dichloroanthracene crystals

Luminescence spectra of β-9,10-dichloroanthracene have been studied at room temperature in the pressure range ≈ 0–10 GPa. The initial shift to lower energies of the excimer emission is linear (≈500 cm⁻¹/GPa) up to ≈3 GPa. Above this pressure a sharper emission grows at the high-energy side at the expense of the excimer luminescence. The suppression of the excimer state at pressures above ≈ 7 GPa is attributed to the strong repulsive interactions between the molecular pair. The new emission is thought to arise from a more loosely bound configuration in the excited state which could originate from a crystal phase change around 3 GPa.     

The role of vacancy defects and holes in the fracture of carbon nanotubes

We present quantum mechanical calculations using density functional theory and semiempirical methods, and molecular mechanics (MM) calculations with a Tersoff–Brenner potential that explore the role of vacancy defects in the fracture of carbon nanotubes under axial tension. These methods show reasonable agreement, although the MM scheme systematically underestimates fracture strengths. One- and two-atom vacancy defects are observed to reduce failure stresses by as much as 26% and markedly reduce failure strains. Large holes – such as might be introduced via oxidative purification processes – greatly reduce strength, and this provides an explanation for the extant theoretical–experimental discrepancies.     

Theoretical study of the photodimerization of pyrimidine bases

The experimental results are correctly interpreted as from the delocalized model of the transition state and the theory of perturbations. The surfaces of the energy of interaction between both molecules in function of different coordinates of reaction make it possible to discuss the concerted nature of photodimerization and permit the two conjugated systems to be added between carbon atoms 5 and 6 of both molecules.

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Zusammenfassung Die experimentellen Resultate werden mit Hilfe eines delokalisierten Modells des Übergangszustandes und der Störungstheorie richtig interpretiert. Die Energiehyperfläche der Wechselwirkung zwischen den beiden Molekülen als Funktion verschiedener Reaktionskoordinaten erlaubt die Diskussion des konzertierten Ablaufs der Photodimerisation und der Addition der beiden konjugierten Systeme, die zwischen den Kohlenstoffatomen 5 und 6 der beiden Moleküle stattfindet.

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Résumé Les résultats expérimentaux sont correctement interprétés à partir du modèle delocalisé de l'état de transition et de la théorie des perturbations. Les surfaces de l'énergie d'interaction entre les deux molécules en fonction aux différentes coordonnées de réaction permettent de discuter le caractère concerté de la photodimerisation et que l'addition des deux systèmes conjugués se réalise entre les carbones 5 et 6 des deux molécules.

     

Temperature dependence of the hole mobility in the alpha phase of 9,10-dichloroanthracene

The mobility of holes in the alpha of 9,10-dichloroanthracene is isotropic with a value of 300 K of 3.8 ± 0.2 cm² V^?1 s^?1 and a temperature dependence given by T^?n where n = 2.4.     

Unsymmetrical photodimerization of a 9-aminomethylanthracene in the crystalline salt

By the salt formation with particular nonaromatic dicarboxylic acids, rapid and selective photodimerization of 9-(N,N-dimethylaminomethyl)anthracene (1) was accomplished in the solid state. For instance, the salt with trans,trans-muconic acid or acetylenedicarboxylic acid was led quantitatively to the 9,10:4',1' photodimer usy-ht-2, the first example of the unsymmetrical [4+4] photodimerization of anthracene in the solid state. The reactions were rationalized by the relevant C...C distances between the reacting carbons.     

Exploring two-state reaction pathways in the photodimerization of cyclohexadiene.

The ground- (S0) and lowest triplet-state (T1) pathways associated with dimerization of cyclohexadiene to give [2+2] and [4+2] cycloadducts have been theoretically studied at the UBLYP and UB3LYP levels of theory with the 6-31G* basis set. The DFT energies were validated by CCSD(T) single-point energy calculations. These cycloaddition reactions follow stepwise mechanisms with formation of bis-allylic biradical (BB) intermediates. In the S0 ground state, the interaction between two cyclohexadiene molecules with formation of BB intermediate IN(S0) has a large activation enthalpy of 32.0 kcal mol(-1). On the other hand, C-C bond-formation in the lowest triplet state (T1) leading to BB intermediate IN(T1) has a low activation enthalpy of 5.0 kcal mol(-1), but the subsequent ring closure involves a very large activation enthalpy of 43.4 kcal mol(-1). Triplet-to-singlet intersystem crossing from IN(T1) to IN(S0) favors cyclization to give the corresponding [2+2] and [4+2] cycloadducts.     

The orientational state of uniaxially stretched PMMA

Uniaxially stretched samples of PMMA were investigated by Brillouin Spectroscopy (BS). From the velocity of hypersound we could determine most of the elastic constants. Using a recently developed analysis [1] it is demonstrated that the properties of this polymer can be well described by the aggregate model. This result offers the possibility of mapping the mechanical properties by birefringence measurements. The dependence of the fourth momentP ₄ on the second momentP ₂ is identical with that determined for PC [2] and follows, in the measured range, that of an affine orientational state. Nevertheless, the dependence on the stretching ratio differs for different molecular weights. Thus the partition of the deformation into an orientational and an elongational contribution, as has been proposed [3], seems to be well founded.The partition depends on the stretching conditions.     

The orientational ordering of benzyl alcohol in phospholipid bilayers

The α-methylene segment of benzyl alcohol has been deuterated at one site and the proton—deuterium dipole spin interaction measured for this compound dissolved in bilayers of dimyristoylphosphatidylcholine. These data, when combined with measured deuterium quadrupole interactions at other sites, allow for a more complete understanding of the orientational ordering and conformation of benzyl alcohol in the bilayers. Models for the rotary motion of the ring are discussed.     

The dynamics of the intramolecular and orientational motions of pinacyanol

The influence of viscosity on the electronic relaxation of pinacyanol in alcoholic solutions has been determined by recording, with picosecond and nanosecond resolution, the fluorescence decay at various temperatures. The results are interpreted with the aid of a model of electronic relaxation implying a molecular conformational change with no internal potential barrier and with a Boltzmann equilibrium distribution of the initial configuration. Furthermore, fluorescence depolarization analysis indicates that the reorientational relaxation of the molecule takes place on a time scale much greater than that characterizing the intramolecular geometrical change.