Pre-equilibrium γ ray emission in different reaction mechanisms at 8 MeV/nucleon

Recent experimental results on particles-γ coincidence measurements on the systems ¹²C +⁶⁴Ni and ³⁵Cl +⁶⁴Ni at about 8 MeV/nucleon are interpreted as due to the effect of a Dipole pre-equilibrium emission produced during the damping of the proton-neutron relative collective motion in very deformed intermediate systems formed in the first instants of the collision. The pre-equilibrium effects are evaluated through semiclassic kinetic theories and through modified statistical approach to include non-stationary effects in Fusion processes, Massive Transfer reactions (in the ¹²C +⁶⁴Ni system) and in Binary Dissipative reactions (in the ³⁵Cl +⁶⁴Ni system). In particular the study performed on the dipole molecular component allows to establish a link between the above phenomenon and the charge and mass transfer process in quasi-peripheral reactions. Received: 15 April 1998 / Revised version: 8 June 1998     

Pre-equilibrium dipole excitation and γ-ray fragment angular correlation in the 32S +74Ge reaction

We report on the results obtained from the study of the ³²S +⁷⁴Ge deep inelastic reaction at incident energy E= 320 MeV. High-energy γ-rays were detected in an array of 6 seven-pack BaF₂ clusters. Coincidence with complex fragments detected in 12 three-stage telescopes ensured the selection of peripheral reaction events. In order to investigate the pre-equilibrium dipole strength excitation two independent analyses were performed. In the first analysis the energy spectra of the γ-rays were evaluated in the statistical model framework while in the second one the γ-ray fragment angular correlation with respect to the nuclear spin vector of the composite system was studied. Both methods indicate the excitation of dipole strength in the highly deformed dinucleus and provide dipole resonance parameter sets that are in good agreement with each other. Received: 24 March 1999 / Revised version: 17 May 1999     

Crystalline phase orientation in biaxially stretched isotactic polypropylene films

The different kinds of crystalline orientations which can occur in industrially relevant isotactic polypropylene (i-PP) films are compared with those generally observed for other semicrystalline polymers. The peculiar bimodality typical of the uniplanar-axial orientation which is present in two-step biaxially stretched i-PP films is rationalized in terms of the two alternative stackings on the (0k0) face of bilayers of macromolecules of the monoclinic unit cell.     

Chirality in the Absence of Rigid Stereogenic Elements: The Absolute Configuration of Residual Enantiomers of C3‐Symmetric Propellers

Two new tris(aryl)phosphane oxides existing as configurationally stable residual enantiomers have been synthesised and their racemates resolved by semipreparative HPLC on a chiral stationary phase (CSP HPLC). One of them, recognised as a conglomerate, could be resolved by fractional crystallisation at a preparative scale level. In this case, the absolute configuration of the propeller‐shaped molecule was determined by anomalous X‐ray scattering. The problem of the correlative assignment of the absolute configuration to all known C₃‐symmetric three‐bladed propeller‐shaped molecules existing as stable residual enantiomers is discussed. The configurational stability of the new chiral phosphane oxides and of the corresponding phosphanes was evaluated by CD signal decay kinetics and dynamic ¹H NMR spectroscopy. The racemisation barriers in phosphanes were found about 10 kcal mol^?1 lower than those found for the corresponding oxides, though geometry and inter‐ring gearing would be very similar in the two series. Configurational stability of residual tris(aryl)phosphanes was found to be influenced by the electronic availability of the phosphorus centre, as evaluated by electrochemical CV experiments.     

Base-displaced intercalated structure of the food mutagen 2-amino-3-methylimidazo[4,5-f]quinoline in the recognition sequence of the NarI restriction enzyme, a hotspot for -2 bp deletions

The solution structure of the oligodeoxynucleotide 5'-d(CTCGGCXCCATC)-3'.5'-d(GATGGCGCCGAG)-3' containing the heterocyclic amine 8-[(3-methyl-3H-imidazo[4,5-f]quinolin-2-yl)amino]-2'-deoxyguanosine adduct (IQ) at the third guanine in the NarI restriction sequence, a hot spot for -2 bp frameshifts, is reported. Molecular dynamics calculations restrained by distances derived from 24 (1)H NOEs between IQ and DNA, and torsion angles derived from (3)J couplings, yielded ensembles of structures in which the adducted guanine was displaced into the major groove with its glycosyl torsion angle in the syn conformation. One proton of its exocyclic amine was approximately 2.8 A from an oxygen of the 5' phosphodiester linkage, suggesting formation of a hydrogen bond. The carcinogen-guanine linkage was defined by torsion angles alpha' [N9-C8-N(IQ)-C2(IQ)] of 159 +/- 7 degrees and beta' [C8-N(IQ)-C2(IQ)-N3(IQ)] of -23 +/- 8 degrees . The complementary cytosine was also displaced into the major groove. This allowed IQ to intercalate between the flanking C.G base pairs. The disruption of Watson-Crick hydrogen bonding was corroborated by chemical-shift perturbations for base aromatic protons in the complementary strand opposite to the modified guanine. Chemical-shift perturbations were also observed for (31)P resonances corresponding to phosphodiester linkages flanking the adduct. The results confirmed that IQ adopted a base-displaced intercalated conformation in this sequence context but did not corroborate the formation of a hydrogen bond between the IQ quinoline nitrogen and the complementary dC [Elmquist, C. E.; Stover, J. S.; Wang, Z.; Rizzo, C. J. J. Am. Chem. Soc. 2004, 126, 11189-11201].     

Origin invariant approaches to the calculation of two-photon circular dichroism

The origin dependence of the expression of two-photon circular dichroism in the length formulation employed by the authors in a recent computational study [B. Jansik et al., Chem. Phys. Lett. 414, 461 (2005)] is discussed in detail, and some inherently origin invariant alternative formulations are introduced. Extensive computational tests on a small reference chiral system, namely, a chiral form of H(2)O(2), are performed at the density functional theory (DFT)/B3LYP level of theory with large diffuse correlation consistent basis sets. The results indicate that the velocity formulation originally proposed by Tinoco, Jr. [J. Chem. Phys. 62, 1006 (1975)] provides the most convenient approach for an origin invariant calculation of two-photon circular dichroism.     

The nuclear-spin-rotation constants of HCY, HSiY, and SiY(2) (Y=F, Cl): an ab initio study

The nuclear-spin-rotation constants of fluoro- (HCF) and chloro- (HCCl) carbene, of the corresponding silylenes (HSiF and HSiCl), and of difluoro- and dichlorosilylene (SiF(2) and SiCl(2)) are quantum-chemically investigated employing the coupled-cluster singles and doubles model augmented by a perturbative treatment of triple excitations together with various sequences of correlation-consistent basis sets. Theoretical best estimates are obtained through consideration of corrections for core correlation and of zero-point vibrational contributions. In addition, nuclear quadrupole coupling constants for the chlorine containing species are determined. A thorough comparison with experiment is made.     

Combined density functional/polarizable continuum model study of magnetochiral birefringence: can theory and experiment be brought to agreement?

The magnetic-field-induced axial birefringence (magnetochirality) of five closed-shell chiral molecules (three substituted oxiranes, carvone, and limonene) is studied at the density functional theory level using Becke's 3-parameter Lee-Yang-Parr functional and frequency-dependent quadratic response theory. The influence of the environment and the conformational distribution on the property is also studied. The environment effects are described by the polarizable continuum model in its integral-equation formulation. The effect of the conformational distribution is investigated by performing calculations on several conformers-for carvone and limonene-followed by Boltzmann averaging. The calculated values for the magnetochiral birefringence are compared to previous ab initio results and experimental data where available. The refined model presented here brings the ab initio values closer to experiment. Still, disagreements remain in some cases and it appears difficult to resolve these discrepancies.     

Advanced materials based on polymer cocrystalline forms

Polymeric “cocrystalline forms,” that is, structures were a polymeric host and a low‐molecular‐mass guest are cocrystallized, were early recognized, and in many cases also well characterized by X‐ray diffraction studies. However, only in the last two decades cocrystalline forms have received attention in material science, due to the ability (of few of them) to maintain an ordered polymer host structure even after guest removal, thus leading to the formation of “nanoporous‐crystalline forms,” for which many applications in the fields of molecular separation and sensors have been proposed. Moreover, in the last decade, an accurate control of the orientation of the polymer cocrystalline phases has been achieved, thus leading to a control of the orientation of the guest molecules, not only in the crystalline phase but also in macroscopic films. In addition, on the basis of this orientation control, in the last few years, cocrystalline films where active molecules are present as guests of polymer cocrystalline phases have been proposed for optical, magnetic and electric applications. In the last few years, it has been also discovered that polymer cocrystallization, when induced by nonracemic guest molecules, can produce stable chiral optical films. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012