Ersin Yurtsever

Comparison of hydrogen bonding in polydimethylsiloxane and polyether based urethane and urea copolymers

Hydrogen bonding in polydimethylsiloxane and polyether based urethane and urea type segmented copolymers was investigated by infrared spectroscopy, differential scanning calorimetry and quantum mechanical calculations. Hydrogen bonding in model urethane and urea compounds was compared with those of the copolymers, in order to determine the extent of interaction and resulting phase mixing between hard and soft segments in these copolymers. Quantum mechanical calculations were also used to determine the interaction energies due to hydrogen bonding in model urethane and urea compounds. Further, similar calculations were also performed to quantify the interactions between silicone and ether type soft segments, and urea and urethane type hard segments. As expected, these calculations clearly indicated the absence of any interaction between silicones and urea groups, while there was substantial hydrogen bonding between urea groups and the oxygen in the ether type soft segments. Results of FTIR studies and quantum mechanical calculations were in good agreement with thermomechanical behavior and mechanical properties of these copolymers.

Hydrogen bonding and polyurethane morphology. I. Quantum mechanical calculations of hydrogen bond energies and vibrational spectroscopy of model compounds

Abstract Advanced quantum mechanical calculations within ab initio molecular orbital theory and density functional theory were performed using gaussian 98 programs in quantitative determination of hydrogen bond (H-bond) energies between various model compound pairs. Model compounds studied contained functional groups or segments that were similar to those in segmented polyurethanes and polyureas. These model compounds included urea, 1,3-dimethylurea, 1,3-dimethylcarbamate, diethyl ether, methyl acetate and ethyl alcohol. Optimized conformations, H-bond energies and H-bond lengths of the complexes were determined. Quantum mechanical calculations indicated that based on relative magnitudes of H-bond energies, appreciable amount of phase mixing between hard and soft segments in polyether or polyester based polyurethanes and polyureas should be expected. Vibrational spectra of individual compounds and their hydrogen-bonded complexes (with themselves and other compounds) were determined. Correlation between theoretical and experimental spectra was found to be very good.

Hydrogen bonding and polyurethane morphology. II. Spectroscopic, thermal and crystallization behavior of polyether blends with 1,3-dimethylurea and a model urethane compound

Abstract Thermal, structural and spectroscopic behavior of the blends of poly(ethylene oxide)glycol (PEO) with a model urethane compound bis(4-butylcarbamatocyclohexyl)methane and 1,3-dimethylurea (DMU) were investigated by differential scanning calorimetry (DSC) and hot-stage optical microscopy (HOM). Blends with a wide range of compositions were prepared in tetrahydrofuran (THF) solutions and dried. DSC results indicated the formation of two-phase structures consisting of a pure polyether phase and a highly mixed DMU–polyether phase. As the amount of polyether in the blends was increased, the melting endotherm of DMU became much broader and shifted to lower temperatures, indicating extensive mixing with PEO. The mixed phase was also crystalline. This was strongly supported by HOM results. While pure PEO and DMU crystals showed spherulitic structures, mixed DMU–PEO phase showed fibrillar crystals. Consecutive heating–cooling cycles of the blends did not result in any changes in the blend morphologies. Formation of strong hydrogen bonding between DMU and PEO was also demonstrated by FTIR spectroscopy from the shifts in (N–H and CO) absorption peaks.

A dft study of polymerization mechanisms of indole

Abstract Polymerization of unsubstituted indoles is studied by accurate density functional theory calculations. Relative stability of all possible dimers of indole is computed in order to understand the thermodynamics of polymerization. It is observed that 2-position is the most likely site to enhance polymerization. A selected set of trimers and tetramers which use a 2-position for linkages are generated to understand the further growth of polyindole. A study of local minima arising from different distributions of the torsional angles reveals that there are two equally probable conformations and the one with the torsional angle changing signs alternatively is slightly favored. The cyclic structures are also investigated and it is shown that it is possible to generate stable three- and four-membered cyclic structures. Finally, the structures of radical cations and intermediate states are fully optimized and the energetics of these metastable species are used to explain the competing mechanisms of radical–radical and radical–neutral pathways.

Composition-induced structural transitions in mixed rare-gas clusters

The low-energy structures of mixed Ar-Xe and Kr-Xe Lennard-Jones clusters are investigated using a newly developed parallel Monte Carlo minimization algorithm with specific exchange moves between particles or trajectories. Tests on the 13- and 19-atom clusters show a significant improvement over the conventional basin-hopping method, the average search length being reduced by more than one order of magnitude. The method is applied to the more difficult case of the 38-atom cluster, for which the homogeneous clusters have a truncated octahedral shape. It is found that alloys of dissimilar elements sAr-Xed favor polytetrahedral geometries over octahedra due to the reduced strain penalty. Conversely, octahedra are even more stable in Kr-Xe alloys than in Kr38 or Xe38, and they show a core-surface phase separation behavior. These trends are indeed also observed and further analysed on the 55-atom cluster. Finally, we correlate the relative stability of cubic structures in these clusters to the glass forming character of the bulk mixtures.

Bosonic helium droplets with cationic impurities : Onset of electrostriction and snowball effects from quantum calculations

Variational Monte Carlo and diffusion Monte Carlo calculations have been carried out for cations such as Li(+), Na(+), and K(+) as dopants of small helium clusters over a range of cluster sizes up to about 12 solvent atoms. The interaction has been modeled through a sum-of-potential picture that disregards higher order effects beyond atom-atom and atom-ion contributions. The latter were obtained from highly correlated ab initio calculations over a broad range of interatomic distances. This study focuses on two of the most striking features of the microsolvation in a quantum solvent of a cationic dopant: electrostriction and snowball effects. They are discussed here in detail and in relation with the nanoscopic properties of the interaction forces at play within a fully quantum picture of the cluster features.

The orthogonal gradient method. A simple method to solve the closed‐shell, open‐shell, and multiconfiguration SCF equations

A new, simple and efficient method of solving closed‐shell, open‐shell, and multiconfiguration SCF orbital equations is presented. It is based on the orthogonal gradient approach introduced originally in conjunction with the maximum overlap principle. Semiempirical test calculations show that the method converges rapidly.

Structural studies of polythiophenes

A theoretical investigation of the electronic structures of the oligomers of thiophene (T) and their derivatives, namely, 2-methylthiophene (2MT), 3-methylthiophene (3MT), 2-cyanothiophene (2CT) and 3-cyanothiophene (3CT), are presented. The most stable forms of the monomer, dimer, trimer and tetramers are obtained by the semi-empirical quantum mechanical methodology using AM1 parametrization. All possible binding sites are investigated in order to understand the bonding in polythiophenes. The 3-substituted molecules form dimers which lie nearly at trans-planar positions but 2CT and 2MT favor cis conformations. The activation energies between these structures and local minima change from 0.2 to 1.8 kcal/mol. Methyl- and cyano-substituted polythiophenes favor linear growth mechanisms. The branching requires 3-6 kcal/mol per substitution. Polythiophenes seem to form fairly flexible chains as understood from the relatively low rotational barriers.

π-Stack Dimers of Small Polyaromatic Hydrocarbons: A Path to the Packing of Graphenes

MP2 calculations of the stacking energy are reported for the dimers of a set of polycyclic aromatic hydrocarbons. The interaction strengths and their dependence on the shape-dependent measures as well as the aromatic character of the monomer are studied. For small systems involving four to six rings, the noncovalent interactions seem to be independent of the shape of the monomers. The most preferred conformations for parallel stacked dimers are not aligned exactly but off-center with small shifts; however, these shifts are on the order of 1 A, and the energy necessary to keep them aligned is less than 0.5 kcal/mol per ring. Small-angle rotations within the molecular planes also do not require much energy, and in some cases they lead to stronger interactions.

Frontiers of chemical dynamics

Theoretical Overview of Chemical Dynamics W.H. Miller. Photodissociation of Weak Bonds: the Spectroscopy and Vibrational Dynamics of Molecular Complex R.E. Miller. Molecular Photoionization Dynamics at Threshold Energies M.G. White. Molecular Interactions from the Dynamical Behaviour of Polyatomic Gaseous Mixtures F.A. Gianturco. Resonances in Molecular Dynamics O. Atabek. Molecules in Laser Fields A.D. Bandrauk. Coherent and Incoherent Laser Control of Photochemical Events M. Shapiro, P. Brumer. Adaptive Feedback Control of Molecular Motion H. Rabitz. Information Theory Approach in Chemical Dynamics R.D. Levine. Trends in Molecular Dynamics Simulation Technique J. Brickmann, et al. Subspace Molecular Dynamics for Long Time Phenomena A. Askar. Vibrational Dynamics at the Adsorbate-Substrate Interface T. Uzer, J.T. Muckerman. Time-Dependent Nuclear Dynamics of Decaying States L.S. Cederbaum, F. Tarantelli. Collisional Energy Transfer New Light on an Old Problem A.J. McCaffery. Angular Momentum Projection in Quantum Molecular Dynamics L. Lathouwers. Three Particle Systems and Hyperspherical Harmonics J. Hinze, et al. Dynamics and Spectroscopy of Highly Excited Molecules F. Borondo, R.M. Benito. Mixed Mode Dynamics within the TDSCF Approximation E. Yurtsever, J. Brickmann.