Abstract: Controlling the stereochemistry of a polymer is highly desirable as this can affect its physical properties, such as its crystallinity, melting point, solubility and mechanical strength. Stereoregular polymers are normally prepared using expensive transition metal catalysts, which typically require demanding reaction conditions, and extending stereochemical-control to free radical polymerization has been a long sought goal. For monomers containing carbonyl groups certain Lewis acids have been shown to be capable of manipulating the stereochemistry, presumably via coordination to the polymer and/or monomer side chains so as to constrain their relative orientations during the propagation step. However, specific mechanistic details have yet to be clarified, and the degree of stereocontrol remains poor. To address these problems, we have been using computational chemistry, supported by polymerization experiments, to study the mechanism of stereocontrol in a variety of free-radical polymerization processes, and to predict the effect of solvents and novel control agents on polymer tacticity. Interestingly we have discovered that many Lewis acids do selectively coordinate to the terminal and penultimate radical side chains in a manner that should, in principle, facilitate control. However, this coordination is driven by the resulting stabilization of the propagating radical, which, ironically, deactivates it toward propagation. At the same time a less energetically favourable, non-controlling coordination mode involving the monomer side chain catalyzes propagation reaction and this provides the dominant reaction path. On this basis we suggest that simultaneous coordination to the monomer and propagating radical using bridging ligands or mixed Lewis acids may provide the way forward.

Abstract: The Fourier Grid Hamiltonian (FGH) Method is used to compute the vibrational eigenvalues and eigenfunctions of bound states of diatomic molecules. For these computations, the Hulburt and Hirschfelder (HH) potential model for diatomics is used. These potential energy functions are used for constructing and diagonalizing the molecular Hamiltonians. The vibrational wave functions for the ground and the excited states are used to calculate the Franck-Condon factors (FCFs), r-Centroids and average internuclear separations which play a significant role in determining the intensity of the bands in electronic transitions. The results of FCFs and r-Centroids for diatomic molecules such as H2, N2, CO, I2 and HF using the FGH method are compared with other methods. The FGH method provides an efficient and accurate alternative to calculate FCFs and other parameters that depend on the vibrational wavefunctions of the ground and exited electronic states. The Franck-Condon profiles indicate a strong correlation between the values of Franck-Condon factors and the mean internuclear separations for the corresponding transitions.

Abstract: The reaction in the human stomach when neutralizing acid with an antacid tablet is simulated and the evolution over time of the concentration of all chemical species present in the reaction medium is obtained. The values of the kinetic parameters of the chemical reaction can be determined by integrating the equation of the reaction rate. This is a classical optimization problem that can be approached with metaheuristic methods. The use of a parallel, parameterized scheme for metaheuristics facilitates the development of metaheuristics and their application. The unified scheme can also be used to implement hyperheuristics on top of parameterized metaheuristics, so selecting appropriate values for the metaheuristic parameters, and consequently the metaheuristic itself. The hyperheuristic approach provides satisfactory values for the metaheuristic parameters and, consequently, satisfactory metaheuristics for the problem of determining the kinetic constants.

Abstract: Molecular simulation describes fluid systems in detailed fashion. In general, they are more accurate and representative than equations of state. However, they require much more computational efforts. Several techniques have been developed in order to speed up Monte Carlo (MC) molecular simulations while preserving their precision. In particular, early rejection schemes are capable of reducing computational cost by reaching the rejection decision for the undesired MC trials at early stages. In this work, the introduced scheme is based on the fact that the energy due to interaction between any couple of Lennard-Jones (LJ) sites cannot be lower than a certain minimum energy that can be easily computed. It is called “non-conservative” as it generates slightly different Markov chains than the ones generated by the conventional algorithms. Nonetheless, the numerical experiments conducted show that these modifications are not significant, and both the proposed and the conventional methods converge to the same ensemble averages. In this study, the non-conservative scheme is first introduced and then compared to the conservative and bond formation early rejection schemes. The method was tested for LJ particles in canonical ensemble at several thermodynamic conditions. Results showed a relation between the thermodynamic conditions and the percentage of the CPU time saved. In principle, more CPU time was saved at conditions with high rejection rates for the MC trials. The non-conservative early rejection scheme was successful in saving more than 45 % of the CPU time needed by the conventional algorithms in canonical ensemble. Finally, this work presents an efficient early rejection method to accelerate MC molecular simulations which is easily extendable to other ensembles and complex molecules.

Abstract: A computational study of the compound containing selenium, 2-selenobarbituric acid, has been carried out. Tautomerism has been studied not only in neutral forms but also in the protonated and deprotonated species. The most stable tautomers for neutral and deprotonated species are equivalent to those obtained by different authors for the analogous barbituric and 2-thiobarbituric acids. However, the most stable tautomer for the protonated 2-selenobarbituric acid differs of that proposed for the analogous compounds. The enthalpy of formation in the gas phase, and the gas-phase acidity and basicity of 2-selenobarbituric acid have been calculated at the G3 and G4 levels, together with the corresponding values for barbituric and 2-thiobarbituric acids. The calculated acidity shows that 2-selenobarbituric acid is a very strong Brønsted acid in the gas phase.

Abstract: Detailed DFT computations and quantum dynamics simulations have been carried out to establish the origin of the extra stability of alloxan.. The effect of solvent, basis set and DFT methods have been examined. Two non-covalent intermolecular dimers of alloxan, namely the H-bonded and the dipolar dimers have been investigated to establish their relative stability. Quantum chemical topology features and NBO analysis have been performed.

Abstract: This study investigates the importance of the p-function used in the computational modeling. The geometric changes of ReH7(PMe3)2 system is used as the model compound. 6-31G, 6-311G and 6-311++G basis sets were used for all elements except Re, which used Christiansen et. al. basis set. Upon removing the p-function on metal, we noticed the geometric changes are minimal as long as triple-zeta basis sets are used for rest of elements. While the relative energy profile of a reaction would still reasonably assemble each other, a direct comparison in energy between the basis set with and without p-function is not recommended

Abstract: The geometry, energetics and dipole moment of the most stable conformers of cytosine in the ground state were calculated at different density functional methods, namely, B3LYP, M06-2X, ωB97-D and PEBPEB methods and the 6-311++G(3df,3pd) basis set. The most stable conformer, the keto-amino conformer is only 1 Kcal/mol more stable than the imino-enol form. The ultrafast radiationless decay mechanism has been theoretically investigated using Complete Active Space Multiconfiguration SCF calculation. The conical intersection seam was searched in the full dimensional space for the vibrational degrees of freedom. A new conical intersection has been identified, a semi-planar conical intersection (SPCI) with main deformations inside the cytosine ring and C=O bond. The g-vector and h-vector for the semi-planar conical intersection were calculated and discussed along with their geometrical parameters. A classical trajectory dynamic simulation has been performed to characterize and identify the evolution of geometry and energy changes of the SPCI with time.