Solvent Induced Shifts in the UV Spectrum of Amides

Solvent effects on the electronic spectra of formamide and trans-N-methylacetamide are studied using four different levels of theory: singly excited configuration interaction (CIS), equations of motion coupled-cluster theory with singles and doubles (EOM-CCSD), completely renormalized coupled-cluster theory with singles and doubles with perturbative triple excitations (CR-EOM-CCSD(T)), and time-dependent density functional theory (TDDFT), employing small clusters of water molecules. The simulated electronic spectrum is obtained via molecular dynamics simulations with 100 waters modeled with the effective fragment potential method and exhibits a blue-shift and red-shift, respectively, for the n → π* and πnb → π* vertical excitation energies, in good agreement with the experimental electronic spectra of amides

Quantum Mechanical Treatment of Variable Molecular Composition: From "Alchemical" Changes of State Functions to Rational Compound Design

"Alchemical" interpolation paths, i.e.~coupling systems along fictitious paths that without realistic correspondence, are frequently used within materials and molecular modeling and simulation protocols for the estimation of relative changes in state functions such as free energies. We discuss alchemical changes in the context of quantum chemistry, and present illustrative numerical results for the changes of HOMO eigenvalues of the He atom due to a linear alchemical teleportation---the simultaneous annihilation and creation of nuclear charges at different locations. To demonstrate the predictive power of alchemical first order derivatives (Hellmann-Feynman) the covalent bond potential of hydrogen fluoride and hydrogen chloride is investigated, as well as the van-der-Waals binding in the water-water and water-hydrogen fluoride dimer, respectively. Based on converged electron densities for one configuration, the versatility of alchemical derivatives is exemplified for the screening of entire binding potentials with reasonable accuracy. Finally, we discuss constraints for the identification of non-linear coupling potentials for which the energy's Hellmann-Feynman derivative will yield accurate predictions

Doctor of Philosophy

dissertationA new quantum chemistry-based, atomic point polarizable dipole potential was developed for molecular dynamic (MD) simulations of poly (ethylene oxide) (PEO) and poly (propylene oxide) (PPO) aqueous solutions employing a modified version of a single water molecule with four interaction sites and Drude polarizability (SWM4-DP). A twoextended charge ether model has been chosen as best describing electrostatic potential of DME. Ether/water interactions were parameterized to reproduce the binding energy of water with 1,2-dimethoxyethane (DME) that was determined from high-level quantum chemistry calculations. The DME/water nonbonded parameters were found to be transferrable to 1,2-dimethoxypropane (DMP). An accuracy of the developed force field was justified by comparing thermodynamics properties obtained from molecular dynamics simulations with experimental data including free energy, enthalpy, and entropy of DME solvation. Free energy of DME solvation in water was obtained employing a new interface transit method (ITM) followed by calculations using perturbation theory. Simulations of DME/water solutions at room temperature using the new polarizable force field yielded enthalpy of solvation in a good agreement with experiment. Simulations of PEO/water and PPO/water solutions improved ability of the new force field to capture, at least qualitatively, low critical solution temperature (LCST) behavior in these solutions. The predicted miscibility of PEO and water as a function of temperature was found to be strongly correlated with the predicted free energy of solvation of DME in water for the various force fields investigated. Intermolecular pair correlations are employed to analyze phase behavior of nonionic polymers in aqueous solution