Simon P. Webb

An Effective Fragment Method for Modeling Solvent Effects in Quantum Mechanical Calculations

An effective fragment model is developed to treat solvent effects on chemical properties and reactions. The solvent, which might consist of discrete water molecules, protein, or other material, is treated explicitly using a model potential that incorporates electrostatics, polarization, and exchange repulsion effects. The solute, which one can most generally envision as including some number of solvent molecules as well, is treated in a fully ab initio manner, using an appropriate level of electronic structure theory. In addition to the fragment model itself, formulae are presented that permit the determination of analytic energy gradients and, therefore, numerically determined energy second derivatives (hessians) for the complete system. Initial tests of the model for the water dimer and water‐formamide are in good agreement with fully ab initio calculations.

Solvent-induced frequency shifts: configuration interaction singles combined with the effective fragment potential method.

The simplest variational method for treating electronic excited states, configuration interaction with single excitations (CIS), has been interfaced with the effective fragment potential (EFP) method to provide an effective and computationally efficient approach for studying the qualitative effects of solvents on the electronic spectra of molecules. Three different approaches for interfacing a non-self-consistent field (SCF) excited-state quantum mechanics (QM) method and the EFP method are discussed. The most sophisticated and complex approach (termed fully self consistent) calculates the excited-state electron density with fully self-consistent accounting for the polarization (induction) energy of effective fragments. The simplest approach (method 1) includes a strategy that indirectly adds the EFP perturbation to the CIS wave function and energy via modified Hartree-Fock molecular orbitals, so that there is no direct EFP interaction with the excited-state density. An intermediate approach (method 2) accomplishes the latter in a noniterative perturbative manner. Theoretical descriptions of the three approaches are presented, and test results of solvent-induced shifts using methods 1 and 2 are compared with fully ab initio values. These comparisons illustrate that, at least for the test cases examined here, modification of the ground-state Hartree-Fock orbitals is the largest and most important factor in the calculated solvent-induced shifts. Method 1 is then employed to study the aqueous solvation of coumarin 151 and compared with experimental measurements.

SOLVATION AND THE EXCITED STATES OF FORMAMIDE

Excited state geometries of formamide have been explored using the multiconfiguration self-consistent-field method. Optimized equilibrium geometries for the S1 and T1 states are nonplanar with the C–O and C–N bond distances substantially increased from the ground state values. The excitation energies at the ground and excited state geometries are calculated to vary dramatically with nonplanar rotation. Raman scattering from the S2 state depends on the transition moment which is shown to vary strongly with geometry. Experimental analyses that project out restricted planar conformations can fit the Raman vibrational pattern but do not inform us about the complicated energy surface for the S2 state which is a resonance embedded in a Rydberg series. Constrained optimizations are used to explore this surface and the variation in the oscillator strength with geometry. Effective fragment potentials (EFP) model the waters in the solvation models. Comparison of the EFP and all-electron structures and energy of bin...

Synthesis of Hydroxy and Methoxy Perylene Quinones, Their Spectroscopic and Computational Characterization, and Their Antiviral Activity¶

Abstract Hydroxy and methoxy perylene quinones are synthesized in an attempt to isolate the essential spectroscopic and biological features of light-induced antiviral agents such as hypericin and hypocrellin. Unlike their naturally occurring counterparts, these synthetic quinones bear the carbonyl, hydroxyl, and methoxy groups in the “bay region.” The hydroxy and methoxy compounds have rich absorption spectra with broad features in the visible (∼450–800 nm) and relatively more intense and narrow features at wavelengths ≤350 nm. High-level ab initio quantum mechanical calculations assign the features in the absorption spectra to electronic transitions from S0 to S2 and to higher-lying electronic states. The calculations indicate that in the ground state the trans dihydroxy isomer is 12.5 kcal/mol lower in energy than the cis dihydroxy isomer and is thus the only species present. The lowest-energy trans methoxy ground state isomer and the lowest-energy cis methoxy ground state isomer are found to be degenerate. An additional cis methoxy isomer 6.3 kcal/mol higher in energy than the global minimum is assumed to contribute to the spectrum and is also considered. Finally, the synthetic compounds exhibit similar light-induced antiviral activity to each other, but significantly less than that of hypericin.

The Effect of Spin-Orbit Coupling on the Magnetic Properties of H2Ti(μ–H)2TiH2

Excited states of singlet and triplet H2Ti(μ−H)2TiH2 have been calculated using multiconfigurational wave functions. The effects of orbital relaxation are determined by optimizing orbitals for all states separately and comparing to state-averaged calculations, and are found to be small. Dynamic electron correlation included through second-order perturbation theory is found to have a considerable effect on excited state relative energies, but not on the ordering of states. Spin-orbit coupling effects are introduced by a one-electron operator which uses an effective nuclear charge to replace two-electron effects. The resulting splittings of the lowest energy triplet state components are 0.027 and 0.199 cm−1, respectively. The former is due to the angular momentum operator which acts along the Ti–Ti axis; the latter is due almost entirely to the angular momentum operator which acts in the direction perpendicular to the plane of the Ti–H–Ti bridge. An overall ferromagnetic effect of 0.660 cm−1 on the ground s...