Tina D. Poulsen

The combined multiconfigurational self-consistent-field/molecular mechanics wave function approach

We present theory and implementation for a new approach for studying solvent effects: the multiconfigurational self-consistent-field/molecular mechanics method. With this method it is possible to describe ground, excited, and ionized states of molecules in solution. The approach is tested by investigating the effect of solvent on H2O in aqueous solution. For the calculated energies we find that polarization effects are significant.

Linear response properties for solvated molecules described by a combined multiconfigurational self-consistent-field/molecular mechanics model

A multiconfigurational self-consistent-field/molecular mechanics (MC-SCF/MM) linear response method that includes polarization effects is presented for calculating molecular properties of solvated systems. Accessible properties include frequency-dependent molecular polarizabilities, electronic excitation energies, and transition moments. Sample calculations are presented for H 2 O in aqueous solution.

A quantum mechanical method for calculating nonlinear optical properties of condensed phase molecules coupled to a molecular mechanics field: A quadratic multiconfigurational self-consistent-field/molecular mechanics response method

An approach for determining nonlinear optical properties within a quantum mechanics/molecular mechanics method is presented. The response equations in the multiconfigurational self-consistent-field/molecular mechanics approach that includes polarization effects are derived and implemented for second order in response theory. The method is employed to calculate frequency-dependent first hyperpolarizabilities and two-photon absorption properties for H2O in aqueous solution. The results are in close agreement with experimental measurements.

Radiative transitions of singlet oxygen: New tools, new techniques and new interpretations

The near‐IR phosphorescence of singlet delta oxygen, O2(a1Δg), has provided a wealth of information since it was first observed in solution‐phase systems. The techniques employed and the quality of the data obtained have unproved significantly over the years that, in turn, presently makes it possible to address a wide variety of problems using both steady‐state and time‐resolved measurements. The development of spectroscopic methods to monitor other transitions in oxygen, specifically those that involve the singlet sigma state, O2(b1σg+), and the incorporation of high‐level computational methods provides access to an even broader range of fundamental issues. The expertise presently available to monitor radiative transitions in oxygen, coupled with the current understanding of the effect of solvent on these transitions as achieved through state‐of‐the‐art theoretical modeling makes it possible to consider the next step forward: the incorporation of spatial resolution and the construction of the singlet oxygen microscope.

The photoisomerization of aqueous ICN studied by subpicosecond transient absorption spectroscopy

The photolysis of aqueous ICN is studied by transient absorption spectroscopy covering the spectral range from 227 to 714 nm with 0.5 ps time resolution. The experimental data show that when ICN(aq) is photolyzed at 266 nm, it dissociates into I and CN and both the I(2P3/2) and I(2P1/2) channels are populated. Approximately half the fragments escape the solvent cage while the remainder recombines within the solvent cage during the first picosecond. The majority of the recombinations form ICN while only a minor fraction produces the metastable INC isomer. INC and ICN relax to the vibrational ground state within 1 ps in good agreement with theoretical estimates based on the golden rule formalism as well as molecular dynamics simulations. Diffusive recombination involving fragments that have escaped the solvent cage further reduces the quantum yield of I and CN to 10% during the following 100 ps. This recombination produces exclusively ICN.

Singlet Sigma: The “Other” Singlet Oxygen in Solution

The lowest excited electronic state of molecular oxygen, O2(a1‐DLg), is often called simply singlet oxygen. This singlet delta state is an acknowledged and well‐studied intermediate in many solution‐phase photosystems. However, the second excited electronic state of oxygen, O2(b1δg+), is also a singlet. It has recently become possible to monitor this singlet sigma state in solution, which, in combination with studies of the singlet delta state, contributes to a better understanding of a variety of general problems in chemistry.

MP2 correlation effects upon the electronic and vibrational properties of polyyne

The linear infinite periodic chain of carbon atoms (polyyne) is studied at the MP2 level employing the crystal orbital approach. The equilibrium structure, the bond length alternation, the energy band gap, the Young Modulus, the force constants, the vibrational frequencies, and the phonon dispersion curves are determined and compared to Hartree–Fock results.

Quadratic response of molecules in a nonequilibrium and equilibrium solvation model: Generalizations to include both singlet and triplet perturbations

Quadratic response theory for equilibrium and nonequilibrium solvation has been extended to include both singlet and triplet perturbations. The approach is tested by investigating the effect of solvent on the phosphorescence lifetime of formaldehyde.

Unrestricted Hartree–Fock band structure calculations for polymers: Application to a cross-talk system

Unrestricted Hartree–Fock calculations for a one-dimensional infinite periodic system have been employed to characterize a cross-talk system between trans-1,4-polybutadiene and a small molecule, O2. The total energy, the energy band structure, and the longitudinal linear polarizability have been investigated. The presence of O2 has been found to influence in a quantitatively as well as a qualitative way the energy band structure of polybutadiene.