The electronic structure of polydiacetylenes: cyclic versus linear models

The electronic structure of polydiacetylenes is investigated with SCF calculations as a function of backbone structure and chain length. The first dipole-allowed excitation (band gap) is red-shifted for the transformation from polydiacetylene (PDA) to polybutatriene (PBT) for cyclic chains The band gap for linear chains is blue-shifted for short chains, for linear chains containing 36 (or more) carbon atoms the band gap is red-shifted.     

Quantitative detection of singlet O2 by cavity-enhanced absorption

A method for the practical determination of the absolute concentration of single (a1delta(g)) oxygen is discussed. The method is based on sensitive off-axis integrated-cavity-output spectroscopy (ICOS). Off-axis ICOS allows narrowband, continuous-wave lasers to be used in conjunction with optical cavities to record sensitive absorption measurements. The details of the method as well as spectroscopic data confirming the first observation of the (1, 0) band of the b1sigma(g)(+) - a1delta(g) Noxon system are presented. The absolute transition probabilities for the b1sigma(g)(+) - a1delta(g) Noxon system, which are not known precisely from experiments, are determined by quantum chemistry theory.     

Visual spatial memory is enhanced in female rats (but inhibited in males) by dietary soy phytoestrogens

Background  

In learning and memory tasks, requiring visual spatial memory (VSM), males exhibit superior performance to females (a difference attributed to the hormonal influence of estrogen). This study examined the influence of phytoestrogens (estrogen-like plant compounds) on VSM, utilizing radial arm-maze methods to examine varying aspects of memory. Additionally, brain phytoestrogen, calbindin (CALB), and cyclooxygenase-2 (COX-2) levels were determined.     

Determining partial differential cross sections for low-energy electron photodetachment involving conical intersections using the solution of a Lippmann-Schwinger equation constructed with standard electronic structure techniques

A method for obtaining partial differential cross sections for low energy electron photodetachment in which the electronic states of the residual molecule are strongly coupled by conical intersections is reported. The method is based on the iterative solution to a Lippmann-Schwinger equation, using a zeroth order Hamiltonian consisting of the bound nonadiabatically coupled residual molecule and a free electron. The solution to the Lippmann-Schwinger equation involves only standard electronic structure techniques and a standard three-dimensional free particle Green's function quadrature for which fast techniques exist. The transition dipole moment for electron photodetachment, is a sum of matrix elements each involving one nonorthogonal orbital obtained from the solution to the Lippmann-Schwinger equation. An expression for the electron photodetachment transition dipole matrix element in terms of Dyson orbitals, which does not make the usual orthogonality assumptions, is derived.     

On the construction of quasidiabatic state representations of bound adiabatic state potential energy surfaces coupled by accidental conical intersections: incorporation of higher order terms

The quadratic vibronic coupling model is an important computational tool for simulating photoelectron spectra involving strongly coupled electronic states in polyatomic molecules. However, recent work has indicated the need for higher order terms, with most of the initial studies focusing on molecules with symmetry-required degeneracies. In this study we report an extension of our approach for constructing fully quadratic representations of bound electronic states coupled by conical intersections, which allows for the inclusion of higher order terms, demonstrated here employing a quartic expansion. Procedures are developed that eliminate unphysical behavior for large displacements, a problem likely to be an endemic to anharmonic expansions. Following work on representing dissociative electronic states, Lagrange multipliers are used to constrain the constructed representation to reproduce exactly the energy, energy gradients, and∕or derivative couplings at specific points, or nodes, in nuclear coordinate space. The approach is illustrated and systematically studied using the four lowest electronic states of triazolyl, (CH)(2)N(3).     

Multiplet splittings in the photoelectron spectra of organic radicals: Trimethylenemethane

Open shell self-consistent-field theory has been applied to systems arising from the photoionization of organic radicals. Specifically, the multiplet splittings of the ... 1a′₂le″² electron configuration of C(CH₂)₃⁺ have been investigated both analytically and numerically. These exchange splittings are found to be quite large and should definitely be considered in the interpretation of the photoelectron spectra of organic radicals.     

On the characterization of three state conical intersections using a group homomorphism approach: mapping the full N-5 dimensional seam space

A method for characterizing the degeneracy preserving seam space in the vicinity of a three state conical intersection is introduced. Second order degenerate perturbation theory is used to construct an approximately diabatic Hamiltonian whose eigenenergies and eigenstates accurately describe the vicinity of the three state conical intersection in its full dimensionality. The perturbative analysis enables the large number, 6(N(int)(N(int)+1)2), of unique second order parameters needed to construct this accurate Hamiltonian to be determined from ab initio data at a limited number of nuclear configurations, with (N(int)+10) being minimal. Using the minimum energy three state conical intersection of the pyrazolyl radical (N(int) = 18), the potential of this approach is illustrated. A Hamiltonian comprised of the ten characteristic (linear) parameters and over 1440 second order parameters is constructed and used to determine the locus of the conical intersection seam as well as to describe the 18 dimensional space in the vicinity of that point of intersection. Our results demonstrate the ability of this methodology to quantitatively reproduce the ab initio potential energy surfaces near a three state conical intersection.