Volume 133, Issue 9, 07 September 2010
Index of content:
band in the absorption spectra of porphyrin nanotubes: Vibronic coupling or exciton effects?" title="Shape of the band in the absorption spectra of porphyrin nanotubes: Vibronic coupling or exciton effects?" />
Absorption and linear dichroism spectra of self-assembled tubular aggregates of porphyrin are studied theoretically with special emphasis on the low energy part of the spectra (the band region) where the coupling with intramolecular vibrations is pronounced. The model Hamiltonian includes both the excitonic coupling between four molecular electronic excited states contributing to the porphyrin and bands as well as the intermediate-strength linear exciton-phonon coupling to one effective high-frequency molecular vibrational mode. Good agreement between the calculated and experimental spectra is obtained. The results allow us to identify the nature of the peaks observed in the band region of the aggregate’s absorptionspectrum; we show that the two most prominent peaks within the band originate from two different excitonic subbands. It is shown that the coupling between the and bands plays an important role and the vibronic coupling affects the details of the absorption lineshape.
Communication: Novel quantum states of electron spins in polycarbenes from ab initio density matrix renormalization group calculations133(2010); http://dx.doi.org/10.1063/1.3476461View Description Hide Description
An investigation into spin structures of poly(-phenylenecarbene), a prototype of magnetic organic molecules, is presented using the ab initio density matrix renormalization group method. It is revealed by achieving large-scale multireference calculations that the energy differences between high-spin and low-spin states (spin-gaps) of polycarbenes decrease with increasing the number of carbene sites. This size-dependency of the spin-gaps strikingly contradicts the predictions with single-reference methods including density functional theory. The wave function analysis shows that the low-spin states are beyond the classical spin picture, namely, much of multireference character, and thus are manifested as strongly correlated quantum states. The size dependence of the spin-gaps involves an odd-even oscillation, which cannot be explained by the integer-spin Heisenberg model with a single magnetic-coupling constant.
Communication: Resolving the vibrational and configurational contributions to thermal expansion in isobaric glass-forming systems133(2010); http://dx.doi.org/10.1063/1.3481441View Description Hide Description
A fundamental understanding of isobaric thermal expansion behavior is critical in all areas of glass science and technology. Current models of glass transition and relaxation behavior implicitly assume that the thermal expansion coefficient of glass-forming systems can be expressed as a sum of vibrational and configurational contributions. However, this assumption is made without rigorous theoretical or experimental justification. Here we present a detailed statistical mechanical analysis resolving the vibrational and configurational contributions to isobaric thermal expansion and show experimental proof of the separability of thermal expansion into vibrational and configurational components for Corning Jade®glass.
Communication: Photodissociation of —Frustrated NN bond breaking causes diffuse vibrational structures133(2010); http://dx.doi.org/10.1063/1.3479391View Description Hide Description
The photodissociation of is studied by wave packet calculations using a global three-dimensional potential energy surface for the first excited state. It is shown that the weak vibrational structures of the absorption cross section are caused by large-amplitude NN stretch motion, combined with strong excitation of the bend as well as the O–NN stretch. Weakening of the NN bond toward the channel is the necessary prerequisite.