A portfolio of photochemical transformations found in a variety areas can be rationalized through consideration either gain 4nπ-electron aromaticity or relief (4n+2) π-electron antiaromaticity excited states ππ* character. These back-of-an-envelope tools provided by Baird's rule serve as compass for excursions electronically states. More information the Review H. Ottosson and co-workers (DOI: 1...

Electronic structure methods are used to estimate differences in reaction barriers for transfer of an electron from singlet ππ* excited 8-aminoguanine (A) or deprotonated 8-aminoguanine anion (A(-)) to a proximal thymine dimer site compared to barriers when ππ* excited 8-oxoguanine (O) or deprotonated 8-oxoguanine (O(-)) serve as the electron donor. It is predicted that the barrier for photoexc...

This article describes our recent experimental studies on internal conversion via a conical intersection using photoelectron spectroscopy. Ultrafast S2(ππ*)-S1(nπ*) internal conversion in pyrazine is observed in real time using sub-20 fs deep ultraviolet pulses (264 and 198 nm). While the photoelectron kinetic energy distribution does not exhibit a clear signature of internal conversion, the ph...

A correct understanding and description of the I = 2 ππ S-wave interaction is important for the extraction of the I = 0 ππ S-wave interaction from experimental data and for understanding the I = 0 ππ S-wave interaction theoretically. With t-channel ρ, f 2 (1270) exchange and the ππ → ρρ → ππ box diagram contribution, we reproduce the ππ isotensor S-wave and D-wave scattering phase shifts and in...

The recent observation of B d → ππ and B̄ d → ππ decay modes allows us to make a fresh isospin analysis of B → ππ transitions. We find that current experimental data can impose some model-independent constraints on the parameter space of direct CP violation in Bd → ππ and Bd → ππ. Furthermore, we establish a direct relationship between the weak phase α and the charge-averaged branching fractions...

We study the exclusive production of ππ and ρπ in hard γγ scattering in the forward kinematical region where the virtuality of one photon provides us with a hard scale in the process. The newly introduced concept of Transition Distribution Amplitudes (TDA) is used to perform a QCD calculation of these reactions thanks to two simple models for TDAs. Cross sections for ρπ and ππ production are ev...

We have searched for two-body charmless hadronic decays of B mesons. Final states include ππ, Kπ, and KK with both charged and neutral kaons and pions; πρ, Kρ, and Kπ; and Kφ, Kφ, and φφ. The data used in this analysis consist of 2.6 million BB̄ pairs produced at the Υ(4S) taken with the CLEO-II detector at the Cornell Electron Storage Ring (CESR). We measure the branching fraction of the sum of...

Using a combination of photoelectron spectroscopy measurements and quantum chemistry calculations, we have identified competing electron emission processes that contribute to the 350-315 nm photoelectron spectra of the deprotonated green fluorescent protein chromophore anion, p-hydroxybenzylidene-2,3-dimethylimidazolinone. As well as direct electron detachment from S0, we observe resonant excit...

The significance of final-state interactions inBd → ππ decays is quantitatively demonstrated by taking elastic ππ ⇀↽ ππ and inelastic ππ ⇀↽ DD̄ rescattering effects into consideration. We find that the present experimental data on B d → ππ can well be understood in this approach without fine-tuning of the input parameters, and large CP-violating asymmetries are expected to manifest themselves in...