Showing papers by "Thomas Bally published in 2014"

Vibronic spectra of the p-benzoquinone radical anion and cation: a matrix isolation and computational study

Abstract: The electronic and vibrational absorption spectra of the radical anion and cation of p-benzoquinone (PBQ) in an Ar matrix between 500 and 40000 cm−1 are presented and discussed in detail. Of particular interest is the radical cation, which shows very unusual spectroscopic features that can be understood in terms of vibronic coupling between the ground and a very low-lying excited state. The infrared spectrum of PBQ˙+ exhibits a very conspicuous and complicated pattern of features above 1900 cm−1 that is due to this electronic transition, and offers an unusually vivid demonstration of the effects of vibronic coupling in what would usually be a relatively simple region of the electromagnetic spectrum associated only with vibrational transitions. As expected, the intensities of most of the IR transitions leading to levels that couple the ground to the very low-lying first excited state of PBQ˙+ increase by large factors upon ionization, due to “intensity borrowing” from the D0 → D1 electronic transition. A notable exception is the antisymmetric CO stretching vibration, which contributes significantly to the vibronic coupling, but has nevertheless quite small intensity in the cation spectrum. This surprising feature is rationalized on the basis of a simple perturbation analysis.

One-Pot Synthesis and AFM Imaging of a Triangular Aramide Macrocycle

Abstract: Macrocyclizations in exceptionally good yields were observed during the self-condensation of N-benzylated phenyl p-aminobenzoates in the presence of LiHMDS to yield three-membered cyclic aramides that adopt a triangular shape. An ortho-alkyloxy side chain on the N-benzyl protecting group is necessary for the macrocyclization to occur. Linear polymers are formed exclusively in the absence of this Li-chelating group. A model that explains the lack of formation of other cyclic congeners and the demand for an N-(o-alkoxybenzyl) protecting group is provided on the basis of DFT calculations. High-resolution AFM imaging of the prepared molecular triangles on a calcite(10.4) surface shows individual molecules arranged in groups of four due to strong surface templating effects and hydrogen bonding between the molecular triangles.

The Primary Steps in Excited‐State Hydrogen Transfer: The Phototautomerization of o‐Nitrobenzyl Derivatives

Abstract: The quantum yield for the release of leaving groups from o-nitrobenzyl “caged” compounds varies greatly with the nature of these leaving groups, for reasons that have never been well understood We found that the barriers for the primary hydrogen-atom transfer step and the efficient nonradiative processes on the excited singlet and triplet surfaces determine the quantum yields The excited-state barriers decrease when the exothermicity of the photoreaction increases, in accord with Bell–Evans–Polanyi principle, a tool that has never been applied to a nonadiabatic photoreaction We further introduce a simple ground-state predictor, the radical-stabilization energy, which correlates with the computed excited-state barriers and reaction energies, and that might be used to design new and more efficient photochemical processes