On the nature of bonding in the photochemical addition of two ethylenes: C-C bond formation in the excited state?

In this work, the 2s + 2s (face-to-face) prototypical example of a photochemical reaction has been re-examined to characterize the evolution of chemical bonding. The analysis of the electron localization function (as an indirect measure of the Pauli principle) along the minimum energy path provides strong evidence supporting that CC bond formation occurs not in the excited state but in the ground electronic state after crossing the rhombohedral S₁/S₀ conical intersection.

Photochemistry in Real Space: Batho- and Hypsochromism in the Water Dimer

The development of chemical intuition in photochemistry faces several difficulties that result from the inadequacy of the one-particle picture, the Born-Oppenheimer approximation, and other basic ideas used to build models. It is shown herein how real-space approaches can be efficiently used to gain valuable insights in photochemistry through a simple example of red and blue shift effects: the double hypso- and bathochromic shifts in the low-lying valence excited states of (H₂ O)₂ . It is demonstrated that 1) the use of these techniques allows the perturbative language used in the theory of intermolecular interactions, even in the strongly interacting short-range regime, to be maintained; 2) one and only one molecule is photoexcited in each of the addressed excited states and 3) the electrostatic interaction between the in-the-cluster molecular dipoles provides a fairly intuitive rationalisation of the observed batho- and hypsochromism. The methods exploited and illustrated herein are able to maintain the individuality and properties of the interacting entities in a molecular aggregate, and thereby they allow chemical intuition in general states, at any geometry and using a broad variety of electronic structure methods to be kept and built.

Theoretical study of the absolute inner-shell photoionization cross sections of the formic acid and some of its hydrogen-bonded clusters

Inner-shell absolute photoabsorption and photoionization cross sections of the formic acid, HCOOH, and its small hydrogen-bonded clusters, i.e., (HCOOH)₂, HCOOH₂ ⁺, HCOHOH⁺, and HCOOH·H₃O⁺, were calculated at the time-dependent density functional theory (TDDFT) level, and the results were used to analyze the effect of the formic acid clustering on the carbon and oxygen K-edge photoionization cross sections. The discrete electronic pseudospectra obtained with square-integrable (L²) basis set calculations were used in an analytic continuation procedure based on continued fraction functions to obtain the photoabsorption cross sections. Symmetry adapted cluster configuration interaction calculations on the small formic acid clusters have also been performed at the oxygen K-edge to assign the discrete transitions and ionization potentials in support to the TDDFT results.