Exotic Bonding Regimes Uncovered in Excited States

Abstract

Real-space tools were employedt os how that the chemical bondings cenario used routinely to understand ground states lacks the necessaryf lexibility in excited states. It is shown that, even for two-center,t wo-electron bonds, the real-space bond orders have exotic values that have never been reported. The nature of these situationsw as uncovered by using electron-counting techniques that provide an appealing statistical interpretationo fb ondingd escrip-tors, together with simple physicalm odels. Bond orders greatert han one as well as negative bond ordersf or a single bonding electron pair emergei ns ituations in which the electrons in the pair show ag regarious (bosonic) instead of the usual lonely (fermionic) behavior.I nt he first case the gregariousp air is intra-atomic, whereas the coupling is interatomic in the second. An umber of examples are used to substantiate these claims.[a] J.

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

“…The reduction in , the covalent contribution of the O−H bonding, contrasts with the increase in the DI of this interaction, , as seen in the first row of Table 1. Despite the close connection between DI(A,B) and , the greater DI in the excited state does not indicate a rise in normal covalency, but indicates instead an increase of the zwitterionic character of the interaction, as we have shown in other cases recently [63, 68] . Very briefly, for a zwitterionic resonance, such as H + −H − ↔H − −H + in H 2 , there are only two possibilities for atomic populations, namely, both electrons are located in any of the two interacting H atoms.…”

“…The local energetic effects of the S 0 !S 1 transition in the H 2 Om onomer are summarised in Ta xc ,t he greater DI in the excited state does not indicate ar ise in normalc ovalency,b ut indicates insteada ni ncreaseo ft he zwitterionic character of the interaction, as we have shown in other cases recently. [63,68] Very briefly,f or az witterionic resonance, such as H + ÀH À $H À ÀH + in H 2 ,t here are only two possibilities for atomic populations, namely,b oth electrons are located in any of the two interacting Ha toms. In such ac ase, the fluctuation of the electron population is maximum and the DI rises to two, even for at wo-centred two-electron (2c,2e) chemical bond.…”

“…Unusually large DIs usually indicatethis kind of ionic resonance. [63,65,68] The decrease of E OÀH class is easier to explain:t he transfer of electron density from the oxygen to hydrogen atom reduces the valueso fj q O j and j q H j ,w hich resultsi nareduction of E OÀH class ,t he classical (ionic-like) contribution of the OÀH bond energy.T hese energetic footprints accompanyt he dipole inversion of the S 0 !S 1 excitation of the H 2 Om olecule ( Figure 1).…”

“…In such a case, the fluctuation of the electron population is maximum and the DI rises to two, even for a two‐centred two‐electron (2c,2e) chemical bond. Unusually large DIs usually indicate this kind of ionic resonance [63, 65, 68] . The decrease of is easier to explain: the transfer of electron density from the oxygen to hydrogen atom reduces the values of | q O | and | q H |, which results in a reduction of , the classical (ionic‐like) contribution of the O−H bond energy.…”

On the Nature of Bonding in the Photochemical Addition of Two Ethylenes: C-C Bond Formation in the Excited State?

“…17,18,19 Although less attention has been put on the study and characterization of bonding nature of the underlying reaction paths, the specificities of changes of electron density or derived quantities involving excited states and conical intersections have been the subject of more recent interests. 20,21,[22][23][24][25][26] The characterization of the nature of chemical bonding is undoubtedly at the heart of our understanding of any chemical process with several practical just based on the link between the topology and the local description of chemical bonding implications. 18,27 In the case of the 2+2 ethylene cycloaddition, it should also be remarked that minimum energy conical intersections (MECI) can be consistently derived from both Valence Bond (VB) analysis and ab-initio CASSCF calculations, 7,28 enabling indeed the characterization of the branching space vectors that lift the degeneracy as well as the details of the qualitative phase change (i.e., nucleus displacement vectors) associated to the MECI point.…”