Energetic Considerations—Resonance

We note that the cyclopropanation of ketene to form cyclopropanone, isoelectronically and otherwise formally related to the cyclopropanation of allene to form methylenecyclopropane, has an altogether different enthalpy of reaction. This value is (63 4) kJmoT favoring ketene and is credible in terms of considerable resonance stabilization of the type CH2=C=0 CH2—C=0 and of a 3-atom n system that includes the double bond and the appropriate, antisymmetric (i.e. difference) combination of the oxygen lone pairs. Further documentation of the stabilization of ketene is seen by comparing the energetics of the hydrogenation reaction... 

Contrary to 6T, no indications for an additional luminescence from deep traps are found in the 4T spectra. The reason for this difference between 4T and 6T may be that in the case of 4T one type of structure is energetically more strongly favored than in the case of 6T, possibly due to the reduced molecular size of 4T. In agreement with the above finding, the 0-0 transition of 4T in photoluminescence (21,025 cm-1) is observed to be in resonance with the lowest line observed in absorption (21,035 cm-1). Since the lines have a considerable overlap, the Stokes shift is below 10 cm-1. 

These energetic relationships are understandable on the basis of rigorous quantum mechanical calculations, which justify the view that in both difluorides there is considerable ionic character. The bonding can be more simply represented by the resonance picture ... 

AuNPs inserted between the electrode surface and redox metalloproteins therefore both work as effective molecular linkers and exert eflfident electrocatalysis. Recent considerations based on resonance turmeling between the electrode and the molecule via the AuNP as a mechanism for enhanced interfadal ET rates suggest that electronic spillover rather than energetic resonance is a hkely origin of the effects (J. Kleis et al., work in progress). Even slightly enhanced spillover compared with a planar Au(lll) surface is enough to enhance the ET rate by the observed amount over a 10-15 A ET distance. 

The bond distances in the hydrazone group indicated a considerable TT-electron delocalization in the system because of the contribution of resonance structure, which are energetically preferred to bicyclic system 59. It was then concluded that a disruption of the furanoid ring follows the introduction of a hydrazone residue at C-2, which induces a rr-electron delocalization in the system. This was due to the sp2 hybridization at C-3, which precludes the necessary fourth valence for ring formation (82MI9). 

When S and T refer to two mixed vibrational levels of the same electronic state, the zero-pressure splittings of levels of S from those of T can be considerable, for instance tens of wavenumbers. Thus any substantial nonresonant process must have an additional term on the right in (3.22) to account for energetically unfavorable circumstances. Despite the differences, the experimental evidence to date points to very rapid collisional transitions between Fermi resonance-mixed vibrational levels in accord with the above theoretical discussions. 

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