Conjugated systems transition

Soret band of the macrocycle, a ti to tt transition, is excited instead. It has been found that the vinyl groups do not participate in the conjugated system [4]. This is based on the fact that the vinyl C=C stretch does not... 

Ultraviolet visible (UV VIS) spectroscopy (Section 13 21) An alytical method based on transitions between electronic en ergy states in molecules Useful in studying conjugated systems such as polyenes... 

The Grignard reagent attacks the unsaturated ketones (3) and (6) from the relatively unhindered ot- or jS-side, respectively, perpendicular to the plane of the conjugated system. An analogous transition state (10) leading to axially substituted 1,6-addition products (11) from A -3-ketones (9) with methylmagnesium halide was suggested by Marshall. ... 

Addition of water across a C=N bond in a conjugated system breaks the conjugation and alters the electronic transitions. The ultraviolet and visible spectra of anhydrous and hydrated species are therefore usually dissimilar, and such differences have been used as the basis for much of the qualitative and quantitative work done on covalent hydration. 

The wavelength necessary to effect the tt — tt transition in a conjugated molecule depends on the energy gap between HOMO and LUMO, which in turn depends on the nature of the conjugated system. Thus, by measuring the UV spectrum of an unknown, we can derive structural information about the nature of any conjugated tt electron system present in a molecule. 

These compounds contain a developed system of conjugated double bonds imparting distinct semiconductor properties on them. Metal ions of variable valency can serve as the central ion M cobalt, nickel, iron, manganese, copper, and so on. In such systems, electron transitions can occur in the conjugated system of the ligands and in the electronic system of the central metal ion. These transitions are the basis for their catalytic activity toward various reactions. 

Ultraviolet-visible spectroscopy (UV = 200 - 400 nm, visible = 400 - 800 nm) corresponds to electronic excitations between the energy levels that correspond to the molecular orbital of the systems. In particular, transitions involving n orbital and ion pairs (n = non-bonding) are important and so UV/VIS spectroscopy is of most use for identifying conjugated systems which tend to have stronger absorptions... 

No new reaction involving cyclic transition states was reported for not fully conjugated systems during the period of time covered. 

With push-pull ethylenes in which the donor part is a cyclic conjugated system with An + 2 ir electrons and/or the acceptor part is one with An tt electrons, the possibility exists for aromatic stabilization of the transition state to C=C rotation. Several such systems with both carbocylic and heterocyclic ring components have been studied. 

Among the various types of possible transitions (a a, a 71, 71 71, 77 —> CT, 77 —> 71 ), the 71 71 transitions are generally the most sensitive to pressure and occur in unsaturated compounds, which are for this reason particularly reactive at high pressure. The lowest energy transition of an unsaturated system is usually a 71 71 transition. The energy shift with pressure of the 71 —> 7t transitions has been reported for several conjugated systems up to 12 GPa [300, 301] and up to 40 GPa, well above the reaction pressure threshold, in the case of benzene and fnran [302, 303]. For anthracene. 

---