Rydberg states, alkenes

This simple orbital model in Figure 12.25 is not entirely consistent with the available experimental data. Indeed, Kropp called the isolated carbon-carbon double bond "one of the most deceptively simple chromophores available to the organic chemist." ° Even though the end absorption apparently includes the n,ii transition, there is evidence that there may also be weak absorption due to tc —> c and Rydberg transitions. In alkenes, the Rydberg state is thought to result from promotion of an electron from the n orbital to a carbon orbital that has the characteristics of a 3s orbital in helium. 

The Rydberg state of alkene is designated as n, R(3s), to specify that one electron from the n bond remains in the n orbital and the other has been shifted to the Rydberg orbital, which resembles the 3s orbital of a helium atom. The Rydberg state of an alkene is considered to have a partially ionic stmcmre such that a net positive charge remains on a carbon atom. The Rydberg state of ethylene is shown below. 

Although the E-Z isomerization through the 7r,7r state is undoubtedly the major decay process of excited alkenes, several byproducts are also derived from the 7t,R(3s) Rydberg and 7i,a states. These states lie closely in... 

Upon direct irradiation in inert solvents, aliphatic alkenes undergo E Z or other isomerization reactions (Sections 6.1.1 and 6.1.2). The same excited states responsible for such transformations, particularly the Rydberg n,R(.is) singlet state, are involved in nucleophilic addition or photoprotonation reactions in protic media.662 663 671 For example, the jt,R(3s) state of tetramethylethene (93), having cation-radical character of the central bond, is readily attacked by a nucleophile (such as methanol) to form a solvated electron and a radical intermediate 94 that disproportionates to a mixture of the ethers 95 (30%) and 96 (37%) (Scheme 6.37).671 The solvated electrons produced are trapped by the solvent molecules. 

The photoisomerization of acyclic alkenes is well documented in both the direct and the photosensitized experiments (79). Of course, the cis-trans process does not need to be discussed here see above and (58). Let us only recall that in direct photochemistry it has been linked to the skew structure of the ( ir,ir ) excited state (80). The internal methyl shift was attributed to the Rydberg [ rr,R(3s)] electronic state. This process is particularly efficient in the gaseous and liquid phase photoirradiation of tetramethylethylene (81-83) ... 

The radial distribution of this orbital is large in comparison with the 2p orbitals that are the basis set for the n orbital. In essence, this is an orbital around the entire molecule. ° ° Moreover, internal conversion of the n,a, Rydberg, and k,tz states of alkenes is slow. These states may therefore exhibit their own characteristic photochemical reactions, so there can be si hcant wavelength effects on the distribution of photochemical products. ... 

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