Polyenylic cations

These results emphasise the important role played by HC1 not only as a catalyst for the dehydrochlorination process but in influencing the distribution of polyene sequences which result from the primary part of the degradation process and the photochemical cross-linking reactions of the polyenylic cations. 

Evidence has accumulated from various sources which supports the idea that polyenylic cations are implicated in other aspects of PVC degradation. Molecular orbital calculations carried out by Starnes (45) show that the charge is better stabilized at the center of the formal delocalized length of the ion than at the end and that since the process represented by equation 16 becomes more favorable with increasing sequence length, this may provide an explanation for the relatively short sequence lengths. 

Sorensen and co-workers242 have studied the stereochemistry of ring closure of arylallyl cations to bicyclic trienyl cations. Similar studies on 1-phenylallyl cations have been carried out by Olah et al.225 Persistent polyenylic cations generated upon incorporation of cinnamyl alcohols in zeolitic matrix have been identified on the basis of IR and UV-visible spectroscopy 243... 

Generation of several other allylic and polyenylic cations as a result of a rather unusual dehydrofluorination reaction of the corresponding polyfluoro-alkane by SbF5 has been recently reported [76] ... 

There are a variety of methods for the computation of the MOs that interact in the transition states of [4+2]-cycloadditions. The LCAO method (linear combination of atomic orbitals) is often employed, and the basic idea is as follows. The MOs of the -systems of alkenes, conjugated polyenes, or conjugated polyenyl cations, radicals, or anions all are built by so-called linear combinations of 2p AOs. In a somewhat casual formulation, one might say that the MOs of these -systems are constructed with the help of the 2pz AOs. These AOs are centered at the positions of the n C atoms that are part of the -system. LCAO computations describe a conjugated -electron system that extends over n s/ 2-hybridized C atoms by way of n Ji-type MOs. 

A retinylic cation (Amilx = 598 nm) was proposed by Blatz et al. [212] to explain the long wavelength absorption maxima of visual pigments. This model was, however, rejected by the same group later [213] on the basis of further experimental data collected on the absorption spectral properties of polyenyl cations. 

II. Electronic Structures, Equilibrium Geometries, and Energetics of the Ground States of Polyenyl Cations and Neutral Polyenes... 

It is this mechanism which forms the primary focus of the present study, and we shall be particularly concerned with the effects of unsaturation on the electronic structures, equilibrium geometries, and energetics of the polyenyl cations and the polyenes derived therefrom. Some of the more practical aspects of the degradation and stabilization of PVC will then be discussed in terms of our results. The question of initiation by defect structures will be considered elsewhere, and we shall be mainly concerned in this work with the propagative or "unzipping phase of the mechanism. 

Most of the limited amount of experimental evidence which is available on the ground states of polyenyl cations comes from NMR solution studies (60,61,62), and even here the investigations have been hampered by competing cyclization to cyclopentenyl cations (60,61,62). 

Geometries. The optimized structures for the polyenyl cations are shown in Table IV the bond length found for the allyl cation is in good agreement with values derived from previous theoretical studies (51-59). Our calculations predict that there is a plane of symmetry which bisects the polyenyl chains at the central carbon atom, perpendicular to the path of conjugation. We find no evidence for the distortion shown in Equation 3,... 

Table IV. STO-3G Calculated Bond Lengths for Linear Polyenyl Cations...

Figure 3. Idealization of resonance forms found for linear polyenyl cations...

---