Models polyenes

A typical example showing that we are able to build macromolecules at will is given by C. P. Pinazzi and co-workers in the first chapter of the second section, Chapter 27. They report how model polyenes can be built and how they react. In Chapter 28 K. F. O Driscoll illustrates the limitations in polymerization. For every vinyl monomer, a ceiling temperature exists, above which depropagation exceeds polymerization. If two vinyl monomers are copolymerized at a temperature at which one depropa-gates, the polymer formed will have an unusual composition and sequence distribution. 

Chemical Reactions of Model Polyenes and Macromolecular Polyenes... 

Fig. 3.1 7r-Bond orders in the S0 (open circles) and T (filled circles) states of model polyenes with the number of conjugated double bonds, n = 5,7, 9, and 11 calculated by the PPP-SD-CI method, the details which are described in Ref. [4].

A great deal of what we know about the low energy excited states of the carotenoids involved in photobiology can be traced to the high-resolution optical spectroscopy of model polyenes. The organization of this chapter parallels the recent evolution of the field and, in large part, follows the... 

The major differences between the optical spectroscopy of model polyenes and short apo-carotenes and the longer, more conjugated carotenoids employed in photobiology are illustrated in Fig. 5. Most striking is the aoss-over from the S,(2 Ag) So(l A ) emission observed for the apo-heptaene (7c) to die dominant, S,( 1 B ) - So( 1 Ag) fluorescence of the analogous apo-nonaene (9). Similar changes... 

In order to examine possible conformational differences between the different classes of poly(RCOT)s, both force-field (MM2) [132] and semiempirical quantum mechanical (AMI) [133] calculations on model polyene oligomers were employed (Fig. 10-23) [34]. Using either routine, it is observed that the single bonds adjacent to the trisubstituted double bond (0] and 2) both strongly deviate from planarity in the models of the soluble polymers. In contrast, in the model of trans- poly (terr-butoxyCOT), an insoluble polymer, j is large, but 2 is not. The good correlation between polymer solubility and calculated chain twist is indicated... 

Figure 10-23 The twist angles, 0, and 2, in the model polyene used for molecular mechanics calculations [34].

We outline the CASVB strategy, which may be used either to generate very compact modem valence bond representations of CASSCF wave functions or to optimize general types of modem VB wave function. Various aspects of the methodology are illustrated by means of applications to the ground state of benzene, to the X A and a A state of FeH, and to the two lowest Ag states of various model polyene systems. 

TABLE V. CASSCF and CASVB results for Ag roots of the model polyene system with c=I0. 

TABLE VI. Overlap integrals between (normalized) modem VB n orbitals for the model polyene system with c=10. Values in the lower triangle relate to the (l) Ag root and those in the upper triangle to the (2) A root. 

Figure 4 Symmetry-unique modem VB 7t orbitals for the (2) Ag root of the model polyene system with ic=10. The representation is analogous to that in Figure 1.

As we have indicated, the CASVB procedures may also be used for the fully-variational optimization of modem VB wave functions. This allows us to investigate for the ground states of these model polyene systems the extent to which the modem VB representations obtained from the CASSCF wave functions by means of the overlap criterion resemble the corresponding flilly-variational spin-coupled results. The energies from such flilly-variational calculations, which include full optimization of the core orbitals, are labelled SC in Table VIII. All of the CASVB wave functions were found to be symmetry pure, without the need to impose any symmetry relationships amongst the orbitals. 

TABLE Vin. CASVB and CASSCF energies (in hartree) for (l) Ag roots of the model polyene systems. 

Synthesis and Reactions.—Carotenoids. Two papers report the synthesis of model polyenes related to naturally occurring carotenoids. Condensation of the two Cio reagents (31) and (32) gave the /3-oxosulphone (33) which was converted... 

Based on the results shown in the table, one can conclude that the largest optical nonlinearity occurs in the longest isomer, system A. This fact confirms the conclusion made for the linear model polyene (Fig. 3.8) and based on the comparison involving its different conformers. The conclusion was generalized for a larger class of condensed aromatic hydrocarbons. 

Szymusiak et al report results on model polyenes with modified indanone groups, studied by DFT, HF/3-21G ab initio and semi-empirical methods. 

The decaene bipolaron delocalization in these series extends over a maximum of 30 atoms and is the longest model polyene bipolaron yet observed. The absorption characteristics for both the neutral and oxidatively doped dithienylpolyenes are listed in Table 9.2. 

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